We address the intervention of classical-like behavior, well documented in experimental studies of strongly correlated electron systems of solids that emerge at temperatures $T$ far below the Debye temperature $T_D$. We attribute this unexpected phenomenon to spontaneous rearrangement of the conventional Landau state beyond a critical point at which the topological stability of this state breaks down, leading to the formation of an interaction-induced flat band adjacent to the nominal Fermi surface. We demonstrate that beyond the critical point, the quasiparticle picture of such correlated Fermi systems still holds, since the damping of single-particle excitations remains small compared with the Fermi energy $T_F=p_F^2/2m_e$. A Pitaevskii-style equation for determination of the rearranged quasiparticle momentum distribution $n_{*}\left(\mathbf{p}\right)$ is derived, which applies to explanation of the linear-in-$T$ behavior of the resistivity $\rho \left(T\right)$ found experimentally.

Engineering of the dielectric environment represents a powerful strategy to control the electronic and optical properties of two-dimensional (2D) materials without compromising their structural integrity. Here we show that the use of high-$\kappa$ dielectrics presents new opportunities for controlling the excitonic states of 2D semiconductors. By solving a 2D Mott-Wannier exciton model for $\mathrm{W}{\mathrm{Se}}_2$ on different substrates using a screened electron-hole interaction obtained from first principles, we demonstrate that the exciton Rydberg series changes qualitatively when the dielectric screening within the 2D semiconductor becomes dominated by the substrate. In this regime, the distance dependence of the screening is reversed and the effective screening increases with exciton radius, which is opposite to the conventional 2D screening regime. Consequently, higher excitonic states become underbound rather than overbound as compared to the hydrogenic Rydberg series. Finally, we derive a general analytical expression for the exciton binding energy of the entire 2D Rydberg series.

We report the discovery of a first-order phase transition at around 125 K in NbCrP, which is a nonsymmorphic crystal with the $Pnma$ space group. From the resistivity, magnetic susceptibility, and nuclear magnetic resonance measurements using crystals made by the Sn-flux method, the high-temperature (HT) phase is characterized to be metallic with a non-negligible magnetic anisotropy. The low-temperature (LT) phase is also found to be a nonmagnetic metallic state with a crystal of lower symmetry. In the LT phase, the spin susceptibility is reduced by $\sim 30\%$ from that in the HT phase, suggesting that the phase transition is triggered by the electronic instability. The possible origin of the phase transition in NbCrP is discussed based on the electronic structure by comparing it with those in other nonsymmorphic compounds, RuP and RuAs.

We study the phase diagram of the interacting two-dimensional electron gas (2DEG) with equal Rashba and Dresselhaus spin-orbit coupling, which for weak coupling gives rise to the well-known persistent spin-helix phase. We construct the full Hartree-Fock phase diagram using a classical Monte Carlo method analogous to that used in Phys. Rev. B 96, 235425 (2017). For the 2DEG with only Rashba spin-orbit coupling, it was found that at intermediate values of the Wigner-Seitz radius $r_s$ the system is characterized by a single Fermi surface with an out-of-plane spin polarization, whereas at slightly larger values of $r_s$ it undergoes a transition to a state with a shifted Fermi surface and an in-plane spin polarization. The various phase transitions are first order, and this shows up in discontinuities in the conductivity, and the appearance of anisotropic resistance in the in-plane polarized phase. In this paper we show that the out-of-plane spin-polarized region shrinks as the strength of the Dresselhaus spin-orbit interaction increases and entirely vanishes when the Rashba and Dresselhaus spin-orbit coupling strengths are equal. At this point the system can be mapped onto a 2DEG without spin-orbit coupling, and this transformation reveals the existence of an in-plane spin-polarized phase with a single displaced Fermi surface beyond $r_s>2.01$. This is confirmed by classical Monte Carlo simulations. We discuss experimental observation and useful applications of the novel phase as well as caveats of using the classical Monte Carlo method.

We describe the structure of the time-harmonic electromagnetic field of a vertical Hertzian electric dipole source radiating over an infinite, translation-invariant two-dimensional electron system. Our model for the electron flow takes into account the effects of shear and Hall viscosities as well as an external static magnetic field perpendicular to the sheet. We identify two wave modes, namely, a surface plasmon and a diffusive mode. In the presence of an external static magnetic field, the diffusive mode combines the features of both the conventional and Hall diffusion and may exhibit a negative group velocity. In our analysis, we solve exactly a boundary value problem for the time-harmonic Maxwell equations coupled with linearized hydrodynamic equations for the flat, two-dimensional material. By numerically evaluating the integrals for the electromagnetic field on the sheet, we find that the plasmon contribution dominates in the intermediate-field region of the dipole source. In contrast, the amplitude of the diffusive mode reaches its maximum value in the near-field region, and quickly decays with the distance from the source. We demonstrate that the diffusive mode can be distinguished from the plasmon in the presence of the static magnetic field, when the highly oscillatory plasmon is gapped and tends to disappear.

