Recent advancements in our understanding of ordered magnets call for a quantification of their entanglement content on an equal footing with classical thermodynamic quantities, such as the total magnetic moment. We evaluate the entanglement entropy (EE) between the two sublattices of a bipartite ordered antiferromagnet, finding it to scale with volume. Thus, the EE density becomes an intensive property and is evaluated to be a universal dimensionality-dependent constant when exchange is the dominant interaction. Our analytic results are validated against the DMRG-based analysis of a one-dimensional (1D) system, finding good agreement. Furthermore, our evaluated EE per bond provides a useful shortcut towards obtaining the central-cut EE in 1D, and the area law in higher-dimensional magnets.

We study a spin-$\frac{1}{2}$ XXZ model with a four-spin interaction on a two-leg ladder. By means of effective field theory and matrix product state calculations, we obtain rich ground-state phase diagrams that consist of eight distinct gapped phases. Four of them exhibit spontaneous symmetry breaking with either a magnetic or valence-bond-solid (VBS) long-range order. The other four are featureless, i.e., the bulk ground state is unique and does not break any symmetry. The featureless phases include the rung singlet (RS) and Haldane phases as well as their variants, the RS* and Haldane* phases, in which twisted singlet pairs $\left(\right|\uparrow \downarrow \rangle +|\downarrow \uparrow \rangle )/\sqrt{2}$ are formed between the two legs. We argue and demonstrate that Gaussian transitions with the central charge $c=1$ occur between the featureless phases and between the ordered phases while Ising transitions with $c=1/2$ occur between the featureless and ordered phases. The two types of transition lines cross at the $\mathrm{SU}\left(2\right)$-symmetric point, where the criticality is described by the $\mathrm{SU}{\left(2\right)}_2$ Wess-Zumino-Witten theory with $c=3/2$. The RS-Haldane* and RS*-Haldane transitions give examples of topological phase transitions. Interestingly, the RS* and Haldane* phases, which have highly anisotropic nature, appear even in the vicinity of the isotropic case. We demonstrate that all the four featureless phases are distinguished by topological indices in the presence of certain symmetries.

Recently developed plasmonic nanostructures are able to generate intense and localized magnetic hot spots in a large spectral range from the terahertz to the visible. However, a direct measurement of the magnetic field at the hot spot has not been performed yet, due to the absence of magnetic field detectors that work at those high frequencies and that fit the hot-spot area. We propose to place a graphene ribbon in the hot spot of a plasmonic nanostructure driven by a laser beam, such that a current is generated due to both the magnetic field at the hot spot and the electric field of the laser. We demonstrate that a nonlinear Hall voltage, which can be measured by standard electrical means, builds up across the ribbon, making it possible to directly probe the magnetic field at the hot spot.

Sizable Rashba spin-orbit coupling (SOC) is of critical importance in potential applications of two-dimensional materials in spintronics devices. However, due to the presence of vertical mirror symmetry, Rashba SOC is absent in 2H transition-metal dichalcogenide monolayers and the spin Hall conductivities are attributed only to Zeeman splitting. We study theoretically the electronic structures and intrinsic spin Hall conductivity of two-dimensional monolayer Janus MoSSe by performing first-principles calculations as well as by using the Kubo formula with Wannier interpolations. We find out that monolayer Janus MoSSe possesses considerable spin Hall conductivities both in conduction and valence bands. In valence bands, the spin Hall conductivity of Janus MoSSe is comparable to that in ${\mathrm{MoS}}_2$ and ${\mathrm{MoSe}}_2$. Moreover, in the conduction bands, the spin Hall conductivities are enhanced up to two orders of magnitude because of strong Rashba SOC. The spin Hall conductivity can be tuned significantly by adjusting the Fermi level or external strains. Our results show that monolayer Janus MoSSe could be a potential candidate to realize two-dimensional flexible spintronics devices.

We theoretically investigate the electron-hole conversions in normal metal-superconductor junctions based on the Kekulé-Y (Kek-Y) patterned graphene. It is shown that, in contrast to the Klein tunneling in the pristine graphene, the perfect Andreev reflection with almost unit efficiency can be extended to a large range of incident angles in the Kek-Y patterned graphene in single-Dirac cone phase with the bias voltage in the gap. The normalized zero-bias conductance is also obtained.

