Proceedings Volume 2696

Spectroscopic Studies of Superconductors

Ivan Bozovic, Dirk van der Marel
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Proceedings Volume 2696

Spectroscopic Studies of Superconductors

Ivan Bozovic, Dirk van der Marel
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 10 June 1996
Contents: 7 Sessions, 70 Papers, 0 Presentations
Conference: Photonics West '96 1996
Volume Number: 2696

Table of Contents

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Table of Contents

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  • Tunneling Spectra: Experimental Studies
  • Raman Spectra
  • Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
  • Infrared Spectra: Theoretical Studies
  • Infrared Spectra: Experimental Studies
  • Tunneling Spectra: Experimental Studies
  • Tunneling Spectra: Theoretical Studies
  • Tunneling Spectra: Experimental Studies
  • Raman Spectra
  • Infrared Spectra: Theoretical Studies
  • Raman Spectra
  • Infrared Spectra: Theoretical Studies
  • Tunneling Spectra: Theoretical Studies
  • Infrared Spectra: Experimental Studies
  • Photoelectron Spectra
  • Raman Spectra
  • Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
  • Infrared Spectra: Experimental Studies
  • Infrared Spectra: Theoretical Studies
  • Tunneling Spectra: Theoretical Studies
  • Photoelectron Spectra
  • Raman Spectra
  • Photoelectron Spectra
  • Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
  • Photoelectron Spectra
  • Tunneling Spectra: Theoretical Studies
  • Photoelectron Spectra
  • Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
  • Infrared Spectra: Experimental Studies
  • Tunneling Spectra: Experimental Studies
  • Raman Spectra
  • Infrared Spectra: Experimental Studies
  • Raman Spectra
  • Tunneling Spectra: Theoretical Studies
  • Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
  • Raman Spectra
  • Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
Tunneling Spectra: Experimental Studies
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Scanning tunneling microscopy and spectroscopy of the chain layer in YBa2Cu3O7-x
Alejandro L. de Lozanne, S. H. Pan, Hal L. Edwards, et al.
We review our recent results on the surface structure and spectroscopy of the chain layer of YBa2Cu3O7-x obtained with a low temperature scanning tunneling microscope. Foremost is the discovery of a long wavelength (approximately 1.3 nm) modulation of the electronic density along the CuO chains, which has now been confirmed by neutron diffraction to also exist in the bulk. Spectroscopically, we observe an energy gap (20 - 25 meV) which disappears near oxygen vacancies. We also give experimental details not previously published.
Josephson tunneling in high-Tc superconductors
Adrian G. Sun, A. S. Katz, Andrew G. Truscott, et al.
Investigation of the overlap of the pair wavefunction of a conventional superconductor with that of a high Tc cuprate should reveal the symmetry of the superconducting order parameter of the cuprate. We have been studying this overlap by Josephson tunneling (tunneling as opposed to SNS or weak link) between conventional superconductors (Pb, Sn) and YBCO. The observation of Josephson coupling in the c direction implies that the order parameter in YBCO is not strictly of dx2-y2 symmetry. We have carried out studies on YBCO in the form of twinned and detwinned single crystals and thin films on various substrates. The magnetic field dependence of the Josephson current Ic(B) allows a direct, dc, and low field measurement of the low temperature penetration depth (lambda) ab. Measurements of (lambda) a and (lambda) b in detwinned single crystals of YBCO give the value of in-plane anisotropy ratio in this material.
Quasi-particle tunneling spectroscopy of high-Tc cuprates
John Zasadzinski, L. Ozyuzer, Z. Yusof, et al.
Superconductor-insulator-normal metal (SIN) and superconductor-insulator-superconductor (SIS) tunnel junctions provide important information on pairing state symmetry and mechanism. Measurements of such junctions on high Tc superconductors (HTS) are reported using mechanical point contacts, which generally display the optimum characteristics that can be obtained from HTS native-surface tunnel barriers. New tunneling data on the infinite-layer cuprate, Sr1-xNdxCuO2 are reported which show a remarkable similarity to another electron-doped cuprate, Nd1.85Ce0.85CuO4. In particular, there is a strong, asymmetric linear background conductance that is indicative of inelastic tunneling from a continuum of states. A discussion is given of the anomalous 'dip' feature found in the tunneling and photoemission data on BSCCO 2212. It is shown that a similar feature is found in many cuprate junctions and that this dip scales with the gap energy over a wide range. New data on the single-layer, tetragonal cuprate, Tl2Ba2CuO6 (Tl2201) are presented and discussed in light of recent published results on the similar compound HgBa2CuO4 (Hg1201). The Hg1201 data display a low, flat sub-gap tunneling conductance which is consistent with a BCS density of states whereas the Tl2201 data display a cusp-like feature at zero bias which is more consistent with dx2-y2 symmetry.
Quasi-particle transport in microjunctions employing normal metal/superconductor interfaces in the presence of a magnetic field
Francesco Tafuri, Alessandro Di Chiara, Fabrizio Fontana, et al.
Quasiparticle transport in superconductor/normal metal-constriction-normal metal (S1/N1-c-N2) microjunctions is investigated. Concepts of proximity effect and Andreev reflection are used in the model we propose. Attention is focused in particular on the influence of the magnetic field on zero bias anomaly (ZBA) phenomena observed in conductance spectra. Among the results, we find that the zero bias conductance is almost insensitive to the applied magnetic field in the metallic regime of a microjunction. Comparisons with experimental results are carried out.
Raman Spectra
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Localized states within 2 eV of Ef in YBa2Cu3O7-d and deduced from the carrier relaxation dynamics
Tomaz Mertelj, Jure Demsar, Bostjan Podobnik, et al.
Temperature dependence of the relaxation of photoexcited (PE) carriers is used as a probe of the electronic structure of the high-temperature superconductor YBa2Cu3O7- (delta ) ((delta) approximately equals 0.1). The relaxation process is studied by 'counting' -- through measurement of the Raman scattering Stokes/anti-Stokes intensity ratio -- the phonons emitted in the process of carrier energy relaxation. The phonon 'shake-off' is found to be strongly temperature dependent, implying that the PE carrier relaxation proceeds via a temperature activated process, which can be understood in terms of hopping between localized states. The long PE carrier lifetime and temperature dependence of the relaxation process implies the existence of localized states within 2 eV of the Fermi energy in optimally doped high-Tc superconductor.
Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
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Defect mechanism of photoinduced superconductivity in YBa2Cu3Ox
Denise C. Chew, John F. Federici, J. Gutierrez-Solana, et al.
Persistent photoconductivity and photoinduced superconductivity have been previously observed in various stoichiometries of YBa2Cu3Ox. The physical mechanism which is responsible for these effects is still under debate. Through an investigation of laser ablated YBa2Cu3Ox thin film compositions, photoluminescence spectra and infrared spectroscopy, the possibility that the mechanism may be defect related has been supported. A correlation between photoluminescence spectra and wavelength dependence support an oxygen defect model of photoinduced persistent conductivity and superconductivity in which the oxygen vacancies act as weakly luminescent F-centers and F+-centers under illumination. Upon infrared illumination the trapped electrons are photoexcited resulting in a partial quenching of the persistent photoconductivity state.
Optical studies of dye-coated superconductor junctions
Steven M. Savoy, Sara J. Eames, David C. Jurbergs, et al.
The preparation and characterization of a new generation of optical sensors fabricated from high-temperature superconductor (HTSC) thin films is reported herein. These new hybrid devices are fashioned using HTSC thin films which are coated with organic dye overlayers. These systems are shown to respond selectively to those wavelengths which are absorbed strongly by the molecular dye. Methods for fabricating the superconductor element and depositing the dye layer are discussed. Moreover, resistivity versus temperature measurements before and after dye deposition are utilized to characterize these hybrid structures. The unique optical response properties of these hybrid sensors are also detailed.
Infrared Spectra: Theoretical Studies
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Gap anisotropy and van Hove singularities in high-Tc superconductors
Julien Bok, Jacqueline Bouvier
We compute the superconducting gap (Delta) k using a simple band structure of the CuO2 planes in the high Tc materials. We suppose that for materials with doping corresponding to maximum Tc, the van Hove singularities lie close to the Fermi level as is confirmed by many photoemission experiments. We use an electron-phonon interaction with weak screening, we find a strong gap anisotropy. For Bi 2212, (Delta) is maximum along the 100 an 010 directions with values between 20 and 30 meV and minimum along 110 with values between 0 and 10 meV. We use this anisotropic gap to compute the quasi-particle excitations density of states and the tunneling current-voltage I(V) characteristic for N-I-S and S-I-S junctions. This model agrees remarkably well with recent experiments of tunneling spectroscopy in high Tc cuprates.
