By using the femtosecond angle-resolved spectroscopic imaging technique, the ultrafast buildup dynamics of room-temperature polariton condensation is explicitly visualized in a ZnO whispering gallery mode microcavity. The buildup time of polariton condensation with respect to the arrival of the femtosecond pump pulse decreases with the increasing pump power and reaches a lower limit of about 4 ps. Simulation results by numerically solving the open-dissipative Gross-Pitaevskii equation coupled to an incoherent reservoir are in quantitative agreement with the experimental observations, showing that the scattering from exciton reservoir to the lower polariton branches and the decay from these branches dominate the buildup process.

We study conditions for the emergence of the preformed Cooper pairs in materials hosting flat bands. As a particular example, we consider a semimetal, with a pair of three-band crossing points at which a flat band intersects with a Dirac cone, and focus on the $s$-wave intervalley pairing channel. The nearly dispersionless nature of the flat band at strong attraction between electrons promotes local Cooper pair formation so that the system may be modeled as an array of superconducting grains. Due to dispersive bands, Andreev scattering between the grains gives rise to the global phase-coherent superconductivity at low temperatures. We develop a mean-field theory to calculate transition temperature between the preformed Cooper pair state and the phase-coherent state for different interaction strengths in the Cooper channel. The transition temperature between semimetal and preformed Cooper pair phases is proportional to the interaction constant, the dependence of the transition temperature to the phase-coherent state on the interaction constant is weaker.

In this paper, we propose a methodology that allows disentangling the interaction of two Stone-Wales defects in a two-dimensional model material, which has the crystalline structure of a honeycomb lattice. The nonaffinity of the material response is represented as a linear superposition of nonaffine displacements caused by every individual defect and a residual nonaffine displacement field. The latter is then identified as the interaction field linked to a weakening or—as also shown—strengthening effect. We perform various numerical deformation tests in the athermal quasistatic limit on selected configurations. In doing so, we introduce a field variable to validate the contribution of the defect interaction field that indicates the mode of material failure, which also explains why some configurations with two Stone-Wales defects have a higher tensile strength than other configurations with only one defect.

In the past decades, considerable efforts have been made to understand the critical features of both classical and quantum long-range (LR) interacting models. The case of the Berezinskii-Kosterlitz-Thouless (BKT) universality class, as in the two-dimensional (2D) classical $XY$ model, is considerably complicated by the presence, for short-range interactions, of a line of renormalization group fixed points. In this paper, we discuss a field-theoretical treatment of the 2D $XY$ model with LR couplings, and we compare it with results from the self-consistent harmonic approximation. These methods lead to a rich phase diagram, where both power law BKT scaling and spontaneous symmetry breaking appear for the same (intermediate) decay rates of LR interactions. We also discuss the Villain approximation for the 2D $XY$ model with power law couplings, providing hints that, in the LR regime, it fails to reproduce the correct critical behavior. The obtained results are then applied to the LR quantum $XXZ$ spin chain at zero temperature. We discuss the relation between the phase diagrams of the two models, and we give predictions about the scaling of the order parameter of the quantum chain close to the transition.

There has been a plethora of studies on domain wall dynamics in magnetic nanostrips, mainly because of its versatile nonlinear physics and potential applications in data storage devices. However, most of the studies focus on out-of-plane domain walls or in-plane head-to-head (tail-to-tail) domain walls. Here, we numerically study the field-driven dynamics of in-plane side-by-side domain walls in ferromagnetic strips, which can be stable in the presence of an in-plane easy-axis anisotropy transverse to the strip. The domain walls move in a rigid-body manner at low field and show complex Walker breakdown behavior at high field. We observe a multistep Walker breakdown through vortex nucleation in wide strips. In the presence of Dzyaloshinskii-Moriya interaction (DMI), the first Walker breakdown field first decreases then increases with interfacial DMI, while it keeps increasing with bulk DMI. These findings complement the current understanding on domain wall dynamics.

