Bulk chromium is the simplest antiferromagnet in our general material database, which has been firmly believed to exhibit a collinear spin structure and a consequent vanishing anomalous Hall effect (AHE). In this work, we report an unexpected weak AHE in polycrystalline thin films of chromium. We attribute this phenomenon to the noncollinear spin textures induced by the local spin frustration and rearrangement in certain areas with high residual strain and defect densities. Moreover, a dominant underlying mechanism of intrinsic Berry phase is speculated. This could be a general feature for all the collinear antiferromagnetic thin-film materials with moderately high defect concentrations, making them promising candidates for emergent antiferromagnetic spintronics.

In $4d$ and $5d$ transition metal compounds, novel properties usually arise from the interplay of electron correlation and spin-orbit interaction based on isolated single-site physics. However, there are also a large number of compounds where the intersite effect plays an important role. Here, we report the electronic and magnetic properties of a hexagonal perovskite ${\mathrm{Ba}}_6{\mathrm{Y}}_2{\mathrm{Rh}}_2{\mathrm{Ti}}_2{\mathrm{O}}_{17}$, which is featured with widely spaced ${\mathrm{Rh}}_2{\mathrm{O}}_9$ spin dimers. Our calculations indicate that ${\mathrm{Ba}}_6{\mathrm{Y}}_2{\mathrm{Rh}}_2{\mathrm{Ti}}_2{\mathrm{O}}_{17}$ is a semiconductor with a small band gap of 0.2 eV, which is consistent with the experimental value of 0.16 eV. In particular, the band gap of ${\mathrm{Ba}}_6{\mathrm{Y}}_2{\mathrm{Rh}}_2{\mathrm{Ti}}_2{\mathrm{O}}_{17}$ is not due to the $J_{\mathrm{eff}}=1/2$ state from isolated single site but due to the joint effect of covalent states and spin-orbit coupling formed by Rh's $t_{2g}$ orbitals inside the ${\mathrm{Rh}}_2{\mathrm{O}}_9$ dimers. Furthermore, we find a giant magnetic anisotropy energy (MAE) of about 20 meV/Rh in ${\mathrm{Ba}}_6{\mathrm{Y}}_2{\mathrm{Rh}}_2{\mathrm{Ti}}_2{\mathrm{O}}_{17}$ with the easy axis being the $c$ axis. Model calculations show that the giant MAE is mainly due to first-order perturbation on the doubly degenerate antibonding states of $e_g^{\pi }$ orbitals. Our work not only reveals the interesting electronic and magnetic properties of ${\mathrm{Ba}}_6{\mathrm{Y}}_2{\mathrm{Rh}}_2{\mathrm{Ti}}_2{\mathrm{O}}_{17}$ but also could stimulate more studies on the materials with $4d$ and $5d$ spin dimers.

SmN is an intrinsic ferromagnetic semiconductor with electrical transport properties that have been of recent experimental interest. Its conductivity is largely dictated by the concentration of nitrogen vacancies that dope the crystal with electrons. Here, we present a density functional theory based study of SmN, variously doped with nitrogen vacancies. We find that the Sm ions which coordinate a nitrogen vacancy take on an intermediate $2–3^{+}$ valence, while the remaining Sm ions in the crystal remain trivalent. Our results show that the localized $4f$ states hybridize with the extended state Sm $5d$ and N $2p$ bands and are involved in electron transport throughout the crystal.

We explore dynamic interactions between the crystal lattice and magnetic degrees of freedom in a frustrated magnetic system using the example of a pyrochlore quantum spin-ice candidate ${\mathrm{Pr}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. Using Raman scattering spectroscopy we demonstrate that crystal electric field excitations of Pr3+, which define the magnetic properties of ${\mathrm{Pr}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$, cannot be understood within a model of a static lattice. We identify vibronic interactions with a phonon which lead to a splitting of a doublet crystal field excitation at around 55 meV. We also observe an unconventional behavior of a splitting of the non-Kramers ground state doublet of ${\mathrm{Pr}}^{3+}$, revealed by observing excitations to the first excited singlet state $E_g^0\to A_{1g}$ at around 10 meV. The splitting has a strong temperature dependence, where the doublet structure is most prominent between 50 and 100 K, and the weight of one of the components strongly decreases on cooling contrary to simple thermal population tendency. We suggest a static or dynamic deviation of ${\mathrm{Pr}}^{3+}$ from the position in the ideal crystal structure can be the origin of the effect, with the deviation strongly decreasing at low temperatures.

