Topological Weyl phase transition in Mo$_x$W$_{1-x}$Te$_2$

TL;DR

This study visualizes a topological phase transition in Mo$_x$W$_{1-x}$Te$_2$, revealing the nucleation of Fermi arcs and Weyl points at a critical composition, advancing understanding of Weyl semimetal transitions.

Contribution

First direct imaging of a topological Weyl phase transition in Mo$_x$W$_{1-x}$Te$_2$ using pump-probe ARPES, showing creation of Fermi arcs and Weyl points at a critical doping level.

Findings

Fermi arc nucleation at x_c ~ 7%

No Fermi arc observed below x_c

Fermi arc extends with increasing x above x_c

Abstract

Topological phases of matter exhibit phase transitions between distinct topological classes. These phase transitions are exotic in that they do not fall within the traditional Ginzburg-Landau paradigm but are instead associated with changes in bulk topological invariants and associated topological surface states. In the case of a Weyl semimetal this phase transition is particularly unusual because it involves the creation of bulk chiral charges and the nucleation of topological Fermi arcs. Here we image a topological phase transition to a Weyl semimetal in Mo$_x$W$_{1-x}$Te$_2$ with changing composition $x$. Using pump-probe ultrafast angle-resolved photoemission spectroscopy (pump-probe ARPES), we directly observe the nucleation of a topological Fermi arc at $x_c \sim 7\%$, showing the critical point of a topological Weyl phase transition. For Mo dopings $x < x_c$, we observe no Fermi arc, while for $x > x_c$, the Fermi arc gradually extends as the bulk Weyl points separate. Our results demonstrate for the first time the creation of magnetic monopoles in momentum space. Our work opens the way to manipulating chiral charge and topological Fermi arcs in Weyl semimetals for transport experiments and device applications.