We have studied theoretically the specific heat of a large number of nonfrustrated magnetic structures described by the Heisenberg model for systems with total angular momentum $J=7/2$, corresponding to the $4f^7$ configuration of ${\mathrm{Gd}}^{+3}$ and ${\mathrm{Eu}}^{+2}$. For a given critical temperature (determined by the magnitude of the exchange interactions), we find that, to a high degree of accuracy, the specific heat is governed by two primary parameters: the effective number of neighbors $z$, which governs the extent of thermal and quantum fluctuations, and the axial anisotropy $K$. The universality of $z$ (its ability to describe specific heat across diverse lattices) holds robustly for systems where exchange interactions do not strongly increase with distance and in the absence of frustration. Otherwise, deviations from universality emerge. Using these two parameters we fit the specific heat of four gadolinium compounds and two europium compounds, achieving a remarkable agreement. The present approach enables the extraction of magnetic interaction parameters not accessible through mean-field theory, offering a powerful tool for interpreting specific heat data in $4f^7$ systems.

Compared to $AB{\mathrm{O}}_3$ oxide perovskites, intrinsic ferroelectricity is less studied in ${ABX}_3$ ($X=\mathrm{Cl}$, Br, I) halide perovskites. Nonetheless, it presents unique opportunities, such as achieving ferroelectric photovoltaics with band gaps suitable for the solar spectrum. In this work, we focus on the $\mathrm{CsGe}{X}_3$ halide perovskites as prototypes for investigating the origin of their ferroelectricity and its relationship with other properties. We find that the presence of stereochemically lone pairs on the perovskite $B$-site is the primary driving force for ferroelectricity, causing off-center displacements of the Ge ions and enhanced covalency with the halide ligands. This contrasts with the behavior of typical ferroelectric oxide perovskites. Our calculations confirm that halide perovskites are intrinsic modest band gap semiconductors. In particular, ${\mathrm{CsGeI}}_3$ exhibits a very favorable band gap of 1.6 eV for photovoltaic applications. Our findings provide valuable insights into the mechanism underlying high-temperature ferroelectricity in halide perovskites with potential optoelectronic applications.

We report a field-induced quantum first-order transition in the quasi-two-dimensional antiferromagnet ${\mathrm{KCoPO}}_4·{\mathrm{H}}_2\mathrm{O}$. Using highly oriented microcrystals with $H\parallel b$, polarization neutron diffraction directly resolves microscopic coexistence between an incommensurate cycloid and a commensurate collinear state on the same specimen, together with a discontinuous jump of the ordering vector from $\mathbf{q}=(\delta ,0,0)$ ($\delta =0.1747$) to $\mathbf{q}=\mathbf{0}$. Magnetization and specific heat show a step and a collapse of the $C/T$ anomaly near ${\mu }_0{H}_c\approx 0.6\mathrm{T}$ with no measurable hysteresis, consistent with strong quantum-fluctuation-induced rounding of first-order signatures in the $T\to 0$ limit. LF-$\mu \mathrm{SR}$ reveals two-component relaxation and loss of the late-time tail around $H_c$, with slow fluctuations persisting to 0.06 K. The $H\text{−}T$ phase diagram exhibits a narrow coexistence region. By the symmetry of space group $Pmn{2}_1$ (Moriya's rules), the Dzyaloshinskii–Moriya vector is constrained predominantly within the $bc$ plane, thereby favoring a spiral with $q\parallel a$. Conversely, XY anisotropy competes with this tendency, producing a near degeneracy between the incommensurate cycloid and the commensurate collinear state and promoting a first-order switch under field sweep. ${\mathrm{KCoPO}}_4·{\mathrm{H}}_2\mathrm{O}$ thus provides a clean insulating platform and a microscopic standard for identifying quantum first-order transitions in frustrated magnets.

Multicomponent Heusler alloys exhibit various magnetic properties arising from their diverse atomic compositions and crystal structures. Identifying the general physical principles that govern these behaviors is essential for advancing their potential in spintronic applications. In this work, we combine density functional theory with atomistic Monte Carlo simulations to investigate the magnetic ground states, finite-temperature magnetic transitions, and electronic structures of fully ordered $L{2}_1$-, $XA$-type, and partially ordered ${\mathrm{V}}_{2}X\mathrm{Al}(X=$ Cr, Mn, Fe, Co, Ni) Heusler alloys. We propose the concept of magnetic motifs, defined as V-$X$-V triangular pathway connected by the nearest-neighbor (NN) exchange interactions $J_{\mathrm{V}-X}$. Within this framework, the magnetic ground states and transition temperatures across the ${\mathrm{V}}_{2}X\mathrm{Al}$ family can be consistently understood. The magnetic order is primarily governed by the NN $J_{\mathrm{V}-X}$ interactions in the triangular motifs, while the transition temperatures are additionally influenced by $J_{X-X}$ couplings. Furthermore, the magnetic motifs are still proven to be effective in our calculations on partially ordered ${\mathrm{V}}_{2}X\mathrm{Al}$ alloys from $L{2}_1$ to $XA$-type structures. Our results suggest that the concept of magnetic motifs provides a unifying principle for understanding magnetic ordering in V-based Heusler alloys and could serve as a candidate guide for exploring magnetism and designing advanced spintronic materials in a broader class of Heusler systems.