Machine learning potentials (MLPs) are becoming powerful tools for performing accurate atomistic simulations and crystal structure optimizations. An approach to developing MLPs employs a systematic set of polynomial invariants including high-order ones to represent the neighboring atomic density. In this study, a formulation of the polynomial invariants is extended to the case of multicomponent systems. The extended formulation is more complex than the formulation for elemental systems. This study also shows its application to the Ti-Al binary system. As a result, an MLP with the lowest error and MLPs with high computational cost performance are selected from the many MLPs developed systematically. The predictive powers of the developed MLPs for many properties, such as the formation energy, elastic constants, thermodynamic properties, and mechanical properties, are examined. The MLPs exhibit high predictive power for the properties in a wide variety of ordered structures. The present scheme should be systematically applicable to other multicomponent systems.

We study the time-dependent circuit complexity of the periodically driven transverse field Ising model using Nielsen's geometric approach. In the high-frequency driving limit the system is known to exhibit nonequilibrium phase transitions governed by the amplitude of the driving field. We analytically compute the complexity in this regime and show that it clearly distinguishes between the different phases, exhibiting a universal linear behavior at early times. We also evaluate the time-averaged complexity, provide evidence of nonanalytic behavior at the critical points, and discuss its origin. Finally, we comment on the freezing of quantum dynamics at specific configurations and on the use of complexity as a tool to understand quantum phase transitions in Floquet systems.

Combining a precise ab initio electron band structure calculation of the ${\mathrm{TiO}}_2$ rutile structure with the temperature evolution of the Ti mean-square displacements, we reproduce a puzzling temperature increase of the electric field gradient at Ti sites in ${\mathrm{TiO}}_2$, observed experimentally. Our method employs a procedure of averaging two quadrupole electron density components ($L=2$) inside a sphere vibrating with the Ti nucleus at its center, where the key factor introducing the temperature dependence is the square root of the Debye-Waller factor. Although the Debye-Waller factor always reduces the corresponding Fourier component, in ${\mathrm{TiO}}_2$ due to the interplay between terms of opposite signs, it results in a net increase of the whole sum with temperature, leading to the growth of the electric field gradient. Quantitatively, we find that the increase of electric field gradient is only half of the experimental value, which we ascribe to anharmonic effects or a strong oxygen position influence. In addition, our method reproduces the unusual temperature dependence of the asymmetry parameter $\eta$, which first decreases with temperature, goes to zero, and then increases.

A general formula for the average vector potential of bulk periodic systems is proposed and shown to set the boundary conditions at magnetic interfaces. For antiferromagnetic materials, the study reveals a unique relation between the macroscopic potential and the orientation-dependent magnetic quadrupole as a result of the different crystalline and magnetic symmetries. In particular, at surfaces and interfaces of a truncated bulk without inversion and time-reversal symmetries, the average vector potential exhibits a discontinuity which results in an interfacial magnetic field. In general, however, due to the surface and interface electronic and atomic relaxations, additional magnetization may result. For the experimentally observed magnetoelectric antiferromagnets, in particular, our symmetry analysis suggests that the relaxation effects could well be a system response to the presence of such a potential discontinuity.

The optical control of magnetism offers an attractive possibility to manipulate small magnetic domains for prospective memory devices on ultrashort time scales. Here, we report on the local deterministic transformation of the magnetic domain pattern from stripes to bubbles in out-of-plane magnetized Co/Pt multilayers controlled only by the helicity of ultrashort laser pulses. Relying on the experimentally determined average size of stripe domains and the magnetic layer thickness, we calculate the temperature and characteristic fields at which the stripe-bubble transformation occurs. Furthermore, we demonstrate that in the narrow range of the laser power, the helicity induces a drag on domain walls.

A nonuniform vorticity of lattice deformation in a surface acoustic wave (SAW) can generate a spin current (SC) in nonmagnetic metals via spin-vorticity coupling (SVC). We demonstrated a strong enhancement of SVC-derived SC generated in Cu and Pt films with increasing the frequency of the SAW by observing the spin-wave resonance (SWR) in an adjacent NiFe film. The comparative amplitudes and high-order frequency variations of SWR in NiFe/Cu and NiFe/Pt bilayers imply that the amplitude of the SC generated via SVC in a SAW is robust against the strength of spin-orbit interaction in nonmagnetic metals.