Van der Waals (vdW) heterostructures consisting of bilayer graphene (BLG) encapsulated within monolayers of strong spin-orbit semiconductor ${\mathrm{WS}}_2$ or ferromagnetic semiconductor ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT), are investigated. By performing realistic first-principles calculations, we capture the essential BLG band structure features, including layer- and sublattice-resolved proximity spin-orbit or exchange couplings. For different relative twist angles (0 or ${60}^{\circ }$) of the ${\mathrm{WS}}_2$ layers and the magnetizations (parallel or antiparallel) of the CGT layers, with respect to BLG, the low energy bands are found and characterized by a series of fit parameters of model Hamiltonians. These effective models are then employed to investigate the tunability of the relevant energy dispersions by a gate field. For ${\mathrm{WS}}_2$/BLG/${\mathrm{WS}}_2$ encapsulation we find that twisting allows to turn off the spin splittings away from the $K$ points, due to opposite proximity-induced valley-Zeeman couplings in the two sheets of BLG. Close to the $K$ points, the electron spins are polarized out of the plane. This polarization can be flipped by applying a gate field. As for magnetic CGT/BLG/CGT structures, we realize the recently proposed spin-valve effect, whereby a gap opens for antiparallel magnetizations of the CGT layers. Furthermore, we find that for the antiferromagnetic orientation the electron states away from $K$ have vanishingly weak proximity exchange, while the states close to $K$ remain spin polarized in the presence of an electric field. The induced magnetization can be flipped by changing the gate field. These findings should be useful for spin transport, spin filtering, and spin relaxation anisotropy studies of BLG-based vdW heterostructures.

Prolonged experimental attempts to find magnetic monopoles (i.e., elementary particles with an isolated magnetic charge in three dimensions) have not yet been successful despite intensive efforts made since Dirac's proposal in 1931. Particle physicists have predicted the possible collision and pair annihilation of two magnetic charges with opposite signs. However, if such annihilation exists, its experimental observation would be difficult because its energy scale is predicted to be tremendously high ($\sim {10}^{16}$ GeV). In the present work, we theoretically predict using the Floquet theory that a pair of slightly gapped Dirac-cone bands in a weakly charge-ordered organic conductor $\alpha -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$, which behave as magnetic charges with opposite signs in the momentum space, exhibit pair annihilation under irradiation with linearly polarized light. This photoinduced pair annihilation is accompanied by a nontopological phase transition to the Floquet normal insulator phase in contrast to the well-known circularly polarized-light-induced topological phase transition to the Floquet Chern insulator phase. We discuss that $\alpha -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$ has a peculiar band structure capable of realizing a suitable experimental condition (i.e., off-resonant condition) and a charge-ordered state providing a required staggered site potential and thereby provides a rare example of materials that can be used to observe the predicted pair annihilation phenomenon. The feasibility of experimental observation is also discussed.

In this paper, we describe the antiferromagnetic and excitonic correlations in the AA-stacked bilayer graphene. We consider the applied external electric field potential to the structure which leads to the electronic charge imbalance between the layers in the system. By using the generalized two-layer Hubbard Hamiltonian, we consider different particle-filling regimes in the layers. We calculate the important energy scales in the system and we establish the conditions for the appearance of the antiferromagnetic order in the system. We consider both large and small Coulomb interaction limits in the layers and the effect of the electric field potential on the calculated order parameters. We discuss the coexistence of antiferromagnetism and excitonic phases and we show that they can coexist only in the regime away from half-filling. In the case away from the half-filling, we show the existence of a critical value $U_{\mathrm{C}}$ of the Coulomb interaction potential at which we establish the transition from the single-valued to the triple-valued excitonic states, governed by the strong electronic reconfiguration, in the system. The zero-temperature limit is considered in the problem.

