The atomic cluster expansion [R. Drautz, Phys. Rev. B 99, 014104 (2019)] is extended in two ways, the modeling of vectorial and tensorial atomic properties and the inclusion of atomic degrees of freedom in addition to the positions of the atoms. In particular, atomic species, magnetic moments, and charges are attached to the atomic positions, and an atomic cluster expansion that includes the different degrees of freedom on equal footing is derived. Expressions for the efficient evaluation of forces and torques are given.

Opposite to the common idea of a magnetic order requirement to obtain spin current propagation, materials with no magnetic ordering have also been revealed to be efficient spin conductors. In this work, we investigate the spin current injection at the interface between a magnetic insulator and a superconductor. We are mainly interested in the paramagnetic insulator/superconductor interface; however, our model also describes the ferromagnetic phase. We used the Schwinger bosonic formalism to describe the magnetic insulator, and standard BCS theory was applied to treat the superconductor layer. In the normal-metal limit, our results are in agreement with the expected ones. For example, we found the correct spin current behavior $I\approx T^{3/2}$ at low temperature. In addition, our model shows a pronounced peak in the spin current injection at temperatures close to the superconductor transition temperature due to the superconducting quasiparticle coherence. The role of magnetic fields in the spin current injection is also investigated.

The magnonic spin Seebeck effect is a key element of spin caloritronics, a field that exploits thermal effects for spintronic applications. Early studies were focused on investigating the steady-state nonequilibrium magnonic spin Seebeck current, and the underlying physics of the magnonic spin Seebeck effect is now relatively well established. However, the initial steps of the formation of the spin Seebeck current are in the scope of recent interest. To address this dynamical aspect theoretically, we propose here an alternative approach to the time-resolved spin Seebeck effect. Our method exploits the supersymmetric theory of stochastics and the Stratonovich-Ito integration scheme. We found that in the early step the spin Seebeck current has both nonzero transversal and longitudinal components. As the magnetization dynamics approaches the steady state, the transversal components decay through dephasing over the dipole-dipole reservoir. The timescale for this process is typically in subnanoseconds, pointing thus to the potential of an ultrafast control of the dynamical spin Seebeck during its buildup.

Spin density waves, based on modulated local moments, are usually associated with metallic materials, but have recently been reported in insulators which display coupled magnetic and structural order parameters. We discuss one such example, the multiferroic ${\mathrm{Cu}}_3{\mathrm{Nb}}_2{\mathrm{O}}_8$, which is reported to undergo two magnetic phase transitions, first to an unknown antiferromagnetic phase at $T_N\approx 26.5\text{K}$, and then to a helicoidal structure coupled to an electric polarization below $T_2\approx 24\text{K}$ [R. D. Johnson et al., Phys. Rev. Lett. 107, 137205 (2011)] which breaks the crystallographic inversion symmetry. By analogy with other complex oxides, one might naturally expect this intermediate phase to be a spin density wave phase. We apply spherical polarimetry to confirm the low-temperature magnetic structure, yet only observe a single magnetic phase transition to helicoidal order. We argue that the reported unknown phase actually supports an imitation spin density wave which originates from a decoupling of the components of the magnetic order parameter, as allowed by symmetry and driven by thermal fluctuations. This provides a mechanism for the magnetic, but not nuclear, structure to break inversion symmetry thereby creating an intermediate phase in the proximity of ${\mathrm{T}}_N$ which imitates a spin density wave. As the temperature is reduced, this intermediate structure destabilizes the crystal such that a structural chirality is induced, as reflected by the emergence of the electric polarization, and the imitation spin density wave relaxes into a generic helicoid. This scenario in which critical fluctuations allow the magnetic structure to break inversion symmetry while the crystal structure remains centrosymmetric might be relevant to other complex multiferroics.

We present how to detect type-I Weyl nodes in a material by inelastic neutron scattering. Such an experiment first of all allows one to determine the dispersion of the Weyl fermions. We extend the reasoning to produce a quantitative test of the Weyl equation, taking into account realistic anisotropic properties. These anisotropies are mostly contained in the form of the emergent magnetic moment of the excitations, which determines how they couple to the neutrons. Although there are many material parameters, we find several quantitative predictions that are universal and demonstrate that the excitations are described by solutions to the Weyl equation. The anisotropic coupling between electrons and neutrons implies that even fully unpolarized neutrons can reveal the spin-momentum locking of the Weyl fermions because the neutrons will couple to some components of the Weyl fermion pseudospin more strongly. On the other hand, in an experiment with polarized neutrons, the scattered neutron beam remains fully polarized in a direction that varies as a function of momentum transfer (within the range of validity of the Weyl equation). This allows measurement of the chirality of Weyl fermions for inversion-symmetric nodes. Furthermore, we estimate that the scattering rate may be large enough for such experiments to be practical; in particular, the magnetic moment may be larger than the ordinary Bohr magneton, compensating for a small density of states.