The electron spin resonance (ESR) of two-dimensional electrons with large effective mass was studied experimentally near even filling factors of the integer quantum Hall effect. Surprisingly, the electron spin resonance did not vanish at the exact even fillings even in the case of large electron densities, where the ground state of the system was previously reported to be paramagnetic. Furthermore, the ESR amplitude was comparable between even and odd fillings. Such anomalous behavior suggests a substantial degree of spin polarization of the even fillings and was observed in two different material systems, namely AlAs quantum wells and ZnO/MgZnO heterojunctions. In AlAs quantum wells, spin resonance tended to split into two well-resolved lines near even fillings whereas it remained a single peak near odd ones. The evolution of ESR with varying filling factor around even fillings is studied in detail. The reported nontrivial findings suggest the modification of the ground state around even filling factors with the aid of strong $e\text{−}e$ interaction and may be viewed as a precursor for the Stoner instability.

Electron mobility in the conduction band of II-VI semiconductors ZnSe, CdTe, ZnTe, and CdSe was studied. Temperature dependence of mobility was calculated using the methodology based on density functional theory calculations of the electronic states, phonon modes, and electron-phonon coupling constants, along with Fourier-Wannier procedure for interpolation to a dense grid in momentum space. The mobilities obtained from calculations within generalized gradient approximation of density functional theory overestimate the experimental mobility several times. The calculation that used improved electronic band structure and high-frequency dielectric constants obtained using a hybrid functional lead to a very good agreement with experimental mobilities for most of the materials studied. It was also found that the Fröhlich model provides a reasonably good estimate of mobilities around room temperature where longitudinal optical phonons provide the dominant scattering mechanism, as expected for these direct gap materials where all relevant electronic states are in the vicinity of the $\mathrm{\Gamma }$ point. The results indicate as well that the long-ranged part of electron-phonon interaction fully determines the electron mobility in the materials studied. For this reason, the approach where only this part of electron-phonon interaction is calculated using the relevant analytical formulas allows for accurate calculation of mobility without the use of the interpolation procedure for electron-phonon coupling constants.

Electric dipole spin resonance (EDSR) is a commonly used tool for manipulation and spectroscopy of quantum-dot-based spin qubits. When an EDSR experiment is embedded in a transport setup and Pauli spin blockade is used as a means for spin-state read-out, then measured resonant responses in the leakage current indeed carry information about the level structure of the system under study. However, the actual line shape of these current resonances differs substantially from experiment to experiment, varying from being symmetric to asymmetric and from being a peak to a dip, a thorough understanding of which is still lacking. Here, we investigate theoretically the detailed line shape of EDSR-induced resonances in the leakage current in the regime of spin blockade, and we connect different line shapes to the different underlying physical mechanisms that can enable the EDSR. We carry out both numerical and analytical investigations, producing simple analytic expressions that give insight into the physics at play. Our results thus provide a means to extract more information about the detailed system parameters of quantum dots hosting spin qubits from an EDSR experiment than just their level structure based on the location of the resonances.

We consider a system of two spins under a scanning tunneling microscope bias and derive its master equation. We find that the tunneling elements to the electronic contacts (tip and substrate) generate an exchange interaction between the spins as well as a Dzyaloshinskii-Moriya interaction in the presence of spin-orbit coupling. The tunnel current spectrum then shows additional lines compared to conventional spin-resonance experiments. When the spins have degenerate Larmor frequencies and equal tunneling amplitudes (without spin orbit), there is a dark state with a vanishing decay rate. The coupling to the electronic environment generates significant spin-spin entanglement via the dark state, even if the initial state is nonentangled.

