Evidence for Surface Andreev Bound states in Cuprate Superconductors from Penetration Depth Measurements

Andreev bound states (ABS) 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 ABS in 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 tun...

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and NCCO. In all experiments the tunneling direction was along the CuO planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak 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-w...

Scanning tunneling spectroscopy of thin epitaxial $SrRuO_{3}/(110)YBa_2Cu_3O_{7-\delta}$ ferromagnet/superconductor bilayers, reveal a clear penetration of the Andreev bound states into the ferromagnetic layer. The penetration is manifested in the density of states of the ferromagnet as a split zero bias conductance peak with an imbalance between peak heights. Our data indicate that the splitting occurs at the superconductor side as a consequence of induced magnetization, con...

We have made in-situ [103] and [110]YBCO/Pb junctions, which display an hysteretic behaviour with both a well-defined Quasi-Particule branch and a Josephson current. A zero energy conductance peak is observed in the tunneling conductance which is a signature of Andreev Bound States (ABS) at a surface of a d-wave superconductor. The observed splitting of this peak at low temperature may confirm the existence of a surface state which breaks time-reversal symmetry. However, up t...

We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb ramp junctions of several geometries. Macroscopically, tunneling occurs in the ab-plane of YBCO either in the (100) and (010) direction, or in the (110) direction. These junctions have a stable and macroscopically well defined geometry. This allows systematic investigations of both quasiparticle and Josephson tunneling over a wide range of temperature and magnetic field. With Nb superconducting, its...

Although considerable studies have been carried out, the true nature of high-Tc superconductors (HTCS) is still not clear. Pseudogap phase at high temperature as well as possible time reversal symmetry breaking at low temperature need further investigations. The need of carefully made samples showing the intrinsic properties of superconductivity is essential to test new theoretical developments. We present in this paper how to control crystallographic orientation in the junct...

We investigate in-plane quasiparticle tunneling across thin film grain boundary junctions (GBJs) of the electron-doped cuprate La$_{2-x}$Ce$_{x}$CuO$_4$ in magnetic fields up to $B=16 $T, perpendicular to the CuO$_2$ layers. The differential conductance in the superconducting state shows a zero bias conductance peak (ZBCP) due to zero energy surface Andreev bound states. With increasing temperature $T$, the ZBCP vanishes at the critical temperature $T_c\approx29 $K if B=0, an...

The formation of bound states at surfaces of materials with an energy gap in the bulk electron spectrum is a well known physical phenomenon. At superconductor surfaces, quasiparticles with energies inside the superconducting gap $\Delta$ may be trapped in bound states in quantum wells, formed by total reflection against the vacuum and total Andreev reflection against the superconductor. Since an electron reflects as a hole and sends a Cooper pair into the superconductor, the ...

We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle tunneling spectra of thin film grain-boundary Josephson junctions made of the electron doped cuprate superconductor LaCeCuO. An applied magnetic field reduces the spectral weight around zero energy and shifts it non-linearly to higher energies consistent with a Doppler shift of the Andreev bound states (ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the onset of superconduc...

This topical review summarizes various features of magnetic penetration depth in unconventional superconductors. Precise measurements of the penetration depth as a function of temperature, magnetic field and crystal orientation can provide detailed information about the pairing state. Examples are given of unconventional pairing in hole- and electron-doped cuprates, organic and heavy fermion superconductors. The ability to apply an external magnetic field adds a new dimension...