Observation of bound surface states in grain-boundary junctions of high-temperature superconductors

Abstract

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the high temperature superconductors (HTS) YBa2Cu3O 7−δ (YBCO), Bi2Sr2CaCu2O 8+δ (BSCCO), La1.85Sr0.15CuO4 (LSCO) and Nd1.85Ce0.15CuO4−y (NCCO). In all experiments the tunneling direction was along the CuO2 planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak (ZBCP) was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.To appear in Physical Review B There is strong evidence that the superconducting order parameter (OP) in the HTS has a dominating d-wave symmetry [1,2]. For this pairing symmetry there is a π-phase shift of the OP in orthogonal k-space directions resulting in a positive and negative sign of the pair potential in those directions. This also means that there are directions with nodes of the pair potential, e. g. for a pure d x 2 −y 2 -symmetry, the nodes are along the [110] direction in the CuO 2 plane. For the tunneling spectra of junctions employing HTS electrode materials with a d-wave symmetry of the OP, a pronounced ZBCP has been predicted originating from mid-gap surface (interface) states or zero energy bound states (ZES) at the Fermi level [3][4][5][6][7][8]. The physical reason for these states originates from the fact that quasiparticles incident and reflecting from the surface propagate through different order parameter fields which leads to Andreev reflection. The constructive interference between incident and Andreev reflected quasiparticles results in bound states. Stable ZES are formed if the scattering induces a change in sign of the OP. For a d x 2 −y 2 -wave symmetry such sign change and, hence, the presence of ZES is possible for all surfaces parallel to the c-axis except for those with the lobe directions perpendicular to the surface, whereas for a s-wave symmetry no ZES are possible. The spectral weight of the ZES for a d x 2 −y 2 -wave symmetry depends on the orientation of the surface with respect to the crystal axis. The maximum spectral weigth is expected for a (110) surface and, hence, a maximum ZBCP is expected for tunneling in the direction of the nodal lines, i. e., the [110] direction. This has been observed recently using low temperature scanning tunneling spectroscopy (LTSTS) [9] and planar type junctions [10]. We note that the ZBCP is sensitive to surface roughness making it difficult to distinguish between the directions in the plane [11][12][13].Initially, the ZBCP in the tunneling spectra...

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

“…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.…”

“…However, there is no problem with the results of tunneling measurements. It has been shown that there is no qualitative difference in the tunneling spectra between NCCO and PCCO for 2 K< T < T c [5,18].…”

“…However, as can already be seen from the differences of the generic phase diagram on the electron-and hole-doped side, the physics of electron-and hole-doped HTS is different. In particular, the order parameter (OP) symmetry of the electron-doped cuprates is most likely of s-wave type [2][3][4][5], in contrast to the d-wave OP symmetry in the hole-doped HTS. To clarify the specific differences and similarities between the electron-and hole-doped HTS a more detailed experimental study of the electron-doped HTS is required.…”

Local and macroscopic tunneling spectroscopy ofY1−xCaxBa<

“…The hallmark of a pure d-wave OP in the tunneling spectra is the zero bias conductance peak (ZBCP) observed for tunneling along and near the nodal [110] direction [5][6][7][8][9][10][11] , reflecting the existence of surface Andreev bound states at the Fermi level [12][13][14][15][16] . In addition, U and V-shaped gaps can be seen, respectively, in the [100] and [001] directions 8, 11 , 15, 17, 18 .…”

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

“…In Chapter 4 we present a qualitative discussion of how a ZBCP may appear in voltage biased d|d junctions. This study was motivated by recent experiments [20,21]. In Chapter 5 we summarize the paper.…”