Pressure Boosts Magnetism in Layered Semiconductor Crystals

Researchers are increasingly focused on exploiting the unique properties of moiré semiconductors to realise novel correlated electronic phases. Pengfei Jiao and Chenghao Qian, both from Shanghai Jiao Tong University, alongside Ning Mao from the Max Planck Institute for Chemical Physics of Solids, and colleagues report a significant advance in controlling these materials through applied pressure. Their work details the development of a cryogenic, dual-gated diamond-anvil cell, using helium as a pressure medium, to reversibly tune twisted bilayer WSe2, enabling magneto-optical spectroscopy under hydrostatic conditions. This innovative approach demonstrates that hydrostatic pressure not only enhances the moiré potential and redshifts excitons, but also stabilises previously absent Stoner ferromagnetism and strengthens the half-filled Chern insulating state, representing a powerful new method for band engineering and correlated magnetism in these two-dimensional heterostructures.

Flat materials just gained a new dimension of control. Applying pressure to layered semiconductors unlocks hidden magnetic properties and exotic electronic states, offering a simple yet powerful method to engineer materials with tailored behaviours for future technologies. Scientists are increasingly focused on manipulating the behaviour of electrons in atomically thin materials to create new quantum phenomena.

Moiré materials, formed by stacking layers with a slight twist, present flat energy bands where electron interactions become dominant, offering a pathway to explore exotic states of matter. Controlling the delicate interlayer coupling, the strength of interaction between stacked layers, has remained a significant challenge, limiting the potential for discovering and tuning correlated electron behaviour.

Researchers have now developed a cryogenic, high-pressure platform to precisely control this interlayer coupling in twisted bilayer tungsten diselenide (WSe2). At the heart of this advance lies a diamond-anvil cell, a device used to generate extreme pressures on microscopic samples. This new setup integrates dual-gated control, allowing for electrical tuning, with magneto-optical spectroscopy, a technique that probes the material’s magnetic and optical properties.

Using helium as a pressure-transmitting medium, the team achieved reversible, uniform pressure up to 2 GPa on the twisted bilayer WSe2, while maintaining cryogenic temperatures down to 1.7 K and applying magnetic fields up to 9 Tesla. This combination enabled detailed observation of how pressure modifies the moiré potential, the energy field experienced by electrons in the twisted structure.

Initial findings reveal that applied pressure not only enhances the moiré potential but also induces a transition to a state exhibiting Stoner ferromagnetism, a type of magnetism not present at the initial twist angle of 3.1 degrees. Simultaneously, the strength of a half-filled Chern insulating state, a topological state of matter with protected edge currents, is amplified, requiring a lower magnetic field for saturation.

Beyond this, observations indicate a phase transition occurring around 2 GPa, shifting the material from a Chern insulator to a Mott insulator, a state where electron interactions prevent conduction. Calculations suggest this transition arises from a switching of the valence band maximum, altering the electronic structure and driving the change in insulating behaviour. Once established, the platform allows for continuous tuning of both magnetism and band structure within the moiré material, though understanding the precise interaction between pressure and electronic correlations remains important.

Hydrostatic pressure tuning and spectroscopic characterisation of twisted bilayer WSe2

A cryogenic dual-gated diamond-anvil cell (DAC) was central to applying and controlling pressure during this research. This DAC configuration allowed for reversible hydrostatic tuning of twisted bilayer WSe2 samples, utilising helium as a pressure-transmitting medium. Employing helium minimises anhydrostaticity, a condition where pressure is not applied equally in all directions, which could distort the sensitive moiré lattice structure.

The DAC chamber underwent screw-compression to generate controlled pressure on the sample, but achieving simultaneous electrical and optical access demanded careful design. Researchers integrated dual-gate control alongside magneto-optical spectroscopy, enabling manipulation of the material’s electronic properties while probing its optical response. By combining reflectance spectroscopy with magnetic circular dichroism (MCD), they monitored changes in the material’s behaviour under increasing pressure and varying magnetic fields.

At temperatures down to 1.7 K and magnetic fields reaching 9 T, the setup facilitated detailed investigation of quantum phenomena. Twisted bilayer WSe2 flakes were mechanically exfoliated onto a substrate before being loaded into the DAC, allowing for tuning of the magnetic field and observation of its effects on the electronic structure.

Stabilised ferromagnetism and a pressure-tuned topological transition in a moiré superlattice

At a twist angle of 3.1 degrees, Stoner ferromagnetism was stabilised, a phenomenon not previously observed without applied pressure. Simultaneously, the strength of the half-filled C = 1 Chern insulating state increased, accompanied by a reduction in its saturation field. Specifically, the saturation field diminished as pressure was applied, indicating a more easily achievable insulating state.

Further experimentation revealed a topological phase transition occurring around 2 GPa, marking a shift from a Chern insulator to a Mott insulator. This transition isn’t abrupt; first-principles calculations pinpoint a Γ, K valence band maximum switching as the driving force, converting an Ising-like K-valley miniband into a spin-degenerate trivial Γ miniband, indicative of the Mott insulating state. Calculations demonstrate that the moiré potential enhances with increasing pressure.