Two-dimensional (2D) Van der Waals compounds exhibit interesting electronic and magnetic properties due to complex intralayer and interlayer interactions, which are of immense importance in realizing exotic physics as well as advanced technology. Various experimental and theoretical studies led to significantly different ground state properties often contrasting each other. Here, we studied a novel 2D material, ${\mathrm{CrSiTe}}_3$, employing magnetic, specific heat, and Raman measurements. Experimental results reveal evidence of incipient antiferromagnetism below 1 kOe concomitant to ferromagnetic order at 33 K. Antiferromagnetic and ferromagnetic interactions coexist at low field in the temperature regime 15–33 K. Low-field data reveal an additional magnetic order below 15 K, which melts on application of external magnetic field and remains dark in the heat capacity data. Raman spectra exhibit anomalies at the magnetic transitions, evidence of strong spin-lattice coupling. Below 15 K, $E_g$ modes exhibit hardening while $A_g$ modes become significantly softer, suggesting weakening of the interlayer coupling at low temperatures which might be a reason for the unusual magnetic ground state and field-induced melting of the magnetic order. These results reveal evidence of exceptional ground state properties linked to spin-lattice coupling and also suggest a pathway to study complex magnetism in such technologically important materials.

We report the observation of a light-induced subpicosecond phase transition in the antiferromagnetic Dirac semimetal EuAgAs, achieved through ultrafast optical excitation. Using ultrafast optical spectroscopy, we probe the nonequilibrium carrier dynamics, discovering distinct fluence-dependent responses in the antiferromagnetic and paramagnetic states, and revealing a possible magnetic order-driven transition between different topological states. Our results demonstrate that EuAgAs, with its highly tunable magnetic structure, possibly offers a unique platform for exploring topological phase transitions. These results underscore the potential of ultrashort optical pulses as powerful tools for the real-time control of topological phases, opening pathways for advances in spintronics, quantum computing, and energy-efficient information technologies.

The elementary excitations of quantum spin systems have generally the nature of weakly interacting bosonic quasiparticles, generated by local operators acting on the ground state. Nonetheless in one spatial dimension the nature of the quasiparticles can change radically, since many relevant one-dimensional $S=1/2$ Hamiltonians can be exactly mapped onto models of spinless fermions with local hopping and interactions. Due to the nonlocal nature of the spin-to-fermion mapping, observing directly the fermionic quasiparticle excitations is impossible using local probes, which are at the basis of all the forms of spectroscopy (such as neutron scattering) traditionally available in condensed-matter physics. Here, we show theoretically that quench spectroscopy for synthetic quantum matter—which probes the excitation spectrum of a system by monitoring the nonequilibrium dynamics of its correlation functions—can reconstruct accurately the dispersion relation of fermionic quasiparticles in spin chains. This possibility relies on the ability of quantum simulation experiments to measure nonlocal spin-spin correlation functions, corresponding to elementary fermionic correlation functions. Our analysis is based on exact results for the quench dynamics of quantum spin chains, and it opens the path to probe arbitrary quasiparticle excitations in synthetic quantum matter.

Recently significant attention has been paid to magnetic field dependent photoluminescence (PL) features of the negatively charged nitrogen-vacancy (NV) centers in diamond. These features are used for microwave-free sensing and are indicative of the spin-bath properties in the diamond sample. Examining the temperature dependence of the PL features allows identifying both temperature-dependent and -independent features, of potential utility in diamond-based quantum sensing and dynamic nuclear polarization applications. Here we study the thermal variability of many different features visible in a wide range of magnetic fields. To this end, we first discuss the origin of the features and tentatively assign the previously unidentified features to cross relaxation of NV center containing multispin systems. The experimental results are compared with theoretically predicted temperature shifts deduced from a combination of thermal expansion and electron-phonon interactions. A deeper insight into the thermal behavior of a wide array of the features may come with important consequences for various applications in high-precision NV thermometry, gyroscopes, solid-state clocks, and biomagnetic measurements.