Plasma oscillations in layered superconductors
Valery L. Pokrovsky
Very soon after publication of the famous BCS work explaining the puzzle of the superconductivity, N. N. Bogolyubov and coworkers proposed their version of the theory. One of the new results they obtained was the discovery of a collective mode -- an oscillation of the Cooper pair density with the energy smaller than 2(Delta) . P. W. Anderson has indicated that this collective mode cannot be found experimentally since the Coulomb forces neglected in the above mentioned work shift their energy to the plasma frequency, i.e., to the high- ultraviolet range in which the superconductivity is inessential. The interest for plasmons in superconductors revived a little in the search for the mechanism of the High-Tc superconductivity. However, a real surge of interest to this problem occurred after experimental observations of the plasma edge in the reflectivity of High-Tc superconductors La2-xSrxCuO4 and YBa2Cu3O8-y. It is worthwhile to mention that a theoretical prediction preceded the experiment. For external reasons, the paper was published only a long time after its completion. Here we present a brief review of the experiments and theoretical developments in the field. The theoretical works are presented in more detail, given the authors' specialization. The content of the review is as follows. In the second section we review the most general relationships for plasma frequency (PF) and dielectric function of the electron plasma in normal metals and their modification for layered normal metals. In the third section we review the experimental observations of the plasma edge in High-Tc superconductors (HTSC). In the fourth section a simple two-fluid model of the plasma oscillations is presented. The fifth section is devoted to the BCS theory of the same phenomenon. Peculiarities of Josephson-linked layered superconductors are considered in Section 6. The experimental observations of the resonance plasma attenuation are discussed in Section 7. Mechanisms of dissipation in the Josephson-linked superconductors are discussed in Section 8. A novel effect of selectional transparency is discussed in Section 9. The conclusions are collected in Section 10.
Infrared Spectra: Experimental Studies
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Doping and charge-carrier density effects in the cuprate superconductors
David B. Tanner, Young-Duck Yoon, Axel Zibold, et al.
The undoped phases of the copper-oxide materials are antiferromagnetic insulators, with a gap of 1.5 - 2 eV. Infrared spectroscopy of these compounds reveals weak absorption, possibly of magnetic origin, in this gap. When the materials are doped, oscillator strength is removed from the charge transfer band. This oscillator strength moves to low frequency, to become midinfrared and free carrier absorption. A systematic study of the electron-doped Nd2- xCexCuO4-y system reveals that the growth of low-frequency oscillator strength with doping concentration x is twice as rapid as in the case of hole-doped materials, such as La2-xSrxCuO4. This behavior is in accord with electronic structure models based on the 3-band Hubbard model and inconsistent with one-band behavior. However, an anomaly occurs for samples which are doped to the critical concentration for superconductivity; these have a greater than expected free-carrier concentration and weaker charge-transfer bands.
Unconventional electrodynamics of high-Tc and organic superconductors
Thomas Timusk, N. Cao, D. N. Basov, et al.
The combination of lowered dimensionality and electron-electron correlations are responsible for the unusual temperature and frequency dependence of the electrical conductivity of the new superconductors. We first review the electrodynamics of two systems, U2Ru2Si2 and Sr2RuO4 where conventional Fermi liquid ideas seem to work. Here transport is by free carriers with strongly renormalized masses. On the other hand the electrodynamics of the high Tc cuprates and the organic charge transfer salts is unconventional. The high Tcs show a Drude peak with an anomalous temperature and frequency dependent scattering rate for the in-plane conductivity, while normal to the planes they are almost insulating. In the organics, the transport currents are carried by a narrow collective mode coupled to phonons.
Determination of the penetration depth in YBa2Cu3O7 thin films from far-infrared transmission
Nicole Bontemps, Louis-Anne de Vaulchier
Transmission measurements in the range 130 GHz - 500 GHz on YBa2Cu3O7- (delta ) epitaxial thin films are reported and discussed. We deduce the absolute value of the electromagnetic penetration depth (lambda) (T) from the frequency variation of the transmission. We concentrate on our highest quality film, which yields (lambda) (OK) equals (1570 plus or minus 200 angstrom), associated with a linear temperature dependence of (lambda) (T) which maps at low temperature (T less than 20 K) the data on high quality single crystals. We derive the real part of the conductivity from the small difference between the film and the single crystal data.
Measurement of c-axis optical absorptivity of La1.87Sr0.13CuO4
John T. Birmingham, Sabrina M. Grannan, Paul L. Richards, et al.
We have directly measured the c-axis optical absorptivity of a large single crystal of La1.87Sr0.13CuO4 between 9 and 80 cm-1 at 2 K using Fourier transform spectroscopy and an absorbed power technique. We seen an absorptivity onset at 41 cm-1, which is 2 - 3 cm-1 higher than that measured at 6 K with reflectance on the same sample, suggesting that a significant fraction of the electrons remain unpaired at 6 K. The absorption increases with frequency below the onset, increasing from 0.3% at 9 cm-1 to 8% at 40 cm-1. Our data are particularly significant at frequencies below the plasma edge, for which the reflectance is close to unity.
Decoupling the superconducting condensate from phonons in the far-infrared spectra of YBa2Cu3O7-x
Ricardo P. S. M. Lobo, Francisco J. Gotor, Philippe Odier, et al.
The infrared reflection spectra of a YBa2Cu3O7-(delta ) single crystal with a large surface containing the c-axis have been recorded from 4 k to room temperature. Thermal treatment of the sample allowed us to obtain spectra for several Tcs, ranging from 50 to 91.3 K on the same sample. The obtained results along the c-axis show an extra band in the superconducting phase which can be attributed to the formation of Cooper pairs. By decoupling this extra absorption from phonons and unpaired charge carries, we have determined the thermal evolution of the London penetration depth and the concentration of pairs along the c-axis as a function of Tc. The observed effects are compared to the BCS theory.
Tunneling Spectra: Experimental Studies
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Phonon structure in tunneling conductance as a possible key to the mechanism of high-Tc superconductivity
A. I. Dyachenko, Vladimir M. Svistunov
An influence of anisotropy on phonon structure in d2I/dV2(V) curve was examined by direct solving of the Eliashberg equations. The studies include the s- and d-types of pairing symmetry as well as scattering on impurities. As is revealed the phonon structure in the second derivative remains isotropic and should be found at eV equals $HBAR(omega) plus <(Delta) > where (omega) is the phonon frequency and the effective gap <(Delta) > is close to the maximum value of (Delta) (k) in the case of strong anisotropy of energy gap (Delta) (k). The smearing of phonon structure could be taken into account by using a complex gap function (Delta) ((omega) +i(Gamma) ) in the Dynes' formula. The influence of the localized states on the phonon structure in d2I/dV2 is discussed.
Tunneling density of states in a-axis YBCO films
Guy Deutscher, Dany Racah
We report a novel and simple method for the fabrication of high quality a-axis YBCO/insulator/normal metal junctions. The insulating layer is formed by in-situ oxidation of a metallic layer having a strong electronegativity (in the present case Al) deposited on the YBCO surface. The junction resistance shows an exponential dependence on the Al thickness over several decades of resistance with values consistent with the known resistance of high quality tunnel junctions, when the Al film is fully oxidized. The thermal and mechanical stability of the junctions, the high yield of the method and its simplicity are very promising for future applications and for the study of the underlying physics. Because some of the oxygen content of the YBCO film is used to oxidize the Al layer, the YBCO film underneath the junction tends to be in the underdoped regime. We discuss this result in terms of strong coupling effects and in light of the theory of Emery and Kivelson, who have predicted that in the underdoped regime there exists a mean field Tc much larger than the measured one, which determines the value of the superconducting gap. We believe that the proposed method is general enough to be used for the preparation of junctions on all high Tc oxides.
Electronic structure of individual flux lines in YBa2Cu3O7-delta
Christophe Renner, I. Maggio-Aprile, A. Erb, et al.
We present images of the flux line lattice and spectroscopic measurements of the vortex core region on the as grown (001) surface of an YBa2Cu3O7-(delta ) single crystal at 4.2 Kelvin. These measurements were done in a magnetic field of 6 Tesla applied parallel to the c-axis using scanning tunneling microscopy. We find an oblique vortex lattice with nearly equal primitive vectors at an angle of 77 degrees. The vortex cores are ellipsoidal, consistent with the ab-plane anisotropy of YBCO. These characteristics can be fit to a distorted square lattice rotated 45 degrees with respect to the YBCO crystal lattice. The tunneling spectroscopy into the vortex core region reveals two bound quasiparticle energy levels with a gap of 11 meV at the Fermi level. Away from the vortex core, the tunneling characteristics are very similar to the zero-field spectra and show a multiple gap-like structure and a large zero-bias conductance.
Tunneling Spectra: Theoretical Studies
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Tunneling spectroscopy of anisotropic superconductors
Satoshi Kashiwaya, Yukio Tanaka, Masao Koyanagi, et al.
Tunneling spectroscopy of normal-insulator-superconductor junction is investigated theoretically. In anisotropic superconductors, different from the case of isotropic superconductor, the effective pair potentials felt by quasiparticles depend on the direction of their motion. By taking this effect into account, it is shown that the conductance spectra strongly depend on the crystal orientation. Using Green's function method, local density of states (LDOS) in superconductor is also calculated. The close relation between conductance spectra and LDOS is presented. The calculation is compared with experimental spectra of high-Tc superconductors.
