Youguo Shi

The periodic Anderson model, traditionally realized in rare-earth compounds with limited tunability, have hindered systematic exploration of correlated quantum phenomena. Here, we introduce a strategy for realizing and engineering this model in incommensurate van der Waals heterostructures by coupling a Mott insulator (${\mathrm{Nb}}_{3}{\mathrm{Cl}}_{8}$) with itinerant electrons (from monolayer graphene), circumventing strict lattice-matching requirements. Through magnetotransport and slave spin mean-field calculations, we demonstrate the hybridization gap ($\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}30\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$), gate-tunable metal-insulator transition, and band-selective electron effective mass enhancement, hallmarks of Kondo coherence. The heterostructure exhibits a nearly order-of-magnitude enhancement in the effective electron mass between hybridized and conventional graphenelike regimes, alongside in-plane magnetic-field-induced metal-insulator transitions. Top gate-temperature phase mapping reveals competing correlated states, including insulating and hidden-order phases. This work establishes an electrically tunable van der Waals platform for studying correlated states generated by coupling a Mott-insulating layer to an itinerant-electron system, providing a materials route for exploring low-dimensional correlated quantum phases.

Kagome lattices host exotic phenomena such as quantum spin liquids, topological Weyl nodes, and flat bands. The kagome ferromagnet Co₃Sn₂S₂ has been a focus of debate due to conflicting reports on its magnetic character. Here, using weakly perturbative probes—primarily neutron scattering and Mössbauer spectroscopy—we determine its ground state and magnetic Hamiltonian. The spin wave spectrum is reproduced by a simple Hamiltonian dominated by a large in-plane nearest-neighbor Heisenberg interaction J and an antisymmetric Dzyaloshinskii-Moriya interaction that is roughly one third of J. This Hamiltonian yields a canted, noncollinear ferromagnetic ground state with an umbrella-type structure, which our data show to be the true ground state at all temperatures below T_C. Additionally, the spin susceptibility above T_C exhibits a reduced critical exponent, akin to other spin-anisotropic systems. Our findings explain the unconventional magnetism in Co₃Sn₂S₂, including the giant anomalous Hall effect and the origin of chiral phonons.

Topological materials are defined by the correspondence between bulk topology and boundary states, yet this correspondence becomes enigmatic on low-symmetry surfaces where bulk and surface periodicities may not coincide within a conventional first bulk Brillouin zone projection. Here we study the (103) surface of the Weyl semimetal NdAlSi and identify Fermi arc interference in the boundary spectrum. Angle-resolved photoemission spectroscopy uncovers loop-like Fermi-arc connectivity and characteristic replica modulations that are not observed on high symmetry surfaces. Crucially, the topological surface states themselves are reconstructed because Fermi arcs from phase-shifted bulk-zone projections overlap and hybridize, producing connectivity patterns unique to low-symmetry facets. We show that these emerge from incomplete bulk projection and multi-cell interference governed by a least-common-multiple framework. Least-common-multiple guided density functional theory and Green’s-function calculations reproduce the reconstructed periodicity and dominant replica structure in the spectra, providing a broadly applicable commensuration guideline. These findings resolve the apparent bulk-boundary correspondence paradox on low-symmetry surfaces and provide an operational route to model and interpret boundary spectra on complex facets.

Interfaces between correlated oxides and strong spin–orbit coupled materials provide a fertile platform for emergent quantum phases, yet controlling these phases through interfacial magnetic reconstruction remains a major challenge. Here, we observe emergent metallicity in ultrathin SrIrO ₃ /SrRuO ₃ heterostructures formed from insulating constituent layers, and this transition is closely associated with interfacial magnetic reconstruction. Combining transport, magnetometry, and angle-resolved photoemission spectroscopy, supported by first-principles calculations, we show that finite spectral weight at the Fermi level and ferromagnetic signatures emerge in the heterostructures. Our calculations further suggest that a staggered Dzyaloshinskii–Moriya interaction at the interface favors ferromagnetic order in SrRuO₃ and collinear antiferromagnetism in SrIrO₃, relative to the collinear and noncollinear antiferromagnetism in isolated ultrathin SrRuO₃ and SrIrO₃, respectively, providing a possible microscopic framework for the coupled magnetic and electronic reconstruction. Our findings identify interfacial magnetic interactions as a powerful tuning parameter for controlling correlated electronic phases in spin-orbit-coupled oxides, opening pathways toward the design of emergent quantum and topological states.

Excitonic insulators host the condensates of bound electron–hole pairs, offering a platform for studying correlated bosonic quantum states and for next-generation device applications. However, how macroscopic coherence emerges from locally collapsed pairing remains elusive. Here, using scanning tunnelling spectroscopy, we report the impurity-induced pair breaking in the excitonic insulator Ta₂Pd₃Te₅. Individual Te vacancies generate a pair of spectral peaks within the excitonic gap. Their energies depend sensitively on the defect configurations and are continuously tunable using tip electric field, indicating controllable impurity scatterings. Spectral mapping shows spatially anisotropic and electronically coupled electron–hole components of the subgap states. These observations, together with mean-field modelling, suggest an excitonic pair-breaking origin. In the strongly electron–hole-imbalanced region, a secondary pair-breaking effect, manifesting an additional pair of subgap states with distinctly lower energies, emerges and displays the interplay of pairing breakings with different excitonic order parameters. Our findings demonstrate the spectroscopic fingerprint of local excitonic depairing at the atomic level, offering a crucial clue to the critical behaviour across excitonic condensation.

BiTeI is a model bulk Rashba semiconductor, with the 3.85 eV Å record-high Rashba parameter among any known materials. A standard Rashba picture features an in-plane chiral spin texture on a pair of concentric Fermi surfaces, with the strict spin-momentum locking of the quasiparticles. However, for the first time, here we observe the unconventional spin textures beyond classical Rashba physics in BiTeI using high-resolution spin- and angle-resolved photoemission spectroscopy with full 3D vectorial spin detection. By simultaneously resolving all three spin components ( P _X , P _Y , and P _Z ), we achieve a band-resolved determination of the complete three-dimensional spin-vector texture for both the inner and outer Rashba-split bands. While the Tellurium-terminated surface largely preserves the canonical tangential spin texture, the Iodine-terminated surface exhibits pronounced radial and sizable out-of-plane spin components, resulting in an unusual spin configuration. These findings reveal that the spin texture in BiTeI can be fundamentally reconfigured by surface termination, establishing surface engineering as a powerful route to control spin textures beyond the ideal Rashba paradigm.

The exploration of tunable superconductivity in strongly correlated electron systems is a central pursuit in condensed matter physics, with implications for both fundamental understanding and potential applications. The Laves phase CeRu₂, a pyrochlore compound, exhibits a three-dimensional (3D) kagome lattice type geometry giving rise to flat bands and degenerate Dirac points, where band structure features intertwine with strong multi-orbital interaction effects deriving from its correlated electronic structure. Here, we combine muon spin rotation (μSR), uniaxial in-plane stress, and hydrostatic pressure to probe the superconducting state of CeRu₂. Uniaxial stress up to 0.22 GPa induces a dome-shaped evolution of the critical temperature T_c, with an initial plateau, successively followed by enhancement and suppression without any structural phase transition. Stress is further found to drive a crossover from anisotropic to isotropic s-wave pairing. In contrast, hydrostatic pressure up to 1.9 GPa leaves T_c largely unchanged but alters the superfluid density from exponential to linear behavior at low temperatures, indicative of nodal superconductivity under hydrostatic pressure. These findings identify CeRu₂ as a prime platform for multifold tuning of superconductivity in a 3D correlated material.