Giant nonlinear Hall effect near the continuous Mott transition in twisted transition metal dichalcogenides

Given an oscillating electric signal, rectification results in a direct-current signal and frequency doubling results in a signal oscillating at twice the frequency of the input. Traditionally, this kind of frequency modulation is achieved by semiconductor junctions or diodes. Their performance is limited by thermal threshold voltages, transition time and many other parameters. In contrast, rectification and frequency doubling through the nonlinear Hall effect are achieved by the Berry curvature dipole, which is an intrinsic property of the material. Therefore, it does not have the limitations faced by traditional methods and can be used in a broad range of technological applications that require rectification or second harmonic generation such as efficient energy harvesting, next-generation wireless techniques, and infrared detectors. To date, however, experimental observations of the nonlinear Hall signals are very week and limited to a small class of non-centrosymmetric materials.

The development of van der Waals assembling techniques offers exciting opportunities to engineer heterostructures with exotic physical properties beyond those of the individual materials. For example, when stacking two monolayer WSe₂ together with a small twist angle, the interlayer hybridization produces flat sub-bands. The modified band structures have led to the observation of emergent phenomena related to electron correlations beyond the expectations from single particle physics, such as continuous Mott transition and possible superconducting phases.

In this study, the research team demonstrates that the twisting technique can provide an additional route for engineering the nonlinear Hall effect. When the first moiré valence band of twisted WSe₂ was half-filled, the nonlinear Hall signal exhibited a sharp peak (see left figure below) with a generation efficiency of 1000 V^-1 that was at least two orders of magnitude greater than those obtained in previous experiments (see right figure below). While the strain-induced nonzero Berry curvature dipole can be used to understand the experimental data away from half-filling, the giant enhancement of the nonlinear Hall signal cannot be understood even when the moiré bands and strained-induced dipole are taken into account. To understand the giant signal, the team studied the temperature-dependent resistance of the system and observed continuous Mott transition near the half-filling of a moiré unit cell. The transition gives rise to a divergent quasiparticle effective mass and enhances the nonlinear Hall signal.

This study demonstrates not only how interaction effects can combine with Berry curvature dipoles to produce novel quantum phenomena, but also the potential of nonlinear Hall measurements as a new tool for studying quantum criticality, which is one of the most challenging and interesting problems in condensed matter physics.