Oxygen vacancies have been identified to play an important role in accelerating grain growth in polycrystalline perovskite-oxide ceramics. To advance the fundamental understanding of growth mechanisms at the atomic scale, classical atomistic simulations were carried out to investigate the atomistic structures and oxygen vacancy formation energies at grain boundaries in the prototypical perovskite-oxide material $\mathrm{SrTi}{\mathrm{O}}_3$. In this paper, we focus on two symmetric tilt grain boundaries, namely, Σ5 (310)[001] and Σ5 (210)[001]. A one-dimensional continuum model is adapted to determine the electrostatic potential induced by charged lattice planes in atomistic structure models containing grain boundaries and point defects. By means of this model, electrostatic artifacts, which are inherent to supercell models with periodic or open boundary conditions, can be considered and corrected properly. We report calculated formation energies of oxygen vacancies on all the oxygen sites across boundaries between two misoriented grains, and we analyze and discuss the formation-energy values with respect to local charge densities at the vacant sites.

We analyze dispersion relations of magnons in ferromagnetic nanostructures with uniaxial anisotropy taking into account inertial terms, i.e., magnetic nutation. Inertial effects are parametrized by the damping-independent parameter $\beta$, which allows for an unambiguous discrimination of inertial effects from Gilbert damping parameter $\alpha$. The analysis of magnon dispersion relation shows its two branches are modified by the inertial effect, albeit in different ways. The upper nutation branch starts at $\omega =1/\beta$, the lower branch coincides with ferromagnetic resonance (FMR) in the long-wavelength limit and deviates from the zero-inertia parabolic dependence $\simeq {\omega }_{\text{FMR}}+D{k}^2$ of the exchange magnon. Taking a realistic experimental geometry of magnetic thin films, nanowires, and nanodiscs, magnon eigenfrequencies, eigenvectors, and $Q$-factors are found to depend on the shape anisotropy. The possibility of phase-matched magnetoelastic excitation of nutation magnons is discussed and the condition was found to depend on $\beta$, exchange stiffness $D$, and the acoustic velocity.

The linear magnetoelectric (ME) effect allows for the selection or switching between two antiferromagnetic (AFM) states via the application of large electric $\left(E\right)$ and magnetic $\left(H\right)$ fields. Once an AFM state is selected, it is preserved by an energy barrier, even when the fields are removed. Using a simple phenomenological model, we find that this energy barrier, which is needed to switch the AFM state, is proportional to the product of the $E$ and $H$ coercive fields ${\left(EH\right)}_{\mathrm{C}}$. We measured the field and temperature dependence of ${\left(EH\right)}_{\mathrm{C}}$ in ${\mathrm{LiCoPO}}_4$ for two different field configurations, and the data show the temperature variation of ${\left(EH\right)}_{\mathrm{C}}\sim {(T_{\mathrm{N}}-T)}^{3/2}$ in good agreement with the model. We also investigated the dynamics of the AFM domain switching using pulsed $E$-field measurements. It was found that the coercive field ${\left(EH\right)}_{\mathrm{C}}$ follows a power-law frequency dependence and is well described in the framework of the Ishibashi-Orihara model, implying one-dimensional character of domain wall propagation.

The breakdown of the celebrated Fermi liquid theory in the strange metal phase is the central enigma of correlated quantum matter. Motivated by recent experiments reporting short-lived carriers, along with the ubiquitous observations of modulated excitations in the phase diagram of cuprates, we propose a phenomenological boson-fermion model for this phase. Along with the fermions, we consider fluctuating bosons emerging from the remnants of a pair density wave as additional current carriers in the strange metal phase. These bosonic excitations are finite-momentum Cooper pairs and thus carry twice the electronic charge, and their net spin can either be zero or one arising from the two spin-$1/2$ electrons. We show in a perturbative calculation within Kubo formalism that such a model displays a $TlogT$ dependence of the resistivity with temperature and manifests the Drude form of the ac conductivity. Furthermore, such bosons are incoherent and hence do not contribute to the Hall conductivity. Finally, the bosons emerging from finite-momentum pairs of spin-triplet symmetry also reproduce the recently observed linear-in-field magnetoresistance in the optimally doped and overdoped cuprates [P. Giraldo-Gallo et al., Science 361, 479 (2018); J. Ayres et al., Nature 595, 661 (2021)].

