We study low-energy properties of spin-$S$ Kitaev models in an anisotropic limit. The effective form of a local conserved quantity is derived in the low-energy subspace. We find this is the same as that of $S=1/2$ case for the half-integer spins but shows a different form for the integer spins. Applying the perturbation theory to the anisotropic Kitaev model, we obtain the effective Hamiltonian. In the integer spin case, the effective model is equivalent to a free spin model under a uniform magnetic field, where quantum fluctuations are quenched. On the other hand, in the half-integer case, the system is described by the toric code Hamiltonian, where quantum fluctuations play a crucial role in the ground state. The boundary effect in the anisotropic Kitaev system is also discussed.

Devising magnetic nanostructures with spatially heterogeneous Dzyaloshinskii-Moriya interaction (DMI) is a promising pathway toward advanced confinement and control of magnetic skyrmions in potential devices. Here we discuss theoretically how a skyrmion interacts with a heterochiral interface using micromagnetic simulations and analytic arguments. We show that a heterochiral interface deflects the trajectory of ferromagnetic (FM) skyrmions, and that the extent of such deflection is tuned by the applied spin-polarized current and the difference in DMI across the interface. Further, we show that this deflection is characteristic of the FM skyrmion, and it is completely absent in the antiferromagnetic (AFM) case. In turn, we reveal that the AFM skyrmion achieves much higher velocities than its FM counterpart, yet experiences far stronger confinement in nanoengineered heterochiral tracks, which reinforces AFM skyrmions as a favorable choice for skyrmion-based devices.

We study the superconducting transition in a two-dimensional electron gas with strong Rashba spin-orbit coupling. We assume low electron density, such that only the majority spin band participates in the transition. We show that the superconducting transition follows either the Bose-Einstein condensation (BEC) or the Bardeen-Cooper-Schrieffer (BCS) scenarios, depending on the position of the chemical potential with respect to the bottom of the majority band, and the strength of the Coulomb repulsion between electrons. Hence, the BEC-BCS crossover in this system can be driven either by the change in the chemical potential, or the distance to a gate.

In the nested limit of the spin-fermion model for the cuprates, one-dimensional physics in the form of half-filled two-leg ladders emerges. We show that the renormalization group flow of the corresponding ladder is towards the $d$-Mott phase, a gapped spin-liquid with short-ranged $d$-wave pairing correlations, and reveals an intermediate $\mathrm{SO}\left(5\right)\times \mathrm{SO}\left(3\right)$ symmetry. We use the results of the renormalization group in combination with a memory-function approach to calculate the optical conductivity of the spin-fermion model in the high-frequency regime, where processes within the hot spot region dominate the transport. We argue that umklapp processes play a major role. For finite temperatures, we determine the resistivity in the zero-frequency (dc) limit. Our results show an approximate linear temperature dependence of the resistivity and a conductivity that follows a nonuniversal power law. A comparison to experimental data supports our assumption that the conductivity is dominated by the antinodal contribution above the pseudogap.

Motivated by the recent experiments that reported signatures of many-body localization of ultracold atoms in optical lattices [M. Schreiber et al., Science 349, 842 (2015)], we study dynamics of highly excited states in the strongly disordered Hubbard model in one dimension. Owing to the $SU\left(2\right)$ spin symmetry, spin degrees of freedom form a delocalized thermal bath with a narrow bandwidth. The spin bath mediates slow particle transport, eventually leading to delocalization of particles. The particle hopping rate is exponentially small in $t/W(t,W$ being hopping and disorder scales) owing to the narrow bandwidth of the spin bath. We find the optimal length scale for particle hopping and show that the particle transport rate depends strongly on the density of singly occupied sites in the initial state. The delocalization rate is zero for initial states with only doubly occupied or empty sites, suggesting that such states are truly many-body localized, and therefore the Hubbard model may host both localized and delocalized states. Full many-body localization can be induced by breaking spin rotational symmetry.

Excitonic complexes in type-II quantum-ring heterostructures may be considered as artificial atoms due to the confinement of only one charge-carrier type in an artificial nucleus. Binding energies of excitons, trions, and biexcitons in these nanostructures are then effectively ionization energies of these artificial atoms. The binding energies reported here are calculated within the effective-mass approximation using the diffusion quantum Monte Carlo method and realistic geometries for gallium antimonide rings in gallium arsenide. The electrons form a halo outside the ring, with very little charge density inside the central cavity of the ring. The de-excitonization and binding energies of the complexes are relatively independent of the precise shape of the ring.

