Kitaev materials often order magnetically at low temperatures due to the presence of non-Kitaev interactions. Torque magnetometry is a very sensitive technique for probing the magnetic anisotropy, which is critical in understanding the magnetic ground state. In this paper, we report detailed single-crystal torque measurements in the proposed Kitaev candidate honeycomb magnet $\alpha \text{−}{\mathrm{RuBr}}_3$, which displays zigzag order below 34 K. Based on angular-dependent torque studies in magnetic fields up to 16 T rotated in the plane normal to the honeycomb layers, we find an easy-plane anisotropy with a temperature dependence of the torque amplitude following closely the behavior of the powder magnetic susceptibility. The torque for the field rotated in the honeycomb plane has a clear sixfold periodicity with a sawtooth shape, reflecting the threefold symmetry of the crystal structure and stabilization of different zigzag domains depending on the field orientation, with a torque amplitude that follows an order parameter form inside the zigzag phase. By comparing experimental data with theoretical calculations we highlight the importance of relevant anisotropic interactions and the role of the competition between different zigzag domains in this candidate Kitaev magnet.

Motivated by the recent observations of various exotic quantum states in the equilateral triangular-lattice phosphates ${\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$ with $J_{\mathrm{eff}}=1/2$ and ${\mathrm{Na}}_2\mathrm{BaNi}{\left({\mathrm{PO}}_4\right)}_2$ with $S=1$, the magnetic properties of spin-5/2 antiferromagnet ${\mathrm{Na}}_2\mathrm{BaMn}{\left({\mathrm{PO}}_4\right)}_2$, their classical counterpart, are comprehensively investigated experimentally. DC magnetization and specific heat measurements on polycrystalline samples indicate two successive magnetic transitions at $T_{\mathrm{N}1}\approx 1.13$ K and $T_{\mathrm{N}2}\approx 1.28$ K, respectively. Zero-field neutron powder diffraction measurement at 67 mK reveals a Y-like spin configuration as its ground-state magnetic structure, with both the $ab$ plane and $c$-axis components of the ${\mathrm{Mn}}^{2+}$ moments long-range ordered. The incommensurate magnetic propagation vector $k$ shows a dramatic change for the intermediate phase between $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$, in which the spin state is speculated to change into a collinear structure with only the $c$-axis moments ordered, as stabilized by thermal fluctuations. The successive magnetic transitions observed in ${\mathrm{Na}}_2\mathrm{BaMn}{\left({\mathrm{PO}}_4\right)}_2$ are in line with the expectation for a triangle-lattice antiferromagnet with an easy-axis magnetic anisotropy.

Recent experiments on 2D superconductors allow the characterization of the critical temperature and of the phase diagram across the BCS-BEC crossover as a function of density. We obtain from these experiments the microscopic parameters of the superconducting state at low temperatures by the BCS mean-field approach. For ${\mathrm{Li}}_x\mathrm{ZrNCl}$, the extracted parameters are used to evaluate the superconducting phase stiffness and the Berezinskii-Kosterlitz-Thouless (BKT) critical temperature throughout the BCS-BEC crossover by implementing the corresponding renormalization group (RG) approach. In this way, we make a quantitative test of the predictive power of the BKT theory for evaluating the critical temperature. The RG flow equations turn out to give a sizable renormalization of the phase stiffness and of the critical temperature, which is crucial to obtain a satisfactory agreement between the BKT theory and the experiments, in particular, in the BCS-BEC crossover regime. We predict the temperature range where phase stiffness renormalization can be measured in ${\mathrm{Li}}_x\mathrm{ZrNCl}$ across the BCS-BEC crossover. Contrary to other microscopic theories of superconductivity, we find that the BKT theory can be exploited to evaluate quantitatively the critical temperature of 2D superconductors in different pairing regimes.

The magnetic proximity effect in superconductor/ferromagnet (S/F) heterostructures with a large number of atomic layers leads to a suppression of the superconducting order parameter and appearance of Zeeman-like spin splitting of the local density of states (LDOS). Here we study the magnetic proximity effects in van der Waals S/F heterostructures with a few atomic layers and demonstrate that the corresponding physics is very different from the classical results. We find that the dependence of the superconducting order parameter exhibits dips as a function of the ferromagnetic exchange field and gating. The number of dips is determined by the number of monolayers in the heterostructure and, in general, the superconductivity is not suppressed by large values of the exchange field. The spin splitting of the LDOS cannot be described by an effective Zeeman field and manifests a multiple peak structure, where each peak is connected to a unique spin splitting of one of the superconducting bands, which also can be tuned by gating.

