The Dzyaloshinskii-Moriya (DM) interaction that originates from relativistic spin-orbit coupling in noncentrosymmetric magnets is a source of topological spin textures. We theoretically investigate the possibility of a skyrmion crystal (SkX) by focusing on the role of the out-of-plane DM interaction that is different from the polar- and chiral-type DM interaction. By performing the simulated annealing for a spin model on a triangular lattice belonging to the $D_{3h}$ point group, we construct low-temperature magnetic phase diagrams in an applied magnetic field. As a result, we find the instability toward different skyrmion crystals under in-plane and out-of-plane magnetic fields, where the key ingredients to stabilize the SkX are different according to the magnetic field direction; the SkX is stabilized under the out-of-plane DM interaction for the in-plane magnetic field, while it is stabilized by additionally introducing an easy-axis anisotropy for the out-of-plane magnetic field. We also discuss the possible multiple-$Q$ states other than the skyrmion crystals in the presence of the out-of-plane DM interaction. Our result indicates that noncentrosymmetric magnetic materials with the out-of-plane DM interaction are potential candidates to host the skyrmion crystals.

Topologically protected whirling magnetic textures could emerge as data carriers in next-generation post-Moore computing. Such textures are abundantly observed in ferromagnets (FMs); however, their antiferromagnetic (AFM) counterparts are expected to be even more relevant for device applications, as they promise ultrafast, deflection-free dynamics while being robust against external fields. Unfortunately, such textures have remained elusive; hence identifying materials hosting them is key to developing this technology. Here, we present comprehensive micromagnetic and analytical models investigating topological textures in the broad material class of $A$-type antiferromagnets, specifically focusing on the prototypical case of $\alpha {\text{-Fe}}_2{\text{O}}_3$—an emerging candidate for AFM spintronics. By exploiting a symmetry-breaking interfacial Dzyaloshinskii-Moriya interaction (iDMI), it is possible to stabilize a wide topological family, including AFM (anti)merons, bimerons, and the hitherto undiscovered AFM skyrmions. While iDMI enforces homochirality and improves the stability of these textures, the widely tunable anisotropy and exchange interactions enable precise control of their core dimensions. We then present a unifying framework to model the scaling of texture sizes based on a simple dimensional analysis. As the parameters required to host and tune homochiral AFM textures may be obtained by rational materials design of $\alpha {\text{-Fe}}_2{\text{O}}_3$, it could emerge as a promising platform to initiate AFM topological spintronics.

Orbital angular momentum (OAM) plays a central role in regulating the magnetic state of electrons in nonperiodic systems such as atoms and molecules. In solids, on the other hand, OAM is usually quenched by the crystal field, and thus has a negligible effect on magnetization. Accordingly, it is generally neglected in discussions around band topology such as Berry curvature and intrinsic anomalous Hall effect (AHE). Here, we present a theoretical framework demonstrating that crystalline OAM can be directionally unquenched in transition metal oxides via energetic proximity of the conducting $d$ electrons to the local magnetic moments. We show that this leads to “composite” Fermi pockets with topologically nontrivial OAM textures. This enables a giant Berry curvature and a resultant intrinsic nonmonotonic AHE, even in collinearly ordered spin states. We use this model to explain the origin of the giant AHE observed in the forced ferromagnetic state of ${\mathrm{EuTiO}}_3$ and propose it as a general scheme for OAM driven AHE.

Recently, a series of two-dimensional nonmagnetic layered materials $X{\mathrm{Si}}_2{Y}_4$ ($X$=transition metals; $Y$ = pnictogens) having similar crystal structures with transition-metal dichalcogenides (TMDs) were proposed for their potential application value. Like TMDs, we propose that chiral phonon involved valley-selective optical circular dichroism can be also obtained in $X{\mathrm{Si}}_2{Y}_4$, and it can be further entangled with multiple Hall effects. However, it is difficult to compare such effect between $X{\mathrm{Si}}_2{Y}_4$ and TMDs due to the nonunified conventions for pseudoangular momentum (PAM) in the previous studies. Here we use ${\mathrm{MoSi}}_2{\mathrm{As}}_4$ and ${\mathrm{MoS}}_2$ as examples to establish a unified convention for both phonon PAM and electronic PAM, together with showing their similarities and differences in crystal structure, band structures, and valley-selective optical circular dichroism. In particular, we find in ${\mathrm{MoSi}}_2{\mathrm{As}}_4$, the chiral phonon emission/absorption, the spin, the valley, and the transverse Hall current, can be modulated by changing either the energy or the handedness of the incident light, which show better performance than TMDs.

