Planar tunneling spectroscopy of high-temperature superconductors: Andreev bound states and broken symmetries (original) (raw)
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Solid State Communications, 1998
Tunneling into high-temperature superconductors is shown to be a powerful spectroscopic probe of the unconventional superconducting state in YBa2Cu307 (YBCO). Planar tunneling is performed on low-leakage, aboriented YBCO/I/Cu junctions, where I is formed with a self-assembled organic monolayer. The reproducible dependence of the tunneling conductance on temperature, magnetic-field, doping and ion-induced disorder confirm that the zero-bias conductance peak (ZBCP) is an Andreev bound state, which arises directly from a d-wave order parameter. At temperatures below-8 K in zero applied magnetic field, the ZBCP splits, signifying a phase transition into a broken time-reversal symmetry state. An applied magnetic field induces further splitting that grows nonlinearly with increasing field. A theoretical model that invokes the formation of subdominant order parameter near the surface shows striking qualitative and quantitative agreement with our data.
Journal of Superconductivity - J SUPERCOND, 2000
An emphasis on reliable materials growth and development of new fabrication techniques has allowed us to investigate the electronic structure of high-temperature superconductors by planar quasiparticle tunneling and electron paramagnetic resonance (EPR) spectroscopies. The quasiparticle (QP) density of states (DoS) is investigated by tunneling into oriented thin films of Y1Ba2Cu3O7 (YBCO) and single crystals of Ba2Sr2Ca1Cu2O8 (BSCCO). Data are obtained as a function of crystallographic orientation, temperature, doping, damage, and applied magnetic field. These data demonstrate that the observed zero-bias conductance peak (ZBCP) is composed of Andreev bound states (ABS) which intrinsically form at a symmetry-breaking interface of an unconventional superconductor, for example, a (110)-surface of d-wave YBCO. Tunneling into doped or ion-damaged YBCO provides a measure of the QP scattering rate below Tc. An applied field causes Doppler shift of the ABS, arising from the scalar product b...
2000
We present a systematic study of the charge transport and quasiparticle tunneling properties of Y 1Ϫx Pr x Ba 2 Cu 3 O 7 thin films. Pr doping increases the resistivity along the copper oxide planes and suppresses the superconducting critical temperature T c , ultimately inducing a superconductor-insulator transition. The tunneling conductance is reproducible and correlated with the crystallographic film orientation. The crystallographic dependence can be divided into two distinct categories: tunneling into ͑001͒-oriented (c-axis͒ films and tunneling into ͑100͒-, ͑110͒-, and ͑103͒-oriented (ab-oriented͒ films. c-axis tunneling data exhibit a conductance dip at zero bias and a broad temperature-dependent peak over ϳ15-40 mV that decreases in magnitude but stays fixed in energy for increasing Pr doping levels. ab-plane tunneling data exhibit a zero-bias conductance peak and a gaplike feature at an energy that scales roughly linearly with T c for xϭ0.0, 0.2, and 0.4. When xϭ0.5, the resistivity is not linear in temperature and a zero-bias conductance dip is observed. The background conductance that ensures conservation of states in the low-temperature ab-plane data exhibits temperature-and doping-dependent structure over ϳ15-40 mV that is very similar to the peak observed in c-axis tunneling. Finally, analysis of the temperature and magnetic field dependence of the zero bias conductance peak indicates that states are conserved to within ϳ20%, supporting its interpretation as a feature of a superconducting density of states.
Atomic site tunneling spectroscopy on high-Tc superconductors
Physica B: Condensed Matter, 1994
Superconducting gap structures of BizSr2CaCu2Oy (BSCCO) and YBa2Cu3Oy (YBCO) have been probed by scanning tunneling microscopy (STM) at cryogenic temperatures. The tunneling conductance curves observed on bulk single crystals of BSCCO and epitaxial thin films of YBCO revealed clear overshooting peaks and flat bottom regions around V= 0 with quite low zero-bias conductances of -1%. Since the electron tunneling process in STM is essentially incoherent, the present observation is favored by the s-wave pairing mechanism. However, the conductance curves were found to be substantially smeared in comparison with the conventional excitation spectra predicted in the BCS (isotropic s-wave) superconductors, suggesting gap anisotropy. 0921-4526/94/$07.00 ~ 1994 Elsevier Science B.V. All rights reserved SSDI: 0921-4526(93)E0192-J
Temperature dependence of tunneling spectra in YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+δ single crystals
Journal of Electron Spectroscopy and Related Phenomena, 2000
During past years, scanning tunneling spectroscopy (STS) investigations of high temperature superconductors have revealed unusual characteristics of the superconducting gap. We present here the temperature dependence of the tunneling conductance spectra on YBa Cu O and Bi Sr CaCu O single crystals. We discuss similarities and differences 2 3 7 2d 2 2 2 81d between these two compounds. In particular we note that while in Bi Sr CaCu O a pseudogap is observed above the 2 2 2 81d critical temperature, even in overdoped samples, we find no indication of a pseudogap in optimally doped YBa Cu O 2 3 7 2d above T. This is consistent with other striking differences between the two systems and supports the interpretation of the c pseudogap in terms of strong superconducting correlations.
