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

Many of the properties of superconductors related to quantum coherence are revealed when the superconducting state is forced to vary in space - in response to an external magnetic field, a proximity contact, an interface to a ferromagnet, or to impurities embedded in the superconductor. Among the earliest examples is Andreev reflection of an electron into a retro-reflected hole at a normal-superconducting interface. In regions of strong inhomogeneity multiple Andreev reflecti...

We study surface states of high-$T_C$ cuprate superconductor YBCO using the bilayer $t-J$ model. Calculations based on the Bogoliubov de Gennes method show that a flux phase that breaks time-reversal symmetry (${\cal T}$) may arise near a (110) surface where the $d_{x^2-y^2}$-wave superconductivity is strongly suppressed. It is found that the flux phase in which spontaneous magnetic fields in two layers have opposite directions may be stabilized in a wide region of doping rat...

We study theoretically the spatial distribution of spontaneous magnetic fields near a (110) surface of cuprate high-$T_C$ superconductor YBCO by treating a bilayer $t-J$ model with the Bogoliubov-de Gennes method. Near a (110) surface where $d_{x^2-y^2}$-wave superconductivity is strongly suppressed, flux phase order occurs locally leading to a time-reversal symmetry broken surface state. Since the spontaneous currents flowing in the flux phase in two layers are antiparallele...

We explore the combined pressure and finite-size effects on the in-plane penetration depth \lambda_{ab} in YBa_2Cu_4O_8. Even though this cuprate is stoichiometric the finite-size scaling analysis of \lambda_{ab}^{-2}(T) uncovers the granular nature and reveals domains with nanoscale size L_{c} along the c-axis. L_{c} ranges from 33.2 Angstrom to 28.9 Angstrom at pressures from 0.5 to 11.5 kbar. These observations raise serious doubts on the existence of a phase coherent macr...

Spin-pumping from a ferromagnetic insulator into a high-$T_c$ superconductor with a $d$-wave superconducting order parameter has recently been experimentally observed. Such unconventional superconducting order is known to produce interfacial bound-states for certain crystallographic orientations. Here, we present a methodology which can be used to study spin-pumping into unconventional superconductors, including the role of interfacial bound-states. As an example, we determin...

We show that interface bound states are formed at isolated graphene-superconductor junctions. These states arise due to the interplay of virtual Andreev and normal reflections taking place at these interfaces. Simple analytical expressions for their dispersion are obtained considering interfaces formed along armchair or zig-zag edges. It is shown that the states are sensitive to a supercurrent flowing on the superconducting electrode. The states provide long range superconduc...

We theoretically describe a thermoelectric effect that is entirely due to Andreev processes involving the formation of Cooper pairs through the coupling of electrons and holes. The Andreev thermoelectric effect can occur in ballistic ferromagnet-superconductor junctions with a dominant superconducting proximity effect on the ferromagnet, and it is very sensitive to surface states emerging in unconventional superconductors. We consider hybrid junctions in two and three dimensi...

The temperature dependence of the London penetration depth \lambda was measured for an untwined single crystal of YBa_2Cu_3O_{7-\delta} along the three principal crystallographic directions (a, b, and c). Both in-plane components (\lambda_a and \lambda_b) show an inflection point in their temperature dependence which is absent in the component along the c-direction (\lambda_c). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture...

The c-axis penetration depth $\lambda_c$ in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (BSCCO) single crystals as a function of temperature has been determined using three high-frequency techniques, namely: (i) measurements of the ac-susceptibility at a frequency of 100 kHz for different sample alignments with respect to the ac magnetic field; (ii) measurements of the surface impedance in both superconducting and normal states of BSCCO crystals at 9.4 GHz; (iii) measurements of the s...

The temperatuer dependence of the magnetic penetration depth of grain aligned YBa2Cu4O8 has been measured along the ab plane and c-axis. Both $\lambda_ab$ and $\lambda_c$ vary as \sqrt{T} up to 0.4Tc implying a square root density of states at low energy. The results are discussed in terms of a proximity model of alternating stacked superconducting and normal layers