Motivated by the observation of Skyrmion-like magnetic textures in two-dimensional (2D) itinerant ferromagnets ${\mathrm{Fe}}_n{\mathrm{GeTe}}_2$ ($n\ge 3$), we develop a microscopic model combining itinerant magnetism and spin-orbit coupling on a triangular lattice. The ground state of the model in the absence of magnetic field consists of filamentary magnetic domain walls revealing a striking similarity with our magnetic force microscopy experiments on ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. In the presence of magnetic field, these filaments were found to break into large-size magnetic bubbles in our experiments. We identify uniaxial magnetic anisotropy as an important parameter in the model that interpolates between magnetic Skyrmions and ferromagnetic bubbles. Consequently, our work uncovers new topological magnetic textures that merge properties of Skyrmions and ferromagnetic bubbles.

Chain-mapping techniques combined with the time-dependent density matrix renormalization group are powerful tools for simulating the dynamics of open quantum systems interacting with structured bosonic environments. Most interestingly, they leave the degrees of freedom of the environment open to inspection. In this work, we fully exploit the access to environmental observables to illustrate how the evolution of the open quantum system can be related to the detailed evolution of the environment it interacts with. In particular, we give a precise description of the fundamental physics that enables the finite temperature chain-mapping formalism to express dynamical equilibrium states. Furthermore, we analyze a two-level system strongly interacting with a super-Ohmic environment, where we discover a change in the spin-boson ground state that can be traced to the formation of polaronic states.

The role of electron-phonon scattering in thermoelectric transport has been paid much attention, especially in multivalley systems. By investigating a two-valley model with electron-phonon coupling, we find three electron transport regimes realized by electron-hole asymmetry of the electron relaxation time due to the strong intervalley scattering. The Seebeck coefficient denotes an electron-hole dichotomy due to this asymmetry. Also, the strong intervalley scattering can enhance the power factor. Our finding sheds light on unexplored thermoelectric transport under strong electron-phonon scattering.

${\mathrm{Fe}}_{n=4,5}{\mathrm{GeTe}}_2$ exhibits quasi-two-dimensional properties as a promising candidate for a near-room-temperature ferromagnet, which has attracted great interest. In this work, we notice that the crystal lattice of ${\mathrm{Fe}}_{n=4,5}{\mathrm{GeTe}}_2$ can be approximately regarded as being stacked by three bipartite crystal lattices. By combining the model Hamiltonians of bipartite crystal lattices and first-principles calculations, we investigate the electronic structure and the magnetism of ${\mathrm{Fe}}_{n=4,5}{\mathrm{GeTe}}_2$. We conclude that flat bands near the Fermi level originate from the bipartite crystal lattices and that these flat bands are expected to lead to the itinerant ferromagnetism in ${\mathrm{Fe}}_{n=4,5}{\mathrm{GeTe}}_2$. Interestingly, we also find that the magnetic moment of the Fe5 atom in ${\mathrm{Fe}}_5{\mathrm{GeTe}}_2$ is distinct from the other Fe atoms and is sensitive to the Coulomb interaction $U$ and external pressure. These findings may be helpful to understand the exotic magnetic behavior of ${\mathrm{Fe}}_{n=4,5}{\mathrm{GeTe}}_2$.

Charge noise in semiconducting quantum dots has been observed to have a $1/f$ spectrum. We propose a model in which a pair of quantum dots are coupled to a 2D bath of fluctuating two level systems (TLS) that have electric dipole moments and that interact with each other, i.e., with the other fluctuators. These interactions are primarily via the elastic strain field. We use a 2D nearest-neighbor Ising spin glass to represent these elastic interactions and to simulate the dynamics of the bath of electric dipole fluctuators in the presence of a ground plane representing metal gates above the oxide layer containing the fluctuators. The interactions between the TLS cause the energy splitting of individual fluctuators to change with time. We calculate the resulting fluctuations in the electric potential at the two quantum dots that lie below the oxide layer. We find that $1/f$ electric potential noise spectra at the quantum dots and cross correlation in the noise between the two quantum dots are in qualitative agreement with experiment. Our simulations find that the cross correlations decrease exponentially with increasing quantum dot separation.