An impurity in a Tomonaga-Luttinger liquid leads to a crossover between the short- and long-distance regime which describes many physical phenomena. However, a calculation of the entire crossover of correlation functions over different length scales has been difficult. We develop a powerful numerical method based on the infinite density matrix renormalization group. By utilizing infinite boundary conditions we can obtain correlation functions within a finite-size window that contains the impurity. For the $S=1/2$ chain, we demonstrate that a full crossover can be precisely obtained, and that their limiting behaviors show a good agreement with field-theory predictions.

Machine learning algorithms provide a new perspective on the study of physical phenomena. In this Rapid Communication, we explore the nature of quantum phase transitions using a multicolor convolutional neural network (CNN) in combination with quantum Monte Carlo simulations. We propose a method that compresses $(d+1)$-dimensional space-time configurations to a manageable size and then use them as the input for a CNN. We benchmark our approach on two models and show that both continuous and discontinuous quantum phase transitions can be well detected and characterized. Moreover, we show that intermediate phases, which were not trained, can also be identified using our approach.

Reliable predictions of electronic levels, excited-state geometric relaxation, and the relative energies of ground and excited levels to host band edges are of paramount importance for $\mathrm{C}{\mathrm{e}}^{3+}$-doped luminescent materials. By combining the constrained occupancy approach and the hybrid density functional calculation in the framework of a generalized Kohn-Sham formalism, we derived a calculation scheme for the band gap of the host material, the equilibrium configurations of ground-state $\mathrm{C}{\mathrm{e}}^{3+}$ and excited-state ($\mathrm{C}{\mathrm{e}}^{3+}$)*, and their relative energies with respect to host band edges in terms of hole capture or electron ionization for $\mathrm{C}{\mathrm{e}}^{3+}$ in $M_2{\mathrm{B}}_5{\mathrm{O}}_9\mathrm{Cl}$ ($M=\mathrm{Ca}$, Sr) charge compensated by $\mathrm{N}{\mathrm{a}}^{+}$. The results of first-principles calculations for $4f\to 5d$ excitations, Stokes shifts, and the relative position of $5d$ levels to conduction-band edge agree well with experiments. The moderate computational cost of the present scheme, which can be applied in efficient prediction of the optical properties of many different Ce-doped materials, is of important value in screening potential lanthanide-doped scintillators and phosphors from minimal information about the host crystal structure.

Topological states were initially discovered in solid state systems and have generated widespread interest in many areas of physics. The advances in cold atoms create novel settings for studying topological states that would be quite unrealistic in solid state systems. One example is that the constituents of quantum gases can be various types of bosons, fermions, and their mixtures. This paper explores interaction-induced topological states in two-dimensional Bose-Fermi mixtures. We propose a class of topological states which have no fractionalized excitations but possess maximally chiral edge states. For previously known topological states, these two features can be found simultaneously in the integer quantum Hall states of fermions and the $E_8$ state of bosons. The existence of some proposed states in certain continuum and lattice models is corroborated by exact diagonalization and density matrix renormalization group calculations. This paper suggests that the Bose-Fermi mixture is a very appealing platform for studying topological states.

Monoatomic metallic glasses, successfully obtained in recent experiments, represent ideal systems to investigate the local atomic structures in glasses and glass-forming processes. By ab initio molecular dynamics we simulate the formation of a metal glass of vanadium and compute different structural, energetic, and electronic properties, including the electrical and thermal conductivities, which are compared to those of vanadium in the standard, solid-state, bcc crystal phase (obtained by adopting a slower quenching rate) and in the liquid phase too. As found for other monoatomic metallic glasses, we show that the fundamental structural process of V glass formation is represented by the tendency of V atoms to form icosahedral structures. This conclusion, together with the analysis of the electronic-charge distribution and the estimate of the electrical conductivity, suggests that the glass state of vanadium can be interpreted as a “frozen” (inherent) liquid configuration.

