Picosecond Ultrasonics in Magnetic Topological Insulator MnBiTe.

MnBiTe is a magnetic topological insulator with layered A-type antiferromagnetic order. It exhibits a rich layer- and magnetic-state dependent topological phase diagram; however, much about the coupling between spin, charge, and lattice remains to be explored. In this work, we report that MnBiTe is an excellent acoustic phonon cavity by realizing phonon frequency combs using picosecond ultrasonics.

Visualizing moiré ferroelectricity via plasmons and nano-photocurrent in graphene/twisted-WSe structures.

Ferroelectricity, a spontaneous and reversible electric polarization, is found in certain classes of van der Waals (vdW) materials. The discovery of ferroelectricity in twisted vdW layers provides new opportunities to engineer spatially dependent electric and optical properties associated with the configuration of moiré superlattice domains and the network of domain walls. Here, we employ near-field infrared nano-imaging and nano-photocurrent measurements to study ferroelectricity in minimally twisted WSe.

Tunable interaction between excitons and hybridized magnons in a layered semiconductor.

The interaction between distinct excitations in solids is of both fundamental interest and technological importance. One such interaction is the coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon, a collective spin excitation. The recent emergence of van der Waals magnetic semiconductors provides a platform to explore these exciton-magnon interactions and their fundamental properties, such as strong correlation, as well as their photospintronic and quantum transduction applications.

Anomalous Second Harmonic Generation from Atomically Thin MnBiTe.

MnBiTe is a van der Waals topological insulator with intrinsic intralayer ferromagnetic exchange and A-type antiferromagnetic interlayer coupling. Theoretically, it belongs to a class of structurally centrosymmetric crystals whose layered antiferromagnetic order breaks inversion symmetry for even layer numbers, making optical second harmonic generation (SHG) an ideal probe of the coupling between the crystal and magnetic structures. Here, we perform magnetic field and temperature-dependent SHG measurements on MnBiTe flakes ranging from bulk to monolayer thickness.

Reversible strain-induced magnetic phase transition in a van der Waals magnet.

Mechanical deformation of a crystal can have a profound effect on its physical properties. Notably, even small modifications of bond geometry can completely change the size and sign of magnetic exchange interactions and thus the magnetic ground state. Here we report the strain tuning of the magnetic properties of the A-type layered antiferromagnetic semiconductor CrSBr achieved by designing a strain device that can apply continuous, in situ uniaxial tensile strain to two-dimensional materials, reaching several percent at cryogenic temperatures.

Interlayer electronic coupling on demand in a 2D magnetic semiconductor.

When monolayers of two-dimensional (2D) materials are stacked into van der Waals structures, interlayer electronic coupling can introduce entirely new properties, as exemplified by recent discoveries of moiré bands that host highly correlated electronic states and quantum dot-like interlayer exciton lattices. Here we show the magnetic control of interlayer electronic coupling, as manifested in tunable excitonic transitions, in an A-type antiferromagnetic 2D semiconductor CrSBr. Excitonic transitions in bilayers and above can be drastically changed when the magnetic order is switched from the layered antiferromagnetic ground state to a field-induced ferromagnetic state, an effect attributed to the spin-allowed interlayer hybridization of electron and hole orbitals in the latter, as revealed by Green's function-Bethe-Salpeter equation (GW-BSE) calculations.

Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator.

Two-dimensional (2D) semiconductors enable the investigation of light-matter interactions in low dimensions. Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials. Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS.