Heavy transition metal compounds with strong spin-orbit coupling have appeared as a platform for d-electron multipolar physics. We report the electronic, magnetic, and structural properties of antifluorite-type tungsten chloride A2WCl6 (A = Cs, Rb, and K), comprising a face-centered cubic lattice of W4+ ions. The 5d 2 configuration of W4+ ions yields a spin-orbit-entangled J = 2 state, which splits into a T2g triplet and a non-Kramers Eg doublet under a cubic crystal field. The spin-orbit manifolds have been discussed to give rise to multipolar ordering such as charge quadrupolar or magnetic octupolar ordering. We found that K2WCl6 undergoes a cubic-to-tetragonal structural transition which reconstructs the low-energy J = 2 manifold, leading to a non-magnetic singlet ground state. By contrast, Rb2WCl6 and Cs2WCl6 show no signs of phase transition and remain non-magnetic down to the lowest temperature measured. At low temperatures, signatures of weak structural anomalies were revealed, which may point to the presence of local distortions of the WCl6 octahedra. We argue that the subtle structural distortion arises from the local quadrupolar component, but the presence of chemical disorder likely prevents the formation of long-range multipolar ordering.
The prospect of merging the paradigms of geometric frustration on a triangular lattice and bond anisotropies in the strong spin-orbit coupling limit holds tremendous promise in the search for exotic quantum materials. Here we identify a new candidate system to realize such physics, the organic quantum antiferromagnet (CD3ND3)2NaRuCl6. We report a combination of thermodynamic, magneto-elastic and neutron scattering experiments on single-crystals to determine the phase diagram in axial magnetic fields H parallel to c and propose a minimal model Hamiltonian. (CD3ND3)2NaRuCl6 displays an ideal triangular arrangement of Ru3+ ions adopting the spin-orbital entangled j eff = 1/2 state. It hosts residual magnetic order below T N = 0.23 K and a highly unusual H - T phase diagram including three different incommensurate states. Spin-waves in the high-field polarized regime are described by a Heisenberg triangular lattice Hamiltonian with a potential sub-leading bond dependent anisotropy term J +/-+/-. We argue that the multi-q ground state in zero magnetic field is a prime candidate for hosting the Z2 vortex crystal proposed on the triangular Heisenberg-Kitaev model. (CD3ND3)2NaRuCl6 is the first member in an extended family of quantum triangular lattice magnets, providing a new playground to study the interplay of geometric frustration and spin-orbit effects.
Vanadium diselenide (VSe2), a layered metallic material with a three-dimensional charge density wave (CDW), has received considerable attention due to the high tunability of its CDW phase. Recently, organic tetrabutyl ammonium (TBA) cations have been intercalated into bulk VSe2, resulting in a novel metal-insulator transition with a new two-dimensional in-plane periodicity lattice modulation. In this study, we employ infrared spectroscopy and first-principles calculations to investigate the electronic structure of both pristine VSe2 and TBA+-intercalated VSe2. Our findings reveal a gradual development of a CDW energy gap in pristine VSe2 during the CDW transition, whereas TBA+-intercalated VSe2 undergoes an abrupt and intricate electronic band reconstruction at the phase transition. The study directly distinguishes between traditional CDW order, characterized by a change in band structure at low energy with the formation of an energy gap, and a first-order phase transition with abrupt band reconstruction over broad energies, as seen in (TBA+)xVSe2. Infrared spectroscopy provides a straightforward method to distinguish between these two scenarios. These findings enhance our understanding of structural phase transitions driven by Fermi surface nesting or alternative mechanisms.
Abstract Floquet engineering uses periodic driving to tailor electronic structures and material properties out of equilibrium. Excitons offer a distinct route to this control: their electron-hole dynamics can act as an internal drive, enabling Floquet engineering beyond direct laser dressing. A microscopic description of this regime requires simultaneous access to excitonic dynamics and their momentum-resolved photoemission signatures, which remains challenging in realistic materials. Here we develop an efficient first-principles framework combining real-time time-dependent density functional theory with a modified long-range-corrected exchange-correlation kernel, Floquet-band analysis, and time- and angle-resolved photoemission simulations. Using monolayer hexagonal boron nitride as a benchmark, we capture a subgap excitonic resonance and show that the resulting oscillatory excitonic field generates clear Floquet sidebands and band renormalization, even without temporal overlap between pump and probe pulses. Our approach establishes a practical and computationally efficient route for predicting nonequilibrium Floquet phenomena driven by collective excitations in quantum materials.
Antiferromagnetic spintronics offers a compelling route toward high-density, ultrafast, and field-insensitive nanoscale devices, yet realizing electrically controllable magnetic textures at room temperature remains challenging. Existing approaches rely on extrinsic skyrmion textures or interfacial Dzyaloshinskii-Moriya interactions, lacking ground-state stability. Although Fe-doped Mn3Sn has demonstrated 6-spin-exchange-driven non-coplanar order, the resulting topological Hall effect vanishes above ~150 K, precluding room-temperature operation. Here we report Mn2.7Co0.3Sn single crystals in which Co substitution stabilizes an intrinsic non-coplanar spin texture persisting above 300 K. Neutron diffraction reveals a 10° spin canting and finite scalar spin chirality at 300 K, generating a topological Hall resistivity of 0.49 μΩ·cm. First-principles calculations confirm the real-space Berry-curvature origin from scalar spin chirality, distinct from the spin-orbit-coupling-dependent momentum-space mechanism. The distinct 3d7 electron count of Co, relative to Fe (3d6), correlates with enhanced thermal stability, demonstrating that substituent-specific electronic structure modulates the exchange landscape beyond generic electron doping. Furthermore, the topological response exhibits bond-directional strain sensitivity: c-axis compression preserves kagome symmetry and modulates the 6-spin strength, whereas ab-plane compression breaks trigonal symmetry. These findings establish substitutional chemistry as a route toward room-temperature topological antiferromagnetic devices.
