We construct exact ground states of the $J_1\text{−}{J}_{3b}$ classical Heisenberg model on the pyrochlore lattice in the presence of a magnetic field. They are noncoplanar multi-$\vec{Q}$ spin configurations with a large magnetic unit cell that generalize the previously found coplanar sublattice pairing states. Using linear spin-wave theory, we show that entropy favors these multi-$\vec{Q}$ states at low temperatures in high magnetic fields. This is confirmed by Monte Carlo simulations, and a phase diagram is constructed. We also calculate the zero-temperature dynamical structure factor. Besides the usual Goldstone modes associated with the ordering $\vec{Q}\mathrm{s}$, we find high intensity gapless modes at momenta where there are no Bragg peaks.

Using a system for measuring magnetization under stress, we revisit the piezomagnetic effect in rutile fluorides, ${\mathrm{MnF}}_2$ and ${\mathrm{CoF}}_2$. We succeed in observing a monotonic increase in the magnetization in response to stress application. The powder-averaged piezomagnetic tensors are estimated to be $Q_{\mathrm{ave}}=\hspace{0.17em}3.7\times {10}^{-4}\mathrm{emu}\text{/}\mathrm{mol}\text{/}\mathrm{MPa}$ for ${\mathrm{MnF}}_2$ and $8.3\times {10}^{-2}$ emu/mol/MPa for ${\mathrm{CoF}}_2$, where the value for ${\mathrm{CoF}}_2$ is two orders of magnitude larger than that for ${\mathrm{MnF}}_2$. We discuss the microscopic mechanism of the piezomagnetic effect in terms of the $g$-tensor anisotropy, the single-ion anisotropy, and the Dzyaloshinskii-Moriya interaction, all of which are associated with the electronic transition via the spin-orbit interaction. The observed larger piezomagnetic tensor components in ${\mathrm{CoF}}_2$ are well interpreted in this formalism. We also observe spontaneous magnetization in ${\mathrm{CoF}}_2$, which exists even at zero stress. We argue that the observed weak ferromagnetism originates from Jahn-Teller distortions caused by orbital degeneracy in ${\mathrm{Co}}^{2+}$ ions.

The generation and control of spin polarization and valley polarization in antiferromagnets (AFMs) have garnered increasing attention due to their potential for enabling faster and more stable multifunctional spintronic and valleytronic memory and logic devices. However, the two primary categories of AFMs—altermagnets and $\mathcal{TP}$-symmetric AFMs ($\mathcal{T}$ is time reversal; $\mathcal{P}$ is spatial inversion)—either lack intrinsic valley polarization or net spin polarization. Here, we propose an effective approach for achieving spontaneous spin-valley polarization in $\mathcal{TP}$-broken layered ferroelectric antiferromagnets (FE-AFMs). The FE-AFMs exhibit lifted spin degeneracy across the entire Brillouin zone, along with uncompensated spin density of states. They combine the benefits of spin polarization in altermagnets with valley polarization in $\mathcal{TP}$-symmetric AFMs. Furthermore, the FE-AFMs feature layer-dependent spin polarization, rooted in their intrinsic ferroelectric property, allowing for the flexible control over spin-valley polarization by interlayer sliding. This tunability facilitates sign-reversible and size-tunable valley Hall and Nernst effects, along with other spin- and valley-dependent transport properties. Our findings are demonstrated in a broad class of $\mathcal{TP}$-broken bilayer antiferromagnets such as ${\mathrm{Nb}}_3{X}_8$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{I}),$ $\mathrm{V}{X}_2$ $(X=\mathrm{S}, \mathrm{Se})$, and ${\mathrm{VSi}}_2{X}_4$ $(X=\mathrm{N}, \mathrm{P})$, underscoring the potential of FE-AFMs for advancing next-generation spin- and valley-based information technologies.

