We report the magnetoresistance (MR), de Haas–van Alphen (dHvA) oscillations, and first-principles calculation studies on topological semimetal candidates ${\mathrm{Ba}X}_4$ $(X=\mathrm{Ga}, \mathrm{In})$. Large unsaturated MR that reaches $3\times {10}^3\%$ in ${\mathrm{BaGa}}_4$ and $4\times {10}^4\%$ in ${\mathrm{BaIn}}_4$ are observed, which is due to the carrier compensations and high mobility. Evident dHvA oscillations are detected with $B//\left[001\right]$ configuration, from which the quantum mobilities are extracted. The light cyclotron effective masses are extracted from the fitting of the thermal damping term in the LK formula. According to the first-principles calculations, ${\mathrm{Ba}X}_4$ possesses several Dirac points in the absence of spin-orbit coupling (SOC), some of which open negligible gaps when SOC is considered. The $Z_2$ invariants of ${\mathrm{Ba}X}_4$ are equal to 1, indicating the nontrivial topological properties.

Macroscopic spin ensembles with brainlike features such as nonlinearity, stochasticity, self-oscillations, memory effects, and plasticity, form attractive platforms for neuromorphic computing. We propose an artificial neural network consisting of electric contacts on conducting films with tunable magnetic textures that is superior to conventional implementations, because it does not require resource-demanding external computations during training. Simulations show that the feedback between anisotropic magnetoresistance and current-induced spin-transfer torque in malleable magnetic textures autonomously trains the network according to the Hebbian learning principle. We illustrate the idea by simulating the pattern recognition by a four-node Hopfield neural network.

Many-electron multiconfigurational ab initio calculations are combined with x-ray spectroscopy to scrutinize a popular model for electron transfer in lanthanide-doped crystals which hypothesizes that the electrons are conveyed by the conduction band of the host. Contrary to this accepted picture, our combined theoretical-experimental effort shows that the reversible electron phototransfer from ${\mathrm{Eu}}^{2+}$ to ${\mathrm{Sm}}^{3+}$ in ${\mathrm{CaF}}_2$ is direct, from metal to metal. It is theoretically predicted and experimentally verified that visible light induces the reverse electron transfer.

The crystal structures of $\mathrm{CaB}\text{−}{\mathrm{H}}_n$ compounds with $n=1–12$ in a pressure range of 50–300 GPa were studied using the genetic algorithm method and first-principles density-function theory calculations. Stable structures with stoichiometry of $\mathrm{CaB}{\mathrm{H}}_6$ and $\mathrm{CaB}{\mathrm{H}}_7$ were predicted in different pressure range. $\mathrm{B}{\mathrm{H}}_4$, $\mathrm{B}{\mathrm{H}}_5$, and $\mathrm{B}{\mathrm{H}}_6$ units were found to be the main motifs in these compounds. Moreover, metastable $Imm2$ $\mathrm{CaB}{\mathrm{H}}_7$ is dynamically stable above 180 GPa, with the formation of tetrahedral $\mathrm{B}{\mathrm{H}}_4$ unit surrounded by Ca atom and ${\mathrm{H}}_3$ unit. Electron-phonon coupling (EPC) calculations reveal that the superconducting properties are closely related to the strong hydrogen-boron bonding of the $\mathrm{B}{\mathrm{H}}_4$ unit in $Imm2$, and $T_c$ can reach ∼200 K at 200 GPa. As the major units, $\mathrm{B}{\mathrm{H}}_4$, $\mathrm{B}{\mathrm{H}}_5$, and $\mathrm{B}{\mathrm{H}}_6$ units exist in the $Imm2$, $P_2$, and $P{2}_1/m$ phases of $\mathrm{CaB}{\mathrm{H}}_7$ at 300 GPa, respectively; the corresponding EPC parameter λ decreases with the increase of hydrogen content in $\mathrm{CaB}{\mathrm{H}}_7$.