We numerically study the dynamics of extended domain walls in homogeneous ferromagnets driven by a uniform magnetic field at zero temperature. Using both micromagnetic Landau-Lifshitz-Gilbert simulations and a collective-coordinate description, we show that flat chiral domain walls become linearly unstable above the Walker breakdown field and below a higher threshold, provided their length exceeds a characteristic scale. This instability is captured by a quasiuniversal spectral stability diagram, parametrized solely by the Gilbert damping, which predicts the onset of deviations from rigid-wall behavior. Beyond the linear regime, large domain walls with bands of unstable modes develop spatiotemporal chaos, intricate Bloch-line dynamics, and deterministic roughening. At a critical field, the system undergoes a dynamical phase transition from a flat to a rough moving phase with universal features. Our results provide a framework for addressing domain-wall dynamics in the presence of thermal fluctuations and quenched disorder by disentangling their effects from intrinsic deterministic instabilities.

We investigate the weak interaction integer quantum Hall (IQH) phase, the intermediate interaction phase identified as a chiral spin liquid (CSL) and the transition between them in the triangular lattice Hofstadter–Hubbard model at a density of one electron per site in an orbital magnetic field corresponding to one-quarter flux per plaquette. Our primary tool is the finite system density matrix renormalization group (DMRG) method with both interaction-strength scan and fixed interaction techniques for cylinders of circumference 3, 5, and 7 and lengths up to 240. For the IQH phase, we use single particle exact diagonalization to clarify finite size effects, including an excess charge on the edges of our cylinders, and the limitations of entanglement spectra degeneracies on small circumference cylinders. For both phases, we use DMRG to study the entanglement spectra, the entanglement entropy, and the effect of flux insertion on charge and spin pumping, all of which show key differences between the two phases. To study the transition, we use interaction-strength scans extending between the two phases, and apply a scaling data collapse of a bond-dimerization order parameter to extract critical exponents. We also extract critical behavior from the divergence of correlation lengths on the IQH side, measuring decay away from edges of both the dimerization order parameter and transverse edge currents. The critical behavior and exponents are consistent with an Ising transition in $1+1$ dimensions. Finally, we obtain excited states in various quantum number sectors finding that the gap to a charge neutral momentum $\pi$ excitation corresponding to fluctuations of the dimerization order parameter closes in the vicinity of the critical point but gaps to other excitations remain large.

Recently, the interplay between quantum magnetism and topology has attracted growing interest, both for its fundamental importance and its technological potential. Topological magnons, quantized spin excitations with nontrivial band topology, hold particular promise for spintronics, offering routes to robust, low-dissipation devices for next-generation information processing and storage. While topological magnons in honeycomb ferromagnets with weak next-nearest-neighbor Dzyaloshinskii-Moriya interactions (DMI) have been extensively investigated, the strong-DMI regime remains largely unexplored. In this work, we examine topological magnetic phases and magnon-phonon hybridization in a two-dimensional magnetic system with strong DMI. We show that strong DMI drives a transition from a ferromagnetic ground state to a ${120}^{\circ }$ noncollinear order. An additional Zeeman field further induces noncoplanar spin textures, giving rise to a diverse set of topological phases. We demonstrate that these topological phases can be directly probed through the transverse magnon transport. Finally, we find that the spin-spin interactions in the strong-$D$ phase enable magnon-phonon coupling that yields hybridized topological bands, whereas such coupling vanishes in the weak-$D$ phase.

We investigate the rich nonequilibrium physics arising in periodically driven open quantum systems, specifically those realized in microcavity resonators, whose dynamics are governed by a non-Hermitian Hamiltonian hosting Floquet exceptional points (FEPs). By introducing a periodically quenched driving protocol, we analytically derive the Floquet effective Hamiltonian and determine the locations of multiple FEPs harbored within the Floquet bulk bands. We demonstrate that the pair production and annihilation of these FEPs can be precisely controlled by fine-tuning the system parameters, and zero and $\pi$ FEPs are topologically characterized by a robust integer-quantized winding numbers. To probe these singularities, we introduce a biorthogonal Floquet fidelity susceptibility whose value exhibits large nonzero peaks at the momentum points hosting FEPs in the Brillouin zone. Furthermore, the momentum-summed susceptibility displays a sharp divergence when the number of FEPs change with respect to the time period of the drive. Our findings also reveal the emergence of Floquet edge states around zero energy and Dirac-like dispersion around $\pi$. Moreover, our model reveals a higher-order skin effect, where the periodically driven Hamiltonian hosts skin modes localized at both edges and corners. These insights offer novel avenues for the Floquet engineering of topological singularities in driven dissipative systems, with significant potential for manipulating light and matter at the microscale.

