Xin-Wei Yi

Curved magnets offer a rich phase diagram and hold great promise for next-generation spintronic technologies. This Letter establishes the paramount significance of high-order vortex states (e.g., 3φ with winding number n≥2) in VSe_{2} nanotubes, which uniquely enable magnonic functionalities fundamentally inaccessible to conventional magnetic systems, and additionally, the superposition of high-order vortex and helical states is identified. These states arise from diameter-dependent competition between the nearest-neighbor ferromagnetic (J_{1}) and longer-range antiferromagnetic (J_{2}/J_{3}) couplings, as jointly validated through density-functional theory calculations and Heisenberg modeling of phase diagrams. Based on the Landau-Lifshitz-Gilbert equation, we show that vortex states exhibit orbital angular momentum (OAM) hybridization governed by selection rules: magnetic anisotropy energy (MAE) and an external magnetic field couple the l mode to l±2(n1) and l±n, respectively. For the 3φ state, MAE couples the l=0 mode with l=±4, producing eight-petal magnon density patterns and providing a natural mechanism for generating high-OAM magnons. These findings establish a predictive theoretical framework for controlling high-order vortex states in curved magnets and highlight VSe_{2} nanotubes as a promising platform for exploring complex magnetism and for the development of future magnonic and spintronic devices.

The cascade of correlated topological quantum states in the newly discovered vanadium-based kagome superconductors, AV₃Sb₅ (A = K, Rb, and Cs), with a Z₂ topological band structure has sparked immense interest. Here, we report the discovery of superconductivity and electronic nematic order in high-quality single-crystals of a new titanium-based kagome metal, CsTi₃Bi₅, that preserves the translation symmetry, in stark contrast to the charge density wave superconductor AV₃Sb₅. Transport and magnetic susceptibility measurements show superconductivity with an onset superconducting transition temperature T_c of approximately 4.8 K. Using the scanning tunneling microscopy/spectroscopy and Josephson scanning tunneling spectroscopy, we demonstrate that the single crystals of CsTi₃Bi₅ exhibit two distinct superconducting gaps. Furthermore, the superconducting gaps break the six-fold crystal rotational symmetry down to two-fold. At low energies, we find that the quasiparticle interference patterns exhibit rotational-symmetry-breaking C₂ patterns, revealing a nematic ordered normal state with the same nematic direction as in the superconducting state. Our findings uncover a novel superconducting state in CsTi₃Bi₅ and provide new insights for the intrinsic electron liquid crystal phases in kagome superconductors.

The Kagome lattice, naturally encompassing Dirac fermions, flat bands, and van Hove singularities, tends to intertwine exotic electronic states. Revealing the characteristics of its Fermi surface will help on clarifying the nature of the complex quantum phenomena in kagome material. Here we report the Fermi surface properties of the new kagome superconductor CsTi₃Bi₅ by the de Haas–van Alphen oscillations. The observed oscillations are clear and consist of six principal frequencies ranging from 214 T to 1013 T. The angular dependence of the frequency implies a quasi-two-dimensional electronic structure. In addition, the Berry phase corresponding to 281 T, determined by the Landau level fan diagram method and full Lifshitz-Kosevich formula fitting, yields a value close to π, which indicates a band structure with nontrivial topological property. These results underscore the potential of CsTi₃Bi₅ as a promising new platform to explore the interplay between topological order, electronic nematicity, and superconductivity.

The recently discovered ATi ₃ Bi ₅ (A=Cs, Rb) exhibit intriguing quantum phenomena including superconductivity, electronic nematicity, and abundant topological states. ATi ₃ Bi ₅ present promising platforms for studying kagome superconductivity, band topology, and charge orders in parallel with AV ₃ Sb ₅ . In this work, we comprehensively analyze various properties of ATi ₃ Bi ₅ covering superconductivity under pressure and doping, band topology under pressure, thermal conductivity, heat capacity, electrical resistance, and spin Hall conductivity (SHC) using first-principles calculations. Calculated superconducting transition temperature ( T _c ) of CsTi ₃ Bi ₅ and RbTi ₃ Bi ₅ at ambient pressure are about 1.85 and 1.92 K. When subject to pressure, T _c of CsTi ₃ Bi ₅ exhibits a special valley and dome shape, which arises from quasi-two-dimensional compression to three-dimensional isotropic compression within the context of an overall decreasing trend. Furthermore, T _c of RbTi ₃ Bi ₅ can be effectively enhanced up to 3.09 K by tuning the kagome van Hove singularities (VHSs) and flat band through doping. Pressures can also induce abundant topological surface states at the Fermi energy ( E _F ) and tune VHSs across E _F . Additionally, our transport calculations are in excellent agreement with recent experiments, confirming the absence of charge density wave. Notably, SHC of CsTi ₃ Bi ₅ can reach up to 226 ℏ ·(e· Ω ·cm) ^–1 at E _F . Our work provides a timely and detailed analysis of the rich physical properties for ATi ₃ Bi ₅ , offering valuable insights for further experimental verifications and investigations in this field.