Motivated by the recently discovered magnonic Hanle effect in an insulating antiferromagnet [T. Wimmer et al., Phys. Rev. Lett. 125, 247204 (2020)], we develop a spin transport theory based on low-energy waves of antiferromagnetic Néel order. These waves have two polarizations, which we describe in analogy to optics using the Stokes vector on the Poincaré sphere. We find that the polarization, which encodes the magnon spin angular momentum, changes periodically with a frequency that is nonlinear in the magnetic field. This explains the observed asymmetry in the Hanle signal as a function of the magnetic field, along with other salient experimental features. By providing an energy-resolved description of the spin injection, our theory combines the kinetic transport of magnons with the coherent dynamics of their polarization in an intuitive way. This opens a general perspective on a coherent control of magnonic spin density in collinear antiferromagnets.

Morphotropic phase boundary (MPB) ferroelectric systems are interesting and attractive for their ability to exhibit enhanced electromechanical response. In general, MPB represents a symmetry-breaking interferroelectric instability in the composition-temperature phase diagram of ferroelectric alloys. Deviating from this conventional trend, the ferroelectric alloy system $(1-x)$ ${\mathrm{BiFeO}}_3-\left(x\right){\mathrm{PbTiO}}_3$ appears to exhibit a rare phenomenon of symmetry preserving tetragonal $\left({\mathrm{T}}_1\right)$ to tetragonal $\left({\mathrm{T}}_2\right)$ isostructural/isosymmetric transition on heating. Here, we resolve this puzzle by studying structural evolution under thermal cycling, the temporal evolution of the structure under isothermal conditions, and Landau-Devonshire phenomenological calculations. We prove that (i) what appears as an isosymmetric ${\mathrm{T}}_2$ at high temperature is not an equilibrium phase, (ii) the equilibrium phase at high temperature is rather of rhombohedral symmetry, and (iii) the rhombohedral phase is precluded during the heating cycle (but stabilized during the cooling cycle) due to internal strain constraints. The understanding developed here has broader implications with regard to the role of internal strain in influencing the stability of phases and, thereby, the structure-sensitive properties of functional materials.

We performed inelastic x-ray and neutron scattering (IXS and INS) experiments for $\left(\mathrm{Ba},\mathrm{Ca}\right)\mathrm{Ti}{\mathrm{O}}_3$ single crystals to elucidate the effect of Ca doping on the atomic dynamics. Dispersion relations of transverse phonons in $\mathrm{BaTi}{\mathrm{O}}_3, \left(\mathrm{B}{\mathrm{a}}_{0.9}\mathrm{C}{\mathrm{a}}_{0.1}\right)\mathrm{Ti}{\mathrm{O}}_3$, and $\left(\mathrm{B}{\mathrm{a}}_{0.8}\mathrm{C}{\mathrm{a}}_{0.2}\right)\mathrm{Ti}{\mathrm{O}}_3$ were determined. The $A$-site–O vibrational mode as well as the acoustic mode were observed both by IXS and INS, whereas the Ti–O soft mode was detected only by INS. It was found that the $A$-site–O vibration exhibits significant softening with increasing Ca concentration, while the Ti–O soft mode is almost unaffected by the Ca doping. This finding is discussed in comparison with the polarization hysteresis loops and piezoelectric coefficients of $\left(\mathrm{Ba},\mathrm{Ca}\right)\mathrm{Ti}{\mathrm{O}}_3$. It is indicated that the softening of the $A$-site–O vibration contributes to the enhancement of the piezoelectricity of Ca-doped $\mathrm{BaTi}{\mathrm{O}}_3$-based materials.

The Mathieu equation occurs naturally in the description of vibrations or in the propagation of waves in media with a time-periodic refractive index. It is known to lead to exponential parametric instability in some regions of the parameter space. However, even in the stable region the matrix that propagates the initial conditions forward in time is non-normal, and therefore, it can result in transient amplification. By optimizing over initial conditions as well as initial time we show that significant transient amplifications can be obtained, going beyond the one simply stemming from adiabatic invariance. Moreover, we explore the monodromy matrix in more depth by studying its $\epsilon$ pseudospectra and Petermann factors, demonstrating that is the degree of non-normality of this matrix that determines the global amplifying features. In the context of wave propagation in time-varying media, this transient behavior allows us to display arbitrary amplification of the wave amplitude that is not due to exponential parametric instability.

