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

Abstract

Using Hall photovoltage measurements, we demonstrate that a linear transverse Hall-voltage can be induced in few layer WTe2 under circularly polarized light illumination. By applying a bias voltage along different crystal axes, we find that the photon-helicity induced Hall effect coincides with a particular crystal axis. Our results are consistent with the underlying Berry-curvature exhibiting a dipolar distribution due to the breaking of crystal inversion symmetry. Using a time-resolved optoelectronic auto-correlation spectroscopy, we find that the decay time of the detected Hall voltage exceeds the electron-phonon scattering time by orders of magnitude but is consistent with the comparatively long spin-lifetime of carriers in the momentum-indirect electron and hole pockets in WTe2. Our observation suggests, that a helicity induced non-equilibrium spin density on the Fermi surface after the initial charge carrier relaxation gives rise to a linear Hall effect.

Light-field and spin-orbit-driven currents in van der Waals materials

“…For monolayer WTe 2 , it was shown that the Berry curvature dipole can be electrically tuned by a displacement field leading to a circular photogalvanic effect when inversion symmetry is broken by an out-of-plane electric field [111]. By contrast, the intrinsically broken inversion symmetry of few-layer WTe 2 is a promising platform for lightwave and Berry phase-driven spin-and current-transport phenomena, e. g., giving rise to a photon-helicity-induced linear Hall effect at room temperature [181].…”

“…In a first step, the semimetal is aligned onto pre-defined Ti/Au electrodes and covered by a thin sheet of hBN using mechanical exfoliation. The orientation of the crystallographic axis a is chosen to match the axis of two opposing electric contacts, which is verified by polarization-resolved Raman spectroscopy [181,182]. A laser (E photon = 1.5 eV) is focused at the center of the WTe 2 -film and an external (longitudinal) bias V bias is applied between source and drain along the crystal a-axis, while the photo-induced (transversal) voltage V photo is measured for different polarizations of the incident light.…”

“…The fast response τ fast of the longitudinal current can be interpreted as an instantaneous increase of photoconductance, limited by the phonon-mediated charge carrier relaxation to the Fermi energy [183,184]. In contrast, the long timescale τ slow is likely to stem from the excited nonequilibrium spin density on the Fermi surface after the initial charge carrier thermalization, which subsequently is driven by an anomalous velocity proportional to the anisotropic Berry curvature [142,181]. Thus, timeintegrated, as well as time-resolved photocurrent spectroscopy facilitate the observation of phenomena related to quantum geometrical phases in Weyl semimetals and topological materials up to room temperature.…”

Ultrafast and Local Optoelectronic Transport in Topological Insulators

“…[ 34,82,83,85 ] A comparable temporal resolution can be achieved by optoelectronic autocorrelation schemes, which, however, rely on saturating the electronic response by the pump pulse and necessitate large pump fluences. [ 36,88 ] Finally, electro‐optic sampling of the THz radiation emitted into the far‐field after optical pumping can be used to infer information on transient currents and terahertz conductivity inside the material. The latter technique enables extremely high temporal resolution down to a few femtoseconds, only limited by the duration of the probe pulse.…”

“…[ 132,133 ] Alternatively, time‐reversal symmetry can effectively be broken by a circularly polarized optical excitation due to the Berry curvature‐dependent chiral selection rules of the Weyl points, which was shown to give rise to a linear photoinduced Hall effect in WTe 2 . [ 88 ] For optoelectronic measurements, the WTe 2 crystal was contacted via four electrodes in a multiterminal configuration (Figure 9c). Between two electrodes an external (longitudinal) bias V bias was applied, and along the perpendicular (transversal) direction a photovoltage V photo was measured for a circularly left‐ and circularly right‐polarized excitations, respectively.…”