Engineering single atomic defects into wide-band-gap materials has become an attractive field in recent years due to emerging applications such as solid-state quantum bits and sensors. The simplest atomic-scale defect is the lattice vacancy, which is often a constituent part of more complex defects such as the nitrogen-vacancy (NV) center in diamond, therefore an understanding of the formation mechanisms and precision engineering of vacancies is desirable. We present a theoretical and experimental study into the ultrafast laser generation of vacancy-interstitial pairs (Frenkel defects) in diamond. In a range of other materials, Frenkel defect formation has previously been linked to the recombination of laser generated excitonic states, however the mechanism in diamond is currently unknown and to date no quantitative agreement has been found between experiment and theory. Here, we find that a model for Frenkel defect generation via the recombination of a bound biexciton as the electron plasma cools provides good agreement with experimental data. The process is described by a set of coupled rate equations of the pulsed laser interaction with the material and of the non-equilibrium dynamics of charge carriers during and in the wake of the pulse.

We establish the theoretical foundation of the Floquet graphene antidot lattice, whereby massless Dirac fermions are driven periodically by a circularly polarized electromagnetic field while having their motion excluded from an array of nanoholes. The properties of interest are encoded in the quasienergy spectra, which are computed nonperturbatively within the Floquet formalism. We find that a rich Floquet phase diagram emerges as the amplitude of the drive field is varied. Notably, the Dirac dispersion can be restored in real time relative to the gapped equilibrium state, which may enable the creation of an optoelectronic switch or a dynamically tunable electronic waveguide. As the amplitude is increased, the ability to shift the quasienergy gap between high-symmetry points can change which crystal momenta dominate in the scattering processes relevant to electronic transport and optical emission. Furthermore, the bands can be flattened near the $\mathrm{\Gamma }$ point, which is indicative of selective dynamical localization. Lastly, quadratic and linear dispersions emerge in orthogonal directions at the $M$ point, signaling a Floquet semi-Dirac material. Importantly, all our predictions are valid for experimentally accessible near-IR radiation, which corresponds to the above bandwidth limit for the graphene antidot lattice. Cycling between engineered Floquet electronic phases may play a key role in the development of next-generation on-chip devices for optoelectronic applications.

There is increasing interest in novel magnetoelectric (ME) materials that exhibit robust ME coupling at room temperature (RT) for advanced memory, energy, spintronics, and other multifunctional device applications, by making use of the ability to control polarization with a magnetic field and/or magnetization via an electric field. Obtaining ME materials with strong ME coupling, understanding the origin, and manipulating its processing along with composition to realize large ME coefficients at RT constitute an important step in multiferroic research. To address this, we have investigated the multiferroic and ME properties of Ni-doped $\mathrm{Pb}\left({\mathrm{Zr}}_{0.20}{\mathrm{Ti}}_{0.80}\right){\mathrm{O}}_3$ (PZT). We find that the ferroelectric $\left(T_{\mathrm{C}}\sim 700\mathrm{K}\right)$ and weak ferromagnetic (∼602 K) phase transitions of Ni-doped PZT are well above room temperature (RT), leading to a strong ME coupling coefficient (${\alpha }_{E,31}$) of $11.7\mathrm{mV}\mathrm{c}{\mathrm{m}}^{–1}{\mathrm{Oe}}^{–1}$ ($H_{\mathrm{ac}}=1\mathrm{Oe}$ and $f=1\mathrm{kHz}$). While x-ray diffraction suggests a single-phase material, high-resolution transmission electron microscopy reveals regions with and without Ni present; thus magnetoelectric coupling between two phases is possible. First-principles calculations suggest the $(\mathrm{N}{\mathrm{i}}_{\mathrm{Pb}}{)}^{\times }$ defect is likely to be responsible for the experimental observed magnetism and ME coupling in Ni-doped PZT. We further demonstrate that Ni-doped PZT exhibits low loss tangent, low leakage current, large saturation polarization, and weak ferromagnetism. Ultimately, our work demonstrates that Ni-doped PZT is a cost-effective RT multiferroic with strong ME coupling.

Nuclear-spin systems in manganites, having separation of ferromagnetic and antiferromagnetic phases, which are manifested as two lines in the NMR spectrum, are studied. Taking into account the Ising nuclear interaction, we obtain an analytical description of Rabi oscillations and free-induction decay in two-component nuclear-spin systems when a radio-frequency pulse nonresonantly excites the nuclei of one magnetic phase and resonantly excites the nuclei of the other phase. It is revealed that the nonlinearity of interaction between the nuclei of these phases results in additional harmonics in the Rabi oscillations of the nonresonantly excited subsystem. We also show that the Ising interaction inside this subsystem forms multiple echoes in the free-induction decay, whereas their nonmonotonic damping and amplitude modulation is caused by the spin coupling between the subsystems.