The homogeneous electron gas is one of the most studied model systems in condensed-matter physics. It is also the basis of the large majority of approximations to the functionals of density-functional theory. As such, its exchange-correlation energy has been studied extensively, and it is well-known for systems of one, two, and three dimensions. Here, we extend this model and compute the exchange and correlation energy, as a function of the Wigner-Seitz radius $r_s$, for arbitrary dimension $D$. We find a very different behavior for reduced dimensional spaces ($D=1$ and 2), our three-dimensional space, and for higher dimensions. In fact, for $D>3$, the leading term of the correlation energy does not depend on the logarithm of $r_s$ (as for $D=3$), but instead scales like a polynomial: $-c_D/r_s^{{\gamma }_D}$, with the exponent ${\gamma }_D=(D-3)/(D-1)$. In the large-$D$ limit, the value of $c_D$ is found to depend linearly on the dimension. In this limit, we also find that the concepts of exchange and correlation merge, sharing a common $1/r_s$ dependence.

This paper presents a study on electron field emission (FE) into ultrahigh vacuum (UHV) from a single, cleaned, graphene sheet where its single sheet nature has been carefully characterized and the emission zones are unambiguously the sheet edges. This definitive approach is in contrast to almost all other studies in the literature and can now guide theoretical work and applications. Our sample characterization starts with transmission electron microcopy imaging, Raman characterization, controlled mounting of individual flakes in a scanning electron microscope followed by in situ high temperature and high field treatments. Detailed FE characterization was carried out including current-voltage plots, FE microscopy, and electron energy spectroscopy during subsequent stages of sample cleaning, at room and liquid air temperatures. The full width at half maximum (FWHM) of the energy peak as well as its relative shift versus current and voltage were explored. A notable result is that the energy spectra are very narrow, ten times less than previously reported.

We performed muon-spin-rotation/-relaxation measurements to investigate the magnetic behavior of $T^{\prime }$-type ${\mathrm{Eu}}_2{\mathrm{CuO}}_4$ (ECO), which is the parent compound of electron-doped cuprate superconductors, and the effects of oxygen-reduction annealing on its magnetism. In as-sintered (AS) ECO, we clarified the development of magnetic correlations upon cooling below $T_{\mathrm{N}1}$ (= 265 K) as well as the coexistence of a dominant fluctuating spin state and partially ordered spin state in the temperature range between $\sim 150$ K and $T_{\mathrm{N}1}$. Upon further cooling, uniform long-range magnetic order was observed below $T_{\mathrm{N}2}=110$ K, which is close to the ordering temperature of 115 K in $T^{\prime }$-type ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (LCO) [Phys. Rev. B 82, 180508(R) (2010)]. For oxygen-reduction-annealed ECO, a similar ordering sequence with the same $T_{\mathrm{N}2}$ was observed but without the partially ordered spin state. Therefore, the fluctuating spin state over a wide temperature range and a $T_{\mathrm{N}2}$ less than the Néel temperature ($T_{\mathrm{N}2}\approx T_{\mathrm{N}1}$) in T-type LCO are common features of the $T^{\prime }$-type parent $R_2{\mathrm{CuO}}_4$ ($R\mathrm{CO}$, $R$: rare-earth ion). The origin of the partially ordered spin state in AS ECO is discussed from the viewpoint of chemical defect. Furthermore, we discuss the roles of electron doping and repairing defect in the observed effect of annealing on the magnetism of $T^{\prime }$-type $R\mathrm{CO}$.

Quantized spin-wave known as magnon, a bosonic quasiparticle, can drag electrons or holes via s-d exchange interaction and boost the thermopower over the conventional diffusive thermopower. P-type magnon-drag thermopower has been observed in both ferromagnetic and antiferromagnetic metallic and degenerate semiconductors. However, it has been less reported for intrinsic or $n$-type magnetic semiconductors; therefore, the impact of magnon-bipolar carrier drag on thermopower has remained unexplored. Here, a theoretical model for magnon-bipolar carrier drag thermopower is derived based on the magnon-carrier interaction lifetimes. The model predicts that the bipolar carrier drag thermopower becomes independent of both the carrier and magnon relaxation times. A proof of concept experiment is presented that confirms this prediction. We also report the observation of magnon-carrier drag thermopower in n-type and intrinsic ferromagnetic semiconductors experimentally. The p-type antiferromagnetic MnTe is doped with different amounts of Cr to produce nondegenerate and n-type semiconductors of various carrier concentrations. Cr dopants have a donor nature and create ferromagnetic-antiferromagnetic clusters due to the $\mathrm{C}{\mathrm{r}}^{3+}$ oxidation state. Heat capacity measurements confirm the presence of magnons in Cr-doped MnTe. It is shown that the magnon-drag thermopower is significantly reduced for 3%–5% Cr-doped samples due to bipolar drag effects and becomes negative for 14% and 20% Cr-doped MnTe due to dominant magnon-electron drag thermopower.