In the quasi-two-dimensional (quasi-2D) copper- and iron-based superconductors, the onset of superconductivity is accompanied by a prominent peak in the magnetic spectrum at momenta close to the wave-vector of the nearby antiferromagnetic state. Such a peak is well described in terms of a spin resonance mode, i.e., a spin-1 exciton theoretically predicted for quasi-2D superconductors with a sign-changing gap. The same theories, however, indicate that such a resonance mode should be absent in a three-dimensional (3D) system with a spherical Fermi surface. This raises the question of the fate of the spin resonance mode in layered unconventional superconductors that are not strongly anisotropic, such as certain heavy-fermion compounds and potentially the newly discovered nickelate superconductor ${\mathrm{NdNiO}}_2$. Here, we use the random phase approximation to calculate the dynamical spin susceptibility of 3D superconductors with a $d_{x^2-y^2}$-wave gap symmetry and corrugated cylindrical-like Fermi surfaces. By varying the out-of-plane hopping anisotropy $t_z/t$, we demonstrate that the appearance of a spin resonance mode is determined by the topology of the hot lines, i.e., lines on the Fermi surface that are connected by the magnetic wave vector. For an in-plane antiferromagnetic wave vector, the hot lines undergo a topological transition from open lines to closed loops at a critical $t_z/t$ value. The closed hot lines cross the nodal superconducting lines, making the spin resonance mode overdamped and incoherent. In contrast, for an out-of-plane antiferromagnetic wave vector, the hot lines remain open and the spin resonance mode remains sharp. We discuss the experimental implications of our results for the out-of-plane dispersion of the spin resonance mode and, more generally, for inelastic neutron scattering experiments on unconventional superconductors.

We formulate the Migdal-Eliashberg theory of electron-phonon interactions in terms of classical spins by mapping the free energy to a Heisenberg spin chain in a Zeeman magnetic field. Spin components are energy-integrated normal and anomalous Green's functions and sites of the chain are fermionic Matsubara frequencies. The Zeeman field grows linearly with the spin coordinate and competes with ferromagnetic spin-spin interaction that falls off as the square of the inverse distance. The spin-chain representation makes a range of previously unknown properties plain to see. In particular, infinitely many new solutions of the Eliashberg equations both in the normal and superconducting states emerge at strong coupling. These saddle points of the free-energy functional correspond to spin flips. We argue that they are also fixed points of kinetic equations and play an essential role in far from equilibrium dynamics of strongly coupled superconductors. Up to an overall phase, the frequency-dependent gap function that minimizes the free energy must be non-negative. There are strong parallels between our Eliashberg spins and Anderson pseudospins, though the two sets of spins never coincide.

In one-dimensional Hermitian tight-binding models, mobility edges separating extended and localized states can appear in the presence of properly engineered quasiperiodical potentials and coupling constants. On the other hand, mobility edges do not exist in a one-dimensional Anderson lattice since localization occurs whenever a diagonal disorder through random numbers is introduced. Here we consider a nonreciprocal non-Hermitian lattice and show that the coexistence of extended and localized states appears with or without diagonal disorder in the topologically nontrivial region. We discuss that the mobility edges appear basically due to the boundary condition sensitivity of the nonreciprocal non-Hermitian lattice.

We investigate the elastic normal modes of two-dimensional media with broken time-reversal and parity symmetries due to a Lorentz term. Our starting point is an elasticity theory that captures the low-energy physics of a diverse range of systems such as gyroscopic metamaterials, skyrmion lattices in thin-film chiral magnets, and certain Wigner crystals. By focusing on a circular disk geometry, we analyze finite-size effects and study the low-frequency shape oscillations of the disk. We demonstrate the emergence of the Rayleigh surface waves from the bottom of the excitation spectrum and investigate how the curvature of the disk-boundary modifies their propagation at long wavelengths. Moreover, we discover a near-cyclotron-frequency wave that is almost completely localized at the boundary of the disk but is distinct from the Rayleigh wave. It can be distinguished from the latter by a characteristic excitation pattern in a small region near the center of the disk.