Tunneling Spectra: Experimental Studies
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Tunneling observation of phonon contribution to the Cooper pairing in high-Tc superconductors
Ryzo Aoki, Hironaru Murakami, Masamitsu Shirai, et al.
Improved tunneling spectroscopy was carried out on high-Tc BSCCO(2212) single crystal, and it provided us a reflection spectrum of the quasi-particle that mediates the strong Cooper pairing. The point contact junction enabled us to observe significant fine structure spectrum beyond the gap edge up to 0.2 eV. The spectrum peaks both in positive and negative bias ranges were found to be in a good one-to-one correspondence to the phonon density-of-states peaks at energy level $HBAR(omega) i reported by the neutron scattering experiment. The significant peaks at higher energies could be also assigned by n(epsilon) O + $HBAR(omega) i (n equals 0.1.2) where (epsilon) O is the breathing mode energy. Numerical computation in terms of the Eliashberg gap equation was carried out and the electron-phonon coupling intensity function (alpha) 2F((omega) ) was elucidated. Most of the peak modes were attributed to the optical phonons due to oxygen oscillations in and around the (CuO2)n plane, wherein the superconducting electronic state takes place. Consequently it turns out that the breathing mode has an essential role in the high-Tc superconductivity in the perovskite type-oxides.
Raman Spectra
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Raman scattering in high-Tc superconductors: electronic excitations
Manuel Cardona, Thomas Strohm, Jens Kircher
Since the discovery of the high Tc superconductors Raman scattering has proven to be an excellent technique to characterize them and to investigate basic physical properties relevant to the elusive mechanism responsible for their superconductivity. We discuss here several aspects of the technique as applied to superconductivity, including scattering by lattice vibrations, magnetic excitations, and electronic excitations, with particular emphasis on the latter, both in the normal and the superconducting state.
Magnetic Raman scattering in cuprate antiferromagnetic insulators and doped superconductors
Girsh Blumberg, Peter M. Abbamonte, Miles V. Klein
We present results of low-temperature two-magnon resonance Raman excitation profile measurements for single layer Sr2CuO2Cl2 and bilayer YBa2Cu3O6.1 antiferromagnets over the excitation region from 1.65 to 3.05 eV. These data reveal composite structure of the two-magnon line shape with peaks at approximately 2.8 and approximately 4 J and strong nonmonotic dependence of the scattering intensity on excitation energy. We analyze these data using the triple resonance theory of Chubukov and Frenkel [Phys. Rev. Lett., 74, 3057 (1995)] and deduce information about magnetic interaction and electronic band parameters in these antiferromagnetic insulators. We study the evolution of the magnetic excitation with hole doping in CuO2 planes of YBa2Cu3O6+(delta ) and YBa2Cu408 single crystals. We find that the spin excitations at energy approximately equal to 3 J, similar to the two-magnon excitations in the insulators, persist with doping and are evidence that antiferromagnetic fluctuations with spatial extent of at least three lattice constants are not overdamped in the underdoped superconductors. The two-magnon resonance study shows the existence of the charge-transfer gap in the underdoped cuprates and provides information about evolution of electronic band parameters with doping. We report a magnetic Raman scattering study of YBa2Cu408 superconductor with substitution of Zn on the Cu(2) site. Two and five-tenths percent of substitution softens the two-magnon excitation frequency from 2900 to 2300 cm-1. This softening is attributed to the absence of a defined spin projection on at least one of eight Cu(2)/Zn sites involved in the superexchange, and it signifies that the Zn- introduced magnetic distortion extends to at lest four Cu sites neighboring a Zn site. The electronic Raman continuum intensity weakens with increasing Zn doping, manifesting a relationship between the Raman continuum and the spin fluctuations.
Electronic Raman scattering in copper-oxide superconductors and related compounds
Rudi U. Hackl, Gaby Krug, Ralf Nemetschek, et al.
We describe Raman-scattering experiments in copper-oxide superconductors with transition temperatures between 12 and 90 K as a function of temperature and polarization. The broad continuum observed in all compounds in both the normal and the superconducting state is attributed predominantly to electronic excitations and will be interpreted in terms of charge- carrier fluctuations. The effect of impurities is studied systematically. The limitations of the present theoretical approach as well as possible generalizations are discussed.
Infrared Spectra: Theoretical Studies
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Optical spectra of high-temperature superconductors
John Ruvalds
The concept of 'free' electrons which yields the Drude description of the conductivity works surprisingly well in conventional metals. By contrast, the infrared reflectivity of the cuprate superconductors deviates dramatically from Drude behavior and thus challenges theory to explain the origin of the anomalous electron damping and the related mass divergence which has implications for the existence of a Fermi surface. The controversial key issue of the carrier concentration in cuprates needs to be resolved by a conserving analysis of the puzzling conductivity. Raman spectra of cuprates also exhibit unconventional electronic contributions over a wide frequency range up to 1 eV, and recent data provide evidence for the symmetry of the superconducting energy gap. A microscopic theory for both the optical conductivity and the Raman anomalies in cuprates derives a linear frequency variation of the damping from electron-electron collisions on a 'nested' Fermi surface that refers to nearly parallel segments of an electron trajectory. Thus the nesting theory links the cuprate anomalies to phenomena in chromium and rare earth metals. Nesting also yields a novel mechanism for d-wave superconductivity that requires a Coulomb repulsion of intermediate strength and key nesting features that distinguish high Tc cuprates from other materials.
Raman Spectra
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Role of symmetry in Raman spectroscopy of unconventional superconductors
Thomas Peter Devereaux
The role of symmetry of the inelastic light scattering amplitude, the superconducting energy gap, and the underlying Fermi surface manifold on the Raman spectra of unconventional superconductors is discussed in detail. Particular emphasis is placed on both single and bi- layer superconductors. It is found that the B1g channel may be the most sensitive to doping due to the role of the Van Hove singularity. Lastly the effect of both disorder and spin fluctuations is considered. The theory imposes strong constraints on both the magnitude and symmetry of the energy gap for the bi-layer cuprates, indicating that a nearly identical energy gap of dx(2-y(2)) symmetry provides a best fit to the data.
Infrared Spectra: Theoretical Studies
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Phonon-assisted IR spectroscopy of quantum antiferromagnets
Jose Lorenzana, R. Eder, George A. Sawatzky
We review recent theoretical results for multimagnon-phonon assisted infrared absorption in antiferromagnetic Heisenberg systems. We show spin wave theory line shapes for 2D spin 1/2 systems (like the parent insulating high-Tc cuprates) 1D spin 1/2 systems and 2D spin 1 systems (like the nickelates) and exact diagonalization results in two-dimensional spin 1/2 systems. The theoretical line shapes are compared with experiments. In the case of cuprates they explain mid-infrared peaks observed in the insulator. In the case of the nickelates a predicted line shape is also shown to agree with the experiments. We discuss the possibility to observe these excitations in other experiments.
Tunneling Spectra: Theoretical Studies
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Impurity scattering in anisotropic superconductors and interband sign reversal of the order parameter
Igor I. Mazin, A. A. Golubov
We discuss various mechanisms that can lead to interband sign reversal of the order parameter in a multi-band superconductor. In particular, we generalize Abrikosov-Gor'kov solution of the problem of weakly coupled superconductor with magnetic and nonmagnetic impurities on the case of arbitrary order parameter anisotropy, including extreme cases as d-pairing or interband sign reversal of the order parameter, and show that interband scattering by magnetic impurities can stabilize an interband sign-reversal state. We discuss a possibility of such state in YBa2Cu3O7 in the context of various experiments: Josephson tunneling, neutron scattering, isotope effect measurements.
Infrared Spectra: Experimental Studies
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Optical studies of magnons, excitons, and polarons in CuO2-layer compounds
Marc A. Kastner, Robert J. Birgeneau
The optical properties of undoped and lightly doped lamellar copper oxides are reviewed. In the undoped materials the absorption below the charge-transfer gap is dominated by magnetic and crystal field excitations of the CuO2 layers. The temperature dependence of the charge-transfer absorption provides evidence that free charged excitations form large polarons. However, the optical ionization energy of holes bound to acceptors is much larger than the thermal ionization energy, indicating that the bound polarons are small. The parameters extracted from optical measurements predict the Hall mobility of holes in lightly doped La2CuO4, with no adjustable parameters, confirming that the carriers are polarons.
Zn-substitution effects on the optical conductivity of YBa2Cu3Oy
Setsuko Tajima, R. Hauff, W.-J. Jang
The Zn-substitution effects on the c-axis optical spectra have been investigated over T-range for YBa2Cu3Oy with y equals 6.6 and 6.9. In the highly doped crystal a large number of unpaired carriers forms a Drude-like spectrum at low frequencies, which buries the gap feature in the conductivity spectrum, keeping the maximum gap frequency unchanged. It strongly suggests an anisotropic superconducting gap. In the underdoped crystal, the anomalous peak around 400 cm-1 which appears in the limited doping and temperature ranges almost disappears by 1.3% Zn-substitution. Considering that the reported strong Zn-effect on T1-1-behavior, this spectral anomaly could be related to the spin gap formation. On the other hand, the low frequency conductivity does not show a remarkable change by Zn-substitution, which indicates less effect of Zn on the interlayer hopping along the c-axis.