We numerically investigate the robustness against various perturbations of measurement-induced phase transition in monitored quantum Ising models in the no-click limit, where the dynamics is described by a non-Hermitian Hamiltonian. We study perturbations that break the integrability and/or the symmetry of the model, as well as modifications in the measurement protocol, characterizing the resulting chaos and lack of integrability through the dissipative spectral form factor. We show that while the measurement-induced phase transition and its properties appear to be broadly insensitive to lack of integrability and breaking of the ${\mathbb{Z}}_2$ symmetry, a modification of the measurement basis from the transverse to the longitudinal direction makes the phase transition disappear altogether.

We propose a mathematical apparatus which converts magnetostatic (Walker) equations into a wave equation by introducing a specific definition of the “magnetic refractive index.” Its value can be manipulated by adjusting spatial distribution of the saturation magnetization or the bias magnetic field. We demonstrate that the latter can be accomplished efficiently by utilizing the bias field produced by magnetization patterns recorded onto a neighboring hard magnetic film using the current magnetic storage technology and derive a specific algorithm by which one can convert a near arbitrary two-dimensional optical device into its magnetic analogue. The wave equation formalism opens the possibility to adopt a number of optics-like analytical and numerical techniques, including those applicable to scattering, near field, diffraction and geometric (Eikonal equation) optics, as well as a generic finite difference time domain (FDTD) algorithm.

The effects of disorder on the robustness of topological magnon states of two-dimensional ferromagnetic skyrmions is investigated. It is diagnosed by evaluating a real space topological invariant, the bosonic Bott index (BI). The disorder simultaneously breaks the axially symmetric shape and the crystalline ordering of the skyrmions array. The corresponding magnonic fluctuations and band spectrum are determined in terms of magnetic field and strength of disorder. We observe the closing of the existing band gaps as the individual skyrmions start to occupy random positions. The analysis reveals that topological states (TSs) persist beyond the perturbative limit when skyrmions reach the glassy phase. In addition, the localization of topologically protected edge states is weakened by the disordered skyrmion structure with increasing localization length. Our findings shed light on the physical understanding of the coexistence of disordered magnetic textures and their topological spin fluctuations.

In the study of orbital angular momentum (OAM), the focus has been predominantly on the intra-atomic contribution. However, recent research has begun to shift towards exploring the interatomic contribution to OAM dynamics. In this paper, we investigate the orbital Edelstein effect (OEE) arising from the interatomic OAM at the edges. We explore the OAM texture within edge states and unveil the OAM accumulation at the edges using several lattice models based on the $s$ orbital. By comparing slabs with differently shaped edges, we not only clarify the role of electron wiggling motion in shaping OAM texture but also highlight the absence of bulk-boundary correspondence in the accumulation process. The topological insulator and higher-order topological insulator models further confirm these findings and provide evidence for the relationship between the higher-order topology and the OEE. Our study advances the comprehension of orbital physics and extends its scope to higher-order topological insulators.

The form of the spin-orbit coupling (SOC) Hamiltonian plays an crucial role in accurately estimating the SOC effect in real systems through first-principle calculations. In this study, we investigate the spin transport properties of $\mathrm{Fe}\left|\mathrm{MgO}\right|\mathrm{Fe}$ magnetic tunnel junctions (MTJs) from atomic first principles, using two commonly employed SOC Hamiltonians and a modified SOC Hamiltonian. Our study reveals substantial enhancement in the tunnel conductance when the SOC effect is considered, inducing resonant states close to the center of two-dimensional Brillouin zone. Moreover, significant discrepancies in the angular-dependent conductances and tunnel magnetoresistance (TMR) of specular $\mathrm{Fe}\left|\mathrm{MgO}\right|\mathrm{Fe}$ MTJs were observed. The findings suggest that careful consideration should be given to the choice of SOC Hamiltonian in first-principle calculations, particularly the magnetic systems with specular interfaces.

We investigate the effects of a Laguerre-Gaussian (LG) beam on the superconducting state. We show that the vortex angular momentum of a LG beam affects the superconducting state and induces currents. The induction of the current by microwave radiation is illustrated on a Josephson loop and SQUID devices. In particular, we establish that coupling a dc SQUID to the ac magnetic flux of a LG beam can stabilize the $\pi$ phase in the SQUID. This can happen via developing a global or local minimum in the effective potential at $\pi$. In the latter case, this happens via the Kapitza mechanism.

Resonant transport occurs when there is a matching of frequencies across some spatial medium, increasing the efficiency of shuttling particles from one reservoir to another. We demonstrate that in a periodically driven, many-body titled lattice, there are sets of spatially fractured resonances. These “emanate” from two essential resonances due to scattering off internal surfaces created when the driving frequency and many-body interaction strength vary, a scattering reminiscent of lens flare. The confluence of these fractured resonances dramatically enhances transport. At one confluence, the interaction strength is finite and the essential resonance arises due to the interplay of interaction with the counter-rotating terms of the periodic drive. We discuss the origin and structure of the fractured resonances, as well as the scaling of the conductance with system parameters. These results furnish a new example of the richness of open, driven, many-body systems.