Magnetic topological materials have attracted much attention due to the interplay between magnetism and topological electronic band structure, which may not only generate new exotic quantum states but also bring great potential applications. Here, we present Seebeck and Nernst effects in the magnetic Weyl semimetal NdAlSi as a function of temperature and magnetic field. We find that the power factor reaches $15\mu \mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ at 300 K and zero field, which compare favorably with those of traditional heavily doped semiconductors. We also find that the anomalous Nernst signals dominate the magneto-thermoelectric properties. In particular, we find that both Seebeck and Nernst effects oscillate clearly as a function of temperature in a constant magnetic field, in addition to the ordinary quantum oscillations with periodicity in inverse magnetic field at a constant temperature. Such new type of temperature-induced oscillations only appears at the narrow field-induced crossover region from a paramagnetic to a polarized paramagnetic phase and arise from the band exchange splitting due to the strong exchange interaction between conductive electrons and $4f$ electrons in NdAlSi. Our findings not only demonstrate the potential application of NdAlSi, but also greatly enrich the available quantum phenomena in thermoelectric response.

We show that a spin wave acquires a polarization-dependent geometric phase along a cyclic trajectory of noncoplanar magnetizations in antiferromagnets. Specifically, we demonstrate that a cyclic set of ${90}^{\circ }$ antiferromagnetic domain walls simultaneously introduces geometric and dynamic phases to spin waves, which thus leads to an asymmetric magnitude of the overall phase for left/right circular components. Based on the polarization-dependent phase, we propose theoretically and confirm by micromagnetic simulations that a Mach-Zehnder interferometer with cyclic ${90}^{\circ }$ domain walls in one arm and a homogeneous domain in the other arm, naturally acts as a spin-wave circular polarizer. Moreover, the circular polarizer has intrinsic nonreciprocity, which filters opposite polarization in the opposite propagation direction.

Recent experiments in the doped cuprates ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ have revealed the emission of anomalous terahertz radiation after impulsive optical excitation. Here, we theoretically investigate the nonlinear electrodynamics of such striped superconductors and explore the origin of the observed radiation. We argue that photoexcitation is converted into a photocurrent by a second-order optical nonlinearity, which is activated by the breaking of inversion symmetry in certain stripe configurations. We point out the importance of including umklapp photocurrents modulated at the stripe periodicity itself, which impulsively drive surface Josephson plasmons and lead to a resonant structure of outgoing radiation, consistent with the experiments. We speculate on the utility of the proposed mechanism in the context of generating tunable terahertz radiation.

We predict a photoinduced Hall effect in an isotropic conventional two-dimensional superconductor with a built-in supercurrent exposed to a circularly polarized light. This second-order with respect to the electromagnetic field amplitude effect occurs when the frequency of the field exceeds double the value of the superconducting gap. It reveals itself in the emergence of a Cooper-pair condensate flow in the direction transverse to the initial built-in supercurrent, which arises to compensate for the light-induced electric current of quasiparticles photoexcited across the gap. The initial supercurrent breaks both the time-reversal and inversion symmetries, while the presence of dilute disorder in the sample provides the breaking of the Galilean invariance. We develop a microscopic theory of the supercurrent Hall effect in the case of weak disorder and show that the Hall supercurrent is directly proportional to the quasiparticle recombination time, which can acquire large values.

The study of the nonlinear anomalous Hall effect (NLAHE) in $\mathcal{P}\mathcal{T}$-symmetric systems has focused on intrinsic mechanisms. Here, we show that disorder contributes substantially to NLAHE and often overwhelms intrinsic terms. We identify terms to zeroth order in the disorder strength involving the Berry curvature dipole, skew scattering, and side jump, all exhibiting a strong peak as a function of the Fermi energy, a signature of interband coherence. Our results suggest NLAHE at experimentally relevant transport densities in $\mathcal{P}\mathcal{T}$-symmetric systems is likely to be extrinsic.

The existence of spontaneous magnetization that fingerprints a ground-state ferromagnetic order was recently observed in two-dimensional (2D) van der Waals materials. Despite progress in the fabrication and manipulation of the atom-thick magnets, investigation of nanoscale magnetization properties is still challenging due to the concomitant technical issues. We propose a promising approach for a direct visualization of the domain walls formed in 2D magnetic materials. By interfacing a 2D magnet with a transition metal dichalcogenide (TMD) monolayer, the strong proximity effects enable pinning the TMD excitons on the domain wall. The emergent localization stems from the proximity-induced exchange mixing between spin-dark and spin-bright TMD excitons due to the local in-plane magnetization characteristic of the domain wall in the magnetic monolayer.

The non-Hermitian skin effect under open boundary conditions is widely believed to originate from the intrinsic spectral topology under periodic boundary conditions. If the eigenspectra under periodic boundary conditions have no spectral windings (e.g., piecewise arcs) or a finite area on the complex plane, there will be no non-Hermitian skin effect with open boundaries. In this article, we demonstrate another scenario beyond this perception by introducing a two-dimensional periodically driven model. The effective Floquet Hamiltonian lacks intrinsic spectral topology and is proportional to the identity matrix (representing a single point on the complex plane) under periodic boundary conditions. Yet, the Floquet Hamiltonian exhibits a second-order skin effect that is robust against perturbations and disorder under open boundary conditions. We further reveal the dynamical origin of these second-order skin modes and illustrate that they are characterized by a dynamical topological invariant of the full time-evolution operator.

