Shengwei Jiang

Superconductivity was recently observed in twisted WSe$_2$ and MoTe$_2$, raising a central question: is the pairing driven by electronic correlations, by phonons, or by both? Answering it requires determining the electron--phonon coupling (EPC) in these moir\'e semiconductors, whose calculation in realistic supercells of thousands of atoms lies beyond the reach of direct first-principles methods. Here we combine filling-dependent Raman spectroscopy with machine-learning first-principles calculations to obtain the EPC mode by mode in supercells of up to tens of thousands of atoms. Raman reveals only a few moir\'e phonons whose frequencies shift strongly with filling; we trace this to an interference selection rule: a phonon couples strongly only when its displacement texture matches the static lattice-reconstruction pattern, and is otherwise suppressed by destructive interference. The rule selects the low- and high-frequency breathing modes seen in Raman and makes the coupling peak at large twist angles, near those at which superconductivity appears. Lattice-reconstruction interference thus emerges as an organizing principle for moir\'e EPC, pointing to a substantial, potentially dominant, phonon contribution to large-angle pairing.

Rhombohedral graphene (RG) offers a highly tunable platform for exploring topology and correlated electronic phases, as its electronic structure can be precisely controlled by carrier density (n) and displacement field (D). In particular, intervalley coherence (IVC) has been proposed as a key instability underlying symmetry breaking and unconventional superconductivity, yet its presence and evolution across the n–D phase diagram have not been experimentally established. Here we study rhombohedral pentalayer graphene using scanning tunnelling microscopy with independent control of carrier density and displacement field. We directly visualize IVC through Kekulé-like modulations in the local density of states and corresponding intervalley scattering peaks in Fourier-transform maps. Systematic mapping across the n–D phase diagram reveals that IVC persists over a broad range of carrier densities and displacement fields, rather than being confined to a narrow region, indicating its coexistence and competition with other correlated states. These results establish a microscopic picture in which IVC emerges as an extended and tunable instability intertwined with other symmetry-breaking orders in RG.

The deterministic control of quantum phases via ultrafast light-matter interaction lies at the frontier of modern physics. While light can trigger hidden phases inaccessible through equilibrium pathways, these photo-induced metastable states are typically transient or highly volatile, limiting their practical utility. Here, we demonstrate a hybrid optical-thermal strategy that utilizes photo-induced topological defects as kinetic gateways to unlock a robust, hidden charge density wave (CDW) phase in the van der Waals antiferroelectric (AFE) In2Se3. Using complementary ultrafast electron diffraction and microscopy, we show that an initial optical quench induces a metastable state characterized by long-lived topological defects via the Kibble-Zurek mechanism; upon thermal cooling, these defects act as heterogeneous nucleation seeds, lowering the kinetic barrier for the system to evolve thermodynamically into a hidden CDW state. Remarkably, this phase transition is fully reversible: a single laser pulse resets the system to a defect-engineered intermediate phase, preserving the topological landscape for subsequent switching. Our results establish a paradigm for using photo-induced topological defects to imprint kinetic pathways, providing a blueprint for the discovery of hidden quantum landscapes and the development of high-speed, non-volatile memory technologies.

Moiré materials—formed by stacking atomically thin crystals with a small twist—create tunable electronic lattices where strong interactions and band topology can generate unusual magnetic and topological phases. In twisted bilayer MoTe₂, ferromagnetism has been closely linked to integer and fractional Chern insulators, while the true zero‑field ground state can be sensitive to twist angle and vertical electric field. Here we show that devices with intermediate twist angle (~3°) may host antiferromagnetic ordered ground states at fixed hole fillings of 1 and 2/3, evidenced by suppressed zero‑field magneto‑optical and Hall responses and field‑driven transitions into quantized Chern states. By tuning only the vertical electric field, we reversibly switch the system from antiferromagnetism to ferromagnetism accompanied by integer or fractional Chern insulating behavior near the layer‑polarization threshold, and then to a paramagnetic regime at larger fields. These results reveal competing correlated ground states in a single moiré platform and establish electric‑field control of magnetism and topology.