We show that when a two-dimensional $\left(2\mathrm{D}\right)$ Dirac fermion moves in disordered environments, the weak time-reversal symmetry breaking by a small mass gives rise to the diffusive wave propagation, i.e., that the wave-packet spread obeys the diffusive law of Einstein, up to a—practically inaccessible—exponentially large length. Strikingly, the diffusion constant is larger than that given by the Boltzmann kinetic theory, and grows unboundedly as the energy-to-mass ratio increases. This diffusive phenomenon is of quantum nature and different from weak antilocalization. It implies a new type of transport in topological insulators at zero temperature.

When a detuned and strong laser pulse acts on an optical transition, a Stark shift of the corresponding energies occurs. We analyze how this optical Stark effect can be used to prepare and control the dark exciton occupation in a semiconductor quantum dot. The coupling between the bright and dark exciton states is facilitated by an external magnetic field. Using sequences of laser pulses, we show how the dark exciton and different superposition states can be prepared. We give simple analytic formulas, which yield a good estimate for optimal preparation parameters. The preparation scheme is quite robust against the influence of acoustic phonons. We further discuss the experimental feasibility of the used Stark pulses. Giving a clear physical picture, our results will stimulate the usage of dark excitons in schemes to generate photons from quantum dots.

Phonon surface scattering has been at the core of heat transport engineering in nanoscale devices. Herein, we demonstrate that this phonon pathway can be the sole mechanism only below a critical, size-dependent temperature. Above this temperature, the lattice phonon scattering coexists along with surface effects. By tailoring the mass disorder at the atomic level, the lattice dynamics in nanowires was artificially controlled without affecting morphology, crystallinity, chemical composition, or electronic properties, thus allowing the mapping of the temperature-thermal conductivity-diameter triple parameter space. This led to the identification of the critical temperature below which the effects of lattice mass disorder are suppressed to an extent that phonon transport becomes governed entirely by the surface. This behavior is discussed based on a modified Landauer-Datta-Lundstrom near-equilibrium transport model. Besides disentangling the main phonon scattering mechanisms, the established framework also provides the necessary input to further advance the design and modeling of heat transport in semiconductor nanoscale systems.

Bound states in the continuum (BICs) enable unique features in tailoring light-matter interaction on nanoscale. These radiationless localized states drive theoretically infinite quality factors and lifetimes for modern nanophotonics, making room for a variety of emerging applications. Here we use the peculiar properties possessed by the so-called $\mathcal{PT}$ symmetric optical structures to propose the novel mechanism for the quasi-BIC manifestation governed by the $\mathcal{PT}$ symmetry breaking. In particular, we study regularities of the spontaneous $\mathcal{PT}$ symmetry breaking in trilayer structures with the outer loss and gain layers consisting of materials with permittivity close to zero. We reveal singular points on the curves separating $\mathcal{PT}$ symmetric and broken-$\mathcal{PT}$ symmetry states in the parametric space of the light frequency and the angle of incidence. These singularities remarkably coincide with the BIC positions at the frequency of volume plasmon excitation, where the dielectric permittivity vanishes. The loss and gain value acts as an asymmetry parameter that disturbs conditions of the ideal BIC inducing the quasi-BIC. Fascinating properties of these quasi-BICs having ultrahigh quality factors and almost perfect transmission can be utilized in sensing, nonlinear optics, and other applications.

Flat-band physics has attracted much attention in recent years because of its interesting properties and important applications. Some typical lattices have been proposed to generate flat bands, such as Kagome and Lieb lattices. The flat bands in these lattices contact with other bands rather than being isolated. However, an ideal flat band should be isolated, because isolation is a prerequisite for a number of important applications. Here, we propose a lattice that can produce isolated flat bands. The lattice is named an interlocking-circles lattice because its pattern seems like interlocking circles. Moreover, the lattice is realized in graphene by hydrogenation. In the hydrogenated graphene, there are two nontrivial isolated flat bands appearing around the Fermi level. Upon hole or electron doping, the flat bands split into spin-polarized bands and then result in a ferromagnetic graphene. Our work not only proposes a type of lattice but also opens a way to find systems with isolated flat bands.