We present a comprehensive computational investigation of electron-phonon interactions in $MX{\mathrm{H}}_3$ hydride compounds, where $M$ represents alkali and post-transition metals, and $X$ denotes $3d$, $4d$, and $5d$ transition metals. Our density functional theory calculations identify 17 dynamically stable compounds. Notably, ${\mathrm{SrAuH}}_3$ and ${\mathrm{SrZnH}}_3$ emerge as theoretical ambient-pressure superconductors with predicted critical temperatures ($T_c$) exceeding 100 K. Analysis of the electronic structure reveals that the $X$ component dominates the density of states at the Fermi level, playing a crucial role in determining electron-phonon coupling strength and superconducting properties. We elucidate the underlying mechanisms governing these properties through detailed examination of the electronic and vibrational spectra. Our findings may challenge the prevailing notion that high-$T_c$ superconductivity in hydrides requires extreme pressures, potentially paving the way for practical applications. This study also provides valuable insights to guide future experimental efforts in the synthesis of ambient-pressure hydride superconductors.

Nonequilibrium driving systems provide fertile ground for exploring intriguing spontaneous symmetry breaking phenomena. In this paper, we report on the intertwined discrete spacetime translational symmetry breaking in a driven-dissipative spin system without pure spatial translational symmetry, resulting in the emergence of an out-of-equilibrium nonmagnetic phase termed the “discrete spacetime crystal.” In contrast to the previously widely studied discrete time crystal, the spins establish a distinctive long-range crystalline order not only in time, but also in the intertwined spacetime direction. We further demonstrate the exponentially long lifetime of the spatial-temporal order against generic weak environmental noise fluctuations. Finally, possible experimental realizations of our model are discussed. Our findings shed light on the existence of exotic phases of matter characterized by intertwined spacetime symmetry breaking, paving the way for future investigations into the properties and potential applications of these exotic states.

Recent advances in spin transport research have highlighted the potential of quantum paramagnets as platforms for exploring novel phenomena and developing next-generation technologies. In this paper, we investigate the Nernst-type thermal spin transport in a quantum paramagnet with spin-orbit couplings. Even though the ground state is paramagnetic, the upper branch parts in this system can carry spin information. By invoking the dispersive crystal electric field (CEF) excitations, the upper branch parts can be described by bosonic quasi-particles with different flavors in the linear flavor-wave regime. To distinguish the magnon Nernst effects in ordered systems, we name this topological spin transport the flavor Nernst effect (FNE). As a proof of principle, we examine the quantum paramagnetic ground state in an effective spin-1 Hamiltonian with Dzyaloshinskii-Moriya interactions and a large hard-axis anisotropy. The flavor Nernst coefficients are calculated using linear response theory. We demonstrate the FNE in a 2D pyrochlore thin film with an all-in-all-out Ising axis configuration, and investigate their dependence on temperature, anisotropy, DM interaction, and external fields. Our results reveal the connection between the FNE and the Berry curvature of the CEF excitations, suggesting potential applications in manipulating thermal spin currents and exploring topological spin transport phenomena in quantum paramagnets.

In quantum many-body systems, interactions play a crucial role in the emergence of information scrambling. When particles interact throughout the system, the entanglement between them can lead to a rapid and chaotic spreading of quantum information, typically probed by the growth in operator size in the Heisenberg picture. In this study, we explore whether the operator undergoes scrambling when particles interact solely through a single impurity in generic spatial dimensions, focusing on fermion systems with spatial and temporal random hoppings. By connecting the dynamics of the operator to the symmetric exclusion process with a source term, we demonstrate the presence of an escape-to-scrambling transition when tuning the interaction strength for fermions in three dimensions. As a comparison, systems in lower dimensions are proven to scramble at arbitrarily weak interactions unless the hopping becomes sufficiently long-ranged. Our predictions are validated using both a Brownian circuit with a single Majorana fermion per site and a solvable Brownian SYK model with a large local Hilbert space dimension. This suggests the universality of the theoretical picture for free fermion systems with spatial and temporal randomness.

The subleading corner logarithmic corrections in entanglement entropy (EE) are crucial for revealing the universal characteristics of quantum critical points (QCPs), but they are challenging to detect. Motivated by recent developments in the stable computation of EE in ($2+1$)-dimensional [($2+1$)D] quantum many-body systems, we have developed a method for directly measuring the corner contribution in EE with lower computational cost. The cornerstone of our approach is to measure the subtracted corner entanglement entropy (SCEE), defined as the difference between the EEs of subregions with the same boundary length for smooth and cornered boundaries during a sign-problem-free quantum Monte Carlo simulation. Our improved method inherently eliminates not only the area-law term of EE but also the subleading log corrections arising from Goldstone modes, leaving the universal corner contribution as the leading term of the SCEE with greatly improved data quality. Utilizing this advanced approach, we calculate the SCEE of the bilayer Heisenberg model on both square and honeycomb lattices across their ($2+1$)D $O$(3) QCPs with different opening angles on the entanglement boundary and obtain accurate values of the corresponding universal corner log coefficients. These findings will encourage further theoretical investigations to access controlled universal information for interacting conformal field theories at ($2+1$) dimensions.