A recent theory by Chen and Zhang [Phys. Rev. Lett. 114, 126602 (2015)] predicts strongly anisotropic damping due to interfacial spin-orbit coupling in ultrathin magnetic films. Interfacial Gilbert-type relaxation, due to the spin pumping effect, is predicted to be significantly larger for magnetization oriented parallel to compared with perpendicular to the film plane. Here, we have measured the anisotropy in the $\mathrm{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/Pt system via variable-frequency, swept-field ferromagnetic resonance (FMR). We find a very small anisotropy of enhanced Gilbert damping with sign opposite to the prediction from the Rashba effect at the FM/Pt interface. The results are contrary to the predicted anisotropy and suggest that a mechanism separate from Rashba spin-orbit coupling causes the rapid onset of spin-current absorption in Pt.

Magnon dark mode, which possesses long coherence time, has been observed in transmission experiments of a microwave cavity loaded with two yttrium iron garnet spheres. Due to two sublattice magnetizations, it seems likely to exhibit magnon dark mode in an isolated antiferromagnet (AFM). To demonstrate such possibility, we developed two distinct approaches based on the magnon-photon Hamiltonian of AFM-embedded cavity and the transfer matrix method, respectively. The numerical and analytical results show that the magnon dark mode can not be obtained in AFM due to different mode polarization of two magnon modes for propagation parallel to applied field and unequal frequencies for propagation perpendicular to applied field. Contrary to previous theoretical work, our calculation includes both mode polarization and propagation direction of waves explicitly, which are important for correctly describing the coupling between cavity photons and AFM magnons. Moreover, it is found that the cavity resonance remains unperturbed even when the magnon-photon coupling is present. This behavior is attributed to the competition between two types of magnon-photon interactions. The results reported here are important for understanding cavity optomagnonics in AFM, ferrimagnet, and other magnets with multiple sublattice magnetizations.

High-precision ultrasonic measurements are used to study magnetoelastic coupling as a function of the in-plane magnetic field orientation in the spin-$1/2$ triangular lattice antiferromagnet ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$. The relevance of this coupling in stabilizing the 1/3 magnetization plateau is explored. The analysis indicates that, while the magnetoelastic coupling in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ is large in comparison to other triangular lattice antiferromagnets, the strength of this coupling is still too small to fully account for the magnetization plateau width in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$. Spin fluctuations are therefore the dominant mechanism inducing and stabilizing the magnetization plateau. Our results also show that the amplitude of the spin fluctuations suddenly drop as the V phase is induced at higher field.

We study theoretically the anisotropic magneto-Peltier effect, which was recently demonstrated experimentally. A first-principles-based Boltzmann transport approach including the spin-orbit interaction shows that Ni has a larger anisotropy of the Peltier coefficient ($\mathrm{\Delta }\mathrm{\Pi }$) than Fe, consistent with experiments. It is clarified that spin-flip electron transitions due to the spin-orbit interaction are the key in the mechanism of the large anisotropic magneto-Peltier effect. Using our method, we further predict several ferromagnetic metals with much larger $\mathrm{\Delta }\mathrm{\Pi }$ than that of Ni.

The spin wave in long-periodic spiral magnetic structures demonstrates the so-called magnetochiral nonreciprocity, i.e., a noninvariance of its dispersion when it propagates parallel and antiparallel to the chiral wave vector. Motivated by recent experiments on systems with magnetochiral long-periodic orders, we studied the effect of static spatially varying magnetization structure on spin wave relaxation. A description of an intrinsic nonlocal magnetization relaxation is given within the scope of a phenomenological equation called the Landau-Lifshitz-Baryakhtar equation (V. G. Bar'yakhtar, Zh. Eksp. Teor. Fiz. 87, 1501 (1984) [Sov. Phys. JETP 60, 863 (1984)]) for crystals of different symmetry. Spiral magnetic structures with a helicity step due to competitive exchange interactions or due to the Dzyaloshinskii-Moriya interaction are considered, and related dissipative functions are obtained. Within the general phenomenological approach, it is demonstrated that magnetochiral nonreciprocity manifests itself in different nonlocal dampings of spin waves with opposite wave vectors, as well. More fundamentally, we show that static but spatially varying magnetization causes a finite damping of the Goldstone mode. Understanding the peculiar nonlocal relaxation of the magnetization precession in materials with spatially varying magnetic order is important for applications of such magnets in magnonics and spintronics.

