A theory for tunneling spectroscopy in normal metal /insulator/triplet superconductor junction is presented. We assume two kinds of non-unitary triplet superconducting states which are the most promising states for Sr2RuO4. The calculated conductance spectra show zero-bias peaks as well as gap structures. The existences of residual components in the spectra reflect the non-unitary properties of superconducting states. 74.50.+r, 74.25.Fy, Recent discovery of superconductivity in Sr 2 RuO 4 1 provides us the first example of a noncuprate layered perovskite material that exhibits superconductivity. Since this compound is isostructural to the cuprate superconductors, the electronic properties in the normal state 2 and superconducting state 3 are highly anisotropic. The rather large residual density of states of quasiparticles at low temperatures is indicated by several experiments.4,5 Furthermore, there are several evidences which support the indications of ferromagnetic spin fluctuations.6 Based on these facts, some theories 7,8 proposed that the non-unitary triplet pairing superconducting states are realized in Sr 2 RuO 4 . Since the triplet pairing states have strong anisotropy in k-space, novel interference effects of the quasiparticles are expected to occur at boundaries and surfaces. To determine the symmetry of the pair potential definitively, it is important to predict the spectra of tunneling experiments which play a significant role to identify the d-wave symmetry in the high-T C superconductors. 9-11

The spatial variations of the local density of states ͑LDOS͒ on the surface of d x 2 Ϫy 2-wave superconductors are calculated using an extended Hubbard model. Our model has the ability to treat the surface geometry with atomic-scale roughness, and gives realistic results beyond those based on the quasiclassical theory. The spatial variation of the LDOS is obtained for various types of surface geometries using a self-consistently determined pair potential. As surface bound states are peculiar to d-wave superconductors, we pay attention to the appearance of zero-energy states ͑ZES's͒. For a flat surface, the formation of a ZES on a ͓110͔ surface and the lack of one on a ͓100͔ surface is obtained, which is consistent with the results of the quasiclassical theory. However, the LDOS shows abundant spatial dependence when the surfaces have roughness. The appearance of a ZES on nominal ͓100͔ surfaces and its disappearance on nominal ͓110͔ surfaces are obtained depending on geometry. The calculations explain the origin of various spectra observed by tunneling spectroscopy on high-T c superconductors. ͓S0163-1829͑98͒04113-7͔

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...

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