Universal conductance fluctuations and direct observation of crossover of symmetry classes in topological insulators

TL;DR

This study directly observes the crossover of symmetry classes in topological insulators through conductance fluctuation measurements, confirming the universal conductance fluctuations and the preservation of time reversal symmetry.

Contribution

First direct measurement of symmetry class crossover in topological insulators using mesoscopic conductance fluctuations, providing clear evidence of symmetry change from symplectic to unitary.

Findings

Observed a factor of two reduction in conductance fluctuations under magnetic field.

Confirmed universal conductance fluctuations as the main source of mesoscopic fluctuations.

Validated phase breaking length consistency across different measurement methods.

Abstract

A key feature of topological insulators (TI) is symplectic symmetry of the Hamiltonian which changes to unitary when time reversal symmetry is lifted and the topological phase transition occurs. However, such a crossover has never been explicitly observed, by directly probing the symmetry class of the Hamiltonian. In this report, we have probed the symmetry class of topological insulators by measuring the mesoscopic conductance fluctuations in the TI Bi$_{1.6}$Sb$_{0:4}$Te$_2$Se, which shows an exact factor of two reduction on application of a magnetic field due to crossover from symplectic to unitary symmetry classes. The reduction provides an unambiguous proof that the fluctuations arise from the universal conductance fluctuations (UCF), due to quantum interference and persists from T = 22 mK to 4.2 K. We have also compared the phase breaking length (l$_\phi$) extracted from both magneto-conductivity and UCF which agree well within a factor of two in the entire temperature and gate voltage range. Our experiment confirms UCF as the major source of fluctuations in mesoscopic disordered topological insulators, and the intrinsic preservation of time reversal symmetry in these systems.