Direct Evidence for Time-Reversal Symmetry Breaking in Topological Superconductor Sr0.1Bi2Se3

TL;DR

This study provides direct experimental evidence of time-reversal symmetry breaking in the topological superconductor Sr0.1Bi2Se3, indicating the presence of a spin-triplet pairing state on the surface, which is significant for understanding unconventional superconductivity.

Contribution

The paper reports the first detection of TRS breaking in Sr0.1Bi2Se3 using μSR measurements, revealing a fully gapped spin-triplet pairing state influenced by structural and spin-orbit interactions.

Findings

TRS breaking detected in Sr0.1Bi2Se3 via μSR.

Evidence for a nodeless, spin-triplet superconducting state.

TRS breaking suggests chiral superconductivity on a topological surface.

Abstract

The single helical Fermi surface on the surface state of three-dimensional topological insulator Bi2Se3 is constrained by the time-reversal invariant bulk topology to possess a spin-singlet superconducting pairing symmetry. In fact, the Cu-doped, and pressure-tuned superconducting Bi2Se3 show no evidence of the time reversal symmetry breaking. We report on the detection of the time reversal symmetry (TRS) breaking in the topological superconductor Sr0.1Bi2Se3 , probed by zero-field (ZF) {\mu}SR measurements. The TRS breaking provides strong evidence for the existence of spin-triplet pairing state. The temperature dependent super-fluid density deduced from transverse-field (TF) {\mu}SR measurement yields nodeless superconductivity with low superconducting carrier density and penetration depth {\lambda} = 1622(134) nm. From the microscopic theory of unconventional pairing, we find that such a fully gapped spin triplet pairing channel is promoted by the complex interplay between the structural hexagonal warping and higher order Dresselhaus spin-orbit coupling terms. Based on Ginzburg-Landau analysis, we delineate the mixing of singlet to triplet pairing symmetry as the chemical potential is tuned far above from the Dirac cone. Our observation of such spontaneous TRS breaking chiral superconductivity on a helical surface state, protected by the TRS invariant bulk topology, can open new avenues for interesting research and applications.