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

Surface Umklapp scattering of quasiparticles, and surface roughness are shown to play essential roles in the formation of the surface bound states in realistic models for YBa_2Cu_3O_{6+x}. The results account for the shape, the impurity dependence of the height, and for a proposed universal width of the zero bias conductance peak.

We report a theoretical study on the phase transition between superconducting states with and without spontaneous surface current. The phase transition takes place due to the formation of surface Andreev bound states in unconventional superconductors. Based on the quasiclassical theory of superconductivity, we examine the influence of atomic-scale surface roughness on the surface phase transition temperature $T_s$. To describe the surface effect, the boundary condition for th...

We report a theoretical study on the deviations of the Meissner penetration depth $\lambda(T)$ from its London value in d-wave superconductors at low temperatures. The difference arises from low-energy surface Andreev bound states. The temperature dependent penetration depth is shown to go through a minimum at the temperature $T_{m0}\sim \sqrt{\xi_0/\lambda_0}T_c$ if the broadening of the bound states is small. The minimum will straighten out when the broadening reaches $T_{m...

A leading contender for the pairing symmetry in the Fe-pnictide high temperature superconductors is extended s-wave $s_\pm$, a nodeless state in which the pairing changes sign between Fermi surfaces. Verifying such a pairing symmetry requires a special phase sensitive probe that is also momentum selective. We show that the sign structure of $s_\pm$ pairing leads to surface Andreev bound states at the sample edge. In the clean limit they only occur when the edge is along the n...

We report measurements of the temperature dependent components of the magnetic penetration depth {\lambda}(T) in single crystal samples of YBa_2Cu_4O_8 using a radio frequency tunnel diode oscillator technique. We observe a downturn in {\lambda}(T) at low temperatures for currents flowing along the b and c axes but not along the a axis. The downturn in {\lambda}_b is suppressed by a small dc field of ~0.25 T. This and the zero field anisotropy of {\lambda}(T) likely result fr...

Vortex entry into (110) oriented YBa$_2$Cu$_3$O$_{7-x}$ films has been studied by tunneling into Andreev - Saint-James bound states, whose energy is shifted by surface currents. At low temperatures, the characteristic field for vortex entry has been found to increase up to values several times higher than that of the Bean-Livingston entry field for conventional superconductors, in agreement with recent theoretical predictions.

A method is presented to measure the absolute value of the London penetration depth, $\lambda$, from the frequency shift of a resonator. The technique involves coating a high-$T_c$ superconductor (HTSC) with film of low - Tc material of known thickness and penetration depth. The method is applied to measure London penetration depth in YBa2Cu3O{7-\delta} (YBCO) Bi2Sr2CaCu2O{8+\delta} (BSCCO) and Pr{1.85}Ce{0.15}CuO{4-\delta}$ (PCCO). For YBCO and BSCCO, the values of $\lambda ...

Electrical transport through a normal metal / superconductor contact at biases smaller than the energy gap can occur via the reflection of an electron as a hole of opposite wave vector. The same mechanism of electron-hole reflection gives rise to low energy states at the surface of unconventional superconductors having nodes in their order parameter. The occurrence of electron-hole reflections at normal metal / superconductor interfaces was predicted independently by Saint Ja...

Ag/BSCCO and Fe/Ag/BSCCO planar tunnel junctions were constructed in order to study experimentally the effect of an exchange potential on the spin polarized current transported through Andreev bound states appearing at the interface with a superconductor with broken time reversal pairing symmetry. The zero bias conductance peak (ZBCP) resulting from the Andreev bound states (ABS) is split into two symmetric peaks shifted at finite energies when the counterlectrode is normal. ...

A computing platform based on low temperature superconductors (LTS) has already been proven both theoretically and experimentally. However, qubits based on high Tc superconductors (HTS) are not yet well understood. Here we study the Andreev bounds states (ABS) in the later materials in order to show that a formal correspondence exists between the Mathieu levels in a Cooper Pair Box qubit built with LTS and the Andreev levels in HTS junctions.