The stabilization of chiral magnetic spin structures in thin films is often attributed to the interfacial Dzyaloshinskii–Moriya interaction (DMI). Very recently, however, it has been reported that the chirality induced by the DMI can be affected by dipolar interactions. These dipolar fields tend to form Néel caps, which entails the formation of a clockwise chirality at the top of the film and a counterclockwise chirality at the bottom. Here we show that engineering an alternating DMI that changes signs across the film thickness, together with the tendency to form Néel caps, leads to an enhanced stability of chiral spin-structures. Micromagnetic simulations for skyrmions demonstrate that this can increase the effective DMI in a prototypical [Pt/Co/Ir] multilayer system by at least $0.6\mathrm{mJ}{\mathrm{m}}^{-2}$. These gains are comparable to what has been achieved using additive DMI, but more flexible as we are not limited to a select set of material combinations. We also present experimental results: By measuring equilibrium domain widths, we quantify the effective DMI in [Pt/Co/Ir] multilayer systems typically used for skyrmion stabilization. Upon introducing an alternating DMI, we demonstrate changes in the effective DMI that agree with our simulations. Our results provide a route toward enhancing the stability of chiral spin structures that does not rely on enlarging the chiral interactions.

The crystalline and magnetic structures of ${\mathrm{Ca}}_2{\mathrm{Ru}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_4$ ($x=0.02$, 0.05, 0.08, and 0.12) have been studied using neutron and x-ray diffraction. The Fe-doping reduces the Ru-O bond length in both apical and planar directions. The smaller Ru($\mathrm{Fe}){\mathrm{O}}_6$ octahedron leads to its reduced distortion. The $Pbca$ space group is maintained in all the Fe dopings, so is the octahedral flattening. Warming has a similar effect on the lattice to that of the Fe doping in releasing the distorted octahedra but precipitates an abrupt octahedral elongation near the $\mathrm{N}\acute{e}\mathrm{el}$ temperature. Two competing antiferromagnetic orders, $A$- and $B$-centered phases have been observed. The Fe-doping-relaxed crystal structure prefers the latter to the former. As the doping increases, the $B$-centered phase continuously grows at the cost of the $A$-centered one and eventually replaces it at $x=0.12$. The absence of the two-dimensional antiferromagnetic critical fluctuations above the magnetic transition temperature and the three-dimensional magnetic correlation below the transition, together with the anomalous lattice response, point to an important role of orbital degree of freedom in driving the magnetic phase competition.

We report on the electrical detection of a hybrid magnon-photon system which comprises a magnetic sample coupled to a planar cavity. While the uniform Kittel mode has the largest coupling strength among all the magnon modes, it generates only a modest voltage signal by means of the inverse spin-Hall effect. We have found that the generated voltage can be significantly enhanced by introducing a higher-order magnon mode, which possesses a much higher spin pumping efficiency, and furthermore, it is nearly degenerated with the Kittel mode. The experimental results can be explained by our theoretical model and suggest that the use of an auxiliary magnon mode can realize the configuration of a magnon-photon system with both strong coupling and large spin current.

Motivated by the recent discovery of near-room temperature superconductivity in high-pressure superhydrides, we investigate from first principles the high-pressure superconducting phase diagram of the ternary Ca-B-H system, using ab initio evolutionary crystal structure prediction, and Density Functional Perturbation Theory. We find that below 100 GPa all stable and weakly metastable phases are insulating. This pressure marks the appearance of several new chemically-forbidden phases on the hull of stability, and the first onset of metalization in ${\mathrm{CaBH}}_5$. Metallization is then gradually achieved at higher pressure at different compositions. Among the metallic phases stable in the Megabar regime, we predict two high-$T_{\text{c}}$ superconducting phases with ${\mathrm{CaBH}}_6$ and ${\mathrm{Ca}}_2{\mathrm{B}}_2{\mathrm{H}}_{13}$ compositions, with critical temperatures of 119 and 89 K at 300 GPa, respectively, surviving to lower pressures. Ternary hydrides will most likely play a major role in superconductivity research in the coming years; our study suggests that, in order to reduce the pressure for the onset of metallicity and superconductivity, further explorations of ternary hydrides should focus on elements less electronegative than boron.