Photoelectron Spectra
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Photoemission, low-dimensionality, and high-temperature superconductivity
Giorgio Margaritondo
Several experiments by different authors have established the existence of an 'anomalous' photoemission effect in one-dimensional systems, including one-dimensional metallic crystals and other examples of one-dimensional metals. The effect consists of the suppression of the photoemission signal at energies close to the Fermi level -- whereas for metals one would expect to see a Fermi edge. Increasing evidence exists, in our opinion, that this phenomenon is due to the decoupling of charge and spin coordinates and to a departure from the Fermi-liquid framework. If confirmed, this conclusion would be extremely relevant to high-temperature superconductivity, since it would pave the way to the use of a similar concept for non-Fermi- liquid theories of high-temperature superconductors.
ARPES studies of the superconducting gap in high-temperature superconductors
H. Ding, M. R. Norman, John Giapintzakis, et al.
Using high resolution angle-resolved photoemission spectroscopy (ARPES), we measure the superconducting gap and its momentum dependence in the high temperature superconductors Bi2Sr2CaCu2O8 (Bi2212) and YBa2Cu3O6.95 (Y123). It is found that umklapp bands due to the incommensurate superlattice in Bi2212 may obscure the intrinsic momentum dependence of the gap. By taking these into consideration, we find a d- wave-like gap function in Bi2212. We also report an unambiguous observation of the superconducting gap in Y123.
Raman Spectra
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Raman scattering and far-infrared reflectivity of Ba5Ta4O15-x and Ba5Nb4O15-x (0.0
Nestor E. Massa, Silvina Pagola, Raul E. Carbonio, et al.
Quite often the literature cites superconductors with high critical temperature as having an anharmonic lattice close to a structural phase transition. It is then of interest to find materials, with distorted perovskite structure, that reveal bare phonon behavior expected for such an environment. Here we discuss layered Ba5Nb4O15 and Ba5Ta4O15-x (0.0 less than or equal to x less than or equal to 0.56) that have four Nb/Ta ions per formula unit. This implies that there is an empty octahedra that in the lattice avoids direct face sharing of the (Nb/Ta)O6 sublattice. As consequence, one expects strong anharmonicities that translate into several subtle anomalous shapes and wide phonon band profiles in the low temperature Raman and infrared spectra. We see that while a centrosymmetric D3d3- P3m1 space group is deduced for both compounds from neutron diffraction measurements, narrow splits of a symmetric stretching Raman mode hint small local departures of that symmetry in Ba5Nb4O15. Weaker structure on the band lower frequency side suggests that in their understanding one has to consider arguments that indicate that the anharmonic contributions to the lattice potential energy are so large that they cannot be treated only by perturbation theory, i.e., there is a lifting of the degeneracy of phonon transitions in the harmonic approximation. Defect induced modes, that are Raman active in Ba5Nb4O15, are understood as local modes due to unrelaxed portions of the lattice that may be related to changes of the phonon damping in infrared reflectivity spectra. It is also noteworthy to point out that the replacement Nb by Ta induces a hardening of most phonons. This is reflected in an increase of frequencies in spite of a heavier Ta ion.
Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
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Thermal difference reflectance spectroscopy of high-Tc superconductors
Matthew J. Holcomb, Catherine L. Perry, James P. Collman, et al.
The temperature dependent thermal difference reflectance (TDR) spectra of thin film samples of Tl2Ba2Ca2Cu3O10, (BiPb)2Sr2Ca2Cu3O10, Tl2Ba2CaCu2O8 and YBa2Cu3O7 have been measured for photon energies between 0.3 and 4.5 eV at temperatures above and below each material's superconducting critical temperature. The amplitude of the characteristic optical structure near the screened plasma frequency of each sample in the normal state TDR spectrum varies approximately linearly with temperature, T, indicating that the temperature dependent optical scattering rate in these materials scales with temperature as T2. From the TDR spectra collected above and below the critical temperature of each sample, the superconducting to normal state reflectance ratio, Rs/RN, has been obtained. In these materials, a feature exists in the experimental Rs/RN spectrum at energies between 1.5 and 2.0 eV which can be accounted for by treating the superconductivity in an Eliashberg model with a coupling function that includes both an electron-phonon interaction and a high energy electron-boson interaction located between 1.6 and 2.1 eV. Good agreement is obtained between theory and experiment based upon this description of the superconducting state.
Infrared Spectra: Experimental Studies
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Residual absorption of high-temperature superconductors at THz frequencies
Philipp Keller, Harry S. Barowski, Martin Prenninger, et al.
We report on a study of the residual absorption of high temperature superconductors (HTS) at THz frequencies. Using transmission Fabry-Perot interferometry, we determined both, real and imaginary part of the conductivity. We especially studied YBCO thin films. We found, that the dynamical conductivity varied strongly from sample to sample. As a main result we find, that the absorptivity increases at frequencies above 1 THz much weaker than quadratically, i.e. the (omega) 2 behavior of the absorptivity at microwave frequencies is not continued in the THz frequency range.
Infrared Spectra: Theoretical Studies
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Infrared optical spectra of high-Tc superconductors: assignment and implications
Mladen Georgiev, Milko Borissov, Lyubomir Mihailov
A multilayer capillary fiber was designed for optical sensor applications and its optical properties were evaluated by using a fluorescent layer immobilized on its inner surface. This fiber structure combines a large interaction surface (the inner wall of the capillary fiber) for binding of fluorescent indicators with evanescent wave fluorescence measurement, whichmay facilitate the design ofpseudohomogeneous assays without separation ofthe bound from nonbound fluorescent indicator. The inner surface of the capillary was derivatized by aminosilanization, followed by biotinylation and addition of streptavidin. This biotin-streptavidin coating facilitates subsequent immobilization of any biotinylated species (e.g. antibodies, antigens etc.) participating in specific molecular recognition. We have evaluated some properties of this capillary fiber design by using fluorescent proteins immobilized on the inner wall of capillary by biotin-avidin-interaction. Fluorescence was excited by a HeNe-laser (fluorescent indicator APC; Aem —660 fllfl) d by a Arlaser (fluorescent indicator RPE, 'em 578 fliT!),and measured with a spectrum analyzer. Key words: capillaryfiber, capillaryfluorescence, opticalfiber sensor, fluorescence sensor, biotinavidin bridge, biotin-streptavidin coating, fluorescent indicator, fluorescent protein, APCfi uorescence, RPE-fluorescence
Tunneling Spectra: Theoretical Studies
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Josephson effect and quasi-particle states in d-wave superconductors
Yukio Tanaka, Satoshi Kashiwaya
A general formula for the Josephson current in a d-wave/insulator/d-wave-superconductor junction is presented by taking account of the zero-energy states formed around the interfaces. For a fixed phase difference between the two superconductors, the current component becomes either positive or negative depending on the injection angle of the quasiparticle. Anomalous temperature dependences are predicted in the maximum Josephson current and in the free energy minima.
Photoelectron Spectra
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Symmetry of order parameter (pair potential) deduced from ARPES and neutron scattering
Jiro Tanaka
The cluster orbitals of complexes containing (formula available in paper) ion are studied by DFT plus HF SCF method. Four different type orbitals composed of (formula available in paper) , two of them are degenerate, are found close to the Fermi level. By using these orbitals, four band models are developed, the top one has a flat energy band around the X and Y points and the next degenerate bands give a wide Fermi surface, the fourth one is fully occupied. The electronic transitions between these bands are allowed and it induces anisotropic order parameter. The symmetry of the order parameter is neither d nor s wave, but it is represented approximately by 2cosX - cosY or 2cosY - cosX. The zero-gap states are searched for the position where the node of the order parameter crosses the Fermi level. Comparison is made with experimental results of angle resolved photoemission spectra on Bi2212 and thermal neutron scattering on LSCO.
Raman Spectra
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Optical detection of the superconducting proximity effect
Laura H. Greene, A. C. Abeyta, Igor V. Roshchin, et al.
We present the first detection of a superconducting proximity effect by optical techniques. Raman scattering on n+-InAs is performed through very thin, high-quality, superconducting Nb films grown directly on the (100)InAs surface. The 6 to 10 nm thick Nb films exhibit Tcs of 2.5 to 5.5 K, as measured by electronic transport, and are flat to approximately 0.5 nm, as measured by x-ray reflectivity. As the Nb/InAs structure is cooled below the superconducting transition temperature, the magnitude of the unscreened LO phonon mode, associated with the surface charge accumulation layer in the InAs, is observed to be enhanced by more than 40%. This reversible change is observed only when the Nb is in good electrical contact with the InAs.
