$^{77}$Se nuclear magnetic resonance of topological insulator Bi$_2$Se$_3$

TL;DR

This study uses $^{77}$Se NMR to explore the bulk electronic properties of Bi$_2$Se$_3$, revealing two distinct resonance lines and strong internuclear coupling, thus providing new insights into topological insulators' bulk behavior.

Contribution

It presents the first detailed NMR analysis of bulk $^{77}$Se in Bi$_2$Se$_3$, uncovering features that challenge previous surface-focused studies.

Findings

Identification of two resonance lines from inequivalent Se sites

Observation of unusual field-independent linewidths

Evidence of strong internuclear coupling mediated by bulk electrons

Abstract

Topological insulators (TIs) constitute a new class of materials with an energy gap in the bulk and peculiar metallic states on the surface. To date, most experiments have focused on probing the surface electronic structure of these materials. Here, we report on new and potentially interesting features resulting from the bulk electronic structure. Our findings are based on a comprehensive nuclear magnetic resonance (NMR) study of $^{77}$Se on Bi$_2$Se$_3$ and Cu$_{0.15}$Bi$_2$Se$_3$ single crystals. First, we find two resonance lines and show that they originate from the two inequivalent Se lattice sites. Second, we observe unusual field-independent linewidths, and attribute them to an unexpectedly strong internuclear coupling mediated by bulk electrons. These results call for a revision of earlier NMR studies and add insight into the bulk electronic properties of TIs.