We demonstrate a method to control the Fermi level around the Van Hove singularity (VHS) in Li-intercalated graphene on the SiC substrate. By angle-resolved photoemission spectroscopy, we observed a clear Lifshitz transition in the vicinity of the VHS when the thickness of graphene exceeds four layers. We calculated the band structure of a multilayer system with different stacking sequences of graphene and Li layer. The so-called stage 2 model reproduces the Lifshitz transition, where Li occupies every other interlayer of graphene. In addition, we found that a sizable Schottky barrier is formed between graphene and the substrate. These properties allow us to explore the electronic phase diagram around the VHS by controlling the thickness.

We report the detection of electron spin resonance (ESR) in individual dimers of the stable free radical 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO). ESR is measured by the current fluctuations in a scanning tunneling microscope (ESR-STM method). The multipeak power spectra, distinct from macroscopic data, are assigned to dimers having exchange and Dzyaloshinskii-Moriya interactions in the presence of spin-orbit coupling. These interactions are generated in our model by interfering electronic tunneling pathways from tip to sample via the dimer's two molecules. This is the first demonstration that tunneling via two spins is a valid mechanism of the ESR-STM method.

We study quantum features of electromagnetic radiation propagating in a one-dimensional superconducting quantum metamaterial composed of an infinite chain of charge qubits placed within two stripe massive superconducting resonators. The quantum-mechanical model is derived assuming weak fields and that, at low temperatures, each qubit is either unoccupied or occupied by a single Cooper pair. We demonstrate the emergence of two bands of single-photon qubit bound states with the energies lying outside the photon continuum—one is above and the second slightly below the linear photon band. The higher energy band varies slowly with the qubit-photon center of mass quasimomentum. It becomes practically flat provided that the electromagnetic energy is far below the Josephson energy when the latter is small compared to the charging energy. The dispersion of the lower band is practically identical to that of free photons. The emergence of bound states may cause radiation trapping indicating possible applicability for the control of photon transport in superconducting qubit-based artificial media.

By using the variational matrix product state method, we numerically study the interacting Kitaev chain with spatially varying periodic and quasiperiodic potentials, and the latter follows the Fibonacci sequence. The edge correlation functions of Majorana fermions and low-lying ground states are computed to explore the robustness of topological superconducting phase. It is found that the original topologically nontrivial phase is separated into two branches by an emergent topologically trivial phase, as a result of the competition among spatially varying potential, electronic, Coulomb interaction, and chemical potential. The analysis of energy gap, occupation number, and intertwined string order parameters together suggests that the lift of degeneracy in the topologically trivial phase is enabled by a potential-induced fracton mechanism, namely, the pairing of four Majorana fermions. The evolution from the emergent fractal structure of population to the original structure of charge density wave is investigated as well.

The Cu-based chalcopyrite compounds are good thermoelectrics for waste heat harvesting, partially due to the dramatic reduction of the thermal conductivity above Debye temperature, whereas the mechanism of this drop is still elusive. By Raman measurements from 7 to 780 K, we have investigated the anharmonicity of the phonon modes ${\mathrm{B}}_2^1, {\mathrm{A}}_1$, and ${\mathrm{B}}_2^3$ in ${\mathrm{CuInTe}}_2$. The fourth-order anharmonicity of ${\mathrm{B}}_2^1$ and ${\mathrm{B}}_2^3$ modes is greatly enhanced with increasing temperature and becomes dominant above 400 K. The phonon dynamics is calculated from 10 to 800 K. The fourth-order anharmonicity is predominant over the third-order ones for the phonon modes at low ($<70{\mathrm{cm}}^{-1}$) and high ($>130{\mathrm{cm}}^{-1}$) frequency domains, confirmed by the enhanced weighted phase space and scattering rate of the four-phonon processes above 300 K. The phonon avoided-crossing is found to take place around 40 ${\mathrm{cm}}^{-1}$ between the lowest-optical mode and acoustic modes. Counting on the fourth-order phonon anharmonicity, the calculated lattice thermal conductivity agrees well with experiment. The results unveil that the dramatic thermal conductivity reduction, about 83% from 300 to 800 K, originates from the enhanced four-phonon process. Our insights on the thermal transport mechanisms might benefit the materials design of thermoelectrics and thermal control.

Strong long-range coupling between distant spins is crucial for spin-based quantum information processing. However, achieving such a strong spin-spin coupling remains challenging. Here we propose to realize a strong coupling between two distant spins via the Kerr effect of magnons in a yttrium-iron-garnet nanosphere. By applying a microwave field on this nanosphere, the Kerr effect of magnons can induce the magnon squeezing, so that the coupling between the spin and the squeezed magnons can be exponentially enhanced. This in turn allows the spin-magnon distance to increase from nano- to micrometer scale. By considering the virtual excitation of the squeezed magnons in the dispersive regime, strong spin-spin coupling mediated by the squeezed magnons can be achieved, and a remote quantum-state transfer, as well as the nonlocal two-qubit iswap gate with high fidelity becomes implementable. Our approach offers a feasible scheme to perform quantum information processing among distant spins.