We investigate the out-of-equilibrium dynamics of a photoexcited charge-density-wave (CDW) state in the square-lattice Holstein model, in a setup similar to a pump-probe experiment. At half-filling, the ground state of this system is characterized by a checkerboard modulation of particle densities, accompanied by a concomitant lattice distortion. An efficient real-space dynamics method integrating the von Neumann equation for electron density matrix and Newton equation for classical lattice dynamics is developed to simulate the dynamical evolution of a photoexcited Holstein system. We find that the energy injected by a short pump pulse results in the reduction of the CDW order and the generation of coherent phonons. At strong photoexcitations, while the CDW order is melted in the sense that the time-averaged order parameter vanishes, a dynamical CDW state is self-sustained by the strong coherent phonon oscillations. Our large-scale simulations further uncover a dynamical regime of intermediate fluence where complex pattern formation is induced by the pump pulse. The emergent spatial textures are characterized by super density modulations on top of the short-range checkerboard CDW order. Our demonstration of pattern formation highlights the significance of dynamical inhomogeneity in quantum many-body systems in pump-probe experiments.

Materials with an ultra-low temperature coefficient of resistivity are desired for the temperature and flow sensors in high-precision electronic measuring systems. In this work, the Kubo-Greenwood formula, implemented in ab initio molecular dynamics simulations, is employed to predict the finite-temperature resistivity of multicomponent alloys with severe lattice distortion. We observe a tiny change in resistivity over a wide temperature range in high-entropy alloys. The electronic resistivity Invar effect in B2 ${\mathrm{Ni}}_{25}{\mathrm{Co}}_{25}{\left(\mathrm{HfTiZr}\right)}_{50}$ Elinvar alloys results from a balance between intrinsic and residual resistivity. This effect is associated with atomic displacements from ideal lattice sites, which are caused by lattice thermal vibrations and chemical disorder-induced lattice distortions. It is further evidenced by a decrease in lattice distortion with temperature and changes in the electronic density of states.

We study stochastic resonance in molecular junctions driven by a periodically varying external field. This is done using the time-dependent Landauer-Büttiker formalism, which follows from exact analytical solutions to the Kadanoff-Baym equations describing the molecular junction subject to an arbitrary time-dependent bias. We focus on a double quantum dot nanojunction and compare the effects of the temperature with the fluctuating bias in the statically driven case. We then consider the combined effect of AC driving and white-noise fluctuations on the rectified current through the nanojunction, and find a stochastic resonance effect, where at certain driving conditions the bias fluctuations enhance the current signal. The study is then extended to include the color noise in the applied bias, so that the combined effect of the color noise correlation time and driving frequency on stochastic resonance is investigated. We thereby demonstrate that photon-assisted transport can be optimized by a suitably tuned environment.

The adaptive variational quantum dynamics simulation (AVQDS) method performs real-time evolution of quantum states using automatically generated parametrized quantum circuits that often contain substantially fewer gates than Trotter circuits. Here we report an improved version of the method, which we call AVQDS(T), by porting the tiling efficient trial circuits with rotations implemented simultaneously technique. The algorithm adaptively adds layers of disjoint unitary gates to the ansatz circuit so as to keep the McLachlan distance, a measure of the accuracy of the variational dynamics, below a fixed threshold. We perform benchmark noiseless AVQDS(T) simulations of quench dynamics in local spin models and compare with an alternative adaptive variational approach on quantum resource requirement. Quantum dynamical simulations implementing realistic noise channels are also reported. Finally, we propose a way to substantially alleviate the measurement overhead of AVQDS(T) while maintaining high accuracy by synergistically integrating quantum circuit calculations on quantum processing units with classical calculations using, e.g., tensor networks to evaluate the quantum geometric tensor. We showcase that this approach enables AVQDS(T) to deliver more accurate results than simulations using a fixed ansatz of comparable final depth for a significant time duration with fewer quantum resources.

