Tunable quantum criticalities in an isospin extended Hubbard model simulator

TL;DR

This paper reports the experimental observation of tunable quantum criticalities in a moiré heterostructure-based simulator, revealing complex quantum phase transitions and critical behaviors involving multiple degrees of freedom.

Contribution

It introduces a highly tunable solid-state quantum simulator using twisted double bilayer graphene to explore quantum critical phenomena beyond traditional models.

Findings

Identification of two distinct quantum critical points during phase transition

Observation of a quantum pseudo criticality under high magnetic field

Evidence of a weak first-order quantum phase transition

Abstract

Studying strong electron correlations has been an essential driving force for pushing the frontiers of condensed matter physics. In particular, in the vicinity of correlation-driven quantum phase transitions (QPTs), quantum critical fluctuations of multiple degrees of freedom facilitate exotic many-body states and quantum critical behaviors beyond Landau's framework. Recently, moir\'e heterostructures of van der Waals materials have been demonstrated as a highly tunable quantum platform for exploring fascinating strongly correlated quantum physics. Here, we report the observation of tunable quantum criticalities in an experimental simulator of extended Hubbard model with spin-valley isospins arising in chiral-stacked twisted double bilayer graphene. Scaling analysis shows a quantum two-stage criticality manifesting two distinct quantum critical points as the generalized Wigner crystal transits to a Fermi liquid by varying the displacement field, suggesting the emergence of a critical intermediate phase. The quantum two-stage criticality evolves into a quantum pseudo criticality as a high parallel magnetic field is applied. In such pseudo criticality, we find that the quantum critical scaling is only valid above a critical temperature, indicating a weak first-order QPT therein. Our results demonstrate a highly tunable solid-state simulator with intricate interplay of multiple degrees of freedom for exploring exotic quantum critical states and behaviors.