Using Monte Carlo techniques, we study a three-orbital ${\mathrm{CuO}}_2$ spin-fermion model for copper-based high-critical-temperature superconductors that captures the charge-transfer properties of these compounds. Our studies reveal the presence of spin order in the parent compound and, more importantly, stripe spin and charge order under hole doping. Due to the $p\text{−}d$ orbital hybridization, the added holes are approximately equally distributed among the two $p$ orbitals of the oxygen atoms and the $d$ orbital of the copper atoms in the unit cell. In rectangular clusters of dimension $16\times 4$ half-filled stripes are observed upon hole doping; namely, when $N_h=2n$ holes are introduced in the system, then $n$ vertical stripes of length 4 along the short periodic direction are formed. The original antiferromagnetic order observed in the parent compound develops a $\pi$ shift across each stripe, and the magnetic structure factor has a peak at wave vector $\mathbf{k}=(\pi -\delta ,\pi )$ with $\delta =2\pi N_h/N=\pi N_h/2L$, where $L=16$. The electronic charge is also modulated, and the charge structure factor is maximized at $\mathbf{k}=(2\delta ,0)$. As electrons are removed from the system, intracell orbital nematicity with $\langle n_{p_x}\rangle -\langle n_{p_y}\rangle \ne 0$ develops in the oxygen sector, as well as intercell magnetic nematicity with $\langle S_{\mathbf{i},d}^z(S_{\mathbf{i}+\mathbf{x},d}^z-S_{\mathbf{i}+\mathbf{y},d}^z)\rangle \ne 0$ in the spin copper sector, in the standard notation. This occurs not only in rectangular but also in square $8\times 8$ lattices. Our results suggest that the essence of the stripe spin and charge distribution experimentally observed in hole-doped cuprates is captured by unbiased Monte Carlo studies of a simple hole-doped charge-transfer insulator ${\mathrm{CuO}}_2$ spin-fermion model.

Multi-Weyl semimetals are new types of topological semimetals which have anisotropic nonlinear energy dispersion and whose topological charge can be 2 or more depending on the value of the winding number $J$. Here we investigate the Kondo effect of a spin-$1/2$ magnetic impurity in a multi-Weyl semimetal of different $J$ by using the variational wave function method. For different $J$, the binding energy is always positive in the presence of broken inversion symmetry, and a bound state is favored to form between the impurity and the host electrons. It is found that the formed bound state is more stable for larger $J$. Due to the spin-orbit coupling, the components of spin-spin correlation functions $J_{uv}\left(\mathbf{r}\right)\left(u,v=x,y,z\right)$ show strong anisotropy in the coordinate space. The spin-spin correlation indicates distinct decay behaviors along different directions due to the anisotropic dispersion in the energy band. In the case of $J=2$ and $J=3$, the rotational symmetries of the spatial spin-spin correlations are displayed. Especially, the components $J_{xx}$ and $J_{yy}$ and $J_{xy}$ and $-J_{yx}$ can be related through a $\pi /\left(2J\right)$ rotation.

Polarization of photonic beams makes a continuous information carrier and thus modifying it dramatically within a short distance is desirable for numerous optical devices. The easiest way to emulate anisotropy, which is a prerequisite for any polarization conversion, is utilizing thin slices of plasmonic multilayers, whose performance is limited by the inevitable losses. Such a difficulty gets overcome by using pairs of these slices in a bilayer configuration and optimizing the direction of their optical axes. In this sense, the losses are balanced by the interplay between fields in each slice and the overall interference scheme yields closer-to-desired transmissions. Several realistic designs giving effective polarization twists of substantial immunity to fabrication defects or misalignment in oscillation frequency and incidence angle, are obtained. These structures are both efficient and minimalistic; thus can be useful in many photonic systems requiring robust polarization engineering and compact packaging.

Phonons are well known to be the main mechanism for the coupling between bright and dark excitons in nonmagnetic semiconductors. Here, we investigate diluted magnetic semiconductors where this process is in direct competition with the scattering at localized magnetic impurities. To this end, a recently developed quantum kinetic description of the exciton spin dynamics in diluted magnetic semiconductor quantum wells is extended by the scattering with longitudinal acoustic phonons. A strong phonon impact is found in the redistribution of exciton momenta on the exciton parabola that becomes especially prominent for high temperatures and exciton distributions further away from the exciton resonance which are optically dark. Despite their impact on the energetic redistribution, acoustic phonons virtually do not affect the exciton spin dynamics as the exciton-impurity interaction always dominates. Furthermore, it turns out that the exciton spin lifetime increases by roughly one order of magnitude for nonequilibrium hot exciton distributions and, in addition, pronounced quantum kinetic signatures in the exciton spin dynamics appearing after resonant optical excitation are drastically reduced.