Andreev bound states in high temperature superconductors

Abstract

Abstract. Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa2Cu3O 7−δ (YBCO), Bi2Sr2CaCu2O 8+δ (BSCCO), and La1.85Sr0.15CuO4 (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd1.85Ce0.15CuO4−y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented.

Anomalous Low Temperature Behavior of SuperconductingNd1.85Ce0.15CuO4−

“…In contrast, our data for both NCCO and PCCO is clearly consistent with an isotropic s-wave symmetry of the OP. We further note that there is still a discrepancy between tunneling measurements suggesting an anisotropic s-wave OP for NCCO [18,21] and the present ∆λ ab (T ) measurements indicating an almost isotropic s-wave OP for both NCCO and PCCO. However, both measurements are clearly not consistent with a d-wave OP in the electron-doped HTS.…”

“…In contrast, the I c (H) patterns of NCCOand PCCO-GBJs are close to a regular Fraunhofer diffraction pattern expected for an ideal Josephson junction [13]. Finally, the tunneling spectra measured for NCCO do not show any zero bias anomaly giving strong evidence for the absence of a sign change in the superconducting pair potential [5,18,21]. These experimental facts exclude both a d x 2 −y 2 -and d xy -symmetry of the OP for NCCO and PCCO.…”

“…It has been both theoretically predicted and experimentally observed that λ ab (T ) can increase with decreasing T due to so-called anti-Meissner currents related to Andreev bound states [18,19]. However, assuming d x 2 −y 2 -symmetry of the OP the maximum spectral weight of such bound states is expected for interfaces close to the (110) orientation.…”

“…However, assuming d x 2 −y 2 -symmetry of the OP the maximum spectral weight of such bound states is expected for interfaces close to the (110) orientation. As a consequence this effect has only been observed in large angle grain boundaries [18], whereas the T dependence of ∆λ ab (T ) for small misorientation angles (θ 24 • ) was found to behave linearly as expected for a d-wave OP [14]. Furthermore, the YBCO-GBJs showing this kind of anomaly in λ ab (T ) also show a I c (H) pattern that strongly deviates from a Fraunhofer pattern and has the maximum I c value shifted to H = 0.…”

Resonant transport through midgap states in voltage-biased Josephson junctions of d-wave superconductors

“…These symmetric processes give a huge contribution to the current, resulting in finite current at low voltages, as in ballistic s|s-juntions. This may explain the zero-bias conductance peaks seen in experiments on grain boundary junctions of HTS superconductors [20,21].…”