Magnetic anisotropy is vital for establishing the long-range magnetic order in two-dimensional systems. Here, based on the density functional theory calculation, we systematically study the magnetic anisotropy of monolayer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, a ferromagnetic insulator. We find that the exchange interaction in monolayer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is nearly isotropic and almost has no contribution to the magnetic anisotropy, as a result of the weak $p-d$ hybridization between Mn and Te atoms. We further reveal that magnetic anisotropy originates from single-ion anisotropy, leading to a ferromagnetic Curie temperature of about 20 K. Interestingly, the emergence of single-ion anisotropy cannot be induced solely by the spin-orbit coupling of Mn atoms, but also involves the spin-orbit coupling of ligand Te, as the spin-orbit coupling of Te atoms can induce changes in the local Mn-$d$ states. This behavior is very different from that in monolayer ${\mathrm{CrI}}_3$ and ${\mathrm{CrGeTe}}_3$, where the anisotropy of the exchange interaction is critical for long-range magnetic order. Our findings may provide a comprehensive understanding of the magnetic behavior in monolayer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and motivate further research on its potential applications.

Double nanocontact (NC) spin transfer vortex oscillator devices, in which NCs of 100-nm diameter have center-to-center separation ranging from 200 to 1100 nm, have been studied by means of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM). The NCs were positioned close to the edge of the top electrical contact so that the magnetization dynamics of the adjacent region could be probed optically. The electrical measurements showed different ranges of frequency operation for devices with different NC separations. For 900-nm NC separation, TRSKM showed magnetic contrast consistent with the formation of a magnetic vortex at each NC, while for 200-nm NC separation a lack of magnetic contrast near the NC region suggests that the magnetization dynamics occur closer to the NC and underneath the top contact. TRSKM also reveals the presence of additional localized dynamical features far from the NCs, which are not seen by electrical measurements; has not been reported previously for double NCs with different separations; and provides insight into how the dynamic state of the phase-locked oscillators is established and stabilized.

An easy-plane ferromagnetic insulator in a uniform external magnetic field and in contact with a phonon bath and a normal metal bath is studied theoretically in the presence of DC spin current injection via the spin Hall effect in the metal. The Keldysh path integral formalism is used to model the magnon gas driven into a nonequilibrium steady state by mismatched bath temperatures and/or electrical injection, and we analyze the magnon system in the normal (uncondensed) state, but close to the instability to Bose-Einstein condensation (BEC), within the self-consistent Hartree-Fock approximation. We find that the steady state magnon distribution function generally has a nonthermal form that cannot be described by a single effective chemical potential and effective temperature. We also show that the BEC instability in the electrically driven magnon system is signaled by a sign change in the imaginary part of the poles for long-wavelength magnon modes and by the divergence of the nonequilibrium magnon distribution function. In the presence of two bath temperatures, we find that the correlation length of the superfluid order parameter fluctuations exhibits nontrivial finite temperature crossover behaviors that are richer than the standard crossover behaviors obtained for the vacuum-superfluid transition in an equilibrium dilute Bose gas. We study the consequences of these thermal crossovers on the magnon spin conductivity and obtain an inverse square-root divergence in the spin conductivity in the vicinity of the electrically induced BEC instability. A spintronics device capable of testing our spin transport predictions is discussed.

${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ is a paradigmatic two-dimensional ferromagnet with strong uniaxial anisotropy. In this work we systematically studied the angular dependence of electrical transport properties in ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ single crystals, and uncovered an unexpected topological Hall effect (THE) in the configuration of large tilted magnetic fields. The Hall resistivity and magnetoresistance varied periodically when the external magnetic field rotated in the $ac$ (or $bc$) plane, while the THE emerged and maintained robust with fields swept across the nearby hard-magnetized $ab$ plane. The approximate in-plane THE covered the whole temperature region below $T_C(\sim 150\mathrm{K})$ and reached the maximum value at 100 K. Emergence of an internal gauge field was proposed to explain the origin of this large THE, which is generated by either the possible topological domain structure of uniaxial ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ or the noncoplanar spin structure forming during the in-plane magnetization. Our results promise to provide an alternative detection method to the in-plane skyrmion formation and may bring a brand new prospective to magnetotransport studies in condensed matter physics.

