Optical properties of the perfectly compensated semimetal WTe

Figure S1. (Color online) The unit cell is shown for the b-c face projected along the a axis. The tungsten atoms are surrounded by tellurium octahedra that are distorted due to the zig-zag nature of the tungsten chains. using both the local spin density approximation (LSDA) as well as the generalized gradient approximation (GGA) using the full-potential linearized augmented plane-wave (FP-LAPW) method [2] with local-orbital extensions [3] in the WIEN2k implementation [4]. The system was assumed to be non-magnetic. An examination of different Monkhorst-Pack k-point meshes indicated that a 9× 5× 2 mesh and Rmtkmax = 7.75 was sufficient for good energy convergence. The geometry of the unit cell was refined through an iterative process whereby the volume was optimized with respect to the total energy while the c/a ratio remained fixed. The atomic fractional coordinates were then relaxed with respect to the total force, typically resulting in residual forces of less than 0.2 mRy/a.u. For both of these procedures spin-orbit coupling is ignored. This approach was repeated until no further improvement was obtained. A comparison of the experimental and calculated (relaxed) unit cell parameters are shown in Table. I. The electronic band structure has been calculated with LSDA and spin-orbit coupling for several different paths between high-symmetry points in the orthorhombic unit cell, shown in Fig. S2. This calculation accurately reproduces the work of Ali et al. [5], as well as an earlier work [6]. The relatively small number of bands crossing the Fermi level ( F) illustrates the semimetallic character of WTe2. In particular, there is an electron-like band and a hole-like band crossing Fermi surface along the Γ − X direction. Figure S2 also reveals that there are several other points in the Brillouin zone at which hole and electron bands just manage to cross the Fermi surface; however, the electronic structure is quite sensitive to the unit cell geometry, and that even a small increase (or decrease) in the Fermi level might move these bands above or below F. While the paths along the high-symmetry directions are useful, the contributions of the hole and electron bands to the transport is best understood