Dynamics of out-of-equilibrium electron and hole pockets in the type-II Weyl semimetal candidate WTe2

Abstract

We present a time-and angular-resolved photoemission (TR-ARPES) study of the transitionmetal dichalcogenide WTe2, a candidate type II Weyl semimetal exhibiting extremely large magnetoresistence. Using femtosecond light pulses, we characterize the unoccupied states of the electron pockets above the Fermi level. Following the ultrafast carrier relaxation in distinct parts of the Brillouin zone, we report remarkably similar decay dynamics for electrons and holes. Our results confirm that charge compensation between electron and hole pockets-a key effect to explain the non saturating magnetoresistance of this material-is a distinctive feature of WTe2 even in an out-of-equilibrium regime.

In-plane anisotropy of the photon-helicity induced linear Hall effect in few-layer WTe2

“…Figure 4 (c) shows the time-resolved auto-correlation of the longitudinal photo-current measured in the direction between the source and drain contacts. In contrast to the transverse photovoltage, the main time-scale dominating the longitudinal photocurrent, which we denote as τ fast is in the order of 2 ps and can be interpreted as an instantaneous photo-conductance increase, which is limited by the phonon-mediated charge carrier-relaxation to the Fermi-energy [18,19,27]. We note, that a slower time-scale is also detectable in the photo-current to a lesser extent.…”

“…We interpret the difference between co-polarized and cross-polarized auto-correlation of the transverse photovoltage as a signature of the photo-excited spin population. Intriguingly, the decay time surpasses the electron-phonon relaxation time of a few ps by orders of magnitude [18,27]. In fact, the slow time-scale τ slow of spin relaxation was reported to be limited by the phonon-assisted recombination of momentum indirect electron-hole pairs, suggesting, that the charge carrierrelaxation to the Fermi-energy does not significantly randomize the spin-polarization [19].…”

Spin- and angle-resolved photoemission on topological materials

“…Combined with the large number of bulk bands, the fact that the Weyl points are partly in the unoccupied energy range, and the presence of "preformed" Fermi arcs this makes the unambigious identification of the type II Weyl semimetal phase through (S)ARPES very difficult. Even with ac- cess to the unoccupied electronic structure any possible identification remains indirect [119][120][121] . The primary candidates for type II Weyl semimetal are MoTe 2 and WTe 2 with much theoretical and experimental studies devoted to both, a review of which goes far beyond the scope of this work.…”

Direct Evidence for Charge Compensation-Induced Large Magnetoresistance in Thin WTe2

“…So far, distinct physical mechanisms, such as electron-hole compensation [13][14][15] and spin texture induced suppression of backscattering 16,17 , have been proposed to explain the physical origin. The charge compensation as the most studied mechanism has been supported by the observed nearly identical electron and hole pockets at Fermi surface by angle-resolved photoemission spectroscopy (ARPES) 15,[18][19][20] . However, these ARPES experiments only offer an indirect evidence.…”