Tunnelling effects on surface bound states in unconventional superconductors

Abstract

Recent studies on high-T c superconductors have aroused new interest in tunnelling effects in unconventional superconductors. Unlike in conventional s-wave superconductors, the d-wave pairing state in these materials has an internal phase of the pair potential. The internal phase as a function of the wavevector of the Cooper pairs has a large influence on the electric properties of tunnelling junctions. Important effects of the internal phase on the Josephson current were first predicted theoretically. The idea has been established through several experiments using high-T c Josephson junctions, which detect π -phase shift between the aand b-axis directions and fractional flux quanta. These results give convincing evidence for d-wave symmetry in high-T c superconductors. In addition, the existence of new interference effects in the quasiparticle states near surfaces and boundaries has been suggested through theoretical predictions. Experimentally, a large number of tunnelling spectroscopy data showed zero-bias conductance peaks (ZBCPs), the origin of which cannot be explained in terms of the classical concept that a tunnelling conductance spectrum is a phase-insensitive probe of the electronic states. It is clarified theoretically that the observed ZBCPs reflect the formation of zeroenergy states on the surface due to the π -phase shift of internal phase in the d-wave pairing symmetry. The formulation developed for tunnelling spectroscopy suggests that tunnelling spectroscopy is essentially phase sensitive. In addition, the formation of the bound states has been shown to have a serious influence on the electrical properties of Josephson junctions. Several anomalous properties including strong enhancement of the Josephson current in the low-temperature region have been predicted theoretically. In this report, recent developments in tunnelling effects on surface bound states in unconventional superconductors are reviewed.

Point-contact spectroscopy of Cu0.2Bi2Se3single crystals

“…One possible interpretation of such conductance structure is that the observed Zero Bias Conductance Peak (ZBCP) is a manifestation of an Andreev bound state originating from the surface of an unconventional superconductor. The peak is not as narrow as in the case of a d-wave superconductor 11 . The shape of the spectrum with the peak and the dips around it, is similar to the expected spectrum in a chiral p-wave superconductor 12 , where the andreev bound state is dispersive and wide.…”

Noise cross-correlation and Cooper pair splitting efficiency in multi-teminal superconductor junctions

“…The Andreev reflection coefficient is 1 and rapidly decays as V increases. In the differential shot noise the peak is split and at zero bias the shot noise is equal to zero [23,24,62,63]. These two results lead to a Fano factor equal to zero at zero bias for this kind of symmetry.…”

Theoretical prediction of the fractional ac Josephson effect in p- and d-wave superconductors

“…For d-wave cuprate superconductors, the midgap Andreev states were predicted theoretically in Ref. [5] and discovered experimentally as a zero-bias conductance peak in tunneling between normal metals and superconductors (see review [6]). For the Q1D organic superconductors, the midgap states were theoretically predicted to exist at the edges perpendicular to the chains [7,8].…”