We consider a three-dimensional non-Hermitian exceptional line semimetal model and take open boundary conditions in the $x$, $y$, and $z$ directions separately. In each case, we calculate the parameter regions where the bulk-boundary correspondence is broken. The breakdown of the bulk-boundary correspondence is manifested by the deviation from unit circles of generalized Brillouin zones (GBZs) and the discrepancy between spectra calculated with open boundary conditions (OBCs) and periodic boundary conditions (PBCs). The consistency between OBC and PBC spectra can be recovered if the PBC spectra are calculated with GBZs. We use both unit-circle Brillouin zones (BZs) and GBZs to plot the topological phase diagrams. The systematic analysis about the differences between the two phase diagrams suggests that it is necessary to use GBZs to characterize the bulk-boundary correspondence of non-Hermitian models.

Photon-assisted tunneling frequently provides detailed information on the underlying charge-transfer process. In particular, the Tien-Gordon approach and its extensions predict that the sideband spacing in bias voltage is a direct fingerprint of the number of electrons transferred in a single tunneling event. Here, we analyze photon-assisted tunneling into subgap states in superconductors in the limit of small temperatures and bias voltages where tunneling is dominated by resonant Andreev processes and does not conform to the predictions of simple Tien-Gordon theory. Our analysis is based on a systematic Keldysh calculation of the subgap conductance and provides a detailed analytical understanding of photon-assisted tunneling into subgap states, in excellent agreement with a recent experiment. We focus on tunneling from superconducting electrodes and into Yu-Shiba-Rusinov states associated with magnetic impurities or adatoms, but we also explicitly extend our results to include normal-metal electrodes or other types of subgap states in superconductors. In particular, we argue that photon-assisted Andreev reflections provide a high-accuracy method to measure small but nonzero energies of subgap states which can be important for distinguishing conventional subgap states from Majorana bound states.

We report the results of the experimental investigation of magnonic Bloch modes in the presence of asymmetric dispersion induced by the interfacial Dzyaloshinskii-Moriya interaction by means of Brillouin light-scattering technique. It was realized in a specially designed ultrathin CoFeB/Pt periodic structure consisting of an array of rectangular nanostrips separated by half-etched grooves. The proposed theory based on the coupled mode approach explains the major features observed experimentally, such as Brillouin zone folding and the asymmetry of the magnonic band-gap points in the reciprocal $k$ space.

We consider the dynamics of maximal quantum Fisher information (MQFI) after sudden quenches for the one-dimensional transverse-field Ising model. Our results show, the same as Loschmidt echo, that there is a universality for the revival times, i.e., they do not depend on the initial state and the size of the quench and are given by integer multiples of $T_{\text{rev}}\simeq \frac{N}{2v_{\text{max}}}$, where $N$ is the system size and $v_{\text{max}}$ is the maximal group velocity of quasiparticles. Critically enhanced and decreased at revival and decay times as $T_{\text{rev}}\equiv T_{\text{dec}}$ are characterized by quenching from the order and disorder phases into the quantum phase transition, respectively, that can be utilized to detect the quantum critical point (QCP). In some quenches crossed from the QCP, nonanalytic behaviors appear at some times due to the turning of the local observable from one direction to another because of identifying the maximum value. We name this phenomenon the dynamical MQFI transitions, occurring at the critical times $t_c$. Interestingly, although no Fisher zero exists in the dynamics of MQFI, the first critical time emerged from the dynamical quantum phase transition is equal to the first time when the logarithm of MQFI is minimum. In addition, we unveil the long-time run of MQFI indicates a signature of a nonequilibrium quantum phase transition at the QCP. We also discuss the probability of the arising of macroscopic superpositions in the nonequilibrium dynamics of the system.

Electron dynamics in the bulk of large band gap dielectric crystals induced by intense femtosecond laser pulses at 800 nm is studied. With laser intensities (a few 10 $\mathrm{TW}/{\mathrm{cm}}^2$) under the ablation threshold, electrons with unexpected energies in excess of 40–50 eV are observed by using the photoemission spectroscopy. A theoretical approach based on the Boltzmann kinetic equation including state-of-the-art modeling for various particles interactions is developed to interpret these experimental observations. A direct comparison shows that both electron heating in the bulk and a further laser field acceleration after ejection from the material contribute equivalently to the final electron energy gain. The laser energy deposition in the material is shown to be significantly driven by a noncollisional process, i.e., direct multiphoton transitions between subbands of the conduction band. This work also sheds light on the contribution of the standard electron excitation/relaxation collisional processes, providing a new baseline to study the electron dynamics in dielectric materials and associated applications as laser material micromachining. To support such applications, a simple expression to evaluate the energy deposition by noncollisional absorption is provided.

