Spectroscopic evidence for type II Weyl semimetal state in MoTe2 (1603.06482v1)

Abstract: In a type I Dirac or Weyl semimetal, the low energy states are squeezed to a single point in momentum space when the chemical potential Ef is tuned precisely to the Dirac/Weyl point. Recently, a type II Weyl semimetal was predicted to exist, where the Weyl states connect hole and electron bands, separated by an indirect gap. This leads to unusual energy states, where hole and electron pockets touch at the Weyl point. Here we present the discovery of a type II topological Weyl semimetal (TWS) state in pure MoTe2, where two sets of WPs (W2+-, W3+-) exist at the touching points of electron and hole pockets and are located at different binding energies above Ef. Using ARPES, modeling, DFT and calculations of Berry curvature, we identify the Weyl points and demonstrate that they are connected by different sets of Fermi arcs for each of the two surface terminations. We also find new surface "track states" that form closed loops and are unique to type II Weyl semimetals. This material provides an exciting, new platform to study the properties of Weyl fermions.

• The paper demonstrates spectroscopic evidence for a type II Weyl semimetal state in MoTe2 by identifying distinct Weyl points through ARPES and DFT analyses.
• It reports the discovery of Fermi arcs and novel track states that vary with surface termination, highlighting key differences in electronic structure.
• The research validates simplified lattice models and theoretical predictions, offering insights that could inform future electronic applications in quantum materials.

Spectroscopic Evidence for Type II Weyl Semimetal State in MoTe$_2$

The paper of topological phases in condensed matter physics has garnered significant interest, with particular emphasis on Dirac and Weyl semimetals due to their unique electronic properties. This paper presents compelling spectroscopic evidence for the existence of a type II Weyl semimetal (TWS) state in the material MoTe$_2$, investigated using angle-resolved photoemission spectroscopy (ARPES), theoretical modeling, density functional theory (DFT), and Berry curvature calculations.

Research Overview

The focal point of this research is the identification of Weyl points (WPs) in MoTe$_2$, a material predicted to exhibit characteristics of a type II Weyl semimetal. Type I and type II Weyl semimetals differ in the nature of their band structures; the latter is characterized by tilted Weyl cones where electron and hole pockets intersect at the WPs, maintaining a finite density of states at the chemical potential.

The methodology implemented involved a comprehensive approach combining ARPES for experimental probing of the electronic structure with detailed theoretical calculations. Two distinguishing characteristics were targeted: the detection of Fermi arcs connecting WPs and the discovery of novel "track states" exclusive to type II TWS.

Key Findings

1. Weyl Point Identification: The paper successfully identified two sets of WPs, denoted as $W_2^\pm$ and $W_3^\pm$, in MoTe$_2$. These WPs are evidenced at the touching points of electron and hole pockets, occurring at different binding energies above the Fermi level. The divergence in WP locations observed in experiments versus DFT calculations is noted, attributed potentially to sensitivity to structural parameters.
2. Fermi Arcs and Track States: The ARPES data revealed Fermi arcs differing between two surface terminations. On surface termination A, arcs connecting $W_2$ and $W_3$ WPs were identified. Surface termination B revealed less distinct arcs due to overlapping bands but indicated the presence of track states creating closed loops—a novel finding unique to type II TWS.
3. Theoretical Correlation: The paper underscores the agreement between simplified lattice models predicting the band dispersion characteristics of type II TWS and observed experimental outcomes, validating the theoretical models pertaining to both the surface and bulk Weyl characteristics.

Implications and Future Directions

The spectroscopic discovery of the type II Weyl semimetal state in MoTe$_2$ offers a significant advancement in understanding topological phases in quantum materials. These results have profound implications for the design and use of materials in electronic applications, where exotic transport phenomena, such as the chiral anomaly or novel magnetoresistance effects, could be harnessed. Furthermore, the identification of "track states" invites further investigation into their physical implications and potential technological applications.

Future research should focus on exploring other candidate materials that exhibit type II Weyl semimetal properties, the stability of these states under various conditions, and potential methods to manipulate these electronic properties for practical applications. Additionally, improving theoretical models to accurately predict the WP positions and analyzing their influence on transport properties will be critical in pushing the frontiers of this research domain.