Using in situ x-ray diffraction measurements in shock-compressed copper (38–276 GPa), we show that the ambient face-centered-cubic (fcc) structure transforms to the body-centered-cubic (bcc) structure at ∼180 GPa. Additionally, stacking fault (SF) abundance in shock-compressed copper increases with compression, reaching ∼10% at the onset of the fcc-bcc phase transition. Both findings are consistent with recent results on shock-compressed gold and silver that showed the importance of SFs in facilitating the fcc-bcc transformation. In contrast to our results, copper was recently reported to retain the fcc structure to over 1000 GPa under ramp compression [Fratanduono et al., Phys. Rev. Lett. 124, 015701 (2020)]. Further studies are needed to understand why the fcc-bcc transformation in copper is observed under shock compression, but not under ramp compression.

The superstructure of $\mathrm{TaT}{\mathrm{e}}_2$ is examined by total high-energy x-ray scattering and large-scale structure modeling over a broad temperature range. At room temperature, it features double zigzag chains of Ta atoms exhibiting metallic bonding. The chains are sandwiched between planes of weakly interacting Te atoms. Below 170 K, the chains appear broken to “butterflylike” clusters and single zigzag chains of covalently bonded Te atoms emerge while Ta-Te interactions remain largely ionic. This is a rare example of a charge density wave compound stabilized by bonding that is a mix of covalent, ionic, and metallic character giving rise to chemically distinct periodic lattice distortions. We argue that the formation of periodic lattice distortions in the Te sublattice in addition to those in the Ta sublattice favors charge delocalization, which may explain the unusually diminished resistivity and increased magnetic susceptibility exhibited by the low-temperature charge density wave phase of $\mathrm{TaT}{\mathrm{e}}_2$. The effect may be common for $5d$ transition metal chalcogenides containing Te because of the extended electronic orbitals of the constituent atoms.

PbTe-based thermoelectric materials remain of great interest due to their peculiar dynamic behavior and high thermoelectric performance. The true structure of pristine PbTe has been widely discussed, since the average rocksalt structure does not adequately account for the low lattice thermal conductivity. Single crystals of PbTe exhibit large amounts of x-ray diffuse scattering, which along with additional experimental observations has led to a series of structural models containing different types of disorder. Recently, analysis of the x-ray diffuse scattering from single-crystal PbTe confirmed the formation of local dynamic dipoles proposed earlier based on inelastic neutron scattering (INS) and one-dimensional pair distribution function (PDF) experiments. Here, we study the single-crystal diffuse x-ray scattering over a wider temperature range from 30 to 622 K in order to investigate the temperature dependence of the dynamic dipole formation. We observe that the longitudinal displacement correlations along the ⟨100⟩ directions form pairwise plateaus which results in a steplike decay of the correlations with increasing interatomic distance. This is consistent with previous observations from single-crystal diffuse scattering experiments on PbTe where the same steplike trend for the correlations was observed. However, upon lowering the temperature we observe a larger relative antiphase displacement contribution to the correlations proposed to originate from the softening of the transverse optical (TO) branch. As the local dipole formation depends on the absolute amplitude of antiphase displacements, and not the relative antiphase contribution to atomic displacements, it is found that the local dipole formation increases upon heating consistent with previous INS and PDF studies. The underlying mechanism responsible for the softening of the TO branch, and thereby the formation of dynamic dipoles, is investigated by comparison to the isostructural KCl system. The different dynamics in KCl and PbTe can be explained from the different bonding mechanism, where the metavalent bonding and ability to dynamically express a stereochemically active lone pair in PbTe gives rise to long-range interactions and TO phonon softening.

