Experimental discovery of a topological Weyl semimetal state in TaP

Abstract: Weyl semimetals are expected to open up new horizons in physics and materials science because they provide the first realization of Weyl fermions and exhibit protected Fermi arc surface states. However, they had been found to be extremely rare in nature. Recently, a family of compounds, consisting of TaAs, TaP, NbAs and NbP was predicted as Weyl semimetal candidates. Here, we experimentally realize a Weyl semimetal state in TaP. Using photoemission spectroscopy, we directly observe the Weyl fermion cones and nodes in the bulk and the Fermi arcs on the surface. Moreover, we find that the surface states show an unexpectedly rich structure, including both topological Fermi arcs and several topologically-trivial closed contours in the vicinity of the Weyl points, which provides a promising platform to study the interplay between topological and trivial surface states on a Weyl semimetal's surface. We directly demonstrate the bulk-boundary correspondence and hence establish the topologically nontrivial nature of the Weyl semimetal state in TaP, by resolving the net number of chiral edge modes on a closed path that encloses the Weyl node. This also provides, for the first time, an experimentally practical approach to demonstrating a bulk Weyl fermion from a surface state dispersion measured in photoemission.

• The paper demonstrates the first experimental identification of Weyl fermion cones and Fermi arc surface states in TaP using ARPES.
• It employs a dual-method ARPES approach to map three-dimensional electronic bands and confirm the bulk-boundary correspondence.
• The findings highlight a non-toxic platform for advancing topological quantum research and potential device applications.

Experimental Discovery of a Topological Weyl Semimetal State in TaP

This paper presents the first experimental verification of a Weyl semimetal state in tantalum phosphide (TaP), as observed through the use of angle-resolved photoemission spectroscopy (ARPES). The study emphasizes the critical role of Weyl fermions in condensed matter physics and details the intricate surface and bulk electronic structures associated with Weyl semimetals.

Summary of Findings

Using ARPES, Weyl fermion cones and nodes were directly identified in the bulk of TaP, and Fermi arc surface states were observed. The authors validate the Weyl semimetal nature of TaP by examining its surface states, which include both topological Fermi arcs and a combination of topologically trivial closed contours near Weyl points. These findings are consistent with first-principle calculations.

Notably, the researchers developed an experimental protocol to establish the bulk-boundary correspondence, which is a hallmark of a Weyl semimetal. This methodology involves resolving the net number of chiral edge modes that encompass the Weyl node, reinforcing TaP as a Weyl semimetal.

Technical Details

• Crystal Structure and Band Topology: TaP crystallizes into a body-centered tetragonal structure, lacking space inversion symmetry, which is advantageous for realizing Weyl semimetal states. The authors describe in detail how, due to spin-orbit coupling (SOC), the system displays unique band crossings that transform into Weyl nodes.
• Chirality: The paper reveals the presence of 24 Weyl nodes categorized as W1 and W2, each with distinct energy offsets due to SOC. W2 nodes, more discernible in the experiment, provide clear verification of the Weyl semimetal phase.
• Experimental Protocols: The study employs both soft X-ray ARPES and ultraviolet ARPES to thoroughly probe the bulk and surface electronic structure of TaP. This dual-method approach allows for comprehensive, three-dimensional mapping of the electronic bands.
• Proposed Methods for Fermi Arc Verification: This work introduces new methodologies to experimentally verify chiral modes by focusing on paths enclosing single Weyl nodes and evaluating chiral edge modes. This proposal represents a practical alternative to merely relying on high-resolution ark observations, which can be technically challenging.

Implications and Future Directions

The discovery and experimental verification of a Weyl semimetal state in TaP, free of toxic elements like arsenic found in similar compounds, is significant for both theoretical research and practical applications. These findings offer a safer alternative for potential future experimental studies and device fabrication. The research demonstrates a replicable methodology for identifying and characterizing Weyl semimetals, paving the way for deeper exploration into similar materials.

Looking forward, further investigations are merited into the interaction between topological Fermi arcs and trivial surface states, particularly under modified surface environments like adsorption or dopant variations. Understanding these interactions could yield insights into topologically protected charge transport and novel quantum device architectures. The theoretical and experimental framework laid out in this work provides a foundation for the ongoing exploration of topologically nontrivial phases in condensed matter systems.