Optical Response of MoTe2 and WTe2 Weyl Semimetals: Distinguishing between Bulk and Surface Contributions

Abstract

A first‐principles investigation of the optical response of the Weyl Semimetals MoTe2 and WTe2 is presented. The approach, based on combining two formulations, allows to both separate the intraband and interband parts of the optical conductivity and to distinguish between the bulk and surface contributions to the optical response. It is found that the response is truly anisotropic, with peaks that can be associated with interband transitions involving either bulk or surface states. The role of the relaxation time, and the relation of the calculated results with available experimental measurements, are also discussed. Furthermore, the approach reported is transferable to any system, topologically trivial or non‐trivial, thus addressing the long‐standing need for comprehensive characterization of the optical response.

Nonmonotonic wavelength dependence of the polarization-sensitive infrared photoresponse of an anisotropic semimetal

“…In the range from 0.2 to 0.8 eV, the light–WTe 2 interaction is determined by transitions near the Weyl points since the interband transitions at other positions in the k -space require much higher photon energies. 31,32,41 In this range, either σ a or σ b increases almost linearly with the photon energy, which is a featured consequence of linear band structures near the Weyl points. 38,40,41 σ a and σ b increase at different rates with the photon energy, as shown in Fig.…”

“…The wavelength-dependent polarization-sensitive photoresponses of our devices are consistent with the dielectric constant spectra extracted from reflection measurements 38 or predicted by first-principle calculations. 31,32,38 Fig. 3(a) shows the real part of optical conductivity Re(σ) = Re(−2πiω[ε(ω) − ε ∞ ]/Z 0 ) (where Z 0 ≈377 Ω) derived from the dielectric constant.…”

“…The polarization selective excitation of these interband transitions is probably attributed to the asymmetric distribution of electron orbitals around the Weyl points, as predicted by theories. 31,32,41 In principle, the crystal symmetry of T d -WTe 2 forbids more transitions associated with the polarization along the b-axis than along the a-axis due to the selection rule. These transitions are not directly at the Weyl points, since the direct transitions between the upper and lower Weyl cones are expected to occur at even lower energy regimes, overlapping the free carrier response range.…”

“…WTe 2 , as a type-II Weyl semimetal, has received much attention due to its unique electronic, magnetic, and ferroelectric properties mostly stemming from the topological singularity near the Weyl points. [21][22][23][24][25][26][27][28] In addition, since WTe 2 exhibits intrinsic inplane anisotropy in optical and electronic properties, [29][30][31][32][33] it has been proposed for infrared polarization detection. We first revealed that the polarization-sensitive photoresponse of WTe 2 has a nonmonotonic wavelength dependence.…”

Anisotropic Optical Response of WTe2 Single Crystals Studied by Ellipsometric Analysis

“…In this context, WTe 2 is intensively studied with comprehensive spectroscopic investigations of the electronic band structure. The uniaxial shaped Fermi surface and inter-band transitions lead to significant intrinsic anisotropy [10,11,31,32] and, together with the 2D layered topological [33][34][35][36] properties, expand the list of degrees of freedom for the optical control and detection of the light wave-vector, the intensity, polarization, phase, frequency, nanoscale confinement and collective charge excitations (plasmonic activity) [37][38][39][40][41]. Optical absorber devices based on WTe 2 have already been developed as part of prototype ultrafast laser systems [42][43][44].…”