The study of curvature-induced effects on the properties of nanostructures has become a cornerstone of magnetism. However, several methodologies usually used for studying nanoscale magnetic systems present difficulties for adequately describing curvature. In this work, we present a method that allows studying, under specific conditions, curved dome/antidome surfaces using an equivalent system without curvature. From the described methodology we obtain the phase diagram between easy-normal and skyrmionic magnetization configurations, as a function of spin-orbit coupling, Dzyaloshinskii-Moriya interaction (DMI), and curvature. The effective DMI of the dome structure increases with the curvature. Nevertheless, the effective anisotropy presents the opposite behavior, decreasing with curvature. These results allow us to conclude that an increase in the skyrmion stability is observed in nanostructures having positive curvature. The presented results propose a route that could facilitate the study of curved nanofilms with intrinsic DMI from comparing them with their planar counterparts.

In ${\mathrm{Ag}}_2{\mathrm{CoO}}_2$, Co forms triangular lattice layers, which are separated by the metallic (${\mathrm{Ag}}_2$) block. The magnetic susceptibility and heat capacity measurements show that this material exhibits an antiferromagnetic transition at $T_{\mathrm{N}}=17.5$ K and the Weiss temperature ($T_{\mathrm{\Theta }}$) and the effective moment are $-274$ K and $1.62{\mu }_{\mathrm{B}}$, respectively, indicating that the Co ion carries spin ($S$) 1/2 and has a strongly frustrated state with $T_{\mathrm{\Theta }}/T_{\mathrm{N}}=15.7$. A density functional theory calculation confirmed that the valence state of the Co ions is $2+$ and the low-spin state with $S=1/2$ is realized at reduced on-site Coulomb interaction on Co. We performed elastic and inelastic neutron scattering experiments in a powder sample of ${\mathrm{Ag}}_2{\mathrm{CoO}}_2$. Although no noticeable magnetic Bragg peaks were observed below $T_{\mathrm{N}}$, distinct magnetic excitations were observed in the inelastic neutron scattering experiments. The excitations are consistent with those expected for the $S=1/2$ Heisenberg triangular lattice antiferromagnet. These results suggest that the ordered moment is reduced due to the quantum effect, which explains the absence of the magnetic Bragg peaks. Our results thus suggest that ${\mathrm{Ag}}_2{\mathrm{CoO}}_2$ is a good candidate to realize a quantum Heisenberg triangular lattice antiferromagnet.

Recent inelastic neutron scattering studies by B. Pan et al. [Nat. Commun. 8, 123 (2017)] find evidence for spin resonances in an iron selenide high-$T_c$ superconductor that persist at energies above the quasiparticle gap. The momenta of such spin excitations form a diamond around the checkerboard wave vector, ${\mathit{Q}}_{\mathrm{AF}}$, that is associated with the square lattice of iron atoms that makes up the system. It has been suggested that the “hollowed-out” spin-excitation spectrum is due to hidden Néel order. We study such a hidden spin density wave (hSDW) state that results from nested Fermi surfaces at the center and at the corner of the unfolded Brillouin zone. It emerges within mean-field theory from an extended Hubbard model over a square lattice of iron atoms that contain the minimal $d_{xz}$ and $d_{yz}$ orbitals. Opposing Néel order exists over the isotropic $d+=d_{xz}+i{d}_{yz}$ and $d-=d_{xz}-i{d}_{yz}$ orbitals. The dynamical spin susceptibility of the hSDW is computed within the random phase approximation, at perfect nesting. Unobservable Goldstone modes that disperse acoustically are found at ${\mathit{Q}}_{\mathrm{AF}}$. A threshold is found in the spectrum of observable spin excitations that forms a “floating ring” at ${\mathit{Q}}_{\mathrm{AF}}$ also. The ring threshold moves down in energy toward zero with increasing Hund's rule coupling, while it moves up in energy with increasing magnetic frustration. Comparison with the normal-state features of the spin-excitation spectrum shown by electron-doped iron selenide is made. Also, recent predictions of a Lifshitz transition from the nested Fermi surfaces to Fermi surface pockets at the corner of the folded Brillouin zone will be discussed.

