Quantum Criticality in Twisted Transition Metal Dichalcogenides

Abstract: In moir\'e heterostructures, gate-tunable insulating phases driven by electronic correlations have been recently discovered. Here, we use transport measurements to characterize the gate-driven metal-insulator transitions and the metallic phase in twisted WSe$_2$ near half filling of the first moir\'e subband. We find that the metal-insulator transition as a function of both density and displacement field is continuous. At the metal-insulator boundary, the resistivity displays strange metal behaviour at low temperature with dissipation comparable to the Planckian limit. Further into the metallic phase, Fermi-liquid behaviour is recovered at low temperature which evolves into a quantum critical fan at intermediate temperatures before eventually reaching an anomalous saturated regime near room temperature. An analysis of the residual resistivity indicates the presence of strong quantum fluctuations in the insulating phase. These results establish twisted WSe$_2$ as a new platform to study doping and bandwidth controlled metal-insulator quantum phase transitions on the triangular lattice.

• The paper identifies continuous metal-insulator transitions in twisted bilayer WSe2 using gate-tunable configurations and activation energy data that shows insulating gaps closing.
• The paper reveals strange metal behavior with low-temperature T-linear resistivity near quantum critical points, challenging conventional Fermi liquid theory.
• The paper demonstrates that displacement field tunability enables systematic control of electronic bandwidth, highlighting the influence of quantum fluctuations on transition behavior.

Analysis of Quantum Criticality in Twisted Transition Metal Dichalcogenides

The study conducted by Ghiotto et al. explores the complex interplay of electronic phases in twisted bilayer WSe$_2$, specifically targeting the structural characterization of metal-insulator transitions driven by gate-tunable configurations. Focusing on the regime near half-filling of the first moiré subband, the researchers have employed transport measurements to assess the nature of these transitions and the subsequent behavior of the metallic state.

Twisted transition metal dichalcogenides (TMDs) have shown a strong propensity for exhibiting unconventional electronic phases, akin to phenomena observed in strongly correlated materials like cuprates and heavy fermion systems. The intrinsic interactions of such heterostructures give rise to varied insulating and metallic behavior, deeply interwoven with quantum criticality and electronic correlations.

Significant Findings

1. Continuous Metal-Insulator Transition: One of the main results from this research is the continuous nature of the metal-insulator transitions both as functions of charge density and displacement field. This continuity is underscored by activation energy data that demonstrates the regression of insulating gaps to zero at the transition boundary.
2. Strange Metal Behavior: Intriguingly, at the transition point between metal and insulator, the paper identifies a low-temperature linear-in-temperature (T-linear) resistivity. This anomalous transport property, commonly referred to as strange metal behavior, aligns with expectations of quantum criticality that defy classical Fermi liquid theory.
3. Quantum Critical Points: Notably, there is a presence of quantum critical fans marked by T-linear resistivity. This is most significant near the metal-insulator transitions on either side of the insulating state, constituting compelling evidence for quantum critical points within the studied system. Moreover, the presence of a nontrivial dependence of the Fermi temperature-like parameter $\alpha_Q$ near these critical points indicates quantum fluctuations dominating the system's behavior.
4. Magnetotransport Anomalies: The study observes an anomaly in magnetoresistance transitioning from quadratic to linear magnetic field dependence, a trait shared with other high-$T_c$ superconductors. This characteristic magnetic response further corroborates the critical behavior noted near transition points.
5. Displacement Field Tunability: The experiments reveal that by manipulating the displacement field, not only do metal-insulator transitions occur, but changes in electronic bandwidth are directly observable, offering a probe into the underlying electronic structure inherently bound to quantum criticality.

Implications and Future Directions

The conclusions put forth in this paper hold significant implications for condensed matter physics. By providing a system where doping and bandwidth can be controlled systematically within a single device, much of the noise and variability from sample-to-sample study are mitigated. This system, thereby, invites further exploration of quantum phases without structural inhomogeneities distorting the intrinsic physics.

Practically, the establishment of WSe$_2$ as a platform for delineating continuous, quantum-critical metal-insulator transitions unlocks possibilities for novel device architectures where quantum fluctuations can be harnessed for electronic tunability. The observed T-linear dependencies and strange metal behaviors might also have implications for understanding high-temperature superconductivity and designing ultra-sensitive electronic sensors.

Theoretically, this work motivates comprehensive modeling efforts to elucidate the precise mechanism enabling such quantum criticality. Exploring the proposed exotic phase, potentially a spin liquid, through more sophisticated computational methods can illuminate additional new states of matter harbored within transition metal dichalcogenide systems.

In summary, the research solidifies twisted bilayer WSe$_2$ as a fertile ground for furthering the understanding of quantum criticality in materials, presenting opportunities to explore the mysteries that lay at the crossroads of metal and insulator phases in low-dimensional materials.