Electronic Raman scattering as a function of doping in high-Tc superconductors
Chris A. Kendziora, Ronald J. Kelley, Marshall Onellion
We report the results of Raman scattering from the electronic continuum in Bi2Sr2CaCu2O8+(delta ) (Bi 2212) and Tl2Ba2CuO6+(delta ) (Tl 2201) high temperature superconductors with variations in the oxygen content, (delta) . Below Tc, a peak develops in the Raman continuum associated with the opening of a superconducting gap, (Delta) (k). By selecting the polarizations of incident and scattered light, we are sensitive to possible anisotropy of the gap within the a-b plane. Near optimal doping, both materials show gap anisotropy, with 2(Delta) /kBTc values of 7.2 (B1g) vs. 5.8 (A1g) in Tl 2201 and 8.5 (B1g) vs. 6.2 (A1g) in Bi 2212. In contrast, both show an isotropic gap at much lower energy shifts when the carrier concentration is raised: 2(Delta) /kBTc equals 3.9 (5.5) for Tl 2201 (Bi 2212) with Tc equals 37 K (57 K). We compare the observed spectra with calculations based on order parameters with d-wave as well as isotropic s-wave symmetry and conclude that raising the doping level reduces the gap anisotropy to near zero.
Superstructure of Bi2Sr2CaCu2O8 superconductors: a Raman scattering study
I. Loa, J. Hofmann, Alexander P. Litvinchuk, et al.
We present results of a Raman scattering study of Bi2Sr2CaCu2O8 single crystals with Tc equals 95 K in a wide temperature (65 - 365 K) and frequency (from 640 down to 10 cm-1) range. The features due to superstructure modulation are found in the low-frequency range. We also investigated additional lines seen in z(xx)z and z(yy)z scattering configurations in the regions of 160 - 240 cm-1 and 300 - 450 cm-1 and analyzed the temperature dependence of the latter.
Raman spectral temperature dependence of Bi2Sr2Ca1-x(Yx)Cu2O8 (x = 0 and 1)
Eric Faulques, V. Perocheau, B. Corraze, et al.
We have investigated the temperature dependence of phonon and magnon Raman scattering of Bi2Sr2CaCu2O8 (2212) and Bi2Sr2YCu2O8 single crystals. We found no significant frequency shifts above and below Tc nor magnons in the yttrium free 2212 compound. The main 460 cm-1 O-Sr phonon is not coupled to an electronic continuum whereas it appears clearly that the 630 cm-1 O-Bi phonon does couple owing to its Fano shape. The issue is whether the oxygen-CuO2 290 cm-1 phonon couples remain open or not. On the contrary, some phonons of the Raman spectra of yttrium 2212 crystals show changes around T* equals 125 K in frequency, linewidth and intensity. For instance, anomalous temperature dependence is evidenced for the frequency and linewidth of the O-Bi lines (628, 670 cm-1) and CuO2 lines (310, 358 cm-1), and the linewidth of the mode at 500 cm-1. A strong and large feature (3060 cm-1) stemming possibly from two-magnon scattering has also been recorded at 8 K and 300 K. The exchange integral J is estimated to be minus 140 meV.
Investigation of low-energy electronic response in high-Tc superconductors by Raman spectroscopy
Akio Yamanaka, Norio Asayama, Takashi Furutani, et al.
Low-energy electronic response due to single-particle excitations has been investigated in high-Tc copper-oxide Bi2Sr2CaCu2O8+(delta ) single-crystals by Raman scattering spectroscopy. We find that the low-energy electronic response in the superconducting phase depends significantly on polarization configuration. For tetragonal B1g the suppression of the low-energy spectral weight of the electronic continuum due to an opening of the superconducting gap occurs abruptly below Tc, whereas that of the B2g-response shows a gradual temperature dependence. The symmetry-dependent superconducting response is basically consistent with the superconducting order parameter having a nodal structure with X2-Y2 symmetry.
Photoelectron Spectra
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Extended van Hove singularity in noncuprate-layered perovskite superconductor Sr2RuO4
Takashi Takahashi, T. Yokoya, A. Chainani, et al.
Low-temperature high-resolution angle-resolved photoemission spectroscopy (ARPES) has been performed on a non-cuprate superconductor Sr2RuO4 (Tc approximately 1 K), which has the same crystal structure as La2CuO4, but with RuO2 layers replacing CuO2. High-resolution ARPES spectra obtained along (Gamma) - Z line (Ru-O bonding direction) in Brillouin zone show the existence of an extended van-Hove singularity (VHS) close to EF (20 plus or minus 2 meV) like high-Tc cuprates, regardless of the character of the electronic states at EF, suggesting that an extended VHS is a general feature of correlated two-dimensional d - electron metals. This requests re-examination of the VHS scenario for superconducting properties of cuprates, while the observed T-linear resistivity seems to be consistent with the present result.
Dependence of the photoelectron spectral function on hole doping in Nd1-xSrxTiO3
S. W. Robey, V. E. Henrich, C. Eylem, et al.
Photoelectron spectroscopy was employed to follow the development of the Ti 3d spectral intensity within approximately 3 eV of the Fermi level (EF) in Nd1-xSrxTiO3 as a function of x. The total intensity in this region is found to correlate linearly with composition as the nominal Ti valence is varied from plus 3 to plus 4. Changes in the spectral structure in this binding energy region are discussed in terms of the one-electron spectral function. For x less than 0.25, (semiconducting compositions) only incoherent intensity associated with the lower Hubbard band is present. Additional intensity, attributed to a coherent quasiparticle contribution, appears at the composition of the metal-insulator transition, x approximately 0.25. The relative intensities of these two components are determined as a function of x, and connections with theoretical models and experimental results on related systems are discussed.
High-resolution photoelectron yield spectroscopy of oxide superconductors
Masashi Kawasaki, Satoru Ohashi, Toshikazu Kitajima, et al.
We have designed and constructed a cryogenic photoelectron yield spectroscopy (PYS) and demonstrated a high energy resolution of 10 meV and a wide dynamic range of 7 orders of magnitude. This system enabled us to measure the absolute value of work function and analyze the electronic structure around the Fermi level (EF) for a Bi2Sr2CaCu2Ox(BSCCO) single crystal and YBa2Cu3O7-(delta ) (YBCO) epitaxial films. When the photoelectron was collected along <001> axis, semiconductive spectrum having a broad tail was observed for both materials. When the measurement was performed from the edge of CuO2 plane, we observed metallic spectra with sharp threshold, of which slope depended on temperature in a way consistent with the Fermi-Dirac distribution function. For BSCCO, a metallic spectrum superimposed a dip structure at 10 K in the vicinity of EF. The data could be well fitted with a curve numerically simulated from the BCS function with a superconducting gap value ((Delta) ) of 20 meV.
Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
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Contribution of ion beam analysis to study the mechanisms of YBaCuO thin film growth and their oxidation kinetics
Julius Siejka, J. Garcia-Lopez
At first a short review of ion beam analysis (IBA) techniques such as Rutherford backscattering analysis, nuclear reaction analysis and of their contribution to the determination of composition and structure of YBaCuO thin films is presented. In the second part, IBA contribution to the measurements of oxygen content and mobility in YBaCuO and to elucidate the mechanisms of high temperature 'in situ' growth of thin films is discussed. The emphasis is on the complementarity of IBA, Raman spectroscopy, TEM and XRD techniques to characterize the YBaCuO thin films in correlation with their physical properties. The results show that fully oxygenated YBaCuO thin films are formed 'in situ' during high temperature T less than or equal to 750 degrees Celsius, reactive sputtering. Their room temperature oxygen content and order is determined by oxygen loss and or uptake during the sample cooling conditions. The physical implications of these findings are analyzed.
Probing the local structure of high-Tc superconductors using XAFS spectroscopy
Frank Bridges, C. H. Booth, Guoguang Li, et al.
X-ray absorption fine-structure (XAFS) is a local structural probe that is complementary to diffraction techniques. We discuss the types of information that can be obtained using this probe and then consider several examples, including the distortion about Co in YBa2Cu3O7 (YBCO), an unusual negative correlation of atom pair displacements in HgBa2CuO4+(delta ), and the distortions about the O(4) atom in thin films and single crystals of YBCO.
X-ray absorption spectroscopic study of new high-Tc superconducting intercalation compound of (HgX2)0.5Bi2Sr2CaCu2Oy(X=Br,I)
Jin-Ho Choy, Nam-Gyu Park, Seong-Ju Hwang, et al.