Under certain conditions, spin waves can be channeled into a broad angular spectrum of wave vectors, where the direction of the group velocity becomes independent of those wave vectors. Such highly focused waves are called caustic waves, whose properties can be manipulated by anisotropies or chiral interactions, like the Dzyaloshinskii-Moriya interaction. In this paper, we theoretically study the focusing features of the spin waves induced by the dipole-dipole interaction in synthetic antiferromagnets. For stacked systems, the dipolar interaction causes a noticeable frequency nonreciprocity when the magnetizations in both films are antiparallelly aligned, and then the focusing properties of the spin waves are enhanced. The role of thicknesses and magnetic graduation along the film's normal are systematically analyzed. We found that the degree of focalization of the spin waves can be manipulated by increasing the layers' thickness. Also, we show that the low- and high-frequency modes exhibit different focalization properties; the low-frequency mode manifests a similar behavior to the heavy-metal/ferromagnet systems with interfacial Dzyaloshinskii-Moriya interaction, while the high-frequency one tends the generate almost reciprocal interference patterns along one axis. In the case of magnetization-graded synthetic antiferromagnets, we demonstrate that the graduation slightly influences the low-frequency mode, while the focusing and nonreciprocal dynamic properties of the high-frequency ones are notoriously altered. The theoretical calculations are compared with micromagnetic simulations, where a good agreement is found between both methods. Our results demonstrate that a synthetic antiferromagnetic system allows for controlling the propagation of spin waves, assisting in the transfer of angular momentum and energy.

The Ising magnet $\mathrm{Sm}{\mathrm{Pt}}_2{\mathrm{Si}}_2$ undergoes an antiferromagnetic (AFM) transition at $T_{\mathrm{N}1}$ = 5.6 K, but the magnetic moments of some Sm ions remain disordered even below $T_{\mathrm{N}1}$. Here, we report the results of measurements of thermal expansion, magnetostriction, and dc magnetization of $\mathrm{Sm}{\mathrm{Pt}}_2{\mathrm{Si}}_2$ in the temperature range from 0.26 to 7 K. The magnetization and thermal expansion results show clear irreversible phenomena at temperatures below $T_{\mathrm{g}}\approx$ 2.4 K. These results suggest that the partially disordered state of the Sm ions becomes a cluster spin glass below $T_{\mathrm{g}}$ and coexists with the AFM long-range order. However, unlike the conventional coexistence of spin glass and antiferromagnetism, the isothermal magnetization process at 0.26 K shows a hysteresis curve with almost no remanent magnetization. This observation suggests that the cluster spin glass state is an unusual state with a short relaxation time at zero field.

The response of monocrystalline boron carbide to planar shock loading has been investigated using molecular-dynamics simulations, along crystal orientations with the highest and lowest elastic moduli. While shocks of intensity lower than the Hugoniot elastic limit (HEL) generate quasilongitudinal and quasitransverse waves, an elastic precursor trailed by inelastic waves is observed when HEL is exceeded. Bond breakage, accompanied by temperature rise, is observed to be the primary mode of deformation under the inelastic waves. Higher shock intensities (particle velocity, $U_p>2.5\mathrm{km}/\mathrm{s}$) generate a deformation shock front, which induces loss of shear strength along with a considerable loss of crystalline order. Boron carbide is also revealed to exhibit two distinct deformation regimes. While shear strength increases with $U_p$ in the first regime $(U_p\le 2.5\mathrm{km}/\mathrm{s})$, a monotonic reduction is observed in the second $(U_p>2.5\mathrm{km}/\mathrm{s})$. The latter regime is also characterized by a convergence in the shock Hugoniot curves obtained along the two loading directions, indicating that the Hugoniot state of boron carbide may be independent of crystal orientation for $U_p>2.5\mathrm{km}/\mathrm{s}$. Finally, it is shown that the simulation-derived hydrostat curve presented in this work is coherent with experimental data up to 140 GPa and provides an accurate basis for assessing residual strength of boron carbide under shock conditions.