Quasi-optical experiments are emerging as a powerful technique to probe magnetic transitions in molecular spin systems. However, the simultaneous presence of the electric- and magnetic-dipole induced transitions poses the challenge of discriminating between these two contributions. In addition, the identification of the spin-electron transitions can hardly rely on the peak intensity because of the current uncertainties on the value of the spin-electric coupling in most molecular compounds. Here, we compute the polarizations required for electric- and magnetic-dipole induced transitions through spin-Hamiltonian models of molecular spin triangles. We show that the polarization allows a clear discrimination between the two kinds of transitions. In addition, it allows one to identify the physical origin of the zero-field splitting in the ground multiplet, a debated issue with significant implications on the coherence properties of the spin qubit implemented in molecular spin triangles.

A theoretical formalism is described for understanding coherent x-ray reflectivity (CXR) from the surface of a semi-infinite crystal having a variable surface topography, described by the height profile $h$($x,y$). The surface topography is imaged as a complex “effective density,” obtained from the phasing and inversion of the coherent x-ray reflectivity data, measured through a rocking scan centered at a vertical momentum transfer $Q_z^0$ and a vertical range $\mathrm{\Delta }{Q}_z$. The formalism predicts that the effective density has an amplitude with a maximum located at the surface height for each position within the surface plane. The phase of the effective density has a lateral variation that is controlled by the surface height and a vertical variation that reflects a combination of the interfacial structure and specific choice of measurement conditions. This understanding enables direct observation of nanometer-scale interfacial topography, i.e., $h\left(x,y\right)c_s$ (where $c_s$ is the vertical substrate lattice parameter) with $\AA$-scale sensitivity to surface height. Numerical simulations illustrate and confirm the theoretical results. These results show how the interpretation of the interfacial density phase obtained by CXR data inversion (i.e., surface topography with respect to a flat surface) is conceptually similar to that previously known for Bragg coherent diffraction imaging (BCDI) measurements of isolated nanoparticles (i.e., lattice displacements with respect to an ideal crystal lattice). This suggests that CXR can be thought of as a form of dark field imaging with respect to the bright field BCDI approach. An implication of these results is that interfacial imaging may bypass some of the significant challenges associated with BCDI imaging of multiple particles having different orientations.

The ability to engineer topologically distinct materials opens the possibility of enabling novel phenomena in low-dimensional nanosystems, as well as manufacturing novel quantum devices. One of the simplest examples, the SSH model with both even and odd number of sites, demonstrates the connection between localized edge states and the topology of the system. We show that the SSH model hosts localized spin centers due to the interplay between the localized edge states and the on-site Hubbard interaction. We further show how one can engineer any number of localized spin centers within the chain by careful addition of defects. These spin centers are paired in spin-singlet or spin-triplet channels within each block separated by the defects, and together they construct an array of spin singlet and/or triplet qubits. The effect of on-site disorder on spin centers is also addressed. As this system is realizable experimentally, our findings describe a way for manipulating and engineering spin qubits and therefore provide a platform for performing many-body quantum simulations on spin excitations like magnons and triplons.

The coexistence of ferromagnetic (FM) state, out-of-plane piezoelectricity, and multiple channels for energy exchange in two-dimensional (2D) materials is highly advantageous for multifunctional ultrathin electronic devices. We predict the stable $\text{MoSe}X$ ($X$ = F, Cl, Br, I) semiconductor monolayers by first principles calculation. The ${\mathit{U}}_{\text{eff}}$ values of Mo atoms are obtained via linear fitting $\mathrm{GGA}+\left(\mathrm{U}\right)$ band gaps to match with HSE results. The magnetic configuration with the lowest energy is the FM state for the four monolayers. Among these monolayers, MoSeI exhibits the largest absolute value of out-of-plane piezoelectric strain coefficient ${\mathit{d}}_{31}$ (−0.55 pm/V). The absolute value is greater than those of the majority of 2D materials. Such as it is 4.2 times higher than ${\mathit{d}}_{31}$ of h-BN (0.13 pm/V) and 18.3 times higher than Janus TMD monolayers (0.03 pm/V). The ${\mathit{d}}_{31}$ of MoSeF and MoSeCl are 0.40 and 0.43 pm/V, respectively. Furthermore, there exists a positive correlation between the out-of-plane piezoelectric stress coefficient ${\mathit{e}}_{31}$ and the electronegativity difference ratio (${\mathit{r}}_{\text{ed}}$), which conforms to the electronegativity difference ratio effect. Intriguingly, MoSeF has multiple channels for energy exchange of holes and electrons in the bands due to the quasiequal extreme point defined in this work. This work presents four multifunctional monolayers that exhibit ferromagnetism, large out-of-plane piezoelectricity, and multiple energy exchange channels. It provides a possible approach for the design of multifunctional energy materials.