Traditional ionic and covalent compound insulators arise from a commensuration between electron count and system volume. On the other hand, conventional topological insulators, outside of quantum Hall effect systems, do not typically display such a commensuration. Instead, they can undergo a phase transition to a trivial insulator that preserves the electron filling. Nevertheless, in some crystalline insulators, termed filling-enforced quantum band insulators (feQBIs), electron filling can dictate nontrivial topology in the insulating ground state. Currently, little is known about the relation between feQBIs and conventional topological invariants. In this work, we study such relations for a particularly interesting example of a half-filling feQBI that is realized in space group 106 with spinless electrons. We prove that any four-band feQBI in space group 106 with filling 2 must have a nontrivial topological invariant, namely, the ${\mathbb{Z}}_2$ glide invariant, and thus must have a quantized magnetoelectric polarizability $\theta =\pi$. We thus have found a three-dimensional example where electron filling and band topology are tied. Such a locking raises intriguing questions about the generality of the band-inversion paradigm in describing the transition between trivial and topological phases.

We perform a detailed study of the phase transitions and mechanisms of electron localization in the extended Hubbard model using the dynamical cluster approximation on a $2\times 2$ cluster. We explore the interplay of charge order and Mott physics. We find that a nearest-neighbor Coulomb interaction $V$ causes “screening” effects close to the Mott phase transition, pushing the phase boundary to larger values of $U$. We also demonstrate the different effects of $V$ on correlations in metallic and insulating regimes, and document the different correlation aspects of charge order and Mott states.

Quantum many-body scars (QMBSs) appear in a flat-band model with interactions on the sawtooth lattice. The flat-band model includes localized eigenstates within compact support, known as compact localized states (CLSs). Some characteristic many-body states can be constructed from the CLSs at a low filling on the flat band. These many-body states are degenerate. Starting with such degenerate states we concretely show how to construct multiple QMBSs with different eigenenergies embedded in the entire spectrum. If the degeneracy is lifted by introducing hopping modulation or weak perturbations, the states lifted by these ways can be viewed as multiple QMBSs. In this work, we focus on the study of the perturbation-induced QMBS. Perturbed states, which are connected to the exact QMBSs in the unperturbed limit, indicate common properties of conventional QMBS systems, that is, a subspace with subvolume- or area-law scaling entanglement entropy, which indicates the violation of the strong eigenstate thermalization hypothesis (ETH). Also for a specific initial state, slow-thermalization dynamics appears. We numerically demonstrate these subjects. The flat-band model with interactions is a characteristic example in nonintegrable systems with the violation of the strong ETH and the QMBS.

To investigate the initial process of Joule heating in semiconductors microscopically and quantitatively, we developed a theoretical framework for the ab initio evaluation of the carrier energy relaxation in semiconductors under a high electric field using a combination of the two-temperature model and the Boltzmann equation. We employed the method for bulk silicon as a typical example. Consequently, we found a remarkable difference in the energy relaxation processes of the electron and hole carriers. The longitudinal acoustic and optical phonons at the zone boundary contribute to the energy relaxation of electron carriers, whereas they contribute negligibly to that of the hole carriers. In addition, at the band edge, the energy relaxation rate is maximized for the electron carriers, whereas it is suppressed for the hole carriers. These differences stem from the presence/absence of intervalley scattering processes and isotropic/anisotropic band structures in electrons and holes. Our results lay the foundation for controlling the thermal generation in semiconductors by material design.

Assisted by general symmetry arguments and a many-body invariant, we introduce a phase of matter that constitutes a topological SO(5) superfluid. Key to this finding is the realization of an exactly solvable model that displays some similarities with a minimal model of superfluid ${}^3\mathrm{He}$. We study its quantum phase diagram and correlations, and find exotic superfluid as well as metallic phases in the repulsive sector. At the critical point separating trivial and nontrivial superfluid phases, our Hamiltonian reduces to the globally SO(5)-symmetric Gaudin model with a degenerate ground manifold that includes quartet states. Most importantly, the exact solution permits uncovering of an interesting non-pair-breaking mechanism for superfluids subject to external magnetic fields. Nonintegrable modifications of our model lead to a strong-coupling limit of our metallic phase with a ground-state manifold that shows an extensive entropy.

