Qiang-Hua Wang

Oxygen defects suppress superconductivity in the bilayer nickelate La${}_{3}$Ni${}_{2}$O${}_{7+\ensuremath{\delta}}$ via a dual mechanism, revealed through density functional theory, dynamical mean-field theory, and functional renormalization group analysis.

The kagome lattice is unique in that Dirac bands, van Hove singularities, and flat band are all present in the band structure. In particular, the Bloch states near the upper van Hove points are sublattice polarized, while they are pair-wise mixed in sublattice contents near the lower van Hove points. These features, combined with correlation effects, may lead to novel electronic orders. We briefly review our theoretical investigations on the kagome lattice near the van Hove fillings by the singular-mode functional renormalization group, quantum Monte Carlo, and effective mean field theory. We establish rich phases of the model under extended Hubbard interactions or Su-Schrieffer-Heeger-like electron-phonon coupling. Among these phases, three types of 2×2 orders are of particular interest. Near the upper van Hove filling, we predicted the 2 × 2 charge bond order in 2013, which is highly relevant in AV ₃ Sb ₅ (A=K,Rb,Cs) kagome materials. In the superconducting channel, we constructed a self-consistent theory for the 2×2 pair density wave, which is made possible by the sublattice polarization and attractive pairing interaction on the nearest neighbor bonds, predicting anisotropic energy gap in momentum space and quantized thermal Hall conductance in the chiral case of the pair density wave, which are consistent with recent photoemission and transport experiments. Near the lower van Hove filling, we further predicted a topological 2 × 2 non-collinear antiferromagnetic state with spins on neighboring sites orthogonal as Cartesian axes in three dimensions, which is a Chern insulator supporting quantized thermal Hall conductance.

UTe₂ is a candidate spin-triplet superconductor, yet its superconducting state remains debated. Using scanning tunneling microscopy/spectroscopy, we observe a novel magnetic vortex with distinct dark-bright contrast in local density of states on UTe₂ (011) surface under perpendicular magnetic fields, resembling the Yin-Yang diagram in Taoism. Each vortex carries a quantized magnetic flux, and the boundary between the two parts aligns with crystallographic a-axis. The vortex states exhibit intriguing behaviors -- the Yang part exhibits a sharp zero-energy conductance peak, whereas the Yin part shows a superconducting gap with pronounced coherence peaks, sharper than those without fields. Theoretical analysis attributes this pattern to vortex-induced asymmetric suppression of topological surface states caused by the tilted Fermi surface, constraining possible d-vector orientations. Our findings constitute distinctive evidence for spin-triplet pairing in UTe₂, and provide new avenues for exploring the interplay between surface states and vortex-bound states in topological superconductors.

The discovery of two-dimensional magnetic materials has provided an ideal platform for exploring physical phenomena in the two-dimensional limit. However, intrinsic two-dimensional antiferromagnetic materials have been rarely reported, limiting systematic studies of their electronic properties. The discovery of novel intrinsic two-dimensional antiferromagnets and the development of robust synthesis strategies, therefore, remain significant challenges. Here, we report the chemical vapor deposition synthesis of CrOCl monolayer films and nanosheets that exhibit excellent air stability. The CrOCl morphology is tunable, ranging from two-dimensional nanosheets to three-dimensional flower-like structures, with lateral sizes ranging from several microns to continuous monolayer films. Structural characterization confirms the material's composition and high crystalline quality. Furthermore, magnetic measurements, supported by theoretical calculations, reveal a Néel temperature for CrOCl of ≈ 14 K. This work provides a reliable route for preparing two-dimensional antiferromagnetic materials.

We uncover a dual mechanism by which oxygen defects suppress superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_{7+δ}$ using density functional theory, dynamical mean-field theory, and functional renormalization group analysis. Apical vacancies and interbilayer interstitials emerge as the dominant low-energy defect species and are further stabilized by orthorhombic domain walls. These two defect classes drive the electronic structure in opposing directions. Vacancy-induced disorder generates local magnetic moments and promotes Anderson localization at moderate concentrations, whereas periodic interstitial ordering yields a coherent but weakly correlated metallic background that fails to support superconductivity. These findings highlight the decisive role of oxygen defects in shaping the superconducting and provide microscopic guidance for improving superconductivity through controlled defect engineering.

In a recent experiment (Hou et al Phys. Rev. X 15, 011027), a new type of necklace-like vortex bound state (VBS) was observed and attributed to disorder induced interference among different Caroli-de Gennes-Matricon (CdGM) states within one single vortex. In this work, we further investigate the possibilities of quantum interference among the CdGM states from different vortices in clean superconductors, which may become significant near the upper critical field. We find a series of interference patterns in the local density of states (LDOS) due to the overlap between spatially separated individual CdGM states. On a vortex lattice, the interference can also lead to a necklace-like LDOS, hence, providing an alternative and intrinsic mechanism to observe the novel necklace-like, or other spatially modulated VBS more generally, in experiments. These results can be understood quite well within an effective tight-binding model constructed from the individual CdGM states, and can be checked in future experiments.