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A Non-Topological Approach to Understanding Weyl Semimetals

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A Non-Topological Approach to Understanding Weyl Semimetals A Non-Topological Approach to Understanding Weyl Semimetals Antonio Levy1,∗ Albert F. Rigosi1, Francois Joint2, and Gregory S. Jenkins3 1National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 2Department of Physics, University of Maryland, College Park, MD 20742, USA and 3Laboratory for Physical Sciences, College Park, MD 20740, USA (Dated: January 31, 2021) In this work, chiral anomalies and Drude enhancement in Weyl semimetals are separately discussed from a semi-classical and quantum perspective, clarifying the physics behind Weyl semimetals while avoiding explicit use of topological concepts. The intent is to provide a bridge to these modern ideas for educators, students, and scientists not in the field using the familiar language of traditional solid- state physics at the graduate or advanced undergraduate physics level. I. INTRODUCTION II. BACKGROUND ON WEYL FERMIONS The concept of Weyl fermions originated from the fu- Weyl fermions (defined below) have historically been sion of two familiar topics. The first is the energy- of interest in answering fundamental questions about momentum relation from relativity expressed as: E2 = the universe, particularly the observation of the matter- 2 (pc)2 + mc2 . The second is the time-dependent antimatter imbalance.1 The family of elementary parti- Schr¨odingerequation from quantum mechanics expressed cles classified as fermions, or particles of half-integer spin, 2 2 } O @ are important in the Standard Model that unifies three as: − (2m) = i} @t . of the four known forces of nature. Within the model To incorporate relativity into a quantum mechani- are twenty-four families of fermions. Almost all of them cal formula, Dirac treated each quantity in the energy- are massive Dirac fermions. Within the family of Dirac momentum relation as an operator acting on a wavefunc- @ fermions lies a subset class known as Weyl fermions, the tion. Using p = −i}O and E = i} @t , the resulting equa- set of fermions that are massless. Those well-versed in tion is: the physics of the weak nuclear force will recall that those interactions are stronger for both left-chiral matter and 2 2 2 right-chiral antimatter than their counterparts of oppo- 1 @ 2 m c − + O = (1) site chirality (chirality summary shown in Fig. 1). c2 @t2 }2 Weyl and 3D Dirac semimetals are the only systems in This is known as the Klein-Gordon equation. It does which signatures of Weyl fermions have been observed. not include spin and is therefore applicable to zero-spin Although they are not fundamental particles, the exci- bosons. tations in these material systems offer a unique play- Dirac transformed this equation by taking the square ground to study Weyl fermion physics like the chiral root of the operators, which requires consideration of all anomaly.2-6 Novel properties predicted for Weyl semimet- three spatial dimensions and the time dependence. The als (WSMs) like significantly reduced scattering, Drude transformation involves 4 × 4 matrices, called the Dirac enhancement along applied magnetic fields, and long- Gamma matrices, that are directly related to the Pauli lived spin-polarized currents in the presence of magnetic matrices −!σ that describe half-spin fermions. The Dirac fields could lead to myriad applications in fields like spin- equation can be written in matrix block form with a tronics and quantum computing. particle-hole (p-h) { or particle-antiparticle { basis: Most introductions to WSMs and Weyl Fermions are mathematically intense, making it difficult to develop an " −! −! c − mc − p · σ p −! −! " = 0; (2) intuitive understanding of their physics. There are a − p · σ − + mc h few works that try to explain the concepts behind Weyl c fermions in an educational context.7-9 This paper aims where " is the energy of the particle. On the other to provide a conceptual introduction accessible to educa- hand, rather than using a particle-hole basis, the Weyl tors, students, and scientists who have some understand- equation uses left- and right-chiral particles as the basis: ing of traditional condensed matter physics. " −! −! mc c − p · σ L " −! −! = 0; (3) c + p · σ mc R Massive particles in the Weyl equation prevent pure chiral eigenstates from emerging out of Equation (3). ∗ [email protected] Pure chiral particles must be massless. The eigenstates 2 the zero-momentum Weyl point. Chirality Due to quantum mechanics, the allowable energies at which electrons are permitted to exist are discrete (and Left-handed Right-handed by extension, are also discrete in k-space via Fourier transformation). These quantized energies make up the band structure of a crystal. For the remainder of this p p paper, it is assumed that the Fermi energy exists solely within the Weyl band and that other bands are suffi- ciently separated in energy so as not to