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Surface States of Weyl Semimetals Magnetic description of the Fermi arc in type-I and type-II Weyl semimetals Serguei Tchoumakov, Marcello Civelli and Mark O. Goerbig1 1Laboratoire de Physique des Solides, Univ. Paris-Sud, Universit´eParis-Saclay, CNRS UMR 8502, F-91405 Orsay Cedex, France (Dated: February 16, 2017) We consider finite-sized interfaces of a Weyl semi-metal and show that the corresponding con- finement potential is similar to the application of a magnetic field. Among the numerous states, which can be labeled by indices n like in Landau levels, the n = 0 surface state describes the Weyl semimetal Fermi arc at a given chemical potential. Moreover, the analogy with a magnetic field shows that an external in-plane magnetic field can be used to distort the Fermi arc and would explain some features of magneto-transport in Weyl semimetals. We derive the Fermi arc for type-I and type-II Weyl semimetals where we deal with the tilt anisotropy by the use of Lorentz boosts. In the case of type-II Weyl semimetals, this leads to many additional topologically trivial surface states at low energy. Finally, we extend the Aharonov-Casher argument and demonstrate the stability of the Fermi arc over fluctuations of the surface potential. I. INTRODUCTION 0 ` x S E One key feature of the interface of a topological insula- E tor put into contact with a normal insulator is the pres- kz ∆ ∆0 kz ky ence of stable surface states [1]. This stability is related ky Bp to the topological Chern number which is based on the existence of a band gap. Surface states can play a crucial Weyl semimetal Interface Insulator ˆ ˆ ˆ role since their metallicity can radically affect the trans- HL(kx, ky, kz) Hs(ˆn, ky, kz) HR(kx, ky, kz) port properties of the topological insulator. There are however topological materials, Dirac semimetals (DSM) FIG. 1. (Color online) Sketch of the smoothly varying in- and Weyl semimetals (WSM), where the band gap is ex- terface between two bulk materials, a Weyl semimetal with two Weyl cones separated by a saddle point at energy ∆ (in actly zero and they can be described as a critical phase 0 lying between normal and topological insulators [2,3]. yellow, on the left) and an insulator with gap ∆ (in white, on the right). Within the interface of width `, the electrons In recent measurements [4-6] on materials of the behave as if they were immersed into an in-plane magnetic (Ta,Nb)-(As,P) family of three-dimensional (3D) WSM, field Bp. surface states with surprising properties were observed and related to the Fermi arc, as described in numerous theoretical studies [7{11]. In particular, the Fermi arc surface states have an open Fermi surface that connects consider that two bulk materials are put into contact the projected bulk Weyl nodes and, furthermore, they are with an interface of width ` (see Fig.1). In agreement spin polarized. These properties are used to characterize with recent semiclassical [11] and numerical [12] studies, genuine WSM [3,5]. Therefore, a better understand- we show, within a constructive quantum-mechanical ap- ing of the Fermi arc properties has been sought in many proach, that the Hamiltonian describing the interface is theoretical investigations. Recently similarities between similar to one in the presence of a magnetic field within a Fermi arcs and Landau bands [11,12] have been pointed particular gauge. This leads to a set of two-dimensional out, although the latter typically arise in the presence of (2D) Landau bands indexed by an integer number n. In a magnetic field. the case of WSM, these states have open Fermi surfaces In various theoretical studies, the crystal potential at as a consequence of the localization of Landau orbits in- the surface of the WSM is sharp because it simplifies the side the region of size `, which is similar to the deriva- derivation of the wavefunction boundary conditions [7{ tion of the degeneracy of Landau bands within the Lan- 10]. In this approach one considers that the width of the dau gauge [14]. This yields a Fermi surface with an odd arXiv:1612.07693v2 [cond-mat.mes-hall] 14 Feb 2017 interface ` is shorter than any other length scale, such number of Fermi arcs with only the n = 0 Landau band as the magnetic or the band bending lengths. In some having a single Fermi arc. This state is the most relevant situations, however, this description is not sufficient. For at low energy and is the main subject of our investiga- example, in Ref. [13], in order to describe the effect of tion. The analogy with the magnetic