Strong and Tunable Electrical Anisotropy in Type‐II Weyl Semimetal Candidate WP₂ with Broken Inversion Symmetry

Abstract

A transition metal diphosphide, WP2, is a candidate for type‐II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP2, spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP2 electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle‐resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first‐principle studies of WP2 are reported. The energies of the observed Raman‐active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first‐principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP2. Moreover, the measured ratio (Rc /Ra ) between the crystalline c‐axis and a‐axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in‐plane electrical resistivity anisotropy among the reported type‐II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first‐principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the Rc /Ra of WP2 at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so‐far‐strongest and magnetic‐field‐tunable electrical resistivity anisotropy found in WP2 can serve as a degree of freedom for tuning the electrical properties of type‐II WSMs, which paves the way for the development of novel electronic applications based on type‐II WSMs.

Effect of topology on quasi-particle interactions in the Weyl semimetal WP$_2$

“…Details of the observed phonon modes of both phases can be found in the Supplement. Our room temperature data are in excellent agreement with a previous study of phonons in β-WP 2 13 .…”

“…This means that the non-topological phase has a larger electron-phonon coupling and accordingly, that this is not essential for transport anomalies of the topological phase. This conclusion is diametrical to a theoretical consideration of phonon scattering processes in β-WP 2 13 . On the other side, the observed anomalies do not contradict the present knowledge of topologically induced phonon anomalies 4,5 .…”

Evidence for dominant phonon-electron scattering in Weyl semimetal WP$_{2}$

“…The full polarization dependence yields the mode assignments shown in Fig. 1a, which are consistent with previous studies 20,21 (Supplemental Table 1). In Fig.…”

Complete Dielectric Tensor and Giant Optical Anisotropy in Quasi-One-Dimensional ZrTe5

“…In general, there are two main methods to obtain the optical constants or dielectric functions. One is to measure the absorption, reflectance, or transmission (ART) spectrum of samples, and then, the complex refractive index can be obtained with an additional Kramers–Kronig (K–K) relation. , In the ART method, the hypotheses on the spectrum beyond the concerned measurement range must be considered when using the K–K relation. Therefore, the accuracy and reliability of the obtained optical constants still need to be improved.…”