Time-reversal symmetry breaking in topological superconductor Sr0.1Bi2Se3

Abstract

The single helical Fermi surface on the surface state of three-dimensional topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ 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 ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ show no evidence of the time-reversal symmetry (TRS) breaking. We report on the detection of the TRS breaking in the topological superconductor ${\mathrm{Sr}}_{0.1}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, probed by zero-field $\ensuremath{\mu}\mathrm{SR}$ measurements. The TRS breaking provides strong evidence for the existence of a spin-triplet pairing state. The existence of TRS breaking is also verified by longitudinal-field $\ensuremath{\mu}\mathrm{SR}$ measurements, which negates the possibility of magnetic impurities as the source of TRS breaking. The temperature-dependent superfluid density deduced from transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements yields nodeless superconductivity with low superconducting carrier density and penetration depth $\ensuremath{\lambda}=1622(134)\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. 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 avenues for interesting research and applications.