field allows us to a magnetic domain wall on Majorana bound states of a explore various aspects of the n = 0 Fermi arc. In par- superconducting chain, it was necessary to introduce a ticular, we show that the effect of an external magnetic smooth gap inversion at the interface. field is to modify the shape of the Fermi arc and that In the present paper, we extend the description of a the combination of tilt and magnetic field can break the smoothly varying interface to the WSM in order to de- arc into two separate pieces. This is investigated in the scribe the coupling to external electromagnetic fields. We case of type-I and type-II WSM [15]. Moreover, in spite 2 of the reduced dimension of the surface compared to the validity of this linear interpolation in Sec.V, where we bulk, we show that the transport properties are widely show that deviations in the form of a fluctuating interface affected by the surface states. Indeed, the n = 0 surface potential do not alter the main conclusions drawn from state acts as a Faraday cage and localizes the electric field the present model. Notice that the wavevector compo- at the surface. We finally prove the topological stability nent kx perpendicular to the interface needs to be treated of the n = 0 Fermi arc, based on a generalization of the as a quantum-mechanical operator such that [x; kx] = i Aharonov-Casher argument [16]. (here, and in the remaining parts of this paper, we use The paper is organized as follows: in Sec. II we explic- a system of units with ~ = 1). We consider situations itly model the interface of a bulk Weyl semimetal with with a linear x-component in the Hamiltonian that is un- an insulating bulk. Section III is devoted to a detailed changed across the interface, such that H^L(kx)−H^L(kx = ^ ^ discussion of the surface and bulk screening. In Sec.IV 0) = HR(kx) − HR(kx = 0) = vxkxσ^x and δh = δh(kk) we describe the effect of the tilt anisotropy on the Fermi with the inplane momentum kk = (0; ky; kz). In order arc structure and relate it to the effect of an electric field to describe the opposite interface one has to permute which is canceled with a proper use of Lorentz boosts. hL $ hR. This corresponds to replacing (hL; δh; `) in We extend the Aharonov-Casher argument in Sec.V, Eq. (1) by (hR; −δh; `) or equivalently by (hR; δh; −`) in originally established to prove the stability of the n = 0 all the derived expressions. Landau level of 2D DSM in a spatially inhomogeneous We rotate the Hamiltonian H^s with angle θ along the magnetic field, to the surface states of 3D WSM in the x−axis with tan(θ) = δhz/δhy in order to simplify the presence of a fluctuating surface potential. We extend the treatment of the non-commuting variables, [x; kx] 6= 0. use of chirality to three-dimensional systems and relate In the new basis, the interface Hamiltonian reads the long range behavior to surface states derived using ^ (θ) iθσ^x=2 ^ −iθσ^x=2 boundary conditions. We present a detailed discussion of Hs = e Hse our results in Sec.VI, discussing possible consequences 2 x − hxi on magneto-transport experiments. = vx kx + x=`x σ^x + sign(vx`) 2 σ^y `S + M(kk)^σz; (2) II. MAGNETIC DESCRIPTION OF THE SURFACE STATES and we introduce the notation δhk(kk) = (0; δhy; δhz) for the in-plane components of δh and Let us first consider a general case of a left and a right hxi=` = −δh · h /δh2 = 1 − δh · h /δh2; bulk, each of which is modeled by a set of parameters k L k k R k p p (see Fig.1). These parameters vary smoothly across `x = jvx`j=jδhxj; `S = jvx`j=jδhkj; (3) an interface of size `, and here we choose to linearly M(kk) = b · hL=jbj = b · hR=jbj; interpolate them between the two bulk Hamiltonians: where b = r × (δhx=`) = e × δh=` plays a role similar H^L = hL(k) · σ^ in the material on the left side (x < 0) x and H^ = h (k) · σ^ in the material on the right side to the magnetic field as we show below. Hamiltonian R R (2) can then be rewritten with the help of the following (x > `), where σ^ = (^σx; σ^y; σ^z) is a vector combining the three Pauli matrices. The interface region x 2 [0; `] ladder operators between the two bulk materials can also model a surface a^ = p`S k + x=`2 − i sign(v `)(x − hxi)=`2 ; in the case where one of the two materials is a trivial 2 x x x S bulk insulator. The discussion of the general model in (4) a^y = p`S k + x=`2 + i sign(v `)(x − hxi)=`2 ; Sec.IIA allows us to investigate the emergence of Fermi 2 x x x S arcs on the surface, while we discuss more explicit ex- y amples in the following two subsections.
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