[Paper Review] A strongly robust Weyl fermion semimetal state in Ta$_{3}$S$_{2}$

Guoqing Chang, Su‐Yang Xu+12 more|arXiv (Cornell University)|Dec 29, 2015

Topological Materials and Phenomena37 references5 citations

TL;DR

This paper proposes Ta₃S₂ as a highly robust type-II Weyl semimetal with the largest momentum-space separation of Weyl nodes among known candidates—approximately twice that of TaAs and twenty times greater than WTe₂—using first-principles calculations and symmetry analysis. The robustness arises from intrinsic structural stability, lack of need for fine-tuning, and a topological metal-to-insulator transition accessible via lattice expansion, making it ideal for probing chiral anomalies and future device applications.

ABSTRACT

Weyl semimetals are extremely interesting. Although the first Weyl semimetal was recently discovered in TaAs, research progress is still significantly hindered due to the lack of robust and ideal materials candidates. In order to observe the many predicted exotic phenomena that arise from Weyl fermions, it is of critical importance to find robust and ideal Weyl semimetals, which have fewer Weyl nodes and more importantly whose Weyl nodes are well separated in momentum space and are located close to the chemical potential in energy. In this paper, we propose by far the most robust and ideal Weyl semimetal candidate in the inversion breaking, single crystalline compound tantalum sulfide Ta$_3$S$_2$ with new and novel properties beyond TaAs. We find that Ta$_3$S$_2$ has only 8 Weyl nodes, all of which have the same energy that is merely 10 meV below the chemical potential. Crucially, our results show that Ta$_3$S$_2$ has the largest $k$-space separation between Weyl nodes among known Weyl semimetal candidates, which is about twice larger than TaAs and twenty times larger than the predicted value in WTe$_2$. Moreover, we predict that increasing the lattice by $<4\%$ can annihilate all Weyl nodes, driving a novel topological metal-to-insulator transition from a Weyl semimetal state to a topological insulator state. We further discover that changing the lattice constant can move the Weyl nodes and the van Hove singularities with enhanced density of states to the chemical potential. Our prediction provides a critically needed robust candidate for this rapidly developing field. The well separated Weyl nodes, the topological metal-to-insulator transition and the remarkable tunabilities suggest Ta$_3$S$_2$'s potential as the ideal platform in future device-applications based on Weyl semimetals.

Motivation & Objective

• To identify and design Weyl semimetals with maximally separated Weyl nodes in momentum space for enhanced experimental observability and robustness.
• To establish a general methodology for predicting robust Weyl semimetal candidates based on crystal structure, symmetry, and band topology.
• To demonstrate that Ta₃S₂ exhibits the largest k-space separation of Weyl nodes among known Weyl semimetal candidates, surpassing TaAs and WTe₂.
• To explore experimentally feasible pathways to induce a topological metal-to-insulator transition in Ta₃S₂ via lattice engineering.
• To validate the experimental feasibility of observing Weyl fermions in Ta₃S₂ through photoemission and transport measurements due to node proximity to the Fermi level.

Proposed method

• Employing first-principles density functional theory (DFT) calculations to compute the electronic band structure and Weyl node positions in Ta₃S₂.
• Analyzing the band dispersion near the Weyl nodes to confirm type-II Weyl fermion behavior, characterized by tilted cones and a saddle-point band structure.
• Identifying the presence of a van Hove singularity at the Weyl node due to the band's saddle-point nature, which enhances the density of states.
• Using symmetry analysis to confirm the absence of inversion symmetry, which protects the Weyl nodes topologically.
• Simulating lattice expansion effects via structural deformation to predict the topological phase transition to a topological insulator state.
• Proposing three experimental routes to increase the b-lattice constant by ~4%: external pressure (~6 GPa), epitaxial strain via lattice-mismatched substrates, and isoelectronic substitution (e.g., S to Se).

Experimental results

Research questions

• RQ1 What is the momentum-space separation of Weyl nodes in Ta₃S₂, and how does it compare to other known Weyl semimetal candidates?
• RQ2 Can Ta₃S₂ host a robust Weyl semimetal state without requiring fine-tuning of chemical composition or external fields?
• RQ3 What is the topological phase transition pathway from a Weyl semimetal to a topological insulator in Ta₃S₂, and is it experimentally accessible?
• RQ4 How does the presence of a van Hove singularity at the Weyl node influence the electronic and transport properties of Ta₃S₂?
• RQ5 Can the Weyl node separation in Ta₃S₂ be experimentally resolved using photoemission or scanning tunneling spectroscopy?

Key findings

• Ta₃S₂ exhibits the largest k-space separation between Weyl nodes among known Weyl semimetal candidates, approximately twice that of TaAs and twenty times greater than WTe₂.
• The Weyl nodes in Ta₃S₂ are located near the Fermi level, enabling direct observation via angle-resolved photoemission spectroscopy (ARPES).
• The band structure near the Weyl nodes forms a saddle point, leading to a van Hove singularity that enhances the density of states and produces a divergence in the first derivative of the DOS.
• A ~4% increase in the b-lattice constant is predicted to annihilate all Weyl nodes, driving the system into a topological insulator phase.
• The required lattice expansion can be achieved experimentally via external pressure (~6 GPa), epitaxial strain on mismatched substrates, or isoelectronic substitution (e.g., Ta₃(S₁₋ₓSeₓ)₂).
• Ta₃S₂ is a single-crystalline, inversion-breaking compound, making it inherently stable and experimentally viable for probing chiral anomalies and monopole physics.