79 research outputs found
Bielectron vortices in two-dimensional Dirac semimetals
Searching for new states of matter and unusual quasiparticles in emerging
materials and especially low-dimensional systems is one of the major trends in
contemporary condensed matter physics. Dirac materials, which host
quasiparticles which are described by ultrarelativistic Dirac-like equations,
are of a significant current interest from both a fundamental and applied
physics perspective. Here we show that a pair of two-dimensional massless
Dirac-Weyl fermions can form a bound state independently of the sign of the
inter-particle interaction potential, as long as this potential decays at large
distances faster than Kepler's inverse distance law. This leads to the
emergence of a new type of energetically-favourable quasiparticle: bielectron
vortices, which are double-charged and reside at zero-energy. Their bosonic
nature allows for condensation and may give rise to Majorana physics without
invoking a superconductor. These novel quasiparticles arguably explain a range
of poorly understood experiments in gated graphene structures at low doping.Comment: 9 pages, 2 figure
Thermal Transport for Probing Quantum Materials
Thermal transport is less appreciated in probing quantum materials in
comparison to electrical transport. This article aims to show the pivotal role
that thermal transport may play in understanding quantum materials: the
longitudinal thermal transport reflects the itinerant quasiparticles even in an
electrical insulating phase, while the transverse thermal transport such as
thermal Hall and Nernst effect are tightly linked to nontrivial topology. We
discuss three types of examples: quantum spin liquids where thermal transport
identifies its existence, superconductors where thermal transport reveals the
superconducting gap structure, and topological Weyl semimetals where anomalous
Nernst effect is a consequence of nontrivial Berry curvature. We conclude with
an outlook of the unique insights thermal transport may offer to probe a much
broader category of quantum phenomena.Comment: A short review article with 6 figures. Comments are welcome
Bielectron vortices in two-dimensional Dirac semimetals
Searching for new states of matter and unusual quasi-particles in emerging materials and especially low-dimensional systems is one of the major trends in contemporary condensed matter physics. Dirac materials, which host quasi-particles which are described by ultra-relativistic Dirac-like equations, are of a significant current interest from both a fundamental and applied physics perspective. Here we show that a pair of two-dimensional massless Dirac-Weyl fermions can form a bound state independently of the sign of the inter-particle interaction potential, as long as this potential decays at large distances faster than Kepler's inverse distance law. This leads to the emergence of a new type of energetically favorable quasiparticle: bielectron vortices, which are double-charged and reside at zero-energy. Their bosonic nature allows for condensation and may give rise to Majorana physics without invoking a superconductor. These novel quasi-particles arguably explain a range of poorly understood experiments in gated graphene structures at low doping
Weyl, Dirac and high-fold chiral fermions in topological quantum materials
Quantum materials hosting Weyl fermions have opened a new era of research in
condensed matter physics. First proposed in 1929 in particle physics, Weyl
fermions have yet to be observed as elementary particles. In 2015, Weyl
fermions were detected as collective electronic excitations in the strong
spin-orbit coupled material tantalum arsenide, TaAs. This discovery was
followed by a flurry of experimental and theoretical explorations of Weyl
phenomena in materials. Weyl materials naturally lend themselves to the
exploration of the topological index associated with Weyl fermions and their
divergent Berry curvature field, as well as the topological bulk-boundary
correspondence giving rise to protected conducting surface states. Here, we
review the broader class of Weyl topological phenomena in materials, starting
with the observation of emergent Weyl fermions in the bulk and of Fermi arc
states on the surface of the TaAs family of crystals by photoemission
spectroscopy. We then discuss some of the exotic optical and magnetic responses
observed in these materials, as well as the progress in developing some of the
related chiral materials. We discuss the conceptual development of high-fold
chiral fermions, which generalize Weyl fermions, and we review the observation
of high-fold chiral fermion phases by taking the rhodium silicide, RhSi, family
of crystals as a prime example. Lastly, we discuss recent advances in Weyl-line
phases in magnetic topological materials. With this Review, we aim to provide
an introduction to the basic concepts underlying Weyl physics in condensed
matter, and to representative materials and their electronic structures and
topology as revealed by spectroscopic studies. We hope this work serves as a
guide for future theoretical and experimental explorations of chiral fermions
and related topological quantum systems with potentially enhanced
functionalities.Comment: To appear in Nature Review Material
- …