Magnetic $\left(\chi \right)$, transport $\left(\rho \right)$, and heat capacity $\left(C_{\mathrm{m}}\right)$ properties of CeIrSi are investigated to elucidate the effect of geometric frustration in this compound with trillium type structure because, notwithstanding its robust effective moment, ${\mu }_{\mathrm{eff}}\approx 2.46{\mu }_{\mathrm{B}}$, this Ce-lattice compound does not undergo a magnetic transition. In spite of that it shows broad $C_{\mathrm{m}}\left(T\right)/T$ and $\chi \left(T\right)$ maxima centered at $T_{\max }\approx 1.5$ K, while a $\rho \propto T^2$ thermal dependence, characteristic of electronic spin coherent fluctuations, is observed below $T_{\mathrm{coh}}\approx 2.5$ K. Magnetic field does not affect significantly the position of the mentioned maxima up to $\approx 1$ T, though $\chi \left(T\right)$ shows an incipient structure that completely vanishes at ${\mu }_{0}H\approx 1$ T. Concerning the $\rho \propto T^2$ dependence, it is practically not affected by magnetic field up to ${\mu }_{0}H=9$ T, with the residual resistivity ${\rho }_0\left(H\right)$ slightly decreasing and $T_{\mathrm{coh}}\left(H\right)$ increasing. These results are compared with the physical properties observed in other frustrated intermetallic compounds.

The static and dynamic properties of spin polarons in La-doped ${\text{CaMnO}}_3$ are explored theoretically, by means of an effective low-energy Hamiltonian. All parameters of the effective Hamiltonian are evaluated from first-principles theory. The Hamiltonian is used to investigate the temperature stability as well as the response to an external applied electric field, for spin polarons in bulk, surface, and as single two-dimensional layers. Technically this involves atomistic spin-dynamics simulations in combination with kinetic Monte Carlo simulations. Where a comparison can be made, our simulations exhibit excellent agreement with available experimental data and previous theory. Remarkably, we find that excellent control of the mobility of spin polarons in this material can be achieved, and that the critical parameters deciding this are the temperature and strength of the applied electrical field. We outline different technological implications of spin polarons, and point to spin polaronics as an emerging subfield of nanotechnology. In particular, we demonstrate that it is feasible to write and erase information on an atomic scale, by use of spin polarons in ${\text{CaMnO}}_3$.

The optical response of superconductors with odd-frequency Berezinskii pairing is studied. By using a simple model with a parabolic dispersion law and a nonmagnetic disorder, the spectral function, the electron density of states, and the optical conductivity are calculated for a few gap Ansätze. The spectral function and the electron density of states clearly reveal the gap for the Berezinskii pairing for the sufficiently strong frequency dependence of the order parameters. It is found that, similarly to the conventional BCS pairing, the odd-frequency gaps induce peaks in the real part of the conductivity, which, however, are sharper than in the BCS case. The magnitude and position of these peaks are determined by the frequency profile of the gap. The imaginary part of the optical conductivity for the Berezinskii pairing demonstrates sharp cusps that are absent in the case of the BCS superconductors. The corresponding results suggest that the Berezinskii pairing might allow for the optical transparency windows related to the onsets of the attenuation peaks in the real part of the conductivity. Thus, the study of the optical response not only provides an alternative way to probe the odd-frequency gaps, but can also reveal additional features of the dynamic superconducting pairing.

Ternary aluminosilicate glasses are of great interest in glass and earth sciences. The structural role of the non-network cations is not fully understood until now. Understanding the structural effect of the non-network cations is necessary for explaining their impact on the macroscopic properties of aluminosilicate glasses. In this work, we use molecular dynamics to investigate physical properties of a series of charge balanced aluminosilicate glasses. Elastic properties and the glass transition temperature were calculated. Our results are in accordance with the experimental data found in the literature. We found that elastic moduli increase with the charge balancing cations field strength (FS), while the glass transition temperature is negatively correlated to FS. The effect of the charge balancing cations field strength on the calculated properties is discussed and explained using the change in the structural properties, energetic environment of atoms, and two-body excess entropy. This allows us to get an overview of the effect of cations nature on the properties of the glass.

Skyrmion-based spintronic devices are considered one of the prime candidates for the next generation of functional devices. The mechanistic understanding of magnetomechanics arising from the intrinsic spin-lattice coupling in skyrmion materials becomes an attractive issue. In this paper, by taking MnSi as an example, we proposed an atomistic simulation model based on the developed spin-lattice dynamics scheme for skyrmion materials. The calculated magnetomechanical properties are examined to be qualitatively consistent with the experimental measurement, which suggests that our model is appropriate for the study of thermodynamics, magnetics, and mechanics of skyrmion materials. In addition, the microscopic understanding of the thermodynamic and kinetic aspects of magnetomechanics in skyrmion materials are presented. Through the intrinsic spin-lattice coupling, the transformation of magnetic ordering by external magnetic field results in the change of mechanical properties, for instance, the global mechanical anisotropy and local distortion of lattice structure. The calculation results indicate the magnetomechanics are the result of spin nonalignment induced by the synergistic effects of Dzyaloshinsky-Moriya interaction, Heisenberg exchange interaction, and Zeeman interaction of interspins and elastic interactions of interlattices. Further, the relaxation time of the lattice structure is calculated to characterize the speed of mechanical response against the magnetic ordering changes, which might provide hints of low-energy consumption and low applied electric current in driving skyrmions. The microscopic understanding provided by current study could help the research and design of skyrmion-based spintronic devices.