With the goal of maximizing the thermoelectric (TE) figure of merit $ZT$, Mahan and Sofo [Proc. Natl. Acad. Sci. USA 93, 7436 (1996)] found that the optimal transport distribution (TD) is a $\delta$ function. Materials, however, have TDs that appear to always be finite and nondiverging. Motivated by this observation, this study focuses on deriving what is the optimal bounded TD, which is determined to be a boxcar function for $ZT$ and a Heaviside function for power factor. From these optimal TDs, upper limits on $ZT$ and power factor are obtained; the maximum $ZT$ scales with ${\mathrm{\Sigma }}_{\max }T/{\kappa }_l$, where ${\mathrm{\Sigma }}_{\max }$ is the TD magnitude and ${\kappa }_l$ is the lattice thermal conductivity. These results help establish practical upper limits on the performance of TE materials and provide target TDs to guide band and/or scattering engineering strategies.

We study the prethermalization and thermalization dynamics of local observables in weakly perturbed nonintegrable systems, with Hamiltonians of the form ${\widehat{H}}_0+g\widehat{V}$, where ${\widehat{H}}_0$ is nonintegrable and $g\widehat{V}$ is a perturbation. We explore the dynamics of far from equilibrium initial states in the thermodynamic limit using a numerical linked cluster expansion (NLCE), and in finite systems with periodic boundaries using exact diagonalization. We argue that generic observables exhibit a two-step relaxation process, with a fast prethermal dynamics followed by a slow thermalizing one, only if the perturbation breaks a conserved quantity of ${\widehat{H}}_0$ and if the value of the conserved quantity in the initial state is $\mathcal{O}\left(1\right)$ different from the one after thermalization. We show that the slow thermalizing dynamics is characterized by a rate $\propto g^2$, which can be accurately determined using a Fermi golden rule (FGR) equation. We also show that during such a slow dynamics, observables can be described using projected diagonal and Gibbs ensembles, and we contrast their accuracy.

Electron spin transport in a disordered ferromagnetic metal is theoretically studied from the hydrodynamic viewpoint, focusing on the role of electron vorticity. The spin-resolved momentum flux density of electrons is calculated microscopically, taking account of the spin-orbit interaction and uniform magnetization, and the expression for the spin motive force is obtained as the linear response to a driving electric field. It is shown that the spin-resolved momentum flux density and motive force are characterized by troidal moments, vector products of the applied external electric field and the spin polarization or magnetization, which act as effective driving fields when the anomalous or spin Hall effects are taken into account. The spin-vorticity and magnetization-vorticity couplings are shown to arise naturally as conservative forces driven by the toroidal moments, and nonconservative forces are also found to exist. Spin accumulation induced by the electron flow is calculated and vorticity-induced torque and spin relaxation are discussed. The vorticity-induced torque, a linear effect of the spin-orbit interaction, is argued to be larger than the conventional relaxation torques acting on magnetization structures such as nonadiabatic ($\beta$) current-induced torque. The direct and inverse spin Hall effects and spin-orbit torque are discussed in the context of spin-vorticity coupling.

The magnetic structures and spin dynamics of ${\mathrm{Na}}_2{\mathrm{Co}}_2\mathrm{Te}{\mathrm{O}}_6$ and ${\mathrm{Na}}_2{\mathrm{Ni}}_2\mathrm{Te}{\mathrm{O}}_6$ are investigated by means of elastic and inelastic neutron scattering measurements and the results are discussed in the context of a generalized Kitaev-Heisenberg model on a honeycomb lattice with strong spin-orbit coupling. The large number of parameters involved in the Hamiltonian model is evaluated by using an iterative optimization algorithm capable of extracting model solutions and simultaneously estimating their uncertainty. The analyses indicate that both ${\mathrm{Co}}^{2+}$ $\left(d^7\right)$ and ${\mathrm{Ni}}^{2+}$ $\left(d^8\right)$ antiferromagnets realize bond-dependent anisotropic nearest-neighbor interactions and support the theoretical predictions for the realization of Kitaev physics in $3d$ electron systems with effective spins $S=1/2$ and 1. By studying the Na-doped system ${\mathrm{Na}}_{2.4}{\mathrm{Ni}}_2\mathrm{Te}{\mathrm{O}}_6$, we show that the control of Na content can provide an effective route for fine-tuning the magnetic lattice dimensionality, as well as to controlling the bond-dependent anisotropic interactions.

