## Topological states of matter

#### Collapse of the transport lifetime at a topological phase transition

Topological insulators are states of matter characterized by an inverted band structure driven by strong spin-orbit coupling. They have a variety of unusual properties including robust surface states protected by the topology of the electron wavefunctions.

Graduate student Liang Wu (Armitage group) conducted a detailed study of the quantum phase transition separating a topological insulator from a conventional one in (Bi_{1−x}In_{x})_{2}Se_{3}. He utilized time-domain terahertz spectroscopy to investigate the low frequency conductivity in a series of these compounds with different indium content *x*. When *x* crossed the critical value of 0.06, the observed transport lifetime of electrons collapsed, indicating the destruction of surface states that distinguish a topological insulator and the onset of a topologically trivial phase.

The collapse was less sudden in thinner samples, where the tell-tale topological surface states are not so easily distinguished from more conventional bulk states. Measuring the width of the quantum phase transition as a function of the sample thickness provided a measure of the extent of the surface states, a few quintuple layers of Se–Bi–Se–Bi–Se.

A paper describing this work has been recently published in Nature Physics.