Quantum Hall edge channels can be combined with metallic regions to fractionalize electrons and form correlated impurity models. We study a minimal device that has been experimentally achieved quite recently, with two floating islands connected to three edge channels via quantum point contacts in the integer quantum Hall regime. At high transparency of the quantum point contacts, we establish a mapping to the boundary sine-Gordon model and thereby reveal the nature of the quantum critical point. We deduce from this mapping universal expressions for the conductance and noise, in agreement with the experimental findings, and discuss the competition between Kondo-like screening of each individual island and the cooperative transfer of electrons between them. We further predict that the device operated at finite voltage bias produces fractional charges $e^{*}=e/3$ and propose a generalization to $N$ islands with the fractional charge $e^{*}=e/(N+1)$.

The critical behavior of the Ising model in three dimensions on a lattice with site disorder is studied by applying Monte Carlo simulation techniques. Two cases for the site disorder are considered: uncorrelated disorder and long-range correlated disorder with a spatial correlation function that decays according to a power law $r^{-a}$. The critical exponents $\beta$ and $\gamma$ as well as updated results for the critical exponent $\nu$ and confluent correction exponent $\omega$ are provided for a variety of different correlation exponents $a$ and disorder concentrations $p_d$. The estimation is done by using finite-size scaling analyses and a global fit procedure which combines the results obtained for different concentrations of defects. From the estimated critical exponents, the validity of hyperscaling relations is studied and finally the critical temperatures are provided for different $a$ and $p_d$.

In band Jahn-Teller (BJT) metals, some lattice distortions with their symmetries different from that of the bulk structure can linearly couple to the BJT electrons, so pressures to produce or enhance such distortions can affect the BJT electrons near the Fermi level. This pressure-electric effect in BJT metals is studied theoretically on the basis of the ${\mathrm{\Gamma }}_{12}$ subband model for the structural transition in A15 compounds. The applied pressure is assumed to consist of a spatially uniform pressure and a local pressure. The former induces or enhances a uniform distortion, which contributes to the redistribution of electrons between the ${\mathrm{\Gamma }}_{12}$ subbands, while the latter produces a local distortion with its gradient, which causes flows of band electrons between different regions in space. The steady electronic state under the uniform and local pressures is obtained by use of the Boltzmann equation and the relaxation-time approximation. It is clarified that the local pressure with its gradient can generate an electromotive power when the degeneracy of the BJT subbands is lifted spontaneously and/or by the external uniform pressure. The obtained pressure-gradient electric effect is predicted to be a probe into BJT effects which may work in some metals with structural transitions.

Different from odd-order harmonics, even-order harmonics are a result of broken inversion symmetry, where the second harmonic generation has long been used in the study of surface magnetism. Their underlying mechanisms are always intriguing. Here we reveal an unreported mechanism of even harmonics from the diamond with a nitrogen-vacancy (NV) center from the viewpoint of group theory. Two different types of interband transition channels are identified. One is the direct transitions between a valence band and a conduction band, called type-I interband transition. Type II is defined as the indirect interband transitions between two conduction bands or two valence bands. Once type-II transitions are involved only through the inversion symmetry breaking, even-order harmonics occur in the NV center in diamond. The physical origin of this new type of interband transitions is rooted in group theory, where the inversion symmetry forbids type-II transitions between two valence or conduction bands with the same irreducible representation. By contrast, the broken inversion symmetry in the NV center lifts this restriction, giving rise to even-order harmonics. Our study represents a paradigm shift in high harmonic generation.

Excitation and propagation of spin waves inside magnetic domain walls (DWs) have received attention because of their potentials in spintronic and communication applications. In addition to wave amplitude and frequency, spin waves have a third character: handedness, whose manipulation is certainly of interest. We propose in this paper that the handedness of low-energy spin-wave excitations can be controlled by tuning the net angular momentum ${\delta }_s$ in a ferrimagnetic (FiM) DW, attributing to the inequivalent magnetic sublattices. The results indicate that the spin-wave dispersion depends on both ${\delta }_s$ and wave handedness. For a positive (negative) ${\delta }_s$, a gapless dispersion is observed for left-handed (right-handed) spin waves, while a frequency gap appears for right-handed (left-handed) spin waves. Thus, a FiM wall could serve as a filter of low-energy spin waves in which only spin waves with a specific handedness can propagate. Furthermore, the energy consumption loss for spin-wave excitation in the wall is much lower than that inside the domain, while the group velocity is much faster too, demonstrating the advantages of DWs serving as spin waveguides. Moreover, the current-induced spin-wave Doppler shift in the FiM wall is also revealed and can be controlled by ${\delta }_s$. In this paper, we unveil the interesting spin-wave dynamics in FiM DWs, benefiting future spin-wave applications.