We have systematically performed x-ray absorption spectroscopic (XAS) studies on the new mercuric halide intercalated superconductors, (HgX2)0.5Bi2Sr2CaCu2Oy with X equals Br and I, in order to investigate the electronic and crystal structure of these compounds. The present Hg LIII-edge EXAFS results indicate that the mercury is coordinated with two bromide or iodide ligands with bond length of 2.46 angstrom for the HgBr2-intercalate and 2.65 angstrom for the HgI2-one. This is the first example of linear molecular salt of HgX2 stabilized in the solid lattice. According to the I LI- and Br K-edge x-ray absorption near edge structure (XANES) spectroscopic studies, a small amount of electron transfer from the intercalant HgX2 to the host lattice could be observed upon intercalation, resulting in the negatively charged state of (HgX2)(delta -). The Cu K-edge EXAFS analysis reveals that the bond distance of Cu-Oaxial is slightly shortened upon intercalation, reflecting the oxidation of (Cu-O) layer, which is well consistent with the I LI- and Br K-edge XANES results.
Photoelectron Spectra
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Occupied and unoccupied electronic states in a layered perovskite superconductor without copper: band dispersions, van Hove singularity, and hole states in Sr2RuO4
Tom R. Cummins, Michael S. Schmidt, Dong Hui Lu, et al.
The occupied and unoccupied electronic structure of high quality single crystals of the copper- free layered perovskite superconductor Sr2RuO4 has been measured for the first time. Angle-resolved photoemission spectroscopy with high energy and angle resolution determined band dispersions along the principle directions of the projected Brillouin zone of the (001) surface. The highlight of these spectra is the observation and characterization of an extended van Hove singularity. The singularity is located 17 meV below the Fermi level and extends around the M point for around 0.2 angstrom-1 along both (Gamma) -M-Gamma and X-M-X. Dispersions of the near-Fermi level states in Sr2RuO4 reveal 3 bands which cross the Fermi level giving rise to 3 Fermi surfaces; 1 electron-like Fermi surface around the (Gamma) point and 2 hole-like surfaces encircling the X point. The topology of the empirically determined Fermi surfaces is found to be in qualitative agreement with local density approximation band structure calculations. Photoemission of the full valence band of Sr2RuO4 are also presented. The total valence band width is approximately 9 eV, the density of states at the Fermi level derived from photoemission is 1.3 states/(eV cell) and has an 80%/20% mix of metal to ligand character. Polarization-dependent O1s NEXAFS performed on Sr2RuO4 demonstrate mainly hole states at the Fermi level in orbitals in the RuO2 planes. Electron correlations among Ru 4d levels in Sr2RuO4 are argued to have a lesser impact than that of 3d levels in the cuprates.
Tunneling Spectra: Theoretical Studies
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Influence of exchange field on d-wave superconductivity
Ljiljana Dobrosavljevic-Grujic, Zoran Radovic
Exchange field effects and d-wave pairing in superconductors may have similar manifestations, as for example (pi) -phase states and spontaneous currents in Josephson devices. Influence of exchange field on the Josephson tunneling between d-wave superconducting electrodes through a ferromagnetic barrier is studied theoretically. Implications of combined exchange field and d-wave pairing effects are discussed in the context of the experimental verification of d-wave superconductivity.
Chain-plane coupling in YBa2Cu3O7
William A. Atkinson, Jules P. Carbotte
The purpose of this work is to discus the role that coupling between the CuO chains and CuO2 planes in YBa2Cu3O7 plays in the superconducting state. The important issue is the structure and magnitude of the superconducting gap in the chains. We discuss a simple pairing interaction -- in which a boson is exchanged between the planes and chains -- which leads to a gap in the chains that is larger than would be found in a simple proximity model of superconductivity. The interlayer pairing interaction has an s-wave symmetry which overwhelms the intrinsic d-wave gap in the planes when the interlayer interaction is large. The limitation that this imposes is important, as it restricts the size of gap in the chains. We compare this model with other models of interlayer pairing.
Photoelectron Spectra
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Quasi-particle spectrum in Sr2CuO2Cl2
The electronic structure in antiferromagnetic insulator Sr2CuO2Cl2 is calculated in the multi band p-d model of CuO2 layer with account for strong electron correlations. The results are in good agreement with recent ARPES data and are compared with the results of the t-J model. Spin fluctuations result in the anisotropy of the effective mass at the top of the valence band.
Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
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Temperature dependence of the dielectric function of laser deposited YBCO thin films at 3392 nm
D. George Walmsley, T. Bade, Patrick G. McCafferty, et al.
We have excited surface plasmons in a YBCO thin film at different temperatures using attenuated total reflection of light. The 300 nm thick c-axis film was fabricated using pulsed laser deposition onto an MgO (100) substrate with 248 nm KrF excimer radiation. Critical temperature of the film was 89.6 K and its roughness, as shown by atomic force microscopy, 20 nm rms, without droplets over areas of 10 micrometer by 10 micrometer. The sample was mounted in Otto geometry on a cooled stage which allowed the temperature to be varied between 300 K and 70 K. An infrared HeNe laser at 3392 nm was used to excite the surface plasmons. The dielectric function of the film was determined between room temperature and 80 K. The imaginary part of the dielectric function decreased substantially with reduction in temperature. Results obtained were: (epsilon) r equals -24.1 plus 0.0013T and (epsilon) i equals 7.7 plus 0.067T where T is the temperature in Kelvin. The ratio (epsilon) i300/(epsilon) i80 at 2.13 is less than the resistance ratio R300/R80 at 2.81. An explanation is offered in terms of two temperature independent mechanisms operative at optical frequencies: enhanced Rayleigh scattering of surface plasmons at grain boundaries and intraband/interband transitions. The real part of the dielectric function, (epsilon) r, was found to be only slightly temperature dependent. It was, however, highly sample dependent when comparison was made with the results of other films, a feature attributed to surface and grain boundary contamination.
In-situ resistivity and Hall effect studies of persistent photoconductivity in oxygen-depleted YBa2Cu3Ox
Wilhelm Markowitsch, C. Stockinger, W. Lang, et al.
We report on in-situ studies of the resistivity and the Hall effect in partially oxygen-depleted, metallic YBa2Cu3Ox (YBCO) thin films during illumination with white light. The measurements were performed at temperatures of 100 K, 200 K, and 290 K and showed that the resistivity as well as the Hall coefficient decreased as a function of the illumination time. The photo-induced reduction of both quantities was largest at 290 K. Evaluation of the results of the Hall effect measurements within a simple one-band model shows that both the carrier mobility and the carrier concentration are enhanced by photodoping at 100 K and 200 K, with the enhancement of the carrier concentration dominating. At 290 K, however, the mobility decreases at large illumination times whereas the carrier concentration increases substantially, over-compensating for the loss of mobility. From the qualitatively different time dependencies of the carrier mobility and the carrier concentration, we conclude that two co-existing mechanisms contribute to photodoping: The first mechanism is related to a change of the electronic structure and is tentatively attributed to photo-assisted oxygen ordering. The second mechanism resembles the photogeneration of carriers in semiconductors and is ascribed to a photo-induced charge transfer. At 290 K, only the charge transfer process drives photodoping, whereas oxygen ordering seems to be hampered by thermal disordering. Additional evidence for the co-existence of the two persistent photoconductivity mechanisms is derived from measurements of the spectral efficiency of photodoping at 253 K. We observe a finite photodoping effect at photon energies above and below the charge transfer gap of YBCO. Above the gap energy (approximately equals 1.6 eV), however, the efficiency of photodoping increases remarkably. Our conclusion is that two mechanisms contribute to photodoping in metallic YBCO. One is related to oxygen ordering and does not involve interband transitions. Additionally, photo-induced charge transfer is responsible for the enhanced spectral efficiency of photodoping when the photon energy of the exciting light exceeds the charge transfer gap.
Infrared Spectra: Experimental Studies
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Studies of the residual absorption of HTSC at submillimeter wavelengths by means of photothermal interference spectroscopy
Harry S. Barowski, Armin Arnold, R. Eder, et al.
The determination of the residual, low temperature absorption of high temperature superconductors is of interest for applications of this new material at submillimeter wavelengths and of basic interest. The photothermal interference spectroscopy allows us to measure the residual, low temperature absorption of a HTSC. For the determination of the residual absorption of a superconductor a far-infrared beam is periodically modulated and focused on the sample. Absorption leads to a periodic change of the temperature of the sample surface and, due to heat diffusion, also in the gas volume adjacent to the sample. This temperature change in the gas is detected via the refractive index change using a two beam interferometer. We studied the residual losses of YBaCuO thin films on various substrates and of BiSrCaCuO (2212) single crystals at submillimeter wavelengths. We find that the frequency dependence of the absorptivity, which shows a frequency squared behavior at microwave frequencies, is less than quadratic at THz-frequencies. The YBaCuO thin films show a plateau between 0.6 THz and 4 THz with an absolute value of the absorptivity of about 10-2. A BiSrCaCuO single crystal shows a plateau between 1 THz and 4 THz with an absorptivity in the order of 10-3.
Effects of Ni doping on the optical conductivity of YBa2Cu3O6.95
Christopher C. Homes, Q. Song, B. P. Clayman, et al.
