Observation of broken inversion and chiral symmetries in the pseudogap phase in single and double layer bismuth-based cuprates

TL;DR

This study uncovers broken inversion and chiral symmetries in the pseudogap phase of bismuth-based cuprates through photogalvanic response measurements, revealing symmetry reductions below the pseudogap temperature.

Contribution

It provides direct evidence of symmetry breaking in the bulk pseudogap phase and refines the possible point group symmetries involved in the phase transition.

Findings

Broken inversion and chiral symmetries observed below T*

Symmetry limited to mm2 or mm2̅1 groups

Bulk symmetry breaking confirmed beyond surface effects

Abstract

We deduce the symmetry of the pseudogap state in the single and double layer bismuth-based cuprate superconductors by measuring and analyzing their circular and linear photogalvanic responses, which are related linearly to the chirality and inversion breaking respectively of the order parameter. After separating out the trivial contribution arising from the surface where inversion symmetry is already broken, we show that both responses start below the pseudogap temperature $T^*$ and grow below it to a sizable magnitude, revealing the broken symmetries in the bulk of the crystal. Through a detailed analysis of the dependence of the signals on the angle of incidence, the polarization of the light, and the orientation of the crystal, we are able to discover that the point group symmetry below $T^*$ is limited to $mm2$ or $mm2\underline{1}$ groups. Taking into account formation of domains and previous measurements, our results narrow down the possible symmetries of the microscopic origin of the phase transition(s) at $T^*$.