Magnon-magnon scattering, which induces magnon thermalization and dissipation, plays a crucial role in magnon transport and has garnered significant attention. Previous theoretical studies have either simplified the scattering time or potential, which limits their applicability to low-temperature regimes, or have approximated the magnon band structure to reduce computational complexity across a wide temperature range. In this work, we derive a formalism in the framework of the nonlinear spin wave theory to determine the magnon scattering rate using the Fermi's golden rule without simplifying the magnon band or scattering potential, where material parameters are acquired from first-principles calculations. We apply this methodology to the two-dimensional van der Waals ferromagnet monolayer ${\mathrm{CrI}}_3$ and obtain a temperature- and momentum-dependent magnon-magnon scattering rate that scales with ${\mathbf{k}}^2$ and $T^3$. We further investigate the magnon transport properties, including magnon spin conductivity, heat conductivity, and the spin Seebeck coefficient. We find that all these properties decrease with temperature, but none follows a simple power law. Our first-principles-based framework provides a robust approach for calculating magnon-magnon scattering rates in a material-specific manner, which is essential for understanding magnon transport.

The domain size $\mathit{l}$ in ferroic films is commonly believed to scale with the film thickness $\mathit{d}$ following Kittel's law $\mathit{l}\propto \sqrt{\mathit{d}}$, as initially proposed by Kittel for ferromagnetic materials. Here, we establish an analytical model for the scaling behavior of alternating ferromagnetic (FM) and antiferromagnetic (AFM) stripe domains having nonidentical widths in the presence of an applied magnetic field. The analytical model is derived based on domain thermodynamics, where the change in the free energy of the configuration is evaluated when a parent AFM single domain structure transforms into the field-induced $\mathrm{FM}+\mathrm{AFM}$ stripe domain structure due to the magnetostatic and interfacial energies arising in the mixed phase structure in addition to the bulk free energy change. Using ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ as an example, an A-type antiferromagnetic material below the temperature of 13 K that undergoes a transition to the $\mathrm{FM}+\mathrm{AFM}$ coexistence phase upon applying a magnetic field, our analytical model predicted, corroborated by phase-field simulations, a significant departure of the thickness scaling of the ferromagnetic and antiferromagnetic domains from Kittel's scaling law. A careful analysis of the domain thermodynamics reveals that for a specific equilibrium FM/AFM phase fraction, the abnormal inverse thickness scaling of domain size at small film thicknesses originates from the thickness dependence of the magnetostatic energy and its competition with the interfacial energy determines the equilibrium domain size at a given film thickness. In addition to the film thickness, the equilibrium domain width of each stripe domain is influenced by the relative equilibrium phase fractions of the FM and AFM phases in the two-phase mixture depending on the strength of the applied magnetic field and temperature. The control of the domain size in an FM/AFM coexistence with reduced dimensionality and magnetic field tuning offers unique possibilities for the realization of exotic topological phases and functional devices based on AFM/FM spintronics.

$A_2\mathrm{Cu}{B}^{\prime }{\mathrm{O}}_6$ double perovskites with $A=\mathrm{Sr}$ and Ba and $B^{\prime }=\mathrm{W}$, Te, and Mo have received a lot of attention in the literature as material candidates for the spin-liquid ground state and frustrated spin lattices. However, Ca analogs have not been reported yet. In this work, the $\mathrm{C}{\mathrm{a}}_2\mathrm{CuW}{\mathrm{O}}_6$ double perovskite was synthesized by a high-pressure, high-temperature method at 6 GPa and between 1370 and 2020 K. The crystal structure was studied by synchrotron x-ray powder diffraction between 100 and 1000 K. $\mathrm{C}{\mathrm{a}}_2\mathrm{CuW}{\mathrm{O}}_6$ crystallizes in the triclinic space group $P\text{−}1$ between 100 and 1000 K with the onset of partial decomposition above 750 K. Room-temperature lattice parameters are $a=5.68435\AA , b=7.51261\AA , c=5.49548\AA , \alpha =90.{0703}^{\circ }, \beta =94.{7634}^{\circ }$, and $\gamma =90.{2311}^{\circ }$. $\mathrm{C}{\mathrm{a}}_2\mathrm{CuW}{\mathrm{O}}_6$ demonstrates low-dimensional magnetic behavior with the broad maximum of the magnetic susceptibility around 60 K caused by the dominant second-neighbor interactions within the deformed square planes of the $\mathrm{C}{\mathrm{u}}^{2+}$ ions. A long-range antiferromagnetic order takes place at $T_{\mathrm{N}}=32$ K. A magnetic field-induced transition was observed at 28 kOe (at 2 and 5 K), 29 kOe (at 10 K), 30 kOe (at 15 K), 32 kOe (at 20 K), and 36 kOe (at 25 K). Solid solutions $\mathrm{C}{\mathrm{a}}_{2\text{−}x}\mathrm{S}{\mathrm{r}}_x\mathrm{CuW}{\mathrm{O}}_6$ with $x=0.5$, 1, and 1.5 were also prepared. The $x=0.5$ and 1 samples crystallize in the space group $P\text{−}1$ at room temperature, while the $x=1.5$ sample has monoclinic symmetry (space group $P{2}_1/n$). They also show low-dimensional magnetic behavior, $T_{\mathrm{N}}=32-34$ K, and magnetic field-induced transitions.

