[Paper Review] Arc-tunable Weyl Fermion metallic state in Mo$_x$W$_{1-x}$Te$_2$

Tay‐Rong Chang, Su‐Yang Xu+14 more|arXiv (Cornell University)|Aug 27, 2015

Topological Materials and Phenomena2 references3 citations

TL;DR

This paper proposes a tunable Weyl metallic state in Mo$_x$W$_{1-x}$Te$_2$ via first-principles calculations, demonstrating that as little as 2% Mo doping stabilizes Weyl fermions at room temperature. The momentum-space separation between Weyl nodes—and thus the length of the Fermi arcs—can be continuously tuned from zero to ~3% of the Brillouin zone by varying Mo concentration, offering a unique, experimentally feasible route to topological Weyl physics beyond the TaAs family.

ABSTRACT

Weyl semimetals may open a new era in condensed matter physics because they provide the first example of Weyl fermions, realize a new topological classification even though the system is gapless, exhibit Fermi arc surface states and demonstrate the chiral anomaly and other exotic quantum phenomena. So far, the only known Weyl semimetals are the TaAs class of materials. Here, we propose the existence of a tunable Weyl metallic state in Mo$_x$W$_{1-x}$Te$_2$ via our first-principles calculations. We demonstrate that a 2% Mo doping is sufficient to stabilize the Weyl metal state not only at low temperatures but also at room temperatures. We show that, within a moderate doping regime, the momentum space distance between the Weyl nodes and hence the length of the Fermi arcs can be continuously tuned from zero to ~ 3% of the Brillouin zone size via changing Mo concentration, thus increasing the topological strength of the system. Our results provide an experimentally feasible route to realizing Weyl physics in the layered compound Mo$_x$W$_{1-x}$Te$_2$, where non-saturating magneto-resistance and pressure driven superconductivity have been observed.

Motivation & Objective

• To identify a new, experimentally accessible platform for realizing Weyl semimetal physics in a layered transition metal dichalcogenide.
• To address the lack of tunability in existing Weyl semimetals, particularly the TaAs family, which lack controllable Fermi arc lengths.
• To demonstrate that Weyl nodes and Fermi arcs can be stabilized and tuned in Mo$_x$W$_{1-x}$Te$_2$ across a range of Mo concentrations.
• To provide a theoretical foundation for observing exotic quantum phenomena such as the chiral anomaly and non-saturating magnetoresistance in a single material system.
• To clarify the connection between constant-energy surface states and Fermi arc connectivity in systems with energy-offset Weyl nodes and type-II Weyl cones.

Proposed method

• First-principles density functional theory (DFT) calculations are used to compute the electronic band structure of Mo$_x$W$_{1-x}$Te$_2$ across varying Mo concentrations.
• The Weyl node positions and chiral charges are identified by analyzing the band degeneracies in momentum space and computing the Berry curvature monopoles.
• Fermi arc connectivity is reconstructed from constant-energy contours at different energies, tracking surface state bands that cross bulk pockets.
• The evolution of bulk Fermi pockets (electron- and hole-like) is analyzed across energy slices to map the topological surface states.
• The system's topological strength is quantified by measuring the momentum-space distance between Weyl nodes, which scales with the Fermi arc length.
• The stability of the Weyl state is assessed across doping levels, including the critical point at ~0.5% Mo doping where the system transitions from a gapped to a Weyl semimetal phase.

Experimental results

Research questions

• RQ1 Can a Weyl metallic state be stabilized in Mo$_x$W$_{1-x}$Te$_2$ with experimentally feasible Mo doping levels?
• RQ2 How does the length of the Fermi arc—used as a measure of topological strength—vary with Mo concentration in Mo$_x$W$_{1-x}$Te$_2$?
• RQ3 Why do constant-energy surface maps in Mo$_x$W$_{1-x}$Te$_2$ appear to show non-intuitive Fermi arc connectivity, and how can this be reconciled with the actual Weyl node pairing?
• RQ4 Is the Weyl semimetal phase in Mo$_x$W$_{1-x}$Te$_2$ robust at room temperature and under moderate doping?
• RQ5 Can the Weyl nodes be annihilated by external pressure, and does this imply a tunable topological phase transition?

Key findings

• A Weyl metallic state is stabilized in Mo$_x$W$_{1-x}$Te$_2$ with as little as 2% Mo doping, even at room temperature.
• The length of the Fermi arc increases continuously from zero to approximately 3% of the Brillouin zone size as Mo concentration increases from the critical point.
• At ~0.5% Mo doping, the system reaches a critical point where the conduction and valence bands touch, marking the onset of Weyl node formation.
• The Fermi arc connects a W1(-) and a W2(+) Weyl node, despite apparent connectivity in constant-energy maps suggesting otherwise, due to energy-dependent band evolution.
• The observed surface state bands in constant-energy contours are shown to reconstruct the true Fermi arc by tracking crossings across multiple energy slices.
• The system's topological strength, quantified by Weyl node separation, is tunable via chemical doping, a feature absent in the TaAs family of Weyl semimetals.