Space-time modulation adds another powerful degree of freedom to the manipulation of classical wave systems. It opens the door for complex control of wave behavior beyond the reach of stationary systems, such as nonreciprocal wave transport and realization of gain media. Here we generalize the transfer matrix method and use it to create a general framework to solve wave propagation problems in time-varying acoustic, electromagnetic, and electric circuit systems. The proposed method provides a versatile approach for the study of general space-time-varying systems, which allows any number of time-modulated elements with an arbitrary modulation profile, facilities the investigation of high-order modes, and provides an interface between space-time-modulated systems and other systems.

We have studied electron spin resonance (ESR) absorption spectra for the nonmagnetically diluted strong-leg spin ladder magnet (${\mathrm{C}}_7{\mathrm{H}}_{10}{\mathrm{N})}_2{\mathrm{Cu}}_{(1-x)}{\mathrm{Zn}}_x{\mathrm{Br}}_4$ (abbreviated as DIMPY) down to 450 mK. Formation of the clusters with nonzero net magnetization is confirmed; the cluster-cluster interaction is evidenced by the concentration dependence of ESR absorption. High-temperature spin-relaxation time was found to increase with nonmagnetic dilution. The ESR linewidth analysis proves that the Dzyaloshinskii-Moriya (DM) interaction remains the dominant spin-relaxation channel in diluted DIMPY. Experimental data indicate that the dilution results in the weakening of the effective DM interaction, which can be interpreted as total suppression of DM interaction in the close vicinity of impurity atom.

We study two charge-ordered states in a $t-J-U$ model with the method of renormalized mean-field theory. Compared with the $t-J$ model, which excludes the existence of one site occupied by two electrons, finite Coulomb interaction $U$ will allow the appearance of doublons and produce more charge fluctuations. The charge density wave states accompanied by Cooper pair density modulations have similar features with the states found in the $t-J$ model. The difference is that the modulation of doublons exists, and it has the same wavelength and phase as holes. Charge-ordered states disappear at the $U/t\le 9$ region when the system does not correspond to the strongly correlated resonating valence-bond state. This feature indicates that local Coulomb interactions are the origin of charge-ordered states observed in cuprates. In addition, we calculate the strength of superconducting coherence and the gap depth of the nodal pair-density-wave state based on the local density of states. These two quantities also vary with lattice sites periodically. The oscillations have same wave vector and phase as the modulation of holes. These results are consistent with recent experiments.

We present a decomposition of the two-particle vertex function of the single-band Anderson impurity model which imparts a physical interpretation of the vertex in terms of the exchange of bosons of three flavors. We evaluate the various components of the vertex for an impurity model corresponding to the half-filled Hubbard model within dynamical mean-field theory. For small values of the interaction almost the entire information encoded in the vertex function corresponds to single-boson exchange processes, which can be represented in terms of the Hedin three-leg vertex and the screened interaction. Also for larger interaction, the single-boson exchange still captures scatterings between electrons and the dominant low-energy fluctuations and provides a unified description of the vertex asymptotics. The proposed decomposition of the vertex does not require the matrix inversion of the Bethe-Salpeter equation. Therefore, it represents a computationally lighter and hence more practical alternative to the parquet decomposition.

A major obstacle to identify exotic quantum phases of matter featuring spin-charge separation above one-dimension is the lack of tailored probes allowing to establish their presence in correlated materials. Here we propose an optoelectronic response that could allow to pinpoint the presence of certain spin-charge separated states with emergent neutral gapless fermions in two and three-dimensional materials. We show that even though these states behave like insulators under static electric fields, they can display clear cyclotron resonance peaks in their light absorption spectrum under static magnetic fields, but typically the principal Kohn mode will be missing in comparison to ordinary metals. This distinctive phenomenon could be tested in materials such as triangular lattice organics, three-dimensional mixed valence insulators ${\mathrm{YbB}}_{12}$ and ${\mathrm{SmB}}_6$, and transition metal dichalcogenides 1T-${\mathrm{TaS}}_2$ and 1T-${\mathrm{TaSe}}_2$.

