Odd-frequency pairs and anomalous proximity effect in nematic and chiral states of superconducting topological insulators

Abstract

We investigate emergent odd-frequency pairs and proximity effect in nematic and chiral states of superconducting topological insulators (STIs), such as ${M}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ ($M=$ Cu, Sr, Nb). The interplay of pairing symmetry, the orbital degrees of freedom, and strong spin-orbit interaction generates a variety of odd-frequency pairs in the bulk and surface of STIs. The nematic and chiral states are the prototypes of topological superconductors with and without time-reversal symmetry, respectively. We find that the Fermi surface evolution from a closed spheroidal to an open cylindrical shape changes the pairing symmetry from the nematic to chiral state, which causes the evolution of the odd-frequency pairings and surface Andreev bound states (SABSs). In addition, spin polarization of odd-frequency pairs and SABSs stems from the nonunitary pairing in the chiral state. The evolution and spin polarization of odd-frequency pairs and SABSs can be captured by tunnel conductance spectroscopy. Furthermore, we study the anomalous proximity effect in various irreducible representations of STIs. The anomalous proximity effect was originally predicted in spin-triplet superconductor junctions without spin-orbit interaction. Odd-frequency spin-triplet $s$-wave pairs penetrate into diffusive normal (DN) metals and induce a pronounced zero-energy peak of the local density of states in the DN region. Here we demonstrate that contrary to the well-known results, the anomalous proximity effect in STIs is not immune to nonmagnetic impurities. The fragility is attributed to the fact that the proximitized odd-frequency even-parity pairs are admixtures of $s$-wave and non-$s$-wave pairs due to strong spin-orbit interaction inherent to the parent materials.