Multipole representation is proposed for the anisotropic Coulomb interactions in solids. Any local interactions can be expressed as the product of two multipole operators, and the interaction parameters are systematically classified based on the point group symmetry. The form of the multipole interactions are restricted not only by the symmetry and Hermiticity but also by the spatial structure of the interaction, which is closely related to the presence or absence of the odd-rank multipoles. As an exemplary demonstration, the screened Coulomb interaction for ${\mathrm{SrVO}}_3$ is considered, where only a few parameters are necessary for its description. By comparing it with the unscreened version, the totally symmetric $A_1$ representation is found to be strongly suppressed, but the $A_1$ component still gives a dominant contribution for the anisotropic part of the interaction. The anisotropic interactions are also applied to the localized two $f$-electron wave functions, which give the same-order contribution as the one-body level splitting estimated by the band structure calculation.

The statistical properties of spectra of quantum systems within the framework of random matrix theory are widely used in many areas of physics. These properties are affected if two or more sets of spectra are superposed, resulting from the discrete symmetries present in the system. The superposition of spectra of $m$ such circular orthogonal, unitary, and symplectic ensembles are studied numerically using higher-order spacing ratios. For given $m$ and the Dyson index $\beta$, the modified index ${\beta }^{\prime }$ is tabulated whose nearest-neighbor spacing distribution is identical to that of $k$-th order spacing ratio. For the case of $m=2$ ($m=3$) in the Circular Orthogonal Ensemble (Circular Unitary Ensemble) a scaling relation between ${\beta }^{\prime }$ and $k$ is given. For COE, it is conjectured that for $k=m+1$ ($m\ge 2$) and $k=m-3$-th ($m\ge 5$) order spacing ratio distribution the ${\beta }^{\prime }$ is $m+2$ and $m-4$, respectively, whereas in the case of the Circular Symplectic Ensemble, for $k=m+1$ ($m\ge 2$) and $k=m-1$-th ($m\ge 3$) the ${\beta }^{\prime }$ is $2m+3$ and $2(m-2)$, respectively. We also conjecture that for given $m$ ($k$) and $\beta$, the sequence of ${\beta }^{\prime }$ as a function of $k$ ($m$) is unique. Strong numerical evidence in support of these results is presented. These results are tested on complex systems like the measured nuclear resonances, quantum chaotic kicked top and spin chains.

Controllable magnetic skyrmion motion represents a highly relevant issue in preparing advanced skyrmion-based spintronic devices. Specifically, magnon-driven skyrmion motion can be easily accessible in both metallic and insulating magnets and thus is highly preferred over electric current control, further for the ultralow energy consumption. In this paper, we investigate extensively the dynamics of skyrmion motion driven by magnons in an antiferromagnet using the collective coordinate theory, focusing on the effect of magnon polarization. It is theoretically revealed that skyrmion Hall motion driven by circularly polarized magnons becomes inevitable generally, benefiting a comprehensive understanding of antiferromagnetic (AFM) skyrmion dynamics. More importantly, the elastic scattering theory and numerical results unveil the strong interdependence between linearly polarized magnons and skyrmion motion, suggesting the complicated dependence of skyrmion motion on the polarization nature of driving magnons. On the reversal, the scattering from the moving skyrmion may lead to decomposition of the linearly polarized magnon into two elliptically polarized magnon bands. Consequently, a net transverse force acting on the skyrmion is generated owing to the broken mirror symmetry, which in turn drives skyrmion Hall motion. Hall motion can be completely suppressed only in a specific condition where the mirror symmetry is preserved. This paper unveils nontrivial skyrmion-magnon scattering behavior in antiferromagnets, advancing AFM spintronics and benefiting high-performance devices.