Hidenori Takagi

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 A ₂ WCl ₆ (A = Cs, Rb, and K), comprising a face-centered cubic lattice of W ⁴⁺ ions. The 5 d ² configuration of W ⁴⁺ ions yields a spin-orbit-entangled J = 2 state, which splits into a T _2g triplet and a non-Kramers E _g 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 K ₂ WCl ₆ 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, Rb ₂ WCl ₆ and Cs ₂ WCl ₆ 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 WCl ₆ 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.

Ta ₂ NiSe ₅ continues to draw interest for its T _c = 326 K phase transition, whose dual electronic and structural nature reflects a complex interplay of electron-hole (excitonic) and electron-lattice interactions. The majority of studies that have attempted to decipher the relative importance of these interactions, particularly through charge tuning, have been focused on bulk samples. Here, we utilize an all-dry exfoliation and transfer protocol to isolate ultrathin flakes of Ta ₂ NiSe ₅ on insulating Al ₂ O ₃ and conducting Au. Using polarized Raman spectroscopy, we uncover the following substrate dependence: Four layers of Ta ₂ NiSe ₅ on Al ₂ O ₃ show a sharp structural and electronic transition that is lowered by roughly 40 K from the bulk T _c . Meanwhile, four layers of Ta ₂ NiSe ₅ on Au undergo a structural and electronic transition that is much more gradual with respect to temperature and finishes roughly 150 K below the bulk T _c . The pronounced broadening points to an atomic-scale interface effect, wherein electrostatic screening and charge transfer from Au produces a T _c gradient perpendicular to the layers of the flake. We discuss the role of excitonic physics and suggest the possibility for interface engineering to pattern nanoscale junctions in Ta ₂ NiSe ₅ .

The kagome lattice of spin-1/2 copper atoms in herbertsmithite is conjectured to sustain a quantum spin liquid state with spinon quasiparticles. Ideally, the kagome crystal planes are each separated by a plane of spinless zinc atoms. However, in real crystals, some spin-1/2 copper atoms substitute randomly onto these inter-kagome zinc sites. Here we reconceptualize such ‘impurity’ atoms as quantum witness spins whose dynamics is designed to probe the spin liquid state. We then introduce spin noise spectroscopy to measure the frequency and temperature dependence of witness spin dynamics, demonstrating that their phenomenology is consistent with extensive interactions between witness spins mediated by propagation of spinons through a quantum spin liquid. Ultimately, a sharp transition occurs at around 260 mK, below which the properties of both spin noise and magnetic susceptibility suggest that the witness spins form a spin glass phase. Among the theoretical models considered, we demonstrate that our observations are only consistent with spinon-mediated interactions between witness spins by either a Z ₂ or U(1) quantum spin liquid, with the former model more closely matching the data. Our work demonstrates that quantum mechanical witness spins may now conceivably be used as a widely applicable probe of quantum spin liquid physics.

A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions are central to bridging these two domains while simultaneously serving as on-off switches for electronic states. Here, we demonstrate how the prototypical material of K ₂ Cr ₈ O ₁₆ undergoes a ferromagnetic metal-insulator transition accompanied by a change in band topology. Through inelastic x-ray and neutron scattering experiments combined with first-principles theoretical calculations, we show that this transition is not driven by a Peierls mechanism, given the lack of phonon softening. Instead, we establish the transition as a topological metal-insulator transition within the ferromagnetic phase with potential axionic properties, where electron correlations play a key role in stabilizing the insulating state. These results reveal how a metal-insulator transition provides a pathway through which magnetism, topology, and electronic correlations interact.

Ta 2 NiSe 5 undergoes a phase transition characterized by the opening of an energy gap and a reduction in its crystal symmetry below T c = 326 K . The primary cause of this transition is debated to be either electron-hole correlations, in line with the excitonic insulator hypothesis, or structural instability. The chain structure of Ta 2 NiSe 5 enables effective control over its electronic and lattice properties by applying uniaxial strain. In this study, we utilize polarization-resolved Raman spectroscopy to demonstrate that the electronic and structural order parameters respond differently to uniaxial pressure applied along the Ta-Ni chains. Compressive strain reduces monoclinic distortions while enhancing excitonic fluctuations. Such disparate behavior suggests that many-body effects may significantly amplify exciton fluctuations even when monoclinic distortions are minimized, thereby supporting the excitonic nature of the phase transition.

We have measured the critical current density, superconducting coherence length, and superconducting transition temperature of single-domain, epitaxially grown Nb ( 110 ) / Au ( 111 ) / Nb ( 110 ) trilayers, all of which show a nonmonotonic dependence on the thickness of the Au layer. These results are compared with the predictions of a relativistic, theory, which incorporates superconducting correlations. We find good agreement with experiment, coming from a rich interplay between superconducting proximity and quantum size effects, mediated by Andreev bound states. These results suggest that quantum size effects could provide a systematic method of controlling the transport properties of superconducting multilayers. Published by the American Physical Society 2025

Although spin and charge stripes in high- T c cuprates have been extensively studied, the exact range of carrier concentration over which they form a static order remains uncertain, complicating efforts to understand their significance. The problem is challenging due to the combined effects of quenched disorder and competition with superconductivity—both significant in cuprates—which add to the inherent difficulty of determining phase boundaries. In La 2 − x Sr x CuO 4 (LSCO) and in zero external magnetic field, static spin stripes are confined to a doping range well below p * , the pseudogap boundary at zero temperature. However, when high fields suppress the competing effect of superconductivity, spin-stripe order is found to extend up to p * . Here, we investigate La 1.8 − x Eu 0.2 Sr x CuO 4 (Eu-LSCO) using La 139 nuclear magnetic resonance and observe field-dependent spin fluctuations suggesting a similar competition between superconductivity and spin order as in LSCO. Nevertheless, we find that static spin stripes are present practically up to p * irrespective of field strength: The stronger stripe order in Eu-LSCO prevents superconductivity from enforcing a nonmagnetic ground state, except very close to p * . Thus, spin-stripe order is consistently bounded by p * in both LSCO and Eu-LSCO, despite their differing balances between stripe order and superconductivity. This indicates that the canonical stripe order, where spins and charges are intertwined in a static pattern, is fundamentally tied to the pseudogap phase, though the exact nature of this connection has yet to be elucidated. Any stripe order beyond the pseudogap endpoint must then be of a different nature: Either spin and charge orders remain intertwined, but both fluctuating, or only spin order fluctuates while charge order remains static. The presence of spin-stripe order up to p * and the pervasive, slow, and field-dependent spin-stripe fluctuations, as well as the electronic inhomogeneity documented in this work, must all be carefully considered in discussions of Fermi surface transformations, putative quantum criticality, and strange metal behavior.

Configurational entropy has been discussed for high entropy alloys and high entropy oxides with multiple metal atoms sharing the same atomic site, where the entropy allows for the stability of a single phase material. Typically, the entropy from individual atoms occupying the same site is considered, but here we expand on this to include the entropy coming from charges on the cations sharing the same site. Considering the valence skip nature of Sn, the charge-entropy picture supports the stability region of rocksalt Ag _x Sn_1−x Se. We find evidence for strong hybridization of the fluctuating Sn state from x-ray photoemission and Mössbauer spectroscopy, where these fluctuations on Sn are maintained to low temperature, well into the superconducting state, due to the lack of long-range order between Ag/Sn atoms in the entropy-stabilized rocksalt structure. Our charge-entropy model expands on the considerations when designing entropy-stabilized materials and opens up the possibility of designing new superconducting materials containing valence-skip elements.