Theory of Tunneling Conductance for Normal Metal/Insulator/ Triplet Superconductor Junction

Abstract

Tunneling conductance spectra of normal metal/insulator/triplet superconductor junctions are investigated theoretically. As triplet paring states we select several types of symmetries that are promising candidates for the superconducting states in UPt 3 and in Sr 2 RuO 4 . The calculated conductance spectra are sensitive to the orientation of the junction which reflects the anisotropy of the pairing states. They show either zero-bias conductance peaks or gap-like structures depending on the orientation of the junctions. The existence of a residual density of states, peculiar to nonunitary states, is shown to have a significant influence on the properties of the conductance spectra. Present results serve as a guide for the experimental determination of the symmetry of the pair potentials in UPt 3 and Sr 2 RuO 4 .KEYWORDS: triplet superconductor, nonunitary pair potential, zero-bias conductance peak, §1. IntroductionSince the discovery of superconductivity in heavy fermion compounds the determination of the symmetry of the pair potential in these materials has been an important issue. 1) Among the heavy fermion superconductors, so far properties of UPt 3 have been studied most extensively. Based on NMR experiments 2) the possibility of odd parity pairing states, i.e., triplet pairing states, has been suggested. Theoretically, several papers propose two-dimensional even parity states, i.e. singlet pairing states, 3,4,5) while others suggest triplet pairing states belonging to the onedimensional representation (A u ) 6, 7) and to the two-dimensional representation (E u ). 8, 9) Although theoretical 10,11,12) and experimental 13,14,15,16,17) spectroscopic studies of the thermal conduction and transverse sound attenuation of UPt 3 have been performed, the symmetry of the pair potentialcould not yet been determined.Recently superconductivity has been discovered in Sr 2 RuO 4 , 18) which is the first example of a noncuprate layered perovskite superconductor. Since this compound is isostructual to the cuprate superconductors the electronic properties in the normal state 19) Phase-sensitive measurements provide the most useful information for the determination of the symmetry of the pair potentials. 27) Recently it was shown that tunneling spectroscopy has the ability to detect the phase of the pair potential, 28,29,30) as follows: In anisotropic superconductors quasiparticles feel different signs of the pair potentials depending on the directions of their motions. 31) At the normal metal/superconductor interface the anisotropy of the pair potential significantly influences the properties of the Andreev reflections. 32, 33) As a result tunneling conductance spectra of the normal metal/insulator/anisotropic superconductor junctions are modified due to the anisotropy of pair potential. 28,29) The most remarkable feature is the existence of zerobias conductance peaks (ZBCP) in the tunneling spectra for d-wave symmetry. The origin of these peaks is the localized zero energy states (ZES) 34) due to the change of sign of the p...

Split of zero-bias conductance peak in normal-metal/d-wave superconductor junctions

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Quasiparticle States near the Surface and the Domain Wall in apx±ipy-Wave Superconductor

“…21,22,23,24,25,26,27) Recently characteristic tunneling conductance properties were examined theoretically for the triplet pairing states. 28,29) Conductance peak features related to the bound states are very sensitive to the angle of the incidence in this case, in contrast to the d-wave superconductor. In this paper we investigate the electronic states near the surface and compare them with those found near domain walls.…”

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Theory of thermal and charge transport in diffusive normal metal/superconductor junctions

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