We investigate how the Kitaev spin liquid state described by Majorana fermions coupled with $Z_2$ gauge fields is affected by non-Kitaev interactions which exist in real candidate materials of the Kitaev magnet. It is found that the off-diagonal exchange interaction referred to as the ${\mathrm{\Gamma }}^{\prime }$ term dramatically changes the Majorana band structure in the case of the antiferromagnetic Kitaev interaction and gives rise to a topological phase transition from a non-Abelian topological phase with the Chern number equal to $\pm 1$ to an Abelian phase with the Chern number equal to $\pm 2$, in which the $Z_2$ vortices behave as Abelian anyons. On the other hand, other non-Kitaev interactions such as the Heisenberg interaction and the $\mathrm{\Gamma }$ term only affect the bandwidth of the Majorana band as long as the spin liquid state is not destabilized.

The revisited theoretical phase diagrams for thin films of cubic helimagnets with the easy-plane anisotropy are shown to have different topology as previously reported [E. A. Karhu et al., Phys. Rev. B 85, 094429 (2012)]. For both in-plane and out-of-plane directions of an applied magnetic field, the phase diagrams exhibit extensive areas of stable skyrmions, which overlap for a wide range of anisotropy parameters. Although the existence of the out-of-plane skyrmions was contradicted within the previous theoretical models, we prove that additional surface twists lead to their stability, while the moderate easy-plane anisotropy increases the stability range of in-plane skyrmions. Moreover, the interplay between the anisotropy and the surface twists gives rise to a stable spiral state canted with respect to the surfaces. Being absent in bulk helimagnets, this oblique spiral occupies vast areas at the phase diagrams in thin-film nanosystems and serves as a connecting-link between cones and helicoids. Our theory gives clear directions for renewed experimental studies of in-plane and out-of-plane skyrmions in epitaxial MnSi(111)/Si(111) thin films.

We report a study of the effect of hydrostatic pressure (P) on the Griffiths-like phase in selected compounds of the $R_5{({\mathrm{Si}}_x{\mathrm{Ge}}_1-x)}_4$ family of alloys (${\mathrm{Tb}}_{4.925}{\mathrm{La}}_{0.075}{\mathrm{Si}}_2{\mathrm{Ge}}_2$ and ${\mathrm{Gd}}_5{\mathrm{Ge}}_4$) which present either the ${\mathrm{Gd}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$-type (monoclinic, M) or the ${\mathrm{Sm}}_5{\mathrm{Ge}}_4$-type [orthorhombic-II, O(II)] structural phases at room temperature. The downward deviation in the inverse low-field dc susceptibility ${\chi }_{dc}^{-1}$ from the Curie-Weiss law below a characteristic temperature $T_G$ indicates that the Griffiths-like phase exists at pressures up to 10 kbar. From the obtained T-P phase diagrams, the pressure coefficient of the Griffiths-like temperature, ${dT}_G/dP$, has been determined. These results are compared with those obtained in ${\mathrm{Dy}}_5{\mathrm{Si}}_3\mathrm{Ge}$ in a previous work. The ${dT}_G/dP$ coefficient is strongly dependent on the nature (first or second order) of the long-range order (FM or AFM) transition. This effect can be ascribed to a different structural character of the clusters within the Griffiths phase. A ratio of $\sim 0.5$ between the ${dT}_G/dP$ and the pressure coefficient of long-range magnetic ordering temperatures, ${dT}_{C,N}/dP$ ($T_C$, ferromagnetic; $T_N$, antiferromagnetic), is found in all the studied compounds.

Artificial kagome spin ice exhibits exotic magnetic correlations driven by a combination of geometric frustration and dipolar interactions that, at low-enough temperature, can result in ordered phases. This order, whether it is the ground state of several kagome rings, or the theoretically predicted long-range order of an extended array, has yet to be experimentally observed. By introducing an interfacial Dzyaloshinskii-Moriya interaction, we are able to reduce the blocking temperature of the individual nanomagnets, allowing a system of 30 kagome nanomagnets to explore its vast manifold of microstates and find its ground state. Furthermore, the extracted magnetic correlations in an extended artificial kagome spin ice are found to exhibit quantitative signatures of long-range charge order, providing evidence of the theoretically predicted continuous phase transition to the charge-ordered state. The significant lowering of the blocking temperature in nanomagnets is important for the exploitation of superparamagnetism in artificial spin systems and devices.

