# Universal Faraday rotation in HgTe wells with critical thickness

## TL;DR

This study demonstrates the universal Faraday rotation close to the fine structure constant in HgTe quantum wells at the critical thickness, revealing robust quantization linked to Dirac states and carrier type changes.

## Contribution

It provides experimental evidence of universal Faraday rotation in HgTe wells at the topological phase boundary, connecting it with Dirac physics and dynamic Hall conductivity.

## Key findings

- Quantized Faraday rotation near the fine structure constant.
- Robustness of the quantization across magnetic fields and gate voltages.
- Carrier type transition from holes to electrons with gate voltage.

## Abstract

The universal value of Faraday rotation angle close to the fine structure constant is experimentally observed in thin HgTe quantum wells with thickness on the border between trivial insulating and the topologically non-trivial Dirac phases. The quantized value of the Faraday angle remains robust in the broad range of magnetic fields and gate voltages. Dynamic Hall conductivity of the hole-like carriers extracted from the analysis of the transmission data shows theoretically predicted universal value of consistent with the doubly degenerate Dirac state. On shifting the Fermi level by the gate voltage the effective sign of the charge carriers changes from positive (holes) to negative (electrons). The electron-like part of the dynamic response does not show quantum plateaus and is well described within the classical Drude model.