Topologically stable field configurations appear in many fields of physics, from elementary particles to condensed matter and cosmology. Deep physical relations and common physical features of a large variety of very different solitonic states in these systems arise from the mathematical similarity of phenomenological equations. During the last decade, chiral liquid crystals and chiral magnets took on the role of model objects for the experimental investigation of topological solitons and the understanding of their nonsingular field configurations. This is directly related to the discovery of particle-like chiral skyrmions, which are also considered as promising ingredients for technological applications. Here we introduce a paradigm of facile skyrmionic networks with mutually orthogonal orientations of constituent isolated skyrmions. On the one hand, such networks are envisioned as a novel concept of spintronic devices based, e.g., on gapless skyrmion motion along each other, and are presumably responsible for precursor phenomena near the ordering temperatures of bulk cubic helimagnets. In particular, we demonstrate an interconversion between mutually orthogonal skyrmions: horizontal skyrmions may swirl into an intermediate spring-like state and subsequently squeeze into vertical skyrmions with both polarities. On the other hand, skyrmion tubes are considered as building blocks for particle-like states with more involved internal structure. A family of target-skyrmions, which includes a so far overlooked type with a multiple topological charge, is formed owing to the tendency to minimize the interaction energy between vertical and horizontal skyrmions. The conical phase serves as a suitable background for considered skyrmion evolution. It substantializes the attracting skyrmion-skyrmion interaction in the skyrmionic networks, shapes their internal structure, and guides the nucleation processes. Alternatively, intricate textural changes of isolated skyrmions result not only in the structural deformations of a host conical state, but may lead to the formation of an exotic skyrmion order with pairs of merons being the core of the game. Generically, the fundamental insights provided by this work emphasize a three-dimensional character of skyrmion metamorphoses and can also be extended to three-dimensional solitons, such as hopfions.

Magnetic van der Waals (vdW) materials, which can serve as ideal platforms to study low-dimensional magnetism and possess potential applications in next-generation spintronic devices, have attracted intensive attention recently. Here, based on density functional theory calculations we predict that the electron-doped $\mathrm{Mn}{\left({\mathrm{ReO}}_4\right)}_2$ (MRO) with an undulating layered structure is a ferromagnetic (FM) vdW material. Our calculations show that the magnetic interaction between two nearest-neighboring Mn atoms in the undoped MRO is very weak due to the long distance between the Mn atoms separated by the nonmagnetic ${\mathrm{ReO}}_4$ tetrahedra. With moderate electron doping from charge injection or K intercalation, the Re atoms become spin polarized. Thus, an effective FM coupling between the Mn atoms can be induced via the Mn-O-Re antiferromagnetic superexchange, while the calculated $T_c$ can exceed room temperature. The electron-doped MRO not only takes the advantage of strong FM interaction as double perovskites, but also avoids the shortage of the antisite disorder. The calculated small cleavage energy and stable phonon spectra indicate that MRO can be exfoliated to thin films, which facilitates its surface modification. Our paper on MRO, thus, provides a template to realize ferromagnetism in vdW materials with multitype transition metals.

Intrinsic magnetic topological insulators (MTIs) $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ and $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4/{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ are expected to realize the high-temperature quantum anomalous Hall effect and dissipationless electrical transport. However, there is still a lack of ideal MTI candidates with magnetic ordering of the ferromagnetic (FM) ground state. Here, we show a MTI sample of $\mathrm{Mn}{\left({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3}\right)}_4{\mathrm{Te}}_7$ which holds the coexistence of a FM behavior state and topological nontriviality. The dramatic modulation of the magnetism is induced by a charge carrier engineering process via the Sb substitution in the $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$ matrix with antiferromagnetic ordering. The evolution of magnetism in $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_4{\mathrm{Te}}_7$ is systematically investigated by our magnetic measurements and theoretical calculations. The clear topological surface states of the FM sample of $\mathrm{Mn}{\left({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3}\right)}_4{\mathrm{Te}}_7$ are further verified by angle-resolved photoemission spectroscopy. The demonstration of the intrinsic FM-MTI of $\mathrm{Mn}{\left({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3}\right)}_4{\mathrm{Te}}_7$ in this paper sheds light on further material optimization of intrinsic MTIs and paves the way for further studies to clarify the relationships between topology, magnetism, and charge carriers in topological materials.

