Linear resistivity at van Hove singularities in twisted bilayer WSe2.

Different mechanisms driving a linear temperature dependence of the resistivity ρ ∼ T at van Hove singularities (VHSs) or metal-insulator transitions when doping a Mott insulator are being debated intensively with competing theoretical proposals. We experimentally investigate this using the exceptional tunability of twisted bilayer (TB) WSe2 by tracking the parameter regions where linear-in-T resistivity is found in dependency of displacement fields, filling, and magnetic fields. We find that even when the VHSs are tuned rather far away from the half-filling point and the Mott insulating transition is absent, the T-linear resistivity persists at the VHSs. When doping away from the VHSs, the T-linear behavior quickly transitions into a Fermi liquid behavior with a T2 relation. No apparent dependency of the linear-in-T resistivity, besides a rather strong change of prefactor, is found when applying displacement fields as long as the filling is tuned to the VHSs, including D ∼ 0.28 V/nm where a high-order VHS is expected. Intriguingly, such non-Fermi liquid linear-in-T resistivity persists even when magnetic fields break the spin-degeneracy of the VHSs at which point two linear in T regions emerge, for each of the split VHSs separately. This points to a mechanism of enhanced scattering at generic VHSs rather than only at high-order VHSs or by a quantum critical point during a Mott transition. Our findings provide insights into the many-body consequences arising out of VHSs, especially the non-Fermi liquid behavior found in moiré materials.

Observation of Multi-Phonon Emission in Monolayer WS2 on Various Substrates.

Transition metal dichalcogenides (TMDs) have unique absorption and emission properties that stem from their large excitonic binding energies, reduced-dielectric screening, and strong spin-orbit coupling. However, the role of substrates, phonons, and material defects in the excitonic scattering processes remains elusive. In tungsten-based TMDs, it is known that the excitons formed from electrons in the lower-energy conduction bands are dark in nature, whereas low-energy emissions in the photoluminescence spectrum have been linked to the brightening of these transitions, either via defect scattering or via phonon scattering with first-order phonon replicas. Through temperature and incident-power-dependent studies of WS2 grown by CVD or exfoliated from high-purity bulk crystal on different substrates, we demonstrate that the strong exciton-phonon coupling yields brightening of dark transitions up to sixth-order phonon replicas. We discuss the critical role of defects in the brightening pathways of dark excitons and their phonon replicas, and we elucidate that these emissions are intrinsic to the material and independent of substrate, encapsulation, growth method, and transfer approach.