We numerically investigate the effect of coupling a two-dimensional many-body localized system to a finite heat bath, using shallow quantum circuits as a variational ansatz. Specifically, we simulate optical lattice experiments with two components of ultracold bosons, where only one species is subject to a random disorder potential and the other acts as a heat bath. We obtain a filling fraction-dependent phase diagram with a critical filling consistent with experiments. We also perform long-time time-dependent variational principle (TDVP) simulations with matrix product states, but find that the quantum circuit approach produces superior results because of the lack of boundary effects. Finally, we present additional results on the two-point correlation functions and the quantum mutual information between sites.

${\mathrm{Cu}}_2\left(\mathrm{pymca}\right){}_3({\mathrm{ClO}}_4$) (pymca: pyrimidine-2-carboxylate) consists of a slightly distorted honeycomb lattice of ${\mathrm{Cu}}^{2+}$ spins, which shows no long-range magnetic order down to 0.6 K. A magnetization study revealed 1/3 and 2/3 plateau phases [A. Okutani et al., J. Phys. Soc. Jpn. 88, 013703 (2019)], which is not expected for regular honeycomb antiferromagnets. Inelastic neutron scattering experiments were performed using a powder sample to investigate the exchange interactions of this material. The spin excitations from the singlet ground state to the first three triplet states, predicted from the antiferromagnetic hexagonal spin cluster interacting with 3.9 meV, were observed. Using the exact diagonalization methods, the intercluster coupling was estimated from the excitation peak width to be about 20% of the intracluster interaction, which is consistent with the previously reported value. Our exchange path model explains the anisotropic exchange interactions in the distorted honeycomb plane.

Motivated by the recent observation of a uniform vector chirality (UVC) magnetic order in the maple-leaf lattice (MLL) realization ${\mathrm{Ho}}_3{\mathrm{ScO}}_6$ via powder neutron scattering experiments, we investigate the classical antiferromagnetic Heisenberg model on the maple-leaf lattice. The MLL features three symmetry-inequivalent nearest-neighbor couplings, $J_d, J_t$, and $J_h$. Previous studies, primarily focused on the case where $J_t=J_h$, identified a staggered vector chirality (SVC) order. Extending beyond this limit, we demonstrate that the SVC order remains stable across a broad parameter regime. However, we also find that the UVC order cannot emerge from the nearest-neighbor model alone. By introducing a farther-neighbor antiferromagnetic interaction $J_x$, we demonstrate that even a weak $J_x$ can cause a first-order phase transition from SVC to UVC order. Using linear spin wave theory, we compute the dynamical spin structure factor, revealing distinct signatures for SVC and UVC orders that can be probed through inelastic neutron scattering experiments. Additionally, we calculate the specific heat, which exhibits qualitative agreement with the experimental data for ${\mathrm{Ho}}_3{\mathrm{ScO}}_6$. Our findings provide a minimal framework for understanding ${\mathrm{Ho}}_3{\mathrm{ScO}}_6$ and related MLL systems, like ${\mathrm{MgMn}}_3{\mathrm{O}}_7·3{\mathrm{H}}_2\mathrm{O}$, suggesting avenues for further experimental and theoretical investigations.

Diamond and its doped materials have garnered widespread attention for their potential applications in superconductivity and superhardness. Here we introduce a class of heavily boron-doped diamond, $X{\mathrm{B}}_4{\mathrm{C}}_4 (X=\mathrm{Be},\mathrm{B},\mathrm{Al},\mathrm{Si},\mathrm{P},\mathrm{Ti})$, stabilized by cation compensation. Using density functional theory, we perform high-throughput screening on 67 compounds and identify 6 that are dynamically and mechanically stable at ambient pressure. These materials offer a wide range of tunable properties through the substitution of guest atoms $X$ in $X{\mathrm{B}}_4{\mathrm{C}}_4$: regulating the Fermi level and electronic density of states to achieve high-temperature superconductors, and altering covalent bond lengths and strengths to obtain superhard materials. Among them, $\mathrm{Be}{\mathrm{B}}_4{\mathrm{C}}_4$ hosts a superconducting critical temperature ($T_{\mathrm{c}}$) of 83 K, with an optimal Van Hove singularity, while ${\mathrm{B}}_5{\mathrm{C}}_4$ and $\mathrm{Si}{\mathrm{B}}_4{\mathrm{C}}_4$ exhibits a hardness of ∼44 GPa. The current research findings establish stable heavy boron-doped diamonds with remarkable properties, paving the way for achieving high-temperature superconductivity and superhard performance at ambient pressure.