We study the quantum thermalization and information scrambling dynamics of an experimentally realizable quantum spin model with homogeneous XX-type all-to-all interactions and random local potentials. We identify the thermalization-localization transition by changing the disorder strength, under a proper all-to-all interaction strength. The scrambling dynamics in the localization phase shows behaviors that are distinct from that of local models. The operator scrambling grows almost equally fast in the two phases. In the thermal phase, we show that fast scrambling is exhibited without appealing to the semiclassical limit. We also briefly discuss the experimental realization of the model using superconducting qubit quantum simulators.

${\mathrm{BiCrO}}_3$ is an antiferroelectric perovskite known to exhibit an unconventional spin reorientation transition between antiferromagnetic structures, accompanied by a large jump in weak ferromagnetism. Using a combination of neutron powder diffraction, magnetometry, and symmetry analysis, we confirm the dominant G-type antiferromagnetic order below $T_{\mathrm{N}}=111$ K and identify the magnetic phase transition with a spontaneous rotation of ${\mathrm{Cr}}^{3+}$ moments from the $b$ axis to a particular direction in the $ac$ plane. We demonstrate the role of antiferroelectric displacements produced by the ${\mathrm{Bi}}^{3+}$ lone-pair electrons and octahedral rotations in establishing spin canting via the antisymmetric Dzyaloshinskii-Moriya interaction. This mechanism results in weak ferromagnetism above and below the spin-reorientation and explains the dramatic increase in net magnetization on cooling.

We present a nanoscopic numerical model of spin pumping, complete with spin-torque effects, as an extension of a spin-diffusion solver. The model is fully integrated in a micromagnetics finite-element framework. It is shown that this model can recreate analytical solutions for standard problems. Furthermore, the Gilbert damping of a propagating magnon can be properly modeled. The model can be used to examine spin-pumping effects, and those caused by it, e.g., inverse spin Hall, on a nanoscopic scale.

Eu-based Zintl-phase materials $\mathrm{Eu}{A}_{2}P{n}_2$ $(A=\mathrm{Mg},\mathrm{In},\mathrm{Cd},\mathrm{Zn};Pn=\mathrm{Bi},\mathrm{Sb},\mathrm{As},\mathrm{P})$ have generated significant recent interest owing to the complex interplay of magnetism and band topology. Here, we investigated the crystallographic, magnetic, and electronic properties of layered Zintl-phase single crystals of ${\mathrm{EuMg}}_2{\mathrm{Sb}}_2$ with the trigonal ${\mathrm{CaAl}}_2{\mathrm{Si}}_2$ crystal structure (space group $P\overline{3}m1$). Electrical resistivity measurements complemented with angle-resolved photoemission spectroscopy (ARPES) studies and density functional theory (DFT) calculations find an activated behavior with intrinsic conductivity at high temperatures indicating a semiconducting electronic ground state with a narrow energy gap of 370 meV. Magnetic susceptibility and zero-field heat capacity measurements indicate that the compound undergoes antiferromagnetic (AFM) ordering at the Néel temperature $T_{\mathrm{N}}=8.0\left(2\right)$ K. Zero-field neutron-diffraction measurements reveal that the AFM ordering is A type, where the Eu spins (${\mathrm{Eu}}^{2+}, S=\frac{7}{2}$) arranged in $ab$-plane layers are aligned ferromagnetically in the $ab$ plane and the Eu spins in adjacent layers are aligned antiferromagnetically. Eu-moment reorientation within the $ab$ planes in the trigonal AFM domains associated with a very weak in-plane magnetic anisotropy is also evident below $T_{\mathrm{N}}$ at low fields of $<0.05$ T. Although isostructural semimetallic ${\mathrm{EuMg}}_2{\mathrm{Bi}}_2$ is reported to host Dirac surface states, the observation of narrow-gap semiconducting behavior in ${\mathrm{EuMg}}_2{\mathrm{Sb}}_2$ implies a strong role of spin-orbit coupling (SOC) in tuning the electronic states of these materials. Our DFT studies also suggest that introducing the more electronegative and smaller Sb in place of Bi, besides reducing the SOC, shifts the low-lying conduction bands along the $\mathrm{\Gamma }-\mathrm{A}$ direction to higher energy, resulting in an indirect bulk band gap between the $\mathrm{\Gamma }$ and $\mathrm{M}$ points for ${\mathrm{EuMg}}_2{\mathrm{Sb}}_2$.