Spin canting and complex spin textures in antiferromagnetic materials can be described in terms of Dzyaloshinskii-Moriya interactions (DMI). Values for DMI parameters are not easily measurable directly, and often inferred from other quantities. In this work, we examine how domain wall dynamics in an antiferromagnetic optomagnonic system can display unique features directly related to the existence of DMI. Our results indicate that the presence of DMI enables spin interactions with cavity photons in a geometry which otherwise allows no magneto-optical coupling, and on the other hand modulates frequencies in a geometry where coupling is already realized in the absence of DMI. This result may be used to measure the DMI constant in optomagnonic experiments by comparing resonances obtained with different polarizations of the exciting field.

The recently introduced topological quantum chemistry (TQC) framework has provided a description of universal topological properties of all possible band insulators in all space groups based on crystalline unitary symmetries and time reversal. While this formalism filled the gap between the mathematical classification and the practical diagnosis of topological materials, an obvious limitation is that it only applies to weakly interacting systems, which can be described within band theory. It is an open question to which extent this formalism can be generalized to correlated systems that can exhibit symmetry-protected topological phases which are not adiabatically connected to any band insulator. In this work, we address the many facets of this question by considering the specific example of an extended version of a Hubbard diamond chain. This model features a Mott insulator, a trivial insulating phase, and an obstructed atomic limit phase. Here we first discuss the nature of the Mott insulator and determine the phase diagram and topology of the interacting model with infinite density matrix renormalization group calculations, variational Monte Carlo simulations, and with many-body topological invariants. We then proceed by considering a generalization of the TQC formalism to Green's functions combined with the concept of a topological Hamiltonian to identify the topological nature of the phases. Here we use cluster perturbation theory to calculate the Green's functions. The results are benchmarked with the above-determined phase diagram, and we discuss the applicability and limitations of the approach and its possible extensions in the diagnosis of topological phases in materials, in contrast to the use of many-body topological invariants.

We investigate the phase diagram of the one-dimensional repulsive Hubbard model with mass imbalance. Using DMRG, we show that this model has a “triplet” paired phase (dubbed $\pi \mathrm{SG}$) at generic fillings, consistent with previous theoretical analysis. We study the topological aspect of $\pi \mathrm{SG}$ phase, determining long-range string orders and the filling anomaly which refers to the relation among the single-particle gap, inversion symmetry, and filling imbalance for open chains. We also find, using DMRG, that at $1/3$ filling, commensurate effects lead to two additional phases: A crystal phase and a trion phase; we construct a description of these phases using Tomonaga-Luttinger liquid theory.

We show that in three-dimensional (3D) topological metals, a subset of the van Hove singularities of the density of states sits exactly at the transitions between topological and trivial gapless phases. We may refer to these as topological van Hove singularities. By investigating two minimal models, we show that they originate from energy saddle points located between Weyl points with opposite chiralities, and we illustrate their topological nature through their magnetotransport properties in the ballistic regime. We exemplify the relation between van Hove singularities and topological phase transitions in Weyl systems by analyzing the 3D Hofstadter model, which offers a simple and interesting playground to consider different kinds of Weyl metals and to understand the features of their density of states. In this model, as a function of the magnetic flux, the occurrence of topological van Hove singularities can be explicitly checked.

Being dispersionless, flat bands on periodic lattices are solely characterized by their macroscopically degenerate eigenstates: compact localized states (CLSs) in real space and Bloch states in reciprocal space. Based on this property, this work presents a straightforward method to build flat-band tight-binding models with short-range hoppings on any periodic lattice. The method consists in starting from a CLS and engineering families of Bloch Hamiltonians as quadratic (or linear) functions of the associated Bloch state. The resulting tight-binding models not only exhibit a flat band, but also multifold quadratic (or linear) band touching points (BTPs) whose number, location, degeneracy, and (non)singularity can be controlled to a large extent. Quadratic flat-band models are ubiquitous: they can be built from any arbitrary CLS, on any lattice, in any dimension and with any number $N\ge 2$ of bands. Linear flat-band models are rarer: they require $N\ge 3$ and can only be built from CLSs that fulfill certain compatibility relations with the underlying lattice. Most flat-band models from the literature can be classified according to this scheme: Mielke's and Tasaki's models belong to the quadratic class, while the Lieb, dice, and breathing kagome models belong to the linear class. Many novel flat-band models are introduced, among which an $N=4$ bilayer honeycomb model with fourfold quadratic BTPs, an $N=5$ dice model with fivefold linear BTPs, and an $N=3$ kagome model with BTPs that can be smoothly tuned from linear to quadratic.