Field induced charge symmetry in topological insulator Bi$_2$Te$_3$ revealed by nuclear magnetic resonance

TL;DR

This study uses nuclear magnetic resonance to reveal a unique charge symmetry property in the bulk of topological insulator Bi$_2$Te$_3$, showing how electric field gradients respond to external magnetic field orientation.

Contribution

It demonstrates the presence of two electric field gradients at $^{209}$Bi in Bi$_2$Te$_3$, revealing a hidden charge symmetry property linked to electronic degrees of freedom.

Findings

Two electric field gradients at $^{209}$Bi observed

One gradient aligns with the lattice, the other with the external magnetic field

Suggests a new quadrupolar relaxation mechanism

Abstract

Nuclear magnetic resonance (NMR) was recently shown to measure the bulk band inversion of Bi$_2$Se$_3$ through changes in the $^{209}$Bi nuclear quadrupole interaction, and the corresponding tensor of the local electric field gradient was found to follow, surprisingly, the direction of the external magnetic field if the sample is rotated. This manifests a hidden property of the charge carriers in the bulk of this topological insulator, which is explored here with another material, Bi$_2$Te$_3$. It is found that two electric field gradients appear to be present at $^{209}$Bi, one rests with the lattice, as usual, while a second follows the external field if it is rotated with respect to the crystal axes. These electronic degrees of freedom correspond to an effective rotation of $j$-electrons, and their level life time is believed to be responsible for a new quadrupolar relaxation that should lead to other special properties including the electronic specific heat.