Electric Fields Drive Two Distinct Material Phase Changes

Researchers report the observation of consecutive topological phase transitions between correlated insulators in a moire MoTe2/WSe2 heterobilayer, a phenomenon of fundamental interest yet rarely seen experimentally. Xumin Chang and Wanghao Tian from Shanghai Jiao Tong University, working with Zui Tao and Bowen Shen from Cornell University, alongside Jenny Hu, Kateryna Pistunova, Tony F Heinz and Kin Fai Mak from Stanford University, Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science, and Tingxin Li, Jie Shan and Shengwei Jiang, demonstrate two electric-field-driven transitions at half filling. This collaborative effort reveals how a geometrically frustrated Mott insulator evolves first into a ferromagnetic quantum anomalous Hall Mott insulator, and subsequently into an antiferromagnetic, valley-coherent Mott insulator, accompanied by a critical metallic state. The ability to tune between these distinct correlated states establishes the MoTe2/WSe2 moire platform as a promising avenue for exploring extended Kane-Mele-Hubbard models and understanding the interplay between correlation and topology.

Molybdenum ditelluride and tungsten diselenide moiré heterobilayers exhibit a sequence of transformations driven by an applied electric field. This achievement opens up exciting possibilities for designing new devices where electron behaviour can be precisely tuned and establishes a versatile platform for exploring exotic states of matter and their potential applications in future technologies.

Research focused on angle-aligned molybdenum ditelluride (MoTe2) and tungsten diselenide (WSe2) moiré heterobilayers reveals a sequence of transformations driven by an applied electric field. These transitions involve shifts in both the symmetry and topological order of the material’s electronic states, a phenomenon rarely observed experimentally. Layer-resolved magnetic circular dichroism, magneto-transport, and compressibility measurements were jointly used to map the complete phase diagram of these transitions.

Notably, the antiferromagnetic state exhibits a metamagnetic-like transition at a critical magnetic field, beyond which a Chern insulating transport response reappears. This intricate interplay between electric and magnetic fields allows for precise tuning of the material’s electronic behaviour. The MoTe2/WSe2 platform is now established as a tunable realization of an extended Kane-Mele-Hubbard model, a theoretical framework describing the behaviour of electrons in these materials, and hosting sequential correlation-topology intertwined transitions.

Realising such controlled transitions demands precise material engineering. The study employed angle-aligned MoTe2/WSe2 moiré heterobilayers, where the slight lattice mismatch, approximately 7%, favors electrical contact formation and transport studies. Wannier orbitals reside primarily in each layer, forming a triangular lattice structure that hybridizes upon application of an electric field.

This hybridization creates an effective honeycomb lattice, allowing for manipulation of band topology through the Stark effect. Focusing on half-band-filling, researchers meticulously examined the longitudinal resistance of the material under varying electric fields and temperatures. At zero magnetic field, a distinct dip in longitudinal resistance confirms the presence of the quantum anomalous Hall state, corroborated by magnetic circular dichroism measurements.

Three distinct regions were identified in the material’s behaviour. Below an electric field of 0.68V/nm, the material exhibits a Mott insulating state, with holes localized within the MoTe2 layer. Increasing the field to between 0.685 and 0.705V/nm induces a transition to the QAH insulator, characterised by chiral edge state transport and a quantized Hall resistance of h/e.

Beyond approximately 0.705V/nm, a new antiferromagnetic Mott insulator emerges, displaying diverging resistance at low temperatures. The first phase transition, from Mott to QAH insulator, appears to be a weak first-order transition, while the second transition, to the antiferromagnetic state, is nearly gapless, indicated by a single crossing point in the resistance isotherms.

Mapping magnetism and electronic transport in moiré heterobilayers reveals Chern insulator behaviour

Layer-resolved magnetic circular dichroism (MCD) measurements allowed precise examination of magnetic properties within the moiré heterobilayer structure. MCD spectroscopy was employed to map the spatial distribution of magnetism across the device, determining the evolution of magnetic order during the electric-field-driven phase transitions and providing critical insight into the emergence of both ferromagnetic and antiferromagnetic states.

Magneto-transport measurements were undertaken to characterise the electronic transport properties of the heterobilayer under applied magnetic fields. Four-point probe measurements accurately determined the electrical conductivity as a function of both electric and magnetic field, revealing the appearance of a Chern insulating transport response at higher magnetic fields.

The sensitivity of the magneto-transport data allowed for the identification of a metamagnetic-like transition at a critical field. Compressibility measurements, performed using a gate-controlled field-effect transistor configuration, were used to probe the density of states and charge carrier behaviour. By monitoring the change in capacitance with applied voltage, researchers tracked the opening and closing of charge gaps associated with the different insulating phases, providing a direct measure of the electronic compressibility and revealing a continuous charge-gap collapse accompanying the transition to the valley-coherent Mott insulator.

Device fabrication began with mechanically exfoliating atomically thin layers of MoTe2 and WSe2 onto hexagonal boron nitride substrates. These layers were then stacked with precise angle alignment to create the moiré heterobilayer. Electrical contacts were fabricated using a standard electron-beam lithography process, employing a titanium and gold stack to ensure low-resistance connections. This careful fabrication process was essential for achieving high-quality devices suitable for sensitive transport and optical measurements.

Electric field control of correlated insulating states and the quantum anomalous Hall effect

Initial measurements reveal a distinct evolution of electronic phases in angle-aligned MoTe2/WSe2 moiré bilayers with applied electric field. Longitudinal resistance (Rxx) measurements at zero magnetic field demonstrate a clear transition between insulating states, with values initially increasing rapidly with decreasing temperature for electric fields below 0.68V/nm.

This behaviour characterizes a Mott insulating state where holes reside solely within the MoTe2 layer. Once the electric field exceeds approximately 0.685V/nm, Rxx exhibits a sharp decrease, accompanied by the emergence of a quantized Hall resistance of h/e, a hallmark of the quantum anomalous Hall (QAH) effect. These findings establish a pathway for tuning between correlated insulating states and a critical metallic state within the MoTe2/WSe2 moiré system, offering a platform to explore the interplay between correlation and topology.

Electric fields induce sequential phase changes in layered materials

Scientists are increasingly focused on manipulating materials at the atomic scale to engineer new electronic properties. Recent work with layered materials, specifically molybdenum ditelluride and tungsten diselenide arranged in a moiré pattern, reveals a remarkable sequence of phase transitions driven by an applied electric field. These aren’t simply changes in conductivity; they represent shifts in the fundamental order of electrons within the material, moving from a frustrated insulating state through magnetism and towards a metallic state.

Such control over correlated electron behaviour has long been a goal, hampered by the difficulty of isolating and manipulating the delicate interplay of electron interactions and material structure. Achieving consecutive topological phase transitions remains a considerable challenge. Previous attempts often required extreme conditions or resulted in unstable phases.

This research demonstrates a pathway to navigate between these states with relative ease, using an electric field as the control parameter. Once a critical field strength is surpassed, the material exhibits a return to an insulating state, suggesting a level of control previously unseen in these layered heterostructures. The emergence of a critical metallic state alongside the final phase transition introduces complexity.

Understanding the precise nature of this metallic state requires further investigation. Beyond this, the long-term stability of these phases and their sensitivity to imperfections in the material remain open questions. The ability to repeatedly cycle through these phases opens doors to new device concepts. Imagine creating electronic switches or memory elements where information is encoded not in charge, but in the material’s topology. Future work might explore stacking multiple moiré layers to create even more complex and controllable electronic systems, potentially leading to entirely new forms of quantum electronics.