Time-reversal symmetry breaking state near the surface of $s_{\pm}$-superconductor

TL;DR

This paper theoretically investigates surface states in $s_{ ext{±}}$-superconductors, revealing conditions for time-reversal symmetry breaking states and their experimental signatures.

Contribution

It introduces the concept of a TRB surface state in $s_{ ext{±}}$-superconductors and analyzes its energetic stability compared to other surface states.

Findings

TRB surface state can be energetically favorable at low temperatures.

Transition from TRB to TR state occurs sharply near $T_c$.

Signatures detectable via local density of states measurements.

Abstract

The structure of superconducting order parameter near the surface of a two-band superconductor with $s_{\pm}$ order parameter in the bulk is theoretically investigated. The main parameter of the surface, which determines the appropriate physics is the coefficient of the interband scattering $R_{12}$. For small $R_{12}$ the superconducting order parameter is only suppressed to some extent near the surface for the both bands. For intermediate and strong interband scattering there are two possible non-trivial surface states of the order parameter: (i) purely real solution, where the symmetry of the superconducting state near the surface is changed from $s_{\pm}$ to conventional $s_{++}$ and (ii) time-reversal symmetry breaking (TRB) state. In this state the order parameters in the two bands acquire phases $\phi_{1,2}(x) \neq (0,\pi)$ upon approaching the surface. We argue that at low temperatures the TRB surface state can be more energetically favorable than the $s_{\pm} \to s_{++}$ time reversal symmetry conserving state (TR). For higher temperatures up to $T_c$ only the TR state can exist. The transition between the two temperature regions is rather sharp. Signatures of the transition between the TRB and the TR surface states can be detected by the measurements of the local density of states and the angle-resolved density of states.