We investigated the topological properties and strain modulation of transition-metal monochalcogenides ${\mathrm{W}}_{2}XY(X,Y=\mathrm{S},\mathrm{Se},\mathrm{Te},X\ne Y)$ by combining the first-principles method and the Wannier-based tight-binding method. The number and chirality of Weyl node, surface spectra, and surface Fermi arcs are calculated to identify their topological properties. The ${\mathrm{W}}_{2}XY$ compounds are nodal line semimetals in the absence of spin-orbit coupling (SOC). When SOC turns on, the nodal lines split into 24 Weyl nodes on the $k_z\ne 0$ planes, which are related by time-reversal, $C_{3\mathrm{z}}$ rotational, and mirror symmetries. Weyl points’ energies are close to the Fermi level and the maximal separation between two Weyl points with opposite handedness in reciprocal space is on the order of magnitude of $0.1{\AA }^{-1}$, which can be accessed by angle-resolved photoemission spectroscopy. Interestingly, the Weyl semimetal phase exhibits great robustness and asymmetric response under uniaxial/biaxial compression and tensile strain. The positions of Weyl points change significantly and lead to a striking modulation of topological properties under in-plane biaxial strain. These findings are important for studying the behavior of Weyl fermions in layered materials and useful for realizing their applications in topological electronic devices.

Recent studies of electrical transport, both theoretical and experimental, near the bandwidth-tuned Mott metal-insulator transition have uncovered apparent quantum critical scaling of the electrical resistivity at elevated temperatures, despite the fact that the actual low-temperature phase transition is of first order. This raises the question whether there is a hidden Mott quantum critical point. Here we argue that the dynamical mean-field theory of the Hubbard model admits, in the low-temperature limit, asymptotically scale-invariant (i.e., power-law) solutions, corresponding to the metastable insulator at the boundary of the metal-insulator coexistence region; these solutions can be linked to the physics of the pseudogap Anderson model. While our state-of-the-art numerical renormalization-group calculations reveal that this asymptotic regime is restricted to very small energies and temperatures and hence is difficult to access numerically, we uncover the existence of a wide crossover regime where the single-particle spectrum displays a different power law. We show that it is this power-law regime, corresponding to approximate local quantum criticality, which is continuously connected to and responsible for the apparent quantum critical scaling above the classical critical end point. We connect our findings to experiments on tunable Mott materials.

The branched pattern typically observed through the scanning gate microscopy (SGM) of two-dimensional electron gases in the presence of weak, smooth disorder has recently been found to be robust against a very large shift in the Fermi energy of the electron gas. We propose a toy model, where the potential landscape reduces to a single localized feature, which makes it possible to recast the understanding of branch formation through the effect of caustics in an appropriate set of classical trajectories, and it is simple enough to allow for a quantitative analysis of the energy and spatial dependence of the branches. We find the energy stability to be extremely generic, as it rests only upon the assumptions of weak disorder, weak scattering, and the proportionality of the SGM response to the density of classical electron trajectories. Therefore the robustness against changes of the electron's Fermi energy remains when adopting progressively realistic models of smooth disorder.

We investigate the Kitaev-Heisenberg (KH) model at finite temperature using the exact environment full update (eeFU) [introduced by P. Czarnik et al., Phys. Rev. B 99, 035115 (2019)], which represents purification of a thermal density matrix on an infinite hexagonal lattice by an infinite projected entangled pair state (iPEPS). We show that, owing to a dynamical mapping from a hexagonal to a rhombic lattice, the eeFU on the hexagonal lattice is as efficient as the simple full update (FU) algorithm. Critical temperatures for coupling constants in the stripy and the antiferromagnetic phase are estimated. They are an order of magnitude less than the couplings in the Hamiltonian. By a duality transformation, these results can be mapped to, respectively, the ferromagnetic and zigzag phases. For the special case of the pure Kitaev model, which is tractable by quantum Monte Carlo but the most challenging for tensor networks, the algorithm is benchmarked against the Monte Carlo results. It recovers accurately the crossover to spin ordering and qualitatively the one to flux ordering.

We present 90 meV resolved Co $2p3d$ resonant inelastic x-ray scattering linear dichroism spectra of strained ${\mathrm{LaCoO}}_3$ films and a ${\mathrm{LaCoO}}_3$ single crystal. A polarization-dependent low-energy excitation is observed at $\sim 0.2\mathrm{eV}$ on the tensile-strained ${\mathrm{LaCoO}}_3/{\mathrm{SrTiO}}_3$ film, while it is not observed in either bulk ${\mathrm{LaCoO}}_3$ or the compressive-strained ${\mathrm{LaCoO}}_3/{\mathrm{LaAlO}}_3$ film. Guided by cluster calculations, we are able to distinguish the spin-state manifolds close to their transition point of ${\mathrm{Co}}^{3+}$ ions in ${\mathrm{LaCoO}}_3$ systems. Through a polarization analysis, we show that the spin state can easily flip from a low-spin ${}^1\mathrm{A}_{1g}$ state in an octahedral symmetry to the high-spin ${}^5\mathrm{B}_{2g}$ or ${}^5\mathrm{E}_g$ states with a small tetragonal distortion. A mixture of spin states suggests that the high-spin ${\mathrm{Co}}^{3+}$ plays an important role in long-range ferromagnetic order on both tensile- and compressive-strained ${\mathrm{LaCoO}}_3$ films.