perturb the Weyl state. With this last concept recalled, we are ready to describe the anomaly. Consider the divergence theorem as applied to a charge Q, charge density ρ and electrical current density j : y dQ @ρ −! −! = + · j d3x (4) σ σ dt @t O FIG. 1. A left-handed chiral particle has a spin σ that is Since charge is conserved, the integral extends over antiparallel to its momentum p. The spin and momentum are the entire volume of the system, leaving both sides of parallel for a right-handed chiral particle. Eq. (4) equal to zero. This expression indicates that the total amounts of charge entering and leaving the system are equal. Combining Eq. (4) with Eq. (3) for m = 0 of massless Weyl particles only involve the momentum under non-zero electromagnetic fields (which is done by −! and a parallel or antiparallel spin as shown in Figure 1. replacing −!p with −!p − e A ), the difference between the In solid state physics, the dispersion relation is a math- c number of right- and left-chiral particles, nR − nL can be ematical description of the available energies for an elec- obtained after considerable effort: tron moving about in a material with a given momentum. It is frequently plotted as energy versus momentum (also y referred to as k-space). Further details on the basics of d −! −! (n − n ) / E · B d3x (5) solid state physics can be found in standard textbooks.10 dt R L The relativistic energy-momentum dispersion relation for massless particles is E = pc . For particle-like excita- Equation (5) shows that if a population of Weyl tions (or quasiparticles) moving through solid materials, fermions is subjected to non-zero applied electric and the velocity can be a constant other than the speed of magnetic fields, the chirality of the population will light while still obeying the Weyl equation. Casting the −! −! change with time. This is equivalent to saying that Weyl momentum in terms of wavenumber p = } k , the oper- fermions of a single chirality will be annihilated and re- ative Weyl dispersion relation becomes E = }kv, where placed with Weyl fermions of the opposite chirality. This the v is the Fermi velocity which plays a role similar to c is the chiral anomaly. in the relativistic equations. The zero-energy is defined The chiral anomaly in a crystal is best understood in- at zero momentum and is called the Weyl point, and its tuitively by considering the effects of applied magnetic presence in Weyl semimetals is responsible for the novel and electric fields on the Fermi surface with linear dis- physics they are predicted to exhibit. persions illustrated in Fig. 2. The momentum direction of any quasiparticle on a spherical Fermi surface is ra- dially outward. In the depicted generic WSM, there are III. CHIRAL ANOMALIES two locations in k-space around which the band struc- ture obeys the Weyl equations. Each location hosts one To appreciate how a chiral anomaly emerges, it will species of chiral particle where the spin is either radially help to recall a few concepts from statistical and solid- inward opposite the momentum (orange Fermi surface state physics. Within any material, electrons and other pocket on the left side of each subfigure) or purely aligned quasiparticles only have access to a limited number of radially outward with the momentum (green Fermi sur- momenta. Mapping out these allowable momenta cre- face pocket on the right side of each subfigure). As will ates what is commonly referred to as k-space (closely be discussed later, including only a single chiral Fermi related to the reciprocal lattice of a crystalline solid). pocket violates the conservation of energy. The highest-energy electrons, in some sense, define the So what happens to the Fermi surface if you apply a bounds of achievable momenta, and these bounds sketch magnetic field? It will distort, expanding or contracting out an object in k-space called a Fermi surface. The by an amount proportional to the dot product of the spin Fermi surface for a Weyl particle is a sphere centered on and the magnetic field (Zeeman effect). To intuitively un- 3 derstand these distortions, consider the left-handed chi- the application of fields. ral (orange) Fermi surface pocket in Figure 2(e-g) at the Consider for simplicity the case with both fields point- point where the spin of a quasiparticle is purely antipar- ing in the −x direction (as in Fig. 2). Then, for the right- allel with applied magnetic field. The quasiparticle's en- chiral location, we track the difference between right- and ergy will decrease with applied field. This decrease in en- left-chiral particles nR(θ) − nL(θ), which is proportional ergy, through the linear energy dispersion, causes the mo- to integrated subtraction of the density of states, ex- mentum to decrease. The decrease in momentum shifts pressed as gR(L)("F ) = g("F +(−)a1Bcos(θ)+a2Ecos(θ)) this point of the Fermi surface toward the center of the for the right and left pockets, where a1 and a2 are con- Fermi pocket sphere.
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