We consider a refined version of the string-net model which assigns a different energy cost to each plaquette excitation. Using recent exact calculations of the energy-level degeneracies we compute the partition function of this model and investigate several thermodynamical quantities. In the thermodynamic limit, we show that the partition function is dominated by the contribution of special particles, dubbed pure fluxons, which trivially braid with all other (product of) fluxons. We also analyze the behavior of Wegner-Wilson loops associated to excitations and show that they obey an area law, indicating confinement, for any finite temperature except for pure fluxons that always remain deconfined. Finally, using a recently proposed conjecture, we compute the topological mutual information at finite temperature, which features a nontrivial scaling between system size and temperature, similar to the one-dimensional classical Ising model.

A fundamental question about the nature of quantum materials such as high-$T_c$ systems remains open to date: it is unclear whether they are (some variety of) Fermi liquids, or (some variety of) non Fermi liquids. A direct avenue to determine their nature is to study the (imaginary part of the) self-energy at low energies. Here we present a method to extract this low-$\omega$ self-energy from experimentally derived spectral functions. The method seems suited for implementation with high-quality angle-resolved photoemission data. It is based on a helpful theorem proposed here, which assures us that the method has minimal (or vanishing) error at the lowest energies. We provide numerical examples showing that a few popular model systems yield distinguishably different low-energy self-energies.

Vorticity in polariton systems has been traditionally investigated within the semiclassical mean-field approach. In this work, we show that the fully quantized picture describes richer dynamics for the vortex core in the limit of single particle excitation. The quantum correlations intrinsic to our formalism account for the emergence of a family of trajectories that differ from the circular paths known in macroscopic vorticity phenomena. An analysis of the system dynamics reveals a correspondence between these nontrivial trajectories and the light-matter correlations, which define the hybrid quasiparticles. Our results provide a direct criterium to benchmark light-matter entanglement in polaritonic systems. Finally, we propose a concrete experimental realization that can be achieved within the current capabilities of experimental condensed matter physics.

The oxyselenide $\mathrm{K}{\mathrm{V}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$, featuring an anti-${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4$-type layered structure, was successfully synthesized using a self-flux method. Comprehensive structural, electrical, and heat transport, as well as magnetism measurements, were performed on both single crystals and polycrystals. At 105 K, a density-wave (DW)-like anomalous transition is revealed in temperature-dependent resistivity and Hall coefficient, signifying possibly the opening of DW gaps. This transition is also confirmed by the temperature dependence of magnetic susceptibility and specific heat as a second-order transition. No superstructural peaks and peak splitting were observed in the conventional x-ray diffraction spectra, indicating either a rather small distortion amplitude or a more complicated mechanism than charge- or spin-density waves. Additionally, measurements on the magnetotransport properties of $\mathrm{K}{\mathrm{V}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$ single crystals under a magnetic field up to 16 T have been conducted. Clear Shubnikov–de Haas (SdH) oscillations are observed, and parameters of the Fermi surface (FS), such as Fermi vector and effective mass, are calculated. Multiple FS sheets contribute to the SdH oscillations with frequencies $F_{\alpha }=5\mathrm{T}, F_{\delta }=62\mathrm{T}, F_{\beta }=220\mathrm{T}, F_{\gamma }=364\mathrm{T}$, and $F_{\eta }=586\mathrm{T}$, respectively. Among them, $F_{\alpha }$ corresponds to a very small FS area and cyclotron effective mass, which is expected for Dirac or Weyl points. Beyond the quantum limit of α-FS, an observation of negative magnetoresistance is noted. A linear fit to the Landau level fan diagrams yields values of Berry phase 0.97π (1.17π) for $F_{\alpha }$ ($F_{\beta }$), indicating the existence of nontrivial band structures in $\mathrm{K}{\mathrm{V}}_2\mathrm{S}{\mathrm{e}}_2\mathrm{O}$.

Efficient and accurate algorithms for partition function, free energy, and thermal entropy calculations is of great significance in statistical physics and quantum many-body physics. Here we present an unbiased but low-technical-barrier algorithm within the quantum Monte Carlo framework, which has exceptionally high accuracy and no systemic error. Compared with the conventional specific heat integral method and Wang-Landau sampling algorithm, our method can obtain a much more accurate result of the subleading coefficient of the entropy consistent with theory. This method can be widely used in both classical and quantum Monte Carlo simulations and is easy to be parallelized on a computer.