We present a systematic canonical quantization procedure for lumped-element superconducting networks by using a redundant configuration-space description. The algorithm is based on an original, explicit, and constructive implementation of the symplectic diagonalization of positive semidefinite Hamiltonian matrices, a particular instance of Williamson's theorem. With it, we derive canonically quantized discrete-variable descriptions of passive causal systems. We exemplify the algorithm with representative singular electrical networks, a nonreciprocal extension for the black-box quantization method, as well as an archetypal Landau quantization problem.

Understanding the nature of Cooper pairs is essential to describe the properties of superconductors. The original proposal of Bardeen, Cooper, and Schrieffer (BCS) was based on electrons pairing with same energy and zero center-of-mass momentum. With the advent of new superconductors, different forms of pairing have been discussed. In particular, Cooper pairs with finite center-of-mass momentum have received large interest. Along with such finite-momentum pairs, pairing of electrons at different energies is also central to understanding some superconductors. Here, we investigate the interplay of finite-momentum and finite-energy Cooper pairs considering two different systems: a conventional $s$-wave superconductor under applied magnetic field and a $d$-wave finite-momentum pairing state in the absence of magnetic field relevant to correlated superconductors. Investigating both these systems, we find finite-energy pairs persisting independently of finite-momentum pairing, and that they lead to odd-frequency superconducting correlations. We contrast this finding by showing that the even-frequency correlations are predominantly driven by zero-energy pairs for most frequencies. We further calculate the Meissner effect and find that odd-frequency correlations are essential for correctly describing the Meissner effect.

The compositional as well as structural asymmetries in Janus transition metal dichalcogenides (J-TMDs) and their van der Waals heterostructures (vdW HSs) induce an intrinsic Rashba spin splitting. We investigate the variation of band gaps and the Rashba parameter in three different Janus heterostructures having AB-stacked $\mathrm{Mo}XY/\text{W}XY$ $(X, Y=\mathrm{S},\mathrm{Se},\mathrm{Te};X\ne Y)$ geometry with a $Y\text{−}Y$ interface, using first-principles calculations. We consider the effect of external electric field and in-plane biaxial strain in tuning the strength of the intrinsic electric field, which leads to remarkable modifications of the band gap and the Rashba spin splitting. In particular, it is found that the positive applied field and compressive in-plane biaxial strain can lead to a notable increase in the Rashba spin splitting of the valence bands about the $\mathrm{\Gamma }$ point. Moreover, our ab initio density functional theory (DFT) calculations reveal the existence of a type-II band alignment in these heterostructures, which remains robust under positive external field and biaxial strain. These suggest novel ways of engineering the electronic, optical, and spin properties of J-TMD van der Waals heterostructures holding a huge promise in spintronic and optoelectronic devices. Detailed $\mathbf{k}·\mathbf{p}$ model analyses have been performed to investigate the electronic and spin properties near the $\mathrm{\Gamma }$ and $K$ points of the Brillouin zone.

