Unsplit superconducting and time reversal symmetry breaking transitions in Sr2RuO4 under hydrostatic pressure and disorder

Vadim Grinenko,Bastian Zinkl,Naoki Kikugawa,Debarchan Das,Yoshiteru Maeno,Clifford W Hicks,Rustem Khasanov,Ritu Gupta,Hans-Henning Klauss,Manfred Sigrist

doi:10.1038/s41467-021-24176-8

Abstract

There is considerable evidence that the superconducting state of Sr2RuO4 breaks time reversal symmetry. In the experiments showing time reversal symmetry breaking, its onset temperature, TTRSB, is generally found to match the critical temperature, Tc, within resolution. In combination with evidence for even parity, this result has led to consideration of a dxz ± idyz order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding TTRSB = Tc, but it has a hard-to-explain horizontal line node at kz = 0. Therefore, s ± id and d ± ig order parameters are also under consideration. These avoid the horizontal line node, but require tuning to obtain TTRSB ≈ Tc. To obtain evidence distinguishing these two possible scenarios (of symmetry-protected versus accidental degeneracy), we employ zero-field muon spin rotation/relaxation to study pure Sr2RuO4 under hydrostatic pressure, and Sr1.98La0.02RuO4 at zero pressure. Both hydrostatic pressure and La substitution alter Tc without lifting the tetragonal lattice symmetry, so if the degeneracy is symmetry-protected, TTRSB should track changes in Tc, while if it is accidental, these transition temperatures should generally separate. We observe TTRSB to track Tc, supporting the hypothesis of dxz ± idyz order.

Highlights

There is considerable evidence that the superconducting state of Sr2RuO4 breaks time reversal symmetry
In a previous ZF-μSR experiment, in-plane uniaxial pressure, which does lift the tetragonal symmetry of the unpressurised lattice, was found to induce a strong splitting between Tc and TTRSB47
The microscopic mechanism yielding the signal observed at TTRSB, a weak enhancement in muon spin relaxation rate, remains unclear: the main proposed mechanism, magnetism induced at defects and domain walls by a time reversal symmetry breaking (TRSB) superconducting order, is unproved experimentally[54,55]

Summary

Introduction

There is considerable evidence that the superconducting state of Sr2RuO4 breaks time reversal symmetry. S ± id and d ± ig order parameters are under consideration These avoid the horizontal line node, but require tuning to obtain TTRSB ≈ Tc. To obtain evidence distinguishing these two possible scenarios (of symmetry-protected versus accidental degeneracy), we employ zero-field muon spin rotation/relaxation to study pure Sr2RuO4 under hydrostatic pressure, and Sr1.98La0.02RuO4 at zero pressure. Unconventional pairing states are distinguished from conventional ones by a non-trivial intrinsic phase structure which causes additional spontaneous symmetry breaking at the superconducting phase transition This can lead, for instance, to a reduction of the crystal symmetry or the loss of time reversal symmetry. There are a few known examples: s and dx2Ày2 are relatively close in energy in iron-based superconductors[11,12], while both (U,Th)Be131,4 and UPt32,3,8 have split Tc and TTRSB

Methods

Results

Conclusion