Weyl semimetals

Weyl semimetals
Name	Weyl semimetals
Category	Topological material
Crystal system	various
Discovery	2015
Properties	topological nodes, Fermi arcs, chiral anomaly

Weyl semimetals are a class of quantum materials hosting quasiparticles that mimic massless Weyl fermions predicted in high-energy physics, producing distinct topological band crossings and surface states. They connect concepts and techniques across condensed matter, particle physics, and materials science, and have been investigated using tools developed in studies of Albert Einstein, Paul Dirac, Hermann Weyl, Isaac Newton, and experimental platforms associated with CERN, Lawrence Berkeley National Laboratory, Max Planck Society, MIT, and Stanford University. Research on these materials involves collaborations among groups at institutions such as Harvard University, Princeton University, Columbia University, University of California, Berkeley, and University of Cambridge.

Introduction

Weyl semimetals arise when crystal symmetries and spin–orbit coupling produce nondegenerate band crossings—Weyl nodes—near the Fermi level, analogous to points in band structures studied in the context of Paul Dirac and Hermann Weyl theoretical work. Early theoretical proposals and experimental confirmations involved teams at Princeton University, Harvard University, Max Planck Institute for Chemical Physics of Solids, Institute of Physics, Chinese Academy of Sciences, University of Tokyo, and University of California, Berkeley. Notable material systems that advanced the field were discovered and characterized by collaborations including researchers from Rice University, University of Pennsylvania, Swiss Federal Institute of Technology in Zurich, Argonne National Laboratory, and Lawrence Berkeley National Laboratory.

Theory and band structure

Theoretical descriptions of Weyl semimetals draw on band topology, Berry curvature, and chirality concepts developed by theorists at Princeton University, Harvard University, University of Cambridge, and University of California, Berkeley. Models showing how broken inversion or time-reversal symmetry yields pairs of Weyl nodes were developed in the offices of researchers linked to Columbia University, Stanford University, University of Toronto, and Massachusetts Institute of Technology. Calculations using density functional theory and tight-binding models are routinely performed at centers like Oak Ridge National Laboratory, Argonne National Laboratory, and Max Planck Institute for Chemical Physics of Solids to predict Weyl node positions and Fermi-arc surface states, building on techniques familiar from studies at Bell Labs, IBM Research, and Los Alamos National Laboratory. The chiral anomaly in transport, a central predicted signature, connects to field-theory discussions from CERN and conceptual frameworks influenced by Paul Dirac.

Materials and synthesis

Candidate Weyl semimetals have been realized in transition-metal monopnictides and chalcogenides by synthesis groups at Princeton University, Rice University, Stanford University, and University of California, Berkeley. Prototypical compounds such as those identified in studies by teams at Harvard University, Max Planck Society, and Swiss Federal Institute of Technology in Zurich were grown using techniques developed at Lawrence Berkeley National Laboratory, Argonne National Laboratory, and university crystal-growth labs at Columbia University and University of Chicago. Thin-film and heterostructure approaches have been pursued in facilities linked to MIT, University of Illinois Urbana-Champaign, University of Pennsylvania, and Johns Hopkins University using molecular beam epitaxy and chemical vapor deposition methods refined at Bell Labs and IBM Research. High-pressure synthesis and angle-resolved photoemission spectroscopy collaborations often involve infrastructure at Hermann von Helmholtz Institute and national synchrotrons used by groups from Brookhaven National Laboratory and SLAC National Accelerator Laboratory.

Experimental signatures and characterization

Key experimental probes include angle-resolved photoemission spectroscopy (ARPES) performed at beamlines associated with SLAC National Accelerator Laboratory, Brookhaven National Laboratory, and Diamond Light Source, quantum oscillation studies common in laboratories at Harvard University and Princeton University, and magnetotransport measurements carried out by teams at University of Chicago, Columbia University, and University of California, Berkeley. Observations of Fermi-arc surface states and momentum-resolved spectroscopy were reported by collaborations involving Max Planck Institute for Chemical Physics of Solids and Swiss Federal Institute of Technology in Zurich, while negative longitudinal magnetoresistance attributed to the chiral anomaly was measured in experiments at Rice University, Argonne National Laboratory, and Stanford University. Scanning tunneling microscopy and quasiparticle interference mapping from groups at University of Oxford and University of California, Los Angeles provided local probes complementary to bulk transport, with theoretical interpretation informed by researchers at Princeton University and Massachusetts Institute of Technology.

Applications and potential devices

Proposed applications leverage topological protection, high mobility, and unusual magnetoelectric responses, motivating device efforts at industry and academic labs including IBM, Intel, Samsung, Qualcomm, MIT, Stanford University, and University of Cambridge. Concepts for low-dissipation interconnects, spintronic elements, and photonic devices take inspiration from related work in Nokia Bell Labs and Bell Labs, while proposals for sensors and energy-harvesting devices have been discussed in collaborations involving Lawrence Berkeley National Laboratory and Argonne National Laboratory. Integration with heterostructures and van der Waals materials studied at Columbia University, University of Manchester, and University of California, Berkeley aims to translate fundamental properties into scalable platforms evaluated by groups at Samsung and Intel research centers.

Open questions and research directions

Outstanding challenges span discovery of robust, easily processed Weyl materials and control of node energetics, pursued by teams at Max Planck Society, Harvard University, Stanford University, and Princeton University. Fundamental questions about interactions, disorder, and correlation effects are being explored theorically at Perimeter Institute for Theoretical Physics, Institute for Advanced Study, and Cornell University, with experimental tests in facilities like National High Magnetic Field Laboratory and synchrotrons such as SLAC National Accelerator Laboratory. Cross-disciplinary efforts bridging condensed matter and high-energy approaches involve collaborations with researchers linked to CERN, Lawrence Berkeley National Laboratory, and Brookhaven National Laboratory, aiming to extend control of topology to devices and to explore emergent phenomena analogous to those in particle physics.

Category:Topological materials