Hyperbolic materials are receiving significant attention due to their ability to support electromagnetic fields with arbitrarily high momenta and, hence, to achieve very strong light confinement. Here, based on first-principles calculations and many-body perturbation theory, we explore the characteristic of two-dimensional plasmon modes and its hyperbolic properties for two phases of single-layer boron hosting tilted Dirac cone, namely, the $hr\text{−}sB$ and 8Pmmn borophene. In-plane anisotropy in borophene is manifested in the structural, electronic, vibrational, and optical properties. We find two hyperbolic regimes for both phases of borophene, where the high-energy one is located in the visible range. The $hr\text{−}sB$ borophene is characterized with an intrinsic high carrier density and it supports strong hyperbolic plasmon modes in the visible part of the spectrum. The 8Pmmn borophene, on the other hand, resembles the prototypical Dirac material graphene and on carrier doping acquires anisotropic Dirac plasmons in the mid-infrared. We have also investigated the impact of the electron-phonon coupling and Landau damping on these hyperbolic plasmon modes. Our results show that borophene, having high anisotropy, intrinsic high carrier concentration, low-loss hyperbolic Dirac plasmon modes, and high confinement can represent a promising candidate for low-loss broadband surface plasmon polariton devices.

van der Waals magnetic heterostructures, consisting of a wide band-gap nitride semiconductor and an intrinsic ferromagnetic semiconductor, are potentially useful for low-dimensional spintronic field-effect transistors. However, there is a significant challenge. For instance, the integration often leads to a decreased Curie temperature, the magnetic anisotropy direction change, and low conductivities. Here, we employ the first-principles density-functional method to systematically investigate the electronic and magnetic properties of the $\mathrm{GaN}/{\mathrm{CrI}}_3$ van der Waals heterostructures under electrostatic doping. Though the easy magnetization axis of the monolayer ${\mathrm{CrI}}_3$ transforms from out-of-plane to in-plane direction over 0.2 electron doping per unit cell, the $\mathrm{GaN}/{\mathrm{CrI}}_3$ van der Waals heterostructure maintains perpendicular anisotropy under electron doping, crucial for high-density information storage. The Curie temperature of the $\mathrm{GaN}/{\mathrm{CrI}}_3$ van der Waals heterostructure can be enhanced under both electron and hole doping. The $\mathrm{GaN}/{\mathrm{CrI}}_3$ van der Waals heterostructure presents half-metallic properties and the spin-up conductivities are much larger than that of the monolayer ${\mathrm{CrI}}_3$ under the same electrostatic doping. Our results indicate that constructing heterostructures with ferromagnets and nonmagnetic semiconductors is an effective strategy for developing high-performance field-effect transistors.

Higher order topological insulators (HOTIs) are a novel form of insulating quantum matter, which are characterized by having gapped boundaries that are separated by gapless corner or hinge states. Recently, it has been proposed that the essential features of a large class of HOTIs are captured by topological multipolar response theories. In this paper, we show that these multipolar responses can be realized in interacting lattice models, which conserve both charge and dipole. In this paper we study several models in both the strongly interacting and mean field limits. In two dimensions we consider a ring-exchange model which exhibits a quadrupole response, and can be tuned to a $C_4$ symmetric higher order topological phase with half-integer quadrupole moment, as well as half-integer corner charges. We then extend this model to develop an analytic description of adiabatic dipole pumping in an interacting lattice model. The quadrupole moment changes during this pumping process, and if the process is periodic we show the total change in the quadrupole moment is quantized as an integer. We also consider two interacting three-dimensional ($3\mathrm{D}$) lattice models with chiral hinge modes. We show that the chiral hinge modes are heralds of a recently proposed “dipolar Chern-Simons” response, which is related to the quadrupole response by dimensional reduction. Interestingly, we find that in the mean field limit both the two-dimensional and $3\mathrm{D}$ interacting models we consider here are equivalent to known models of noninteracting HOTIs (or boundary obstructed versions). The self-consistent mean field theory treatment provides insight into the connection between free-fermion (mean field) theories having vanishing polarization and interacting models where dipole moments are microscopically conserved.