The two-dimensional nature of graphene Josephson junctions offers the possibility of creating effective superconductor-ferromagnet-superconductor junctions with tunable Zeeman splitting caused by an in-plane magnetic field. Such junctions would be able to alternate between a conventional superconducting ground state and a ground state with an intrinsic phase difference, making them controllable 0-$\pi$ Josephson junctions. However, in addition to the Zeeman splitting, an in-plane magnetic field will in general also produce an orbital effect because of height variations in graphene, colloquially known as ripples. Both the Zeeman and orbital effect will thus affect the critical current, so to be able to identify 0-$\pi$ transitions it is necessary to understand their combined effect. From both analytical and numerical solutions of the Usadel equation we find that ripples can in fact produce a current response similar to that which is characteristic of a 0-$\pi$ transition. Hence, additional analysis is required in order to reveal the presence of a 0-$\pi$ transition caused by spin splitting in graphene with ripples. We provide a closed form analytical expression for the critical current in the presence of exchange field and ripple effects as well as an expression for the scaling of critical current zeros with junction parameters.

We consider a dynamic protocol for quantum many-body systems, which enables us to study the interplay between unitary Hamiltonian driving and random local projective measurements. While the unitary dynamics tends to increase entanglement, local measurements tend to disentangle, thus favoring decoherence. The competition of the two drivings is analyzed at quantum transitions, where the presence of critical correlations substantially changes the impact of local measurements. We identify a particular regime (dynamic scaling limit) within a dynamic scaling framework, where the two mechanisms develop a nontrivial interplay and peculiar scaling behaviors. This is supported by a numerical analysis of a measurement-driven quantum Ising chain. The local measurement process generally tends to suppress quantum correlations, even in the dynamic scaling limit. The power law of the decay of the quantum correlations turns out to be enhanced at the quantum transition.

Because the iron-ladder compounds ${\mathrm{BaFe}}_2{X}_3$ ($X$ = S or Se) possess highly anisotropic crystal, electronic, and magnetic structures, the order in certain degrees of freedom leads to interesting interplays among them. Here we present a systematic study of the dielectric behavior of ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ and ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ along all crystallographic directions revealing the conduction mechanism and its relation to the underlying magnetic ordering. The temperature dependence of the DC conductivity indicates that the antiferromagnetic order is accompanied by a transition in the conduction mechanism from a simple activated behavior above $T_{\text{N}}$ to three-dimensional variable range hopping below $T_{\text{N}}$. Although the magnetic structures that develop on ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ and ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ are rather different—despite nearly identical crystal structures—we do not find significant difference in the conduction mechanism.

The growth rate and crystal quality of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ thin films are limited by suboxide (${\mathrm{Ga}}_2\mathrm{O}$) desorption. It has been reported that the growth of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ by molecular beam epitaxy and metalorganic chemical vapor deposition can be improved by an additional indium supply. In order to reveal the mechanism by which In impacts growth, we use density functional theory to explore the adsorption of Ga and In adatoms on the ${\mathrm{Ga}}_2{\mathrm{O}}_3$(010) surface and the effect of In on the growth rate. We find that the most stable configuration under an extreme Ga-rich condition is the surface with four Ga adatoms per unit cell. In the presence of indium, Ga adatoms become more strongly bound to the surface. Coadsorption of In and Ga thus increases the stability of the metal-covered surface and suppresses suboxide desorption; this explains the higher growth rate under Ga and In flux in experiments.

A major obstacle to reaching the wide applications of valleytronics operations is the lack of suitable anomalous valley Hall (AVH) materials, which should be easily fabricated and exhibit spontaneous valley polarization. Here, we identify an excellent two-dimensional AVH material, namely single-layer $\mathrm{N}{\mathrm{b}}_3{\mathrm{I}}_8$, based on first principles. Single-layer $\mathrm{N}{\mathrm{b}}_3{\mathrm{I}}_8$ is a robust ferromagnetic semiconductor with a moderate band gap. More importantly, due to the intrinsic breakings of both time-reversal symmetry and inversion symmetry, it exhibits the long-sought spontaneous valley polarization, without requiring any external tuning. Spontaneous valley polarization can reach as large as 107 meV, which is beneficial for practical operations. Furthermore, the experimental exfoliation of single-layer $\mathrm{N}{\mathrm{b}}_3{\mathrm{I}}_8$ is highly feasible as the stable layered bulk phase exists. Our findings provide an ideal platform for exploring the AVH effect and future valleytronic applications.