Discovery of a new type of topological Weyl fermion semimetal state in Mo$_x$W$_{1-x}$Te$_2$

TL;DR

This paper reports the discovery of a new Type II Weyl semimetal state in Mo$_x$W$_{1-x}$Te$_2$, identified through ARPES measurements and theoretical calculations, expanding the class of topological semimetals.

Contribution

It presents the first experimental observation of a Type II Weyl semimetal in Mo$_x$W$_{1-x}$Te$_2$, demonstrating topological Fermi arcs and Weyl points at the Fermi level.

Findings

Observation of topological Fermi arc above the Fermi level

Identification of Weyl points at the Fermi level

Confirmation of Type II Weyl semimetal state

Abstract

The recent discovery of a Weyl semimetal in TaAs offers the first Weyl fermion observed in nature and dramatically broadens the classification of topological phases. However, in TaAs it has proven challenging to study the rich transport phenomena arising from emergent Weyl fermions. The series Mo$_x$W$_{1-x}$Te$_2$ are inversion-breaking, layered, tunable semimetals already under study as a promising platform for new electronics and recently proposed to host Type II, or strongly Lorentz-violating, Weyl fermions. Here we report the discovery of a Weyl semimetal in Mo$_x$W$_{1-x}$Te$_2$ at $x = 25\%$. We use pump-probe angle-resolved photoemission spectroscopy (pump-probe ARPES) to directly observe a topological Fermi arc above the Fermi level, demonstrating a Weyl semimetal. The excellent agreement with calculation suggests that Mo$_x$W$_{1-x}$Te$_2$ is the first Type II Weyl semimetal. We also find that certain Weyl points are at the Fermi level, making Mo$_x$W$_{1-x}$Te$_2$ a promising platform for transport and optics experiments on Weyl semimetals.