We study a one-dimensional supersymmetric hard-core fermion model first proposed by Fendley, Schoutens, and de Boer [Phys. Rev. Lett. 90, 120402 (2003)]. We focus on the full Hilbert space instead of a restricted subspace. Exact diagonalization shows the degeneracy of zero-energy states scales exponentially with size of the system, with a recurrence relation between different system sizes. We solve the degeneracy problem by showing the ground states can be systematically constructed by inserting immobile walls of fermions into the chain. Mapping the counting problem to a combinatorial one and obtaining the exact generating function, we prove the recurrence relation on both open and periodic chains. We also provide an explicit mapping between ground states, giving a combinatorial explanation of the recurrence relation.

Preparing the ground state of the Fermi-Hubbard model is challenging, in part due to the exponentially large Hilbert space, which complicates efficiently finding a path from an initial state to the ground state using the variational principle. In this work, we propose an approach for ground state preparation of interacting models by involving a classical reservoir, simplified to the instantaneous-response limit, which can be described using a Hamiltonian formalism. The resulting time evolution operator consists of spin-adapted nearest-neighbor hopping and on-site interaction terms similar to those in the Hubbard model, without expanding the Hilbert space. We can engineer the coupling to rapidly drive the system from an initial product state to its interacting ground state by numerically minimizing the final state energy. This ansatz also closely resembles the Hamiltonian variational ansatz, offering a fresh perspective on it.

We investigate the emergence and transformation of pinch-point singularities in the excitation spectrum of electronic flat-band systems on kagome and pyrochlore lattices with spin-orbit coupling (SOC) and Coulomb interactions. While pinch points are widely recognized as signatures of classical spin liquids, they also appear in electronic flat-band systems when there is a singular band-touching point to dispersive bands. We explore how SOC modifies the pinch-point structure in the chiral spin flat-band metallic state, which we term spinor-ice. The pinch-point profile can rotate or redistribute its spectral weight, governed by a prefactor in the spectral function that primarily depends on the direction of the ground-state spin polarization. We show that the spinor-ice state could be experimentally probed by rotating the spin polarization of the injected electron to infer internal magnetic structures. These observations are discussed in conjunction with angle-resolved photoemission spectroscopy and the application to the potential SOC flat-band material ${\mathrm{CsW}}_2{\mathrm{O}}_6$. We also demonstrate the persistent residual pinch-point features under Coulomb interactions and deviations from the ideal flat-band limit.

The interplay of Rashba and quantum spin Hall effects in non-centrosymmetric systems presents both challenges and opportunities for spintronic applications. While Rashba spin-orbit coupling can disrupt the quantum spin Hall phase, their coexistence may enable additional spintronic functionalities by coupling spin-momentum locking in the bulk with topologically protected edge states. Using the ${\mathrm{ZnIn}}_2{\mathrm{Te}}_4$ monolayer, a predicted polar two-dimensional topological insulator, as a case study, we investigate how intrinsic and Rashba spin-orbit coupling compete within a single material. Our results identify key conditions under which sizable Rashba spin splitting can coexist with a stable quantum spin Hall phase, offering guidance for engineering quantum spin Hall insulators with enhanced spintronic capabilities.

Seeking to unravel the enigmatic Verwey transition and its interplay with magnetism, we have conducted comprehensive measurements on the temperature-dependent electrical resistivity and magnetic moment of stoichiometric and doped-magnetite single crystals at temperatures reaching 1000 K. These investigations have allowed us to identify the Curie temperature, $T_{\mathrm{C}}$, and other characteristic temperatures of the electrical resistivity. Remarkably, we have identified correlations between these temperatures and the Verwey temperature, $T_{\mathrm{V}}$, indicating that the electrical transport properties and the mechanism of the Verwey transition are closely related to the magnetic properties.