In crystalline materials, low lattice thermal conductivity is often associated with strong anharmonicity, causing significant deviations from the expected Lorentzian lineshape of phonon spectral functions. These deviations, occurring in an overdamped regime, raise questions about the applicability of the Boltzmann transport equation. Furthermore, strong anharmonicity can trigger structural phase transitions with temperature that cannot be adequately described by the standard harmonic approximation. To address these challenges, we propose an approach for computing lattice thermal conductivity. Our method combines the Green-Kubo linear response theory with the stochastic self-consistent harmonic approximation. The latter describes the temperature-dependent evolution of the crystal structure, including first- and second-order phase transitions, as well as the vibrational properties in highly anharmonic materials. The Green-Kubo method considers the full lineshapes of phonon spectral functions in the calculation of lattice thermal conductivity, thus eliminating the questionable use of phonon lifetimes in the overdamped regime and naturally including coherent transport effects. Additionally, we extend our theory to model complex dynamical lattice thermal conductivity, enhancing understanding of time-dependent thermoreflectance experiments. As a practical application, we employ this approach to calculate lattice thermal conductivity of ${\mathrm{CsPbBr}}_3$, a complex crystal known for its anomalous thermal transport behavior and a complex phase diagram. Our method determines the thermal conductivity across different phases in good agreement with experiments.

Exactly solvable models in quantum many-body dynamics provide valuable insights into many interesting physical phenomena and serve as platforms for rigorously investigating fundamental theoretical questions. Nevertheless, they are extremely rare and existing solvable models and solution techniques have serious limitations. In this paper, we present a family of exactly solvable unitary circuits that model quantum many-body dynamics in discrete space and time. Unlike many previous solvable models, one can exactly compute the full quantum dynamics initialized from any matrix product state in this family of models. The time evolution of local observables and correlations, the linear growth of Rényi entanglement entropy, spatiotemporal correlations, and out-of-time-order correlations are all exactly computable. A key property of these models enabling the exact solution is that any time-evolved local operator is an exact matrix product operator with a finite bond dimension, even at arbitrarily long times, which we prove using the underlying ${\mathbb{C}}^{*}$-(weak) Hopf algebra structure along with tensor network techniques. We lay down the general framework for construction and solution of this family of models, and give several explicit examples. In particular, we study in detail a model constructed out of a ${\mathbb{C}}^{*}$-weak Hopf algebra that is very close to a Floquet version of the PXP model, and the exact results we obtain may shed light on the phenomenon of quantum many-body scars, and more generally, Floquet quantum dynamics in constrained systems.

