Continuous Mott transition in semiconductor moiré superlattices (2103.09779v1)

Abstract: The evolution of a Landau Fermi liquid into a nonmagnetic Mott insulator with increasing electronic interactions is one of the most puzzling quantum phase transitions in physics. The vicinity of the transition is believed to host exotic states of matter such as quantum spin liquids, exciton condensates and unconventional superconductivity. Semiconductor moir\'e materials realize a highly controllable Hubbard model simulator on a triangular lattice, providing a unique opportunity to drive a metal-insulator transition (MIT) via continuous tuning of the electronic interactions. Here, by electrically tuning the effective interaction strength in MoTe2/WSe2 moir\'e superlattices, we observe a continuous MIT at a fixed filling of one electron per unit cell. The existence of quantum criticality is supported by the scaling behavior of the resistance, a continuously vanishing charge-gap as the critical point is approached from the insulating side, and a diverging quasiparticle effective mass from the metallic side. We also observe a smooth evolution of the low-temperature magnetic susceptibility across the MIT and find no evidence of long-range magnetic order down to ~ 5% of the Curie-Weiss temperature. The results signal an abundance of low-energy spinful excitations on the insulating side that is further corroborated by the presence of the Pomeranchuk effect on the metallic side. Our results are consistent with the universal critical theory of a continuous MIT from a Landau Fermi liquid to a nonmagnetic Mott insulator in two dimensions.

• The paper demonstrates continuous tuning of electron interactions using a dual-gate FET to achieve a metal-to-nonmagnetic Mott insulator transition.
• It reports resistance scaling with power-law behavior and diverging quasiparticle mass near a critical electric field, confirming quantum criticality.
• The study reveals nonmagnetic insulating states with extensive spin excitations, offering a promising platform for investigating exotic 2D quantum phases.

Continuous Mott Transition in Semiconductor Moiré Superlattices

This paper investigates a fundamental quantum phase transition, specifically the continuous Mott transition, in moiré superlattices composed of transition metal dichalcogenides (TMDs), MoTe$_2$/WSe$_2$. The research revolves around controlling electronic interactions to transform a Landau Fermi liquid into a nonmagnetic Mott insulator. Moiré superlattices based on semiconductors offer a set of parameters analogous to the Hubbard model on a triangular lattice, serving as an effective platform to explore the Mott transition via highly tunable systems.

Key Experimental Findings

The authors employ a dual-gate field-effect transistor (FET) structure to manipulate the electronic interaction strength (U/W), which is critical for traversing the metal-insulator transition. By adjusting the out-of-plane electric field, they achieve continuous tuning of the electronic states at a constant filling factor of one electron per unit cell. The paper reports several pivotal observations:

1. Resistance Scaling and Diverging Quasiparticle Mass: Near the critical electric field ($E_c ≈ 1.304 \, \text{V/nm}$), resistance scaling adheres to power-law behavior. The findings suggest a continuous evolution without hysteresis, a hallmark of a continuous Mott transition. Additionally, the quasiparticle effective mass diverges as the transition is approached from the metallic side. This observation is consistent with quantum criticality theories, indicating a complete disappearance of the electronic Fermi surface.
2. Magnetic Susceptibility Evolution: The paper reveals the absence of long-range magnetic order down to very low temperatures (~5% of the Curie-Weiss temperature), pointing to a nonmagnetic insulator state. Further, the system's magnetic properties exhibit a smooth transition across the Mott transition without abrupt changes, suggesting strong quantum fluctuations that prevent symmetry-breaking orders.
3. Pomeranchuk Effect and Spinful Excitations: The observations indicate extensive low-energy spin excitations in the insulating state, substantiated by the presence of the Pomeranchuk effect on the metallic side. This is characterized by an increased spin entropy with rising temperature, in line with behavior observed in quantum liquids like helium-3.

Implications and Future Directions

The results underscore a near-perfect realization of a continuous Mott transition within a moiré superlattice framework. This experiment showcases how non-magnetic disorder plays a perturbative role due to the high doping density compared to disorder density, thereby isolating the interactions as the primary driver of the Mott transition. This setup allows further exploration of 2D quantum critical phenomena beyond the current understanding, such as the emergence of quantum spin liquids.

Looking ahead, further investigations at lower temperatures might unveil new topological phases or exotic matter states facilitated by quantum fluctuations inherent to the moiré lattice. There is potential for this research to substantiate theoretical frameworks predicting such phases, possibly impacting areas such as high-temperature superconductivity and the paper of quantum materials.

In conclusion, this paper provides significant experimental evidence supporting continuous interaction-driven Mott transitions in two-dimensional moiré superlattices, offering a platform rich for both theoretical exploration and practical application in the evolution of quantum materials.