We analyze both the stationary and time-dependent properties of molecular states in atomic chains on a surface, some of which are composed of atomic states decoupled from the substrate—a phenomenon analogous to dark states in quantum dot systems. To illustrate this effect at the atomic scale, we performed scanning tunneling microscopy (STM) experiments on short silicon chains fabricated on a Si(553)-Au surface. In contrast to quantum dots, which typically involve characteristic energies in the meV range or lower, the atomic chains studied here operate in a high-energy regime, with energies in the eV range. Furthermore, we demonstrate that the local density of states of the chains carries clear signatures of these decoupled states, which significantly affect STM imaging. The topography becomes highly sensitive to the bias polarity, to the extent that some atomic sites may appear nearly invisible to the STM tip. Our time-resolved theoretical analysis reveals that these decoupled states emerge over a finite time interval. This oscillatory dynamical evolution, primarily driven by nearest-neighbor interactions, suggests a universal relaxation mechanism that is largely insensitive to the length of the atomic chain.

We report on optical studies of the Ruddlesden-Popper nickelates ${\mathrm{La}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$, with $n=2$ (${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$), $n=3$ (${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$), and $n=\infty$ (${\mathrm{LaNiO}}_3$). As the number of the ${\mathrm{NiO}}_6$ octahedra layers $n$ grows, the ratio of the kinetic energy determined from the experimental optical conductivity and that from band theory $K_{\text{exp}}/K_{\text{band}}$ increases, suggesting a reduction of electronic correlations. While the bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ lies on the verge of the Mott insulating phase, the trilayer ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and infinite-layer ${\mathrm{LaNiO}}_3$ fall into the regime of correlated metals. The evolution of the electronic correlations in ${\mathrm{La}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ is likely to be dominated by the Ni $d_{z^2}$ orbital. Our results provide important information for understanding the superconductivity in Ruddlesden-Popper nickelates.

By means of a specific heat, susceptibility, and high-pressure electrical resistivity study, we show that the local magnetic moments of the intercalated V ions in ${\mathrm{V}}_5{\mathrm{S}}_8$ realize a prototype of Kondo lattice system, where an antiferromagnetic order of the moments coexists with a Fermi liquid in the ${\mathrm{VS}}_2$ layers with intermediate heavy fermion properties. Our data analysis suggests a simultaneous suppression of Fermi-liquid behavior and of antiferromagnetic order at a critical pressure, $P_c=10$ GPa, the signature of a quantum critical point. This result supports a Kondo lattice scenario and raises the question whether, in the paramagnetic phase at higher pressures, the heavy quasiparticles survive or form a non-Fermi liquid phase governed by the Kondo interaction.

We investigate the transition between elastic and viscous regimes for time-reversal broken Weyl semimetals. In these materials, Hall transport occurs through two parallel channels: the Fermi sea and the Fermi surface. The Fermi sea part remains unaffected by electron-electron scattering, whereas the Fermi surface is influenced by it. We model the disorder by dilute impenetrable spherical impurities. We analyze the flow of electronic fluid with a finite odd viscosity in the presence of such disorder and compute the conductivity tensor. We find that in the generic case of finite intrinsic conductivity, the Hall angle in the viscous regime is parametrically suppressed compared to the elastic regime. In the special case where the intrinsic conductivity vanishes, the ratio between the transverse and the longitudinal resistivities matches the ratio between the odd and even components of the viscosity tensor.

Studies relying on hydrodynamic theory and Kardar-Parisi-Zhang (KPZ) scaling have found that in the one-dimensional Hubbard model spin and charge transport are for all temperatures $T>0$ anomalous superdiffusive at zero magnetic field, $h=0$, and zero chemical potential, $\mu =0$, respectively. However, this contradicts recent exact results that at very low temperature charge transport rather is normal diffusive. In this Letter we identify the mechanisms that control the different types of temperature dependence of the $h=0$ spin and $\mu =0$ charge transport and find that the latter is normal diffusive for all finite temperatures $T>0$, in contrast to the hydrodynamic theory and KPZ scaling predictions.