Weyl semimetal

Weyl semimetal. A Weyl semimetal is a novel topological phase of quantum matter characterized by the existence of emergent, massless quasiparticles known as Weyl fermions. These quasiparticles arise at singular points in the material's Brillouin zone, where non-degenerate electronic bands cross linearly. This phase represents a three-dimensional analog to graphene and is distinguished by its gapless bulk electronic structure and topologically protected surface states, leading to exotic electronic and optical properties. The experimental discovery of materials exhibiting this phase has opened a new frontier in condensed matter physics and materials science.

Definition and basic properties

A Weyl semimetal is fundamentally defined by the presence of band crossings, or Weyl nodes, in its three-dimensional momentum space. These nodes act as sources and sinks of Berry curvature, analogous to magnetic monopoles in momentum space, and always appear in pairs of opposite chirality. The low-energy excitations near these nodes are described by the Weyl equation, originally formulated by Hermann Weyl in the context of particle physics. Key electronic properties include extremely high electron mobility, negative magnetoresistance due to the chiral anomaly, and the existence of open Fermi arcs on the material's surface. The stability of the Weyl nodes requires the breaking of either time-reversal symmetry or inversion symmetry in the crystal lattice.

Theoretical background

The theoretical foundation for Weyl semimetals stems from early work in quantum field theory and the mathematical description of massless spin–½ particles. The concept was translated to condensed matter systems through the study of topological band theory, notably building upon the classification of topological insulators by researchers like Xiao-Liang Qi and Shou-Cheng Zhang. Seminal theoretical proposals for specific material realizations were advanced by groups including those of M. Zahid Hasan and Andrei Bernevig, identifying candidate compounds such as TaAs and Na₃Bi. These proposals leveraged sophisticated first-principles calculations and symmetry analysis to predict the existence of robust Weyl points protected by crystal symmetry.

Experimental realizations

The first experimental confirmations of Weyl semimetal phases were reported nearly simultaneously in 2015 by several international research teams. Using angle-resolved photoemission spectroscopy at facilities like the Advanced Light Source, groups led by M. Zahid Hasan at Princeton University and Yulin Chen at the University of Oxford directly observed the characteristic Fermi arcs on the surface of TaAs crystals. Concurrently, transport measurements by the group of Nai Phuan Ong, also at Princeton University, detected signatures of the chiral anomaly through negative magnetoresistance in Na₃Bi and Cd₃As₂. Further confirmations have since been made in other materials, including the magnetic Weyl semimetal Co₃Sn₂S₂ studied by the Max Planck Institute for Chemical Physics of Solids.

Topological characteristics

The topology of a Weyl semimetal is encoded in its electronic band structure and the global properties of its wave function. Each Weyl node carries a topological charge, or Chern number, of ±1, making them stable against perturbations that do not cause them to annihilate in pairs. This topological nature mandates the existence of connecting Fermi arc surface states, which are open contours that terminate at the projections of the bulk Weyl nodes, as definitively imaged by ARPES experiments on TaAs. The system exhibits a bulk-boundary correspondence similar to that in the quantum Hall effect, but extended to three dimensions. Furthermore, in materials like HgCr₂Se₄, the interplay with magnetic order can lead to anomalous Hall effect and other topological responses.

Potential applications

The unique electronic properties of Weyl semimetals suggest several potential technological applications, though most remain in the exploratory research phase. Their high mobility and low dissipation make them candidates for novel low-power electronics and spintronics devices. The strong response of their chiral carriers to external fields could be harnessed in sensitive magnetic sensors or in quantum computing architectures for fault-tolerant information processing. The large, non-saturating magnetoresistance observed in materials like WTe₂ is of interest for data storage technologies. Furthermore, their nonlinear optical properties are being investigated for advanced photonics and terahertz radiation generation, with research ongoing at institutions like the Massachusetts Institute of Technology and the Paul Scherrer Institute.

Category:Condensed matter physics Category:Topology Category:Materials science