The correlation between electronic disorder (quantified by the Urbach energy $E_U$) and phonon linewidth $\left(\mathrm{\Gamma }\right)$ is known to be linear. However, both parameters exhibit temperature dependence, raising the question of whether the slope of the $\mathrm{\Gamma }$ vs $E_U$ plot remains constant with temperature. In this study, we investigate temperature dependence of the slope, denoted as $\xi$, which represents the sensitivity of phonon mode to changes in disorder. By applying theoretical models for the temperature dependence of $\mathrm{\Gamma }$ and $E_U$, we derive an expression for $\xi \left(T\right)$, revealing its nonlinear behavior at varying temperatures. Our analysis, based on Raman and optical absorption spectra of ${\mathrm{EuFeO}}_3$, demonstrates the explicit temperature dependence of $\xi$. This provides insights into the disorder sensitivity of vibrational modes, which is crucial for understanding phonon dynamics and transport properties. The findings are further supported by additional investigations on ${\mathrm{TiO}}_2$, emphasizing the utility of combined optical and Raman spectroscopic techniques in probing phonon-resolved disorder sensitivity in materials with finite bandgap where $E_U$ can be observed.

We investigate the fundamental topological nature of electrons on a periodically $1/6$-depleted honeycomb lattice, which we call the Bishamon-kikko lattice after a Japanese traditional pattern, by adopting the Haldane model and comprehensively calculating the band structure and topological phase diagram. We also extend our study to the model connecting the Bishamon-kikko and honeycomb lattices. We show that these models exhibit richer topological characteristics compared to the original Haldane model on the honeycomb lattice, such as topological insulating states with higher Chern numbers, metallic states with nontrivial band topology even at commensurate electron fillings, and metal-insulator transitions between them. Our findings offer a playground of correlated topological phenomena and stimulate their realization in a variety of two-dimensional systems, such as van der Waals materials, graphene nanostructures, and photonic crystals.

We present a Floquet Schrieffer-Wolff transform (FSWT) to obtain effective Floquet Hamiltonians and micromotion operators of periodically driven many-body systems for any nonresonant driving frequency. The FSWT perturbatively eliminates the oscillatory components in the driven Hamiltonian by solving operator-valued Sylvester equations with systematic approximations. It goes beyond various high-frequency expansion methods commonly used in Floquet theory, as we demonstrate with the example of the driven Fermi-Hubbard model. In the limit of high driving frequencies, the FSWT Hamiltonian reduces to the widely used Floquet-Magnus result. We anticipate this method will be useful for designing Rydberg multiqubit gates, controlling correlated hopping in quantum simulations in optical lattices, and describing multiorbital and long-range interacting systems driven in-gap.

Predicted as the first large-gap quantum spin Hall insulator due to the intrinsic spin-orbit coupling, monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ undergoes a rich variety of topological phase transitions under the interplay of exchange fields, staggered sublattice potential, and Rashba spin-orbit coupling (RSOC). Aiming at the various topological phases, we explore the anomalous thermoelectric transport modulated by diverse Berry phase effects. It is found that the presence or absence of RSOC is important. In the case without RSOC, the topological phases can be characterized by detecting the number of sign reversals in the spin and valley Nernst conductivities at room temperature, which is different from that in Xene materials. Additionally, the anomalous thermoelectric coefficient can be enhanced by modulating the magnetic exchange field. In the case with RSOC, due to the spin flips, the spin Nernst conductivity is ill-defined. However, the topological feature of valley-polarized quantum anomalous Hall phases can be extracted from the valley orbital magnetization, which is constant in the $K$ valley, and linear dependence in the $K^{\prime }$ valley when the Fermi level is in the gap. Furthermore, the RSOC can be used to enhance the Nernst signal in the topologically nontrivial valley. Our predictions can be experimentally validated and are expected to pave the way for the applications of monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ in spin and valley thermoelectronics.

Genuine multipartite entanglement (GME) is not only fundamentally interesting for the study of the quantum-to-classical transition, but also is essential for realizing universal quantum computing and quantum networks. Here, we investigate the multipartite entanglement (ME) dynamics in a linear chain of $N LC$ resonators interacting optomechanically with a common thermal acoustic reservoir. By presenting the exact analytical solutions of the system evolution, we predict the periodic generation of non-Gaussian ME, including discrete and continuous variable entanglement. Interestingly, GME is obtained even though the system is in a heat bath. The mechanism relies on a special acoustic environment featuring a frequency comb structure. More importantly, our proposed model also allows for the periodic generation of entangled multipartite cat states, i.e., a typical Greenberger-Horne-Zeilinger state, with high fidelity. This work fundamentally broadens the fields of ME, and has wide applications in implementing thermal-noise-resistant quantum information processing and many-body quantum simulation.