The optical conductivity ((sigma) 1) of twinned and untwinned single crystals of YBa2(Cu1-xNix)3O6.95 has been studied using infrared reflectance spectroscopy for x equals 0, 0.75 and 1.4% ('pure,' 'light' and 'heavily' doped). The optical properties have been calculated from a Kramers-Kronig analysis of the reflectance. Nickel acts as a point defect scatterer in the planes, but has only a small effect on Tc. (At the heaviest Ni doping, Tc is only suppressed by approximately equal to 4 K). In the pure, twinned samples for T very much less than Tc, there is a narrow, 'Drude'-like band at low frequency; the conductivity is observed to drop to a minimum of approximately equal to 1000 (Omega) -1 cm-1 at approximately equal to 500 cm-1 before gradually rising to join a temperature-independent midinfrared band. In the heavily- doped sample, there is a large amount of low-frequency residual conductivity; at 6 K some of the residual conductivity has been suppressed and a shoulder in (sigma) 1 is visible at approximately equal to 300 cm-1. In the lightly-doped sample, the approximately equal to 300 cm-1 shoulder is visible already at 12 K. However, in the same detwinned sample, the shoulder in (sigma) 1 is observed only for E (parallel) b, and is present in the normal state indicating that this feature is associated with the chains, and is not related to the superconductivity in these materials. The origin of this feature is discussed, and it is proposed that the shoulder at approximately equal to 300 cm-1 may be related to the pseudogap observed along the c axis in the oxygen-underdoped materials.
Far-infrared reflectivity of Bi2Sr2CaCu2O8+delta single crystals with different oxygen contents
Martin Prenninger, Robert G. Buckley, Philipp Keller, et al.
Results are reported of an infrared reflectivity study of two high Tc superconducting Bi2Sr2CaCu2O8 plus delta single crystals. One of the samples was optimally- doped (Tc equals 91 K) and the other over-doped (Tc equals 73 K). It is observed that the reflectivity of the optimally-doped crystal is near unity at low temperature (10 K) and at frequencies below 400 cm-1. The reflectivity of the over-doped crystal is significantly less than unity. The dynamical conductivity and the penetration depth of the optimally-doped crystal are similar to values for optimally-doped YBa2Cu3O7- delta single crystal. However, the over-doped crystal displays significantly larger low frequency absorption. The results can be understood within a d-wave picture in which pair breaking increases with increasing hole doping and contributes to a significant quasiparticle density below Tc.
Tunneling Spectra: Experimental Studies
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LT-STM/STS observation of definite superconducting gap states on the multistage crystal surface of Bi2Sr2CaCu2O8+X
Hironaru Murakami, Ryzo Aoki
Low temperature STM/STS observations have been carried out on cleaved BSCCO crystal surfaces. We have succeeded in detection of a special layer, probably a CuO2 or Ca layer exposed on the surface. The STS spectrum which was reproducibly observed on this special site shows a considerably anisotropic but distinct superconducting gap structure with a definite and flat gap bottom region. This gap structure shows significantly different characteristic from another gap structure observed on the BiO layer, which shows a rounded shape at the gap bottom region without any indication of a finite gap state.
Tunneling investigation of the gap-like feature in superconducting Y1-xPrxBa2Cu3O7 thin films
Mark W. Covington, Laura H. Greene
We have measured the conductance of Y1-xPrxBa2Cu3O7/Pb planar tunnel junctions fabricated on thin films where x equals 0, 0.2 and 0.4. When tunneling into the ab-plane, there is a weak decrease in conductance at eV approximately 2kBTc, where a superconducting gap is expected. With increasing Pr concentration, the energy where this gap-like feature appears is reduced by an amount commensurate with the reduction in Tc. The gap-like structure in the c-axis tunneling conductance becomes weaker, but displays no change in energy when YBCO is doped with Pr.
Origin of fine structure in tunneling spectra of BiSrCaCuO films
Vladimir M. Svistunov, Yu. F. Revenko, D. N. Afanassyev, et al.
Tunneling spectroscopy studies have been carried out on BSCCO textured films (2212 and 2223 phases) at T equals 4.2 K using the point junction technique with a silver or lead needle as an injector electrode. The tunneling spectra recorded in superconductive state have demonstrated the gap structures corresponding to S-I-S and S-I-N tunneling junctions. The energy gap peaks of the metaloxide correspond to the tunneling along ab plane in the CuO$0- 2) layer. The effect of the series resistance of the deteriorated surface layer which is responsible for the gap broadening in addition to that due to the finite lifetime of quasiparticles is discussed in connection with the determination of the energy gap. The tunneling spectra of Bi-2212 revealing the sharp gap peaks and a reproducible structure above the gap have been investigated in detail. The gap of Pb was also observed in the spectra of BSCCO-Pb junctions suggesting that a S-I-N junction has been formed between the needle and the normal surface of the film. Above the gap, some fine structure independent on magnetic field up to 0.5 T was observed. The most distinct feature was a pronounced dip at the energy of 70 meV which amplitude varied with junction resistance and bias polarity. This feature is consistent with the results of studies on single crystals. We assume this dip is due to the effects of layered structure of metaloxide, the interlayer pairing. The observed V-shaped conductance curve in sub-gap region is also consistent with this model. The remaining weak peculiarities we consider to be due to the electron-phonon interaction. The problem of Eliashberg function reconstruction from these data is discussed.
Tunneling conductance of metaloxide junctions
Vladimir M. Svistunov, A. I. Khachaturov, M. A. Belogolovskii
The differential conductance behavior of metal/insulator/metaloxide and metaloxide/insulator/metaloxide tunnel structures has been studied. It is found that because of small Fermi energies of metaloxides a number of universally accepted principles of tunneling spectroscopy of conventional materials cease to be valid. First, the shape of tunneling characteristics of a metaloxide is unusually sensitive to barrier parameters: the thickness of the insulating layer and the barrier height. If the barrier height is quite large or the thickness is small the dependence of tunnel conductance on voltage for a metaloxide/insulator/metaloxide structure manifests a zero-bias 'peak resistance' anomaly. On further increasing the height or decreasing the thickness the dependence of conductance (sigma) versus voltage V decreases throughout the entire range of voltages. Second, the parabola-like dependence of (sigma) (V) for a metal/insulator/metaloxide structure calculated for a symmetrical rectangular potential barrier appears not to be symmetrical. Its minimum occurs at a finite voltage. Finally, in contrast to metal/oxide/metal contacts the metaloxide tunnel characteristics calculated in the WKB-approximation differ considerably from the corresponding ones obtained in the 'sharp boundaries' model.
Raman Spectra
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Carrier relaxation and localisation in Bi2Sr2Ca1-yYyCu2O8 and YBa2Cu3O7-d: a study by femtosecond time-resolved spectroscopy
Chris J. Stevens, T. N. Thomas, S. Choudhary, et al.
Ultrafast time-resolved optical absorption has been measured as a function of doping and temperature in the high temperature superconductors Bi2Sr2Ca1-yYyCu2O8 (0 less than y less than 1) and YBa2Cu3O7-(delta ) (0.06 less than 1). A fast transient response is observed, which changes sign as a function of y in Bi2Sr2Ca1-yYyCu2O8 and in YBa2Cu3O7-(delta ). Careful consideration of the possible mechanisms for the observed behavior lead us to suggest that two independent mechanisms are operating, whereby the two observed positive photoinduced absorption components are due to the electrodynamic response of photoinduced carriers, while the negative signal arises due to spectral hole burning. In the metallic samples a long-lived component is observed, which we suggest comes from photoinduced localized states intrinsic only to the superconducting species of the high-Tc materials. A very significant increase in the scattering rate (1/(tau) ) at temperatures just above Tc signifies coupling to charge and possibly spin fluctuations just above Tc. A large reduction in scattering rate is observed below Tc, similar to that observed previously in YBa2Cu3O6.9 and Tl2Ba2CaCu2O8.
Infrared Spectra: Experimental Studies
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HTS Fabry-Perot resonators for the far infrared
Philipp Keller, Martin Prenninger, Evgeny V. Pechen, et al.
We report on far infrared (FIR) Fabry-Perot resonators (FPR) with high temperature superconductor (HTS) thin films as mirrors. For the fabrication of FPR we use two parallel MgO plates covered with YBa2Cu3O7-delta thin films on adjacent sides. We have measured the far-infrared transmissivity at 10 K with a Fourier transform infrared spectrometer. Very sharp resonances can be observed for frequencies below 6 THz where the MgO is transparent. The finesse (width of the first order resonance) is comparable to the FPR with metallic meshes as reflectors that are applied in the FIR spectroscopy and astronomy. We have also shown that thin films of gold are not adequate substitute to HTS thin films and not suitable for the fabrication of high-quality FPR due to the ohmic losses.
Raman Spectra
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Intramolecular phonons of the beta and beta* phases of the unlabeled and 13C-labeled (BEDT-TTF)2I3 organic superconductor above and near Tc
Eric Faulques, C. Girault, P. Batail, et al.