We report on the crystal field level splitting and magnetic ground state of the $J_{\mathrm{eff}}=1/2$ square lattice antiferromagnets ${\mathrm{YbBi}}_2{\mathrm{ClO}}_4$ and ${\mathrm{YbBi}}_2{\mathrm{IO}}_4$ using powder inelastic neutron scattering (INS) and neutron diffraction measurements. Both compounds exhibit a well-isolated ${\mathrm{\Gamma }}_7$ doublet ground state under a tetragonal crystal field environment, confirming a robust $J_{\mathrm{eff}}=1/2$ picture with slight XY-type anisotropic character in the $g$-tensor. Notably, the ground state wave functions closely resemble the ${\mathrm{\Gamma }}_7$ doublet expected in the perfect cubic limit, consistent with the nearly cubic ligand configuration of eight ${\mathrm{O}}^{2-}$ ions surrounding ${\mathrm{Yb}}^{3+}$. Below $T_{\mathrm{N}}$ = 0.21 K, ${\mathrm{YbBi}}_2{\mathrm{IO}}_4$ exhibits a stripe long-range magnetic order characterized by an ordering wave vector ${\mathbf{q}}_{\mathrm{m}}$ = (1/2, 0, 0) or its symmetry-equivalent (0, 1/2, 0), with magnetic moments aligned along ${\mathbf{q}}_{\mathrm{m}}$. The ordered moment is approximately 79% of the classical prediction, significantly larger than expected from the isotropic $J_1-J_2$ model, suggesting the possible involvement of exchange anisotropy in explaining this observation. We show that symmetry-allowed XXZ and bond-dependent anisotropic exchange terms in a square lattice can play a critical role in stabilizing the stripe order and suppressing the moment reduction as observed. These findings establish ${\mathrm{YbBi}}_2{\mathrm{ClO}}_4$ and ${\mathrm{YbBi}}_2{\mathrm{IO}}_4$ as unique platforms for exploring rich $J_{\mathrm{eff}}=1/2$ magnetism from two less investigated perspectives: (i) on a square lattice and (ii) within a (nearly) cubic ligand environment.

Linear magnon-phonon coupling hybridizes magnon and phonon bands at the same energy and momentum, resulting in an anticrossing signature. This hybrid quasiparticle benefits from a long phonon lifetime and efficient magnon transport, showing great potential for spintronics and quantum information science applications. In this paper, we present an efficient and accurate first-principles approach for calculating linear magnon-phonon couplings. We first calculate the magnon spectra from linear spin-wave theory with spin Hamiltonian and first-principles exchange constants, which compared well with time-dependent density-functional theory. We then obtain the magnon-phonon coupling from the derivative of off-diagonal exchange constants in real space, calculated from the Hellmann-Feynman forces of the spin-constrained configurations, avoiding the use of cumbersome finite-difference methods. Our implementation allows calculating coupling coefficients at an arbitrary wave vector in the Brillouin zone in a single step, through Fourier interpolation of real-space supercell calculations. We verify our implementation through two-dimensional magnetic systems, monolayer ${\mathrm{CrI}}_3$, in agreement with experiments, and extend its application to monolayer ${\mathrm{CrTe}}_2$. We emphasize the role of nonmagnetic atoms in superexchange interactions and magnon-phonon coupling, which have been overlooked previously. We suggest effective tuning of magnon-phonon coupling through strain, doping, and terahertz excitations, for spintronics and quantum magnonics applications.

The discovery of high critical temperature ($T_c$) superconductivity in pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ has ignited renewed excitement in the search for novel high-$T_c$ superconducting compounds with $3d$ transition metals. Compared to other ambient- pressure superconductors, such as copper-oxide and iron-oxypnictides, unraveling the mechanisms of the pressure-induced superconductivity poses significant and unique challenges. A critical factor in this phenomenon seems to be related to the electronic configuration of $3d$ orbitals, which may play a fundamental role in driving high-$T_c$ superconductivity. However, the pressure effects on the mixed-valence states of $3d$-orbital cations and their influence on the emergence of high-$T_c$ superconductivity remain poorly understood. Here, we use high-pressure ($P$) and low-temperature synchrotron x-ray absorption spectroscopy to investigate the influence of pressure on the mean valence (ν) change of Ni ions in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. Our results demonstrate that at a low temperature of 20 K, ν remains relatively stable across the pressures range from 1 atm to 40 GPa. Based on analyzing the absorption data, we find that at a critical pressure, the ambient-pressure ordered phases disappear and both the structural and the superconducting phase transition occur. The pressure-induced structural phase transition revealed by our absorption results is consistent with that determined by x-ray diffraction, offering information for a comprehensive understanding on the pressure-induced superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$.