We introduce a continuous-time model of many-body quantum dynamics based on infinitesimal random unitary operations, combined with projective measurements. We consider purification dynamics in this model, where the system is initialized in a mixed state, which then purifies over time as a result of the measurements. By mapping our model to a family of effective 1D quantum Hamiltonians, we are able to derive analytic expressions that capture how the entropy of the system decays in time. Our results confirm the existence of two distinct dynamical phases, where purification occurs over a timescale that is exponential versus constant in system size. We compare our analytic expressions for this microscopic model to results derived from field theories that are expected to capture such measurement-induced phase transitions and find quantitative agreement between the two.

${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, the first confirmed intrinsic antiferromagnetic topological insulator, has garnered increasing attention in recent years. Here we investigate the energy correction and lifetime of magnons in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ caused by magnon-magnon interaction. First, a calculation based on the density functional theory was performed to get the parameters of the magnetic Hamiltonian of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Subsequently, the perturbation method of many-body Green's function was employed and the first-order self-energy $\left[{\mathrm{\Sigma }}^{\left(1\right)}\left(\mathit{k}\right)\right]$ and second-order self-energy $\left[{\mathrm{\Sigma }}^{\left(2\right)}(\mathit{k},{\varepsilon }_{\mathit{k}})\right]$ of magnon were obtained. Numerical computations reveal that the corrections from both ${\mathrm{\Sigma }}^{\left(1\right)}\left(\mathit{k}\right)$ and ${\mathrm{\Sigma }}^{\left(2\right)}(\mathit{k},{\varepsilon }_{\mathit{k}})$ strongly rely on momentum and temperature, with the energy renormalization near the Brillouin zone (BZ) boundary being significantly more pronounced than that near the BZ center. Furthermore, our findings indicate the occurrence of dip structures in the renormalized magnon spectrum near the $\mathrm{K}$ and $\mathrm{M}$ points. These dip structures are determined to be attributed to the influence of ${\mathrm{\Sigma }}^{\left(2\right)}(\mathit{k},{\varepsilon }_{\mathit{k}})$.

The coupling between magnons and phonons modulates the transport properties of magnons, resulting in magnon polarons and magnon Seebeck effects, etc. Magnons possess well-defined angular momentum, while phonons are usually composed of linear atomic oscillations lacking angular momentum. However, in chiral systems, phonons become elliptical or circular; thus they will carry angular momentum. In this paper, we emphasize angular momentum and its role in the magnon-phonon coupling. Using a coupled-magnon-phonon model on the hexagonal lattice, we investigate the coupling of achiral and chiral phonons with three types of magnons, namely a magnon on one sublattice, ferromagnetic magnons, and antiferromagnetic magnons. We calculate the magnon-phonon dispersion relations and the strength of magnon-phonon coupling in the reciprocal space. We are particularly interested in the $K$ and $K^{\prime }$ points at the corner of the first Brillouin zone where the normal modes involve circular rotations of magnetization and displacement. We find a selective coupling wherein the coupling between magnons and phonons vanishes either when the magnitude of the magnon or phonon modes is zero or when the circular motion of magnetization is opposite to that of the displacements. These findings pave the way towards manipulating magnon polaron devices based on phonon angular momentum.

The influence of different Dzyaloshinskii-Moriya interaction (DMI) tensor components on the static and dynamic properties of domain walls (DWs) in magnetic nanowires is investigated using one-dimensional collective coordinates models and micromagnetic simulations. It is shown how the different contributions of the DMI can be compactly treated by separating the symmetric traceless, antisymmetric, and diagonal components of the DMI tensor. First, we investigate the effect of all different DMI components on the static DW tilting in the presence and absence of in plane (IP) fields. We discuss the possibilities and limitations of this measurement approach for arbitrary DMI tensors. Secondly, the interplay of different DMI tensor components and their effect on the field driven dynamics of the DWs are studied and reveal a nontrivial effect of the Walker breakdown field of the material. It is shown how DMI tensors combining diagonal and off-diagonal elements can lead to a nonlinear enhancement of the Walker field, in contrast with the linear enhancement obtainable in the usual cases (interface DMI or bulk DMI).