We present a temperature dependence investigation of the C equals C intramolecular phonon for both the normal and inter-ring C* equals C* labeled salts (BEDT-TTF)2I3 superconducting at Tc equals 1.5 K and 8 K. C equals C bond frequencies are found at 1467, 1493 cm-1 and 1420, 1488 cm-1 for the unlabeled and labeled crystals, respectively. We have also computed the infrared and Raman frequencies of the unlabeled and labeled BEDT-TTF molecules. In the low frequency range we found in special operating conditions a tri-iodide I3-line at 106 cm-1 in addition to those found at 36, 116, and 146 cm-1. At 8 K the Raman profile of the intense tri-iodide line at 116 cm-1 can hardly be fitted with a Lorentzian. On the other hand, we also did not succeed to fit correctly the low temperature profile of the inter-ring C equals C line with a single Lorentzian; however Fano line shapes gave satisfactory fits. Taking into account that recent results of Kini, Ferraro and Williams provided evidence of participation of lattice phonons in the pairing mechanism, our results show that intramolecular C equals C vibrations must also be considered as a possible mediation factor for superconductivity.
Raman spectra of Y(Pr)Ba2Cu3O6+delta thin films and oxygen disorder
Eric Faulques, C. Gonzalez
The materials used in this study are a (perpendicular) or c (perpendicular) oriented thin films of Y(Pr)Ba2Cu3O6+(delta ) with thickness of 1400 angstrom to 3000 angstrom. The temperature dependence of the Raman spectrum of YBa2Cu3O6.9 (YBCO) is presented using the excitation wavelength at 676.4 nm (red line). At 10 K all phonons exhibit Fano asymmetries. The phonon frequency and the linewidth of the bands at 335 and 449 cm-1 show anomalous temperature dependence in the vicinity of Tc. Thin films with oxygen stoichiometry near to (delta) equals 0.9 are then investigated. Correlation is made between the appearance of strong Raman features at 230 and 596 cm-1 and the oxygen disorder of the films.
Tunneling Spectra: Theoretical Studies
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Model of non-Josephson oscillations in the resistive state of superconducting films
Gurami Alexandrov Gogadze
A new mechanism of inelastic quasiparticle scattering in the resistive state of superconducting films (the bremsstrahlung effect) is proposed, which stimulates quantum transitions of quasiparticles (with simultaneous 'photon' emission) from the resonance level (epsilon) 1 to the edge of the superconductor energy gap (Delta) . The phase slip center (PSC) is simulated with a S-N-S contact, in which the N-layer width is dN equals 2(1E(xi) )1/2 and the electrochemical potentials of the superconducting 'banks' are shifted (1E is the penetration depth of the longitudinal electric field, (xi) (T) is the coherence length). We believe, that the genesis of the level (epsilon) 1 is associated with interference effects between quasiparticles scattered by the phase boundaries. Electrons pick up the energy (epsilon) 1 in the electric PSC field due to the multiple Andreev reflection. On the NS boundary of PSC there exists nonstationary perturbation addition to the order parameter potential with amplitude (delta) approximately (Delta) (Gamma) 3/2 ((Gamma) is the depairing factor of the superconductor). In terms of model developed the non- Josephson generation effect is only possible in the case of effective overbarrier quasiparticle scattering by (delta) , when the condition (epsilon) 1 - (Delta) >= (delta) is fulfilled. The generation frequency is found from the condition (epsilon) 1 - (Delta) equals $HBARwo on the order of (Delta) (Gamma) 3/2. This gives the characteristic frequencies wsn approximately 10 MHz for tin and wAl approximately MHz for aluminum, which is in good agreement with experiment. The other consequences of the model proposed are discussed, which were supported experimentally.
Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
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ESR spectroscopy research of superconductive signs in glasses enriched with Fe or Ni ions
Anatoly I. Veinger, Andrej G. Zabrodskii, Tatjana V. Tisnek, et al.
Magnetosensitive microwave absorption has been studied in oxide glasses enriched with iron or nickel ions. Conditions have been found under which magnetosensitive microwave absorption has the superconducting form, namely, it increases with field in weak magnetic fields, with the derivative of absorption with respect to magnetic field being at a maximum at fields of the order of 100 Oe. For glasses enriched with Fe ions, these conditions are the following: reduction of Fe3+ ions to Fe2+ and presence of Mg ions in the glass. For glasses enriched with Ni ions, the primary condition is formation in these glasses of tetrahedral structural complexes [Ni04]. In our opinion, such an absorption is due to the presence in these glasses of small-sized superconducting inclusions. Ways of increasing the concentration of these inclusions are discussed.
Determination of surface atomic layer of RBa2Cu3Oy thin films and SrTiO3 (001) single crystal by glancing-incidence-exit x-ray diffuse scattering
Ziyuan Liu, Seiya Ogota, Tadataka Morishita
The surface atomic layer of c-axis oriented RBa2Cu3Oy (R equals Nd, Y) thin films deposited by laser ablation process and SrTiO3 (001) single crystal has been studied by using the method of glancing incidence-exit x-ray diffuse scattering (GIEXS). Utilizing the GIEXS we have correctly identified the topmost layer of SrTiO3 (001) single crystal with a well defined surface terminating layer. It demonstrated the effectiveness of GIEXS to distinguish the surface atomic layer from its underlying layered bulk crystal. The observations on the RBa2Cu3Oy thin films using GIEXS proved that the surface of NdBa2Cu3Oy thin films is predominantly terminated with a CuO chain plane, while a different layer covered the surface of YBa2Cu3Oy thin films. The results on the surface of SrTiO3 (001) single crystal indicated that the surface termination layer of SrTiO3 treated by the chemical method was TiO2 plane, while the annealed SrTiO3 surface was mainly terminated with TiO2 plane with a small portion covered by SrO plane.
Experimental studies of the high-temperature superconducting cuprates using thermal difference reflectance spectroscopy
Catherine L. Perry, Matthew J. Holcomb, James P. Collman, et al.
We have previously measured the temperature dependent thermal difference reflectance (TDR) spectra of several high-temperature superconducting thin films. From the TDR spectra collected above and below the critical temperature of each sample, the superconducting to normal state reflectance ratio, RSRN, has been obtained. We observe significant deviations from unity in this reflectance ratio at photon energies on the order of 2.0 eV. Both the temperature dependence and location of this structure in the RS/RN spectra may be described by solving the Eliashberg integral equations with an electron-boson coupling function that includes both an electron-phonon interaction and an interaction located at approximately the energies of known charge transfer excitations in these materials (approximately 2.0 eV). We found remarkably good agreement between the experimental data and the results of our calculations based upon this description of the superconducting state. We also use the technique of TDR spectroscopy to probe the superconducting proximity effect. By measuring the TDR spectra of the silver layer in a Ag-(BiPb)2Sr2Ca2Cu3O10 bi-layer junction or the indium layer in a In-Ag-(BiPb)2Sr2Ca2Cu3O10 tri-layer junction, at temperatures above and below the critical temperature of the high temperature superconductor, we hope to observe structure due to a proximity induced gap function in the normal metal.
Raman Spectra
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Elastic and inelastic light scattering in the Zn- and Ni-doped YBa2Cu3O7
Milan J. Konstantinovic, Rados Gajic, Jovan Konstantinovic
We present the measurements of the elastically scattered 'plasma' lines intensities as well as the Raman spectra of YBa2(Cu1-xZnx)3O7 and YBa2(Cu1-xNix)3O7 as a function of the temperature and Zn or Ni content. In the case of elastic scattering we found two anomalies at the temperatures T equals Tc and T equals 150 K. This behavior is related to superconducting transition and indicates either the existence of charge fluctuation in temperature region from Tc to T equals 150 K or some electronic or surface phenomena. Furthermore, we analyzed the temperature dependence of the two Raman modes at 340 cm-1 and 440 cm-1. It is found that Zn substantially suppresses the superconductivity-induced phonon anomaly, whereas Ni does not affect it. Moreover, the superconductivity-induced phonon stiffening at the 440 cm-1 mode completely disappeared with Zn doping. This behavior might be explained considering Zn as a magnetic pair breaker.
Other Spectra (XAFS, RBS, ESR, M Sbauer, Thermoreflectance, etc.)
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Tuning of the Fermi level at the shape resonance in a superlattice of quantum wires: a mechanism for high Tc
Antonio Bianconi, A. Lanzara, A. Perali, et al.
We have measured the amplitude of the 1D lattice modulation forming the stripe structure in the CuO2 plane of Bi2Sr2Ca2Cu2O8+y single crystals by EXAFS. The period of the 1D modulation of the CuO2 plane has been measured by Cu anomalous diffraction. The large anharmonic content of the 1D modulation clearly shows the formation of stripes of undistorted LTO lattice. The large amplitude of the local structure modulation in plane indicates the formation of sizable potential barrier due to the linear domain walls in the plane. The size of the stripes L is such that the Fermi level is tuned to the maximum of the density of states formed by the second subband. The tuning of the Fermi level at the shape resonance of the superlattice of quantum wires is proposed to be the mechanism for the amplification of the critical temperature in high Tc cuprate superconductors.