Magnetic domain walls play a critical role in the nanoscale evolution of magnetic devices. Despite early efforts, complete understanding of the micromagnetic evolution of the width $\mathrm{\Delta }$ and the type of magnetic domain walls has still remained lacking. Here, we report a combined analytical and micromagnetic simulation study and establish the scaling of the magnetic domains as a function of the exchange stiffness $A$, uniaxial perpendicular magnetic anisotropy $K_{\mathrm{u}}$, saturation magnetization $M_{\mathrm{s}}$, Dzyaloshinskii-Moriya interaction (DMI), and shape anisotropy of the magnetic device. We find that $\mathrm{\Delta }$ of both Bloch and Néel walls scales excellently with the analytical prediction of $\mathrm{\Delta }=C\sqrt{A/\left[K_{\text{u}}-{\mu }_0{M}_{\text{s}}^2(N_z-N_x{cos}^2\phi )/2\right]}$ (where $C$ is a constant depending on the definition of $\mathrm{\Delta }, \phi$ is the azimuth angle of the magnetic moment at the domain wall center, and $N_x$ and $N_z$ are the longitudinal and perpendicular demagnetization factors). The DMI is found to have little influence on the domain wall width but strongly affect the type of the domain wall. The domain wall has a Bloch configuration at zero DMI and gradually transitions to the Néel configuration upon an increase of the DMI. The shape anisotropy of the magnetic domain wall also affects the domain wall width. These results establish a comprehensive, conclusive understanding of the magnetic domain walls within spintronic devices.

Conventional spin and charge Nernst effects emerge from the spin-orbit coupling mechanism. Here we present a theoretical and computational investigation of Nernst responses in anisotropic ferromagnets, systematically analyzing contributions originating from both electrons and magnons. Through symmetry analysis, we identify giant intrinsic Nernst conductivities in MnAl(101) that persist at room temperature. Remarkably, the calculated spin and charge Nernst conductivities at 300 K reach values of $6.7\hslash \text{/}2e \mathrm{A}{\left(\mathrm{mK}\right)}^{-1}$ and 33.7 $\mathrm{A}{\left(\mathrm{mK}\right)}^{-1}$, respectively, which even exceed the spin-orbit coupling driven mechanism. Our findings establish low-$Z$ transition metal alloys as a promising materials platform for spin-caloritronic applications, while providing fundamental insights into the interplay between crystalline symmetry and transverse thermoelectric spin transport.

We present an effective description for the crystal structures of pnictogen elemental solids. In these materials, each atom contains three valence electrons in $p$ orbitals. They are shared between neighboring atoms to form valence bonds. We propose a trivalent network model on the simple cubic lattice. As a generalization of a dimer model, we impose a constraint that three dimers must touch every site. We argue that intraorbital Coulomb repulsion prohibits the formation of two adjacent, parallel dimers. This leads to a tripod-like local configuration at every site. More importantly, it forces every line of the cubic lattice to have alternating dimers and blanks. There is no dynamics as dimers cannot be locally rearranged. A bulk-boundary mapping emerges whereby bonds in the interior are fully described by Ising variables on three bounding planes—a simple example of holography that may be realized in real materials. To describe the energetics of bonding, we formulate a minimal model in terms of boundary Ising spins. Symmetries reduce the problem to that of three identical, independent, two-dimensional Ising models. An antiferromagnetic Ising ground state corresponds to the A7 structure seen in antimony and gray arsenic. An antiferromagnetic phase within a bilayer describes the structure of phosphorene. By stacking such bilayers, we obtain the A17 structure of black phosphorus. The stripe phase of the Ising models describes the cubic gauche structure of nitrogen. As a testable signature, we demonstrate that single impurities will induce long-ranged domain walls.

The interplay between quantum measurement and localization significantly alters the spreading of quantum information. In this work, we investigate monitored free fermion chains with localized potentials. With the aid of the quantum trajectory method and finite-size analysis, we numerically reveal that the Berezinskii-Kosterlitz-Thouless transition, which emerges in monitored free fermion chains, is robust in the presence of localization. To understand this, we construct a phase diagram that diverges at a boundary described by entanglement propagation. We find that the monitored system with Stark-localized decays quickly under small measurement strength, whereas the Anderson-localized system decays more slowly. By incorporating localization effects, this work advances our understanding of how measurement competes with the coherent spreading of quantum information. Our findings can potentially be realized in cold atom systems, trapped ions, and quantum dot arrays.