Oxygen octahedral tilting significantly reduces the symmetry of perovskites and changes the transport properties of perovskite magnetic tunnel junctions (MTJs). We investigated ${\mathrm{SrRuO}}_3/{\mathrm{CaTiO}}_3/{\mathrm{SrRuO}}_3$ MTJs by first-principles simulation and predicted an intriguing coupling between oxygen octahedral tilting and tunneling magnetoresistance. We obtained higher spin polarization at the Fermi level for the ${\mathrm{SrRuO}}_3$ electrodes with tilted octahedra, which increases the tunneling magnetoresistance value. Transmission and complex band structure analysis shows that the majority spin tunneling current in the ${\mathrm{CaTiO}}_3$ barrier with tilted octahedra is suppressed by wave function symmetry filtering, thus producing a negative spin filtering with an efficiency close to −100% and further enhancing the TMR effect. Both phenomena disappear in the junction without oxygen octahedral tilting, hence they have resulted from lattice distortions. The effective control of the tunneling magnetoresistance via oxygen octahedral tilting could act as a way to improve the performances of perovskite spintronic devices.

Multipolar magnetic phases in correlated insulators represent a great challenge for density functional theory (DFT) due to the coexistence of intermingled interactions, typically spin-orbit coupling, crystal field and complex noncollinear and high-rank intersite exchange, creating a complected configurational space with multiple minima. Although the $+\mathrm{U}$ correction to DFT allows, in principle, the modeling of such magnetic ground states, its results strongly depend on the initially symmetry breaking, constraining the nature of order parameter in the converged $\mathrm{DFT}+\mathrm{U}$ solution. As a rule, $\mathrm{DFT}+\mathrm{U}$ calculations starting from a set of initial on-site magnetic moments result in a conventional dipolar order. A more sophisticated approach is clearly needed in the case of magnetic multipolar ordering, which is revealed by a null integral of the magnetization density over spheres centered on magnetic atoms, but with nonzero local contributions. Here we show how such phases can be efficiently captured using an educated constrained initialization of the on-site density matrix, which is derived from the multipolar-ordered ground state of an ab initio effective Hamiltonian. Various properties of such exotic ground states, like their one-electron spectra, become therefore accessible by all-electron $\mathrm{DFT}+\mathrm{U}$ methods. We assess the reliability of this procedure on the ferro-octupolar ground state recently predicted in ${\mathrm{Ba}}_{2}M{\mathrm{OsO}}_6(\mathrm{M}=\mathrm{Ca},\mathrm{Mg},\mathrm{Zn})$ [Phys. Rev. Lett. 127, 237201 (2021)].

We derive a “Wannier-Hubbard” model consisting of an array of overlapping atomic orbitals interacting via a local Coulomb interaction. Transforming to an orthogonal Wannier basis set, the resulting Hamiltonian displays long-range hopping and interactions, with new terms such as correlated hopping and ferromagnetic direct exchange, among others. We numerically study the one-dimensional version of the model at half-filling using the density matrix renormalization group method, unveiling a rich phase diagram as a function of the interaction $U$ and the overlap $s$ with metallic and ferromagnetic phases separated by a ferrimagnetic region. Our results indicate a path toward understanding emergent phases under pressure and beyond standard model Hamiltonians.

Using high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structure of the chiral helimagnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ and its temperature evolution. The comparison with ${\mathrm{NbS}}_2$ suggests that the electronic structure of ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ is strongly modified by the intercalation of Cr atoms. Our ab initio calculation, consistent with experimental results, suggests strong hybridization between Nb- and Cr-derived states near the Fermi level. In the chiral helimagnetic state (below the Curie temperature, $T_c$), we observe exchange splitting of the energy bands crossing the Fermi level, which follows the temperature evolution of the magnetic moment, suggesting a strong interaction between the conduction electrons and Cr spin moments. Interestingly, the exchange splitting persists far above $T_c$ with weak temperature dependence, in drastic contrast to the itinerant ferromagnetism described by the Stoner model, indicating the existence of short-range magnetic order. Our results provide important insights into the interplay between the electronic structure and magnetism in ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$, which is helpful for understanding the microscopic mechanism of chiral helimagnetic ordering.