Giant nonlinear Hall effect in twisted WSe$_2$ (2006.05615v3)

Abstract: The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism to generate second harmonic electrical Hall signals under time-reversal-symmetric conditions. Here, we introduce a new approach to engineering NHE by using twisted moir\'e structures. We find that the twisted WSe$_2$ bilayer exhibits a NHE when tuning the Fermi level to the moir\'e flat bands. Near half-filling of the first moir\'e band, the nonlinear Hall signal shows a sharp peak with the generation efficiency at least two orders of magnitude larger than those in previous experiments. We propose that the giant NHE and diverging generation efficiency originate from a mass-diverging type continuous Mott transition, which is evidenced by resistivity measurements. This work demonstrates not only how interaction effects can couple to Berry curvature dipoles to produce novel quantum phenomena, but also what NHE measurements can provide for developing a new tool to study the quantum criticality.

Nonlinear Hall Effect in Twisted Bilayer WSe₂

This paper presents an exploration of the nonlinear Hall effect (NHE) induced by continuous Mott transitions within twisted bilayer WSe₂ structures. The nonlinear Hall effect, distinct from conventional linear Hall effects, arises intrinsically from Berry curvature dipoles (BCD) moments in two-dimensional (2D) materials. It is characterized by the generation of second harmonic electrical Hall signals under time-reversal-symmetric conditions, offering promising possibilities for various technological applications such as energy harvesting, wireless communication, and infrared detection.

Experimental Insights into NHE

This paper demonstrates the manipulation of NHE using twisted moiré structures, particularly in WSe₂ bilayers, by exploiting electron correlation effects that induce giant nonlinear signals. Near the half-filling of the first moiré band, a pronounced second harmonic Hall voltage of approximately 15 mV was recorded, driven by a longitudinal voltage of 3.5 mV. The nonlinear Hall generation efficiency of 10 V−1 observed surpasses two orders of magnitude beyond previous reported values for non-twisted materials, indicating the significant impact of electron interactions on the NHE.

Moreover, the paper highlights the role of a mass-diverging type continuous Mott transition in this enhancement, underpinned by resistivity measurements. These findings illustrate the inherent coupling between interaction effects and Berry curvature dipoles, underlining the utility of NHE measurements in furthering the understanding of quantum criticality.

Implications for Condensed Matter Physics

The paper provides compelling evidence for the occurrence of giant NHE near the metal-to-Mott-insulator transition, a critical area of interest in condensed matter physics. The demonstration of a continuous phase transition, characterized by a divergent quasiparticle effective mass, contributes a novel perspective on the nature of quantum phase transitions, particularly in strongly correlated electron systems. This represents a new experimental approach to studying the complex quantum phenomena associated with metal-insulator transitions.

Technical Considerations and Device Fabrication

Using a dual-gate configuration with precise engineering of twist angles, the authors were able to systematically induce strain and symmetry breaking, essential components for realizing the NHE in twisted bilayer WSe₂. The electromicroscopy and theoretical modeling presented corroborate the observed symmetry-breaking effects, supporting the findings of enhanced nonlinear properties due to BCD.

Future Directions in Nonlinear Hall Studies

This research opens up new avenues for the development and engineering of advanced materials utilizing Berry curvature dipoles. Enabling the manipulation of NHE through moiré engineering platforms like WSe₂ could pave the way for developing sophisticated 2D material-based devices in various fields, including sensing, optoelectronics, and quantum computing.

Conclusion

In summary, this paper presents a compelling paper of how twisted bilayer WSe₂ can exhibit a significantly enhanced nonlinear Hall effect through the interplay of electron correlation and quantum criticality. The findings outlined herein contribute to the rich tapestry of knowledge surrounding Mott transitions and offer an experimental methodology to explore the underlying quantum mechanisms.