Moiré semiconductors offer flat bands where Coulomb interactions and band topology intertwine, while the interlayer coupling plays a core role in the formation of moiré potential. However, the limited interlayer coupling strength and the lack of efficient tuning methods hinder further exploration of correlated phenomena in moiré semiconductors. We introduce a cryogenic dual-gated diamond-anvil platform using helium as a pressure medium, enabling reversible hydrostatic tuning together with magneto-optical spectroscopy in twisted bilayer WSe 2 . Pressure enhances the moiré potential, redshifts excitons, and stabilizes Stoner ferromagnetism otherwise absent at a 3.1° twist. Simultaneously, the half-filled C = 1 Chern insulating state strengthens, exhibiting a reduced saturation field. Moreover, we observed a topological phase transition from Chern insulator to Mott insulator at around 2 GPa. First-principles calculations reveal that a Γ − K valence band maximum switching drives this transition by converting an Ising-like topological K -valley miniband into a spin-degenerate trivial Γ miniband. Our findings demonstrate hydrostatic pressure as a powerful, continuous control axis for correlated magnetism and topological band engineering in moiré materials.

Consecutive topological phase transitions (TPTs) between strongly correlated electronic phases that differ simultaneously in symmetry breaking and topological order are of fundamental interest in condensed matter physics, yet, rarely realized experimentally. We report two consecutive electric-field-driven TPTs at half filling (ν=1) in angle-aligned MoTe_{2}/WSe_{2} moiré heterobilayers. With increasing out-of-plane displacement field, a geometrically frustrated Mott insulator evolves into a ferromagnetic quantum anomalous Hall (QAH) Mott insulator, i.e., a spin-polarized topological Mott insulator without an observable charge-gap closure, and subsequently into an antiferromagnetic, valley-coherent Mott insulator (VC-AFM) accompanied by a continuous charge-gap collapse and the emergence of a critical metallic state. Layer-resolved magnetic circular dichroism (MCD), magneto-transport, and compressibility measurements jointly determine the phase diagram. The high-field evolution of the antiferromagnetic state reveals a metamagnetic-like transition at a critical field B^{*}, above which a Chern insulating transport response reappears. Our results establish the MoTe_{2}/WSe_{2} moiré platform as a tunable realization of an extended Kane-Mele-Hubbard model hosting sequential correlation-topology intertwined transitions.

Moiré semiconductors built on angle-aligned transition metal dichalcogenide (TMD) heterobilayers provide a physical realization of the Kondo lattice model, in which one TMD layer is prepared in a Mott insulating state supporting a lattice of local magnetic moments and the other layer in a metallic state supporting itinerant carriers. The artificial Kondo lattice enables the exploration of exotic states of matter near a continuously tunable Kondo breakdown. Here we report the emergence of a symmetry-broken Chern insulator at a moiré hole filling factor 4/3 in angle-aligned MoTe2/WSe2 moiré bilayers, which realize a chiral Kondo lattice. The symmetry-broken Chern insulator, which exhibits integer quantized Hall conductance at a fractional moiré filling, breaks the translational symmetry of the lattice spontaneously; it also appears only near a magnetic field-induced Kondo breakdown in the mixed-valence regime of the material. We further demonstrate that the magnetic field required to induce the Kondo breakdown and to stabilize the symmetry-broken Chern insulator is twist angle dependent. The results present new opportunities for exploring the subtle interplay between topology and Kondo interactions in moiré semiconductors.

Moiré semiconductors offer flat bands where Coulomb interactions and band topology intertwine, while interlayer coupling plays a central role in forming the moiré potential. However, limited interlayer coupling strength and the lack of efficient tuning methods hinder further exploration of correlated phenomena in moiré semiconductors. Here we introduce a cryogenic dual-gated diamond-anvil platform using helium as a pressure medium, enabling reversible hydrostatic tuning together with magneto-optical spectroscopy in twisted bilayer WSe2. Pressure enhances the moiré potential, redshifts excitons, and stabilizes Stoner ferromagnetism otherwise absent at a 3.1-degree twist. Simultaneously, the half-filled C = 1 Chern insulating state strengthens, exhibiting a reduced saturation field. Moreover, we observe a topological phase transition from a Chern insulator to a Mott insulator at around 2 GPa. First-principles calculations reveal that a Gamma-to-K valence-band-maximum switching drives this transition by converting an Ising-like topological K-valley miniband into a spin-degenerate trivial Gamma miniband. Our findings demonstrate hydrostatic pressure as a powerful, continuous control axis for correlated magnetism and topological band engineering in moiré materials.