Transmission enhancement of sound waves across the interface between media with different impedances is a classical problem in acoustic communication, for instance in the case of the water-air interface. Different strategies, including bubble-embedded metasurfaces or impedance-matched metamaterials, have been developed to render the interface transparent. Transmedia wave enhancement is up to now either broadband or incorporating customized wavefront manipulation. In this paper, we explore the use of transformation acoustics and gauge transformation to realize both goals simultaneously. Transformation acoustics is first argued to provide lossless transmedia sound enhancement, although at the expense of adapting the cross section to respect power-flow conservation. It is specifically proven that a gradient anisotropic metamaterial provides perfect transmission between two media in two dimensions and that perfect steering of sound waves is allowed as well. Gauge transformation theory is then adapted from transformation elasticity to provide one-dimensional perfect sound transmission via a gradient Willis-like acoustic metamaterial. Finally, the two transformations are combined together to implement broadband perfect acoustic transmission and focusing at the water-air interface. The lens designed by the proposed method is achromatic. This paper is expected to provide alternative solutions for transmedia communication.

It is now well established that a high-frequency electromagnetic dressing field within the off-resonance regime can significantly modify the electronic, transport, and optical properties of Dirac materials. Here, using light with circular polarization and in the terahertz range, we investigate its effect on the energy spectrum (dressed states), polarization function, plasmons, and their lifetimes due to Landau damping of tilted monolayer $1T^{\prime }{\text{-MoS}}_2$ which acquires two energy gaps associated with up and down pseudospin. We can adjust its electronic properties over a wider range by varying these two band gaps in contrast with graphene. Unlike graphene and other two-dimensional materials, the plasmons are Landau damped in the long-wavelength limit and emerge from the particle-hole continuum at finite wave vector $q$ and frequency $\omega$ even in the absence of irradiation. This damping arises from intraband transitions. With the use of the Lindhard approach for the frequency-dependent polarizability propagator, we have developed a rigorous theoretical formalism for employing the Floquet energy spectrum for investigating the many-body effects on the plasmon excitations, their lifetimes due to Landau damping and the exchange energy of tilted monolayer $1T^{\prime }{\text{-MoS}}_2$ under the normal incidence of electromagnetic radiation at arbitrary temperature. The dressed states at very low temperatures corresponding to circular polarization suppress the response of the system to the external probe. In the long-wavelength limit, the $\sqrt{qT}$-dependent plasmons are restored at high temperatures. Our calculations have shown that the exchange energy decreases exponentially as a function of chemical potential for finite dopings. Its behavior is also influenced by the indirect momentum space pseudospin gap, the tilting of the energy bands, and anisotropy. The results for the exchange could be useful in calculating the compressibility and scattering rates of the tilted gapped monolayer Dirac material $1T^{\prime }{\text{-MoS}}_2$.

Estimating force constants for crystal structures is crucial for calculating various phonon-related properties. However, this task becomes particularly challenging when dealing with a large number of atoms or when third- and higher-order force constants are required. In this study, we propose an efficient approach that involves constructing a complete orthonormal basis set for the force constants. We formulate this approach using projection matrices and their eigenvectors to meet the requirements for the force constants. This basis set enables precise inference of the force constants from displacement-force data sets. Our efficient algorithms for basis-set construction and force-constant estimation are implemented in the symfc code. Furthermore, several applications demonstrated in this study indicate that the current approach facilitates the efficient and accurate determination of force constants.

We investigated the magnetic transitions in ${\mathrm{BiFeO}}_3$ at low temperature (5–300 K) and observed nearly ${90}^{\circ }$ rotation of magnetic domains (imaged by vertical magnetic force microscopy) across 150 K in an epitaxial thin film of thickness $\sim 36$ nm. It offers a clear evidence of spin reorientation transition. It also corroborates the transition observed below $\sim 150$ K in the zero-field-cooled and field-cooled magnetization versus temperature data. The field-driven ${180}^{\circ }$ domain switching at room temperature, on the other hand, signifies presence of ferromagnetism. Since bulk antiferromagnetic ${\mathrm{BiFeO}}_3$ does not exhibit such a transition, this observation in ferromagnetic thin film of ${\mathrm{BiFeO}}_3$ indicates a radical effect because of epitaxial strain. Density functional theory-based first-principles calculations also reveal that combined in- and out-of-plane epitaxial strain induces magnetic transition from G- to C-type structure in ${\mathrm{BiFeO}}_3$.