Spin-$1/2$ triangular lattice Heisenberg antiferromagnet has been accepted as an ideal system for quantum magnetism studies and quantum simulations. This system, for which the classical ground state degeneracy is lifted by quantum fluctuations, exhibits a series of novel spin structures for a field applied in-plane and out-of-plane. It has been found that both anisotropy and interlayer interaction play an important role in the stabilization of the spin configurations in a magnetic field. Conversely, the phase transitions and spin-state evolution in a field along various orientations can provide a deep insight into physics of the triangular lattice Heisenberg antiferromagnet system. While the quantum magnetization process in an in-plane field has been studied extensively, the ground state evolution in the field along the $c$ axis requires further investigation. Here we performed high field magnetization and neutron scattering investigations on a model system of spin-$1/2$ triangular lattice Heisenberg antiferromagnet ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ with field along $c$ axis and with a small offset angle. For $H\parallel c$, the magnetization reveals a narrow plateau prompting a UUD-like phase, which could be suppressed by tilting the field away from the $c$ axis. From the neutron data, a phase transition ${\mu }_0{H}_{c1}\sim 12$ T is detected and interpreted as a transition from an umbrella to a coplanar phase. Around about 22.5 T (${\mu }_0{H}_{c2}$) for $H\parallel c$, another transition is observed which might be attributed to a transition between the coplanar $V$ and $V^{\prime }$ phases based on a comparison with the calculations and previous results. Theoretical calculations using the large-size cluster mean-field plus scaling method predicts a similar phase evolution as the previous semiclassical analysis, and agree with experiment well. The discrepancies between theory and experiment are also discussed, suggesting the physics of a triangular lattice Heisenberg antiferromagnet in a field along $c$ axis has not been fully unraveled.

We report the magnetic susceptibilities $\chi \left(T\right)$, magnetization $M\left(H\right)$, transport properties $\rho (T,$ $H)$, and heat capacity $C_P\left(T\right)$ of DySbTe single crystal. DySbTe is an isostructural compound with the nonmagnetic Dirac nodal line (DNL) semimetal ZrSiS, in which Dy spins show a long-range antiferromagnetic (AFM) ordering below Néel temperature $T_N=7\mathrm{K}$. A steplike magnetization curve and a 1/2 magnetization plateau were observed at 2 K for a magnetic field $H$ applied parallel to the $ab$ plane. The temperature dependence of electrical resistivity of DySbTe shows a bad-metal-like state and a peak at about 7 K associated with the AFM phase transition. The transverse magnetoresistance exhibits a crossover at a critical field from the semiclassical weak-field $H^2$ dependence to the high-field linear dependence, due to the Dirac nodal line states in DySbTe. The first principles calculations of band structures illustrate that DySbTe is a DNL semimetal and turns to be a weak topological insulator if the spin-orbit coupling is considered. DySbTe will be a new topological compound exhibiting the complex interplay between magnetism, topology, and electron correlations.

We provide a theoretical background for electric-field-assisted thermally activated writing and deleting of magnetic skyrmions in ultrathin transition-metal films. We apply an atomistic spin model, which includes the exchange interaction, the Dzyaloshinskii-Moriya interaction, and the magnetocrystalline anisotropy energy. The strengths of the magnetic interactions are taken from density functional theory (DFT) calculations for a Pd/Fe bilayer on the Ir(111) surface. We systematically vary all magnetic interactions up to $\pm 10\%$ treating the magnetoelectric effect in linear response. The critical magnetic fields marking the onset of the skyrmion phase and the field-polarized phase shift considerably upon varying the interaction constants due to the electric field. Based on harmonic transition state theory, we calculate the transition rates for skyrmion nucleation and annihilation, which are in good agreement with experimental values for Pd/Fe/Ir(111). The field-dependent variation of energy barriers and preexponential factors leads to large changes of the transition rates, which are accompanied by changes in skyrmion radii. Finally, we simulate the electric-field-dependent writing and deleting of magnetic skyrmions in Pd/Fe/Ir(111) based on the master equation and transition rates obtained using the magnetic interactions calculated via DFT for electric fields of $\mathcal{E}=\pm 0.5$ V/Å. The magnetic-field-dependent skyrmion probability follows a Fermi-Dirac distribution function of the free energy difference of the skyrmion state and the ferromagnetic (FM) state. The probability function for the opposite electric field directions is in striking agreement with experimental results [Romming et al., Science 341, 636 (2013)].