We investigate the electronic and magnetic properties of the synthesized double perovskites ${\mathrm{Y}}_2{\mathrm{NiIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{NiIrO}}_6$, using density functional calculations, crystal field theory, superexchange pictures, and Monte Carlo simulations. We find that both systems are antiferromagnetic (AFM) Mott insulators, with the high-spin ${\mathrm{Ni}}^{2+}{t}_{2g}^6{e}_g^2$ ($S=1$) and the low-spin ${\mathrm{Ir}}^{4+}{t}_{2g}^5$ ($S=1/2$) configurations. We address that their lattice distortion induces $t_{2g}\text{−}{e}_g$ orbital mixing and thus enables the normal ${\mathrm{Ni}}^{+}\text{−}{\mathrm{Ir}}^{5+}$ charge excitation with the electron hopping from the Ir ‘$t_{2g}$’ to Ni ‘$e_g$’ orbitals, which promotes the AFM ${\mathrm{Ni}}^{2+}\text{−}{\mathrm{Ir}}^{4+}$ coupling. Therefore, the increasing $t_{2g}\text{−}{e}_g$ mixing accounts for the enhanced $T_{\mathrm{N}}$ from the less distorted ${\mathrm{La}}_2{\mathrm{NiIrO}}_6$ to the more distorted ${\mathrm{Y}}_2{\mathrm{NiIrO}}_6$. Moreover, our test calculations find that in the otherwise ideally cubic ${\mathrm{Y}}_2{\mathrm{NiIrO}}_6$, the ${\mathrm{Ni}}^{+}\text{−}{\mathrm{Ir}}^{5+}$ charge excitation is forbidden, and only the abnormal ${\mathrm{Ni}}^{3+}\text{−}{\mathrm{Ir}}^{3+}$ excitation gives a weakly ferromagnetic behavior. Furthermore, we find that, owing to the crystal field splitting, Hund exchange, and broad band formation in the highly coordinated fcc sublattice, ${\mathrm{Ir}}^{4+}$ ions are not in the $j_{\mathrm{eff}}=1/2$ state but in the $S=1/2$ state carrying a finite orbital moment by spin-orbit coupling. This paper clarifies the varying magnetism in ${\mathrm{Y}}_2{\mathrm{NiIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{NiIrO}}_6$ associated with the lattice distortions.

In this work, on the basis of first-principles calculations we demonstrate that magnetic exchange frustration caused spin spiral can generate an electric polarization $\left(31.46\mu \mathrm{C}/{\mathrm{m}}^2\right)$ in two-dimensional (2D) $\mathrm{Mn}{\mathrm{Re}}_2{\mathrm{O}}_8$, which, therefore, can be identified as a very rare type-II multiferroic material with robust magnetoelectric coupling. It is of high interest that the ferroelectricity is out of plane, explained by the mechanism of $\mathit{P}\propto {\mathit{S}}_{\mathrm{a}}\times {\mathit{S}}_{\mathrm{b}}$ in the generalized spin-current model. We propose that compressive strain can improve the Néel temperature (from 31 to 63 K) as well as the electric polarization in terms of enhancing the supersuperexchange interaction for this in-plane noncollinear ${120}^{\circ }$-ordered antiferromagnetic order. The spin spiral chirality can be flexibly switched by a vertical electric field, that is, the polarization-chirality locking effect. It can be confirmed conclusively that our findings deepen the understanding on the magnetoelectric physics, and open a different avenue for magnetoelectric and magneto-optical applications based on 2D type-II multiferroic materials.

Investigating hydrogen bond symmetrization in hydroxyl compounds necessitates the precise determination of crystal microstructures. Nonetheless, H atoms are nearly indistinguishable in experiments, posing a challenge to unraveling the formation mechanism of this phenomenon. A deep learning potential model was used for classical molecular dynamics simulations of the hydrogen bond symmetrization process of $\varepsilon \text{-FeOOH}$ in this study. Through calculations of the H–O bond length, it has been determined that the system undergoes hydrogen bond symmetrization when the pressure reaches 40.25 GPa. The volume thermal expansion curve of $\varepsilon \text{-FeOOH}$ exhibits anomalies due to the proton-disordering phase transition, and the pressure for this transition shows a negative correlation with temperature. The calculated results of the ${\mathrm{O}}_1\cdots {\mathrm{O}}_2$ bond length indicate that an increase in temperature will lead to an increase in the critical pressure for hydrogen bond symmetrization while reducing the distinction between the hydrogen bond symmetrization structure and the proton-disordered structure. In addition, the spin transition of Fe atoms at lower temperatures is unrelated to hydrogen bond symmetrization. However, with increasing temperature, the spin transition may potentially promote hydrogen bond symmetrization.