Local tunneling probe of (110) Y_{0.95}Ca_{0.05}Ba_{2}Cu_{3}O_{7−δ} thin films in a magnetic field
Physical Review B, 2010
Scanning tunneling spectroscopy was performed on (110)-oriented thin films of Ca-overdoped Y0.95Ca0.05Ba2Cu3O 7−δ at 4.2K, to probe the local evolution of Andreev−Saint-James surface states in a c-axis magnetic field. In zero field, we observed conductance spectra with spontaneously-split peaks and spectra with unsplit zero-bias peaks. The former showed enhanced splitting with field, and the latter showed threshold splitting above finite fields. Although both field evolutions can be described in terms of screening and orbital supercurrents, within the framework of d ± iα pairing (d=d x 2 −y 2 ; α=dxy,s), the enhanced splitting is consistent with only the d+iα state. Our results have direct implications on the stability of broken time-reversal symmetry in cuprate superconductors.
Physica C: Superconductivity, 2001
Scanning tunneling spectroscopy (STS) studies reveal long-range (~100 nm) spatial homogeneity in optimally and underdoped superconducting YBa 2 Cu 3 O 7−δ (YBCO) single crystals and thin films, and macroscopic spatial modulations in overdoped (Y 0.7 Ca 0.3)Ba 2 Cu 3 O 7−δ (Ca-YBCO) epitaxial films. In contrast, STS on an optimally doped YBa 2 (Cu 0.9934 Zn 0.0026 Mg 0.004) 3 O 6.9 single crystal exhibits strong spatial modulations and suppression of superconductivity over a microscopic scale near the Zn or Mg impurity sites, and the global pairing potential is also reduced relative to that of optimally doped YBCO, suggesting strong pair-breaking effects of the non-magnetic impurities. The spectral characteristics are consistent with d x 2 −y 2 pairing symmetry for the optimally and underdoped YBCO, and with (d x 2 −y 2+s) for the overdoped Ca-YBCO. The doping-dependent pairing symmetry suggests interesting changes in the superconducting ground state, and is consistent with the presence of nodal quasiparticles for all doping levels. The maximum energy gap ∆ d is non-monotonic with the doping level, while the (2∆ d /k B T c) ratio increases with decreasing doping. The similarities and contrasts between the spectra of YBCO and of Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) are discussed.
Europhysics Letters (EPL), 2003
Tunneling spectroscopy of epitaxial (100) oriented YBa2Cu3O 7−δ films was performed using an STM at 4.2 K. On atomically smooth areas, tunneling spectra revealing clear U-shaped gaps with relatively low zero bias conductance were measured. These spectra can be well fitted to the tunneling theory into a d-wave superconductor only when introducing a strong dependence of the tunneling probability on the wave vector k. Possible origins for this k-selectivity in STM measurements will be discussed. On other areas, V-shaped gaps as well as zero bias conductance peaks are observed, indicating relaxation of k-selectivity and the effect of nanofaceting, respectively.
Tunneling and infrared spectroscopy on high Tc superconductors
Journal of Physics and Chemistry of Solids, 1993
We have investigated single crystais of Bi$k2Cat,YxC$Oe (x=0 -0.35) by far infrared transmission spectroscopy and by break junction tmmeling. The junctions were made at low temperatures in vacuum, and SIS tunneling was measured at various temperatures and magnetic fields. 'IEe tmmeling resuits are interpreted in terms of a Josephson current and a single electron tunneling density of states (DOS). Although the DOS is found to he zerc at the Fermi level, we do not see a fully developed gap even at temperatures well below the critical temperature.
Tunneling Into High TC Superconductor Thin Films in the Presence of High Magnetic Fields
2006
We measured the tunneling conductance of a YBa2Cu3O7−x high Tc superconductor in the presence of high magnetic fields. Using planar indium junctions, we studied the superconductor density of states and the order-parameter symmetry. Typical planar tunneling conductance into the CuO2 planes exhibits two main characteristics: a Zero-Bias Conductance Peak (ZBCP) and a gap-like feature (GLF). The first is a distinct mark for the d -wave order-parameter symmetry, while the later marks the maximum superconducting gap energy scale. In this work we studied the evolution of both the ZBCP and GLF in the presence of high magnetic fields. The ZBCP splits into two peaks under applied magnetic fields perpendicular to the CuO2 planes. We show that, while keeping the magnetic field parallel to the film surface, the ZBCP splitting is remarkably different for films with different crystallographic orientations. We discuss the different mechanisms responsible for the ZBCP splitting. In (110) oriented fi...