The vortex distribution in a high-quality stoichiometric superconductor ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ is studied using low-temperature magnetic force microscopy. Across the entire range of magnetic fields up to 80 Oe, the vortex structures lack long-range order due to vortex pinning. As the vortex density increases, the ordering of the vortex lattice gradually improves, characterized by an increased proportion of sixfold coordination and ${60}^{\circ }$ angle. Continuous-field cooling experiments reveal a high density of equivalent pinning centers in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. In localized regions, vortex chains induced by surface steps are observed, from which the freezing temperatures of the vortex configurations are inferred and found to be consistent with direct experimental observations. Analysis of vortex interaction energies and pinning force distributions, based on the relative positions of vortices, suggests that the strong pinning in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ originates from nanoscale spatial fluctuations in the superfluid density, which is closely linked to the unique structural characteristics of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ single crystals. In this paper, we illustrate the relationship between disordered vortex patterns, critical current density, and nanoscale spatial fluctuations of superfluid density, providing insights into strong pinning in iron-based superconductors.

Significant nonsaturating magnetoresistance and quantum oscillations have been observed across various materials. Here, NdBi with a Néel temperature $T_N$ of 24 K has been successfully synthesized by the flux method and characterized by electric and magnetic measurements. The magnetoresistance reaches a remarkable value of $6.88\times {10}^4$% at 16 T and 2 K, without showing any signs of saturation. The angle-dependent Shubnikov-de Haas oscillations have been performed, leading to experimentally mapping the Fermi surface and extracting the physical properties for each Fermi pocket. The Fermi surface is composed of three distinct types of pockets: an inner spherelike hole pocket denoted as $\beta$, an outer diamondlike hole pocket $\delta$, both located at the Brillouin zone's $\overline{\mathrm{\Gamma }}$ point, and three equivalent ellipsoidlike electron pockets $\gamma$ located at the face centers of the cubic Brillouin zone. The ratio of electron and hole carrier densities, inferred from the volume of these pockets, is 0.96, suggesting that the compensation is responsible for the substantial magnetoresistance observed in NdBi. This finding is further corroborated by the Hall resistivity data, whose carrier densities are $n_h=5.04\times {10}^{20}{\mathrm{cm}}^{-3}$ and $n_e=4.98\times {10}^{20}{\mathrm{cm}}^{-3}$. In addition, the spin splitting is also observed between ${15}^{\circ }$ and ${65}^{\circ }$, providing more information on the Fermi surface.

We present extensive new ab initio path integral Monte Carlo (PIMC) simulation results for the chemical potential of the warm dense uniform electron gas (UEG), spanning a broad range of densities and temperatures. This is achieved by following two independent routes, (i) based on the direct estimation of the free energy [Dornheim et al., Phys. Rev. B 111, L041114 (2025)] and (ii) using a histogram estimator in PIMC simulations with a varying number of particles. We empirically confirm the expected inverse linear dependence of the exchange–correlation (XC) part of the chemical potential on the simulated number of electrons, which allows for a reliable extrapolation to the thermodynamic limit without the necessity for an additional finite-size correction. We find very good agreement (within $\mathrm{\Delta }{\mu }_{\text{xc}}\lesssim 0.5\%$) with the previous parametrization of the XC-free energy by Groth et al. [Phys. Rev. Lett. 119, 135001 (2017)], which constitutes an important cross validation of current state-of-the-art UEG equations of state. In addition to being interesting in its own right, our study constitutes the basis for the future PIMC based investigation of the chemical potential of real warm dense matter systems starting with hydrogen.

Current-induced spin magnetization (CISM) specific to chiral crystals is microscopically analyzed using multipole theory to identify the necessary hopping and spin-orbit couplings (SOCs). Tight-binding models capturing the essence of chiral crystals are introduced to investigate the multipole degrees of freedom possessed by the Hamiltonian. The results reveal that chiral SOC has a multipole degree of freedom specific to chiral crystals. Subsequently, the CISM is evaluated numerically and analytically. The results show that in addition to the chiral SOC, hopping along the $z$ axis, which is irrelevant from a multipole perspective, is crucial for CISM. This hopping is required to break the combined symmetry of wave-vector translation and spin flipping, which we refer to as spin-glide symmetry. This confirms that hopping irrelevant to chirality can play a crucial role in physical properties arising from chirality without contradicting the framework of multipole theory.