Hidden spin polarization (HSP) emerges in centrosymmetric crystals where visible spin splittings in the real space can be observed because of the lack of inversion symmetry in each local sector. Starting from tight-binding models, we introduce nonsymmorphic antiferroelectric (AFE) crystals as a new class of functional materials that can exhibit strong local spin polarization. Such AFE crystals can be basically classified as in-plane and out-of-plane AFE configurations and can be reversibly switched to the ferroelectric phase by an electric field to manifest global spin splittings, enabling a nonvolatile electrical control of spin-dependent properties. Based on first-principles calculations, we predict the realization of strong HSP in the AFE phase of a newly discovered two-dimensional material, quintuple-layer (QL) ${\mathrm{LiBiO}}_2$. Furthermore, the spontaneous electric polarization ($\sim 0.3$ nC/m) and the transition barrier as well as the tunable spin polarization of QL ${\mathrm{LiBiO}}_2$ are discussed.

We study the ballistic-to-diffusive transition induced by the weak breaking of integrability in a boundary-driven XXZ spin chain. Studying the evolution of the spin current density ${\mathcal{J}}^s$ as a function of the system size $L$, we show that, accounting for boundary effects, the transition has a nontrivial universal behavior close to the XX limit. It is controlled by the scattering length $L^{*}\propto V^{-2}$, where $V$ is the strength of the integrability-breaking term. In the XXZ model, the interplay of interactions controls the emergence of a transient “quasiballistic” regime at length scales much shorter than $L^{*}$. This parametrically large regime is characterized by a strong renormalization of the current which forbids a universal scaling, unlike the XX model. Our results are based on matrix product operator numerical simulations and agree with perturbative analytical calculations.

Magnetic properties and underlying magnetic models of the synthetic $A_2{\mathrm{Cu}}_3\mathrm{O}{\left({\mathrm{SO}}_4\right)}_3$ fedotovite ($A$ = K) and puninite ($A$ = Na) minerals, as well as the mixed euchlorine-type ${\mathrm{NaKCu}}_3\mathrm{O}{\left({\mathrm{SO}}_4\right)}_3$ are reported. We show that all three compounds contain magnetic ${\mathrm{Cu}}_6$ hexamer units which at temperatures below about 100 K act as single spin-1 entities. Weak interactions between these magnetic molecules lead to long-range order below $T_N=3.4\mathrm{K}$ ($A$ = Na), 4.7 K ($A$ = NaK), and about 3.0 K ($A$ = K). The formation of the magnetic order is elucidated by ab initio calculations that reveal two-dimensional interhexamer interactions within crystallographic $bc$ planes. This model indicates the presence of a weakly distorted square lattice of $S=1$ magnetic ions and challenges the earlier description of the $A_2{\mathrm{Cu}}_3\mathrm{O}{\left({\mathrm{SO}}_4\right)}_3$ minerals in terms of Haldane spin chains.

We analyze the phenomenon of back hopping in spin-torque induced switching of the magnetization in perpendicularly magnetized tunnel junctions. The analysis is based on single-shot time-resolved conductance measurements of the pulse-induced back hopping. Studying several material variants reveals that the back hopping is a feature of the nominally fixed system of the tunnel junction. The back hopping is found to proceed by two sequential switching events that lead to a final state P′ of conductance close to—but distinct from—that of the conventional parallel state. The P′ state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P′ state involves a switching of the sole spin-polarizing part of the fixed layers. The analysis of literature indicates that back hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system, which justifies a posteriori the mitigation strategies of back hopping that were implemented empirically in spin-transfer-torque magnetic random access memories.

As skyrmion lattices are attracting increasing attention owing to their properties driven by real-space topology, properties of magnetic Weyl semimetals with complex $k$-space topology are moving into the focus of research. We consider Hall transport properties and orbital magnetism of skyrmion lattices imprinted in topological semimetals by employing a minimal model of a mixed Weyl semimetal which, as a function of the magnetization direction, exhibits two Chern insulator phases separated by a Weyl state. We find that while the orbital magnetization is topologically robust and Hall transport properties exhibit a behavior consistent with that expected for the recently discovered chiral Hall effect [F. R. Lux et al., Phys. Rev. Lett. 124, 096602 (2020)], their evolution in the region of the Chern insulator gap is largely determined by the properties of the so-called mixed topology ring states, emerging in domain walls that separate the skyrmion core from the ferromagnetic background. In particular, we show that these localized ring states possess a robust orbital chirality which reverses sign as a function of the skyrmion radius, thereby mediating a smooth switching dynamics of the orbital magnetization. We speculate that while the emergent ring states can possibly play a role in the physics of Majorana states, probing their properties experimentally can provide insights into the details of skyrmionic spin structures.