Zhiwen Shi

Rhombohedral multilayer graphene hosts layer-polarized flat bands, providing an intriguing platform for correlated and topological states; however, how layer number governs symmetry breaking and surface screening remains elusive. Here, we prepare rhombohedral graphene multilayers and systematically conduct electrical transport measurements. We uncover an unconventional layer dependence of phase transitions: the critical displacement field Dc for the layer-antiferromagnetic-to-semimetal transitions remains nearly constant across tetralayer to hexalayer graphene, whereas Dc for the semimetal-to-layer-polarized-insulator transition increases with layer number, defying unscreened Coulomb interaction models. In hexalayer graphene, surface-state-dominated transport emerges with Landau levels and resistive peaks predominantly controlled by adjacent gates, revealing strong interlayer screening. High magnetic fields further reveal valley-layer-locked Landau levels and dissipative states associated with interlayer backscattering, highlighting the presence of decoupled surface states. Our findings establish the layer number as a key tuning knob for engineering correlated and topological phases in rhombohedral graphene multilayers.

Phonon polaritons, with their strong field confinement, enable extreme miniaturization of photonic devices. While optical loss is conventionally perceived as a fundamental limit to be compensated, here we demonstrate a novel scheme by harnessing leakage-induced loss as a degree of freedom for mode manipulation. By coupling 2D hyperbolic polaritons (e.g., α-MoO ₃ ) with 3D bulk leaky channels (e.g., β-Ga ₂ O ₃ ), we achieve a topological reconstruction into quasi-one-dimensional (quasi-1D) polaritons over a broad frequency range. In the frequency domain, out-of-plane phonon-driven leakage in the substrate potently controls the propagation direction of quasi-1D polaritons, enabling continuous frequency-dependent rotation and culminating in an abrupt transition at the phonon resonance. In real space, the quasi-1D isofrequency contours enable on-chip reflectionless propagation across wide angles (>170°) due to their extreme momentum-space collimation. This approach is further generalized to other 2D/3D heterostructures (e.g., α-MoO ₃ /CdWO ₄ ), demonstrating operability across various discrete infrared spectral windows (total Δλ ~ 7 μm). Our findings not only introduce a loss-engineering framework for precise polariton control but also open new possibilities for zero-reflection on-chip optical routing and advanced optical information processing.

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.

Graphene multilayers exhibit electronic spectra that depend sensitively on the layer number and stacking order. Here, we investigate ABCBC-stacked pentalayer graphene, a non-centrosymmetric mixed stacking that combines a rhombohedral trilayer-like cubic band and a Bernal bilayer-like parabolic band. Transport measurements reveal an intrinsic gap at charge neutrality that evolves strongly asymmetrically under a perpendicular displacement field, evidencing built-in layer polarization from broken inversion and mirror symmetry. By tuning the displacement field and carrier density, we drive multiple Lifshitz transitions and observe Landau levels with distinct degeneracies arising from the multi-flatband structure. Remarkably, a ν = -6 quantum Hall state emerges at an exceptionally low magnetic field of ∼20 mT. These results demonstrate that mixed-stacked multilayer graphene provides a tunable platform in which non-centrosymmetric symmetry breaking, multiple flatbands, and unusual quantum Hall physics coexist, opening opportunities to explore emergent correlated and topological electronic states.

The fractional quantum Hall effect at half-filled Landau levels provides a promising route to unusual topological phases that may host non-Abelian excitations, but these states are often fragile and difficult to control experimentally. Here, we report the observation of a cascade of even-denominator fractional quantum Hall states at fillings ν = −5/2, −7/2, −9/2, −11/2 and −13/2, alongside numerous odd-denominator states in mixed-stacked pentalayer graphene—a system characterized by intertwined quadratic and cubic band dispersions. These even-denominator states emerge from two distinct intra-zeroth Landau levels and exhibit displacement-field tunability. At half fillings, continuous quantum phase transitions between even-denominator states, magnetic Bloch states, and composite Fermi liquids are clearly identified upon tuning external fields. Numerical calculations support possible Moore–Read type pairing for even-denominator states, although direct probes of their exchange statistics remain important for future experiments. These results establish mixed-stacked graphene as a versatile platform for tunable correlated topological phases.

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.