By reverse engineering from exact solutions, we obtain Hartree-exchange-correlation (Hxc) potentials for a double quantum dot subject to generic density-density interactions and Hund's rule coupling. We find ubiquitous step structures of the Hxc potentials that can be understood and derived from an analysis of stability diagrams. We further show that a generic Hxc potential can be decomposed into four basic potentials which allows for a straightforward parametrization and paves the road for the construction of Hxc potentials for interacting multiorbital systems. Finally, we employ our parametrization of the Hxc potential in density functional theory calculations of multiorbital quantum dots and find excellent agreement with exact many-body calculations.

We investigate the spin-dependent thermoelectric effect of graphene flakes with magnetic edges in the ballistic regime. Employing static, respectively, dynamic mean-field theory we first show that magnetism appears at the zigzag edges for a window of Coulomb interactions that increases significantly with increasing flake size. We then use the Landauer formalism in the framework of the nonequilibrium Green's function method to calculate the spin and charge currents in magnetic hexagonal graphene flakes by varying the temperature of the junction for different flake sizes. While in nonmagnetic gated graphene the temperature gradient drives a charge current, we observe a significant spin current for hexagonal graphene flakes with magnetic zigzag edges. Specifically, we show that in the “meta” configuration of a hexagonal flake subject to weak Coulomb interactions, a pure spin current can be driven just by a temperature gradient in a temperature range that is promising for device applications. Bigger flakes are found to yield a bigger window of Coulomb interactions where such spin currents are induced by the magnetic zigzag edges, and larger values of the current.

We study the atomic and electronic structures of the commensurate double moiré superlattices in fully relaxed twisted bilayer graphene (TBG) nearly aligned with the hexagonal boron nitride (BN). The single-particle effective Hamiltonian (${\widehat{H}}^0$) taking into account the relaxation effect and the full moiré Hamiltonian introduced by BN has been built for TBG/BN. The mean-field (MF) band structures of the self-consistent Hartree-Fock (SCHF) ground states at a different number ($\nu$) of filled flat bands relative to the charge neutrality point (CNP) are obtained based on ${\widehat{H}}^0$ in the plane-wave-like basis. The single-particle flat bands in TBG/BN become separated by the opened gap at CNP due to the symmetry breaking in ${\widehat{H}}^0$. We find that the broken $C_2$ symmetry in ${\widehat{H}}^0$ mainly originates from the intralayer inversion-asymmetric structural deformation in the graphene layer adjacent to BN, which introduces spatially nonuniform modifications of the intralayer Hamiltonian. The gapped flat bands have finite Chern numbers. For TBG/BN with the magic twist angle, the SCHF ground states with $\left|\nu \right|=1$–3 are all insulating with narrow MF gaps. When the flat conduction bands are filled, the gap at $\nu =1$ is smaller than that at $\nu =3$, suggesting that the nontrivial topological properties associated with the flat Chern bands are more likely to be observed at $\nu =3$. This is similar for negative $\nu$ with empty valence bands. The dependence of the electronic structure of TBG/BN on positive $\nu$ is roughly consistent with recent experimental observations.

We propose an approach called self-learning hybrid Monte Carlo (SLHMC), which is a general method to make use of machine learning potentials to accelerate the statistical sampling of first-principles density-functional-theory (DFT) simulations. The trajectories are generated on an approximate machine learning (ML) potential energy surface. The trajectories are then accepted or rejected by the Metropolis algorithm based on DFT energies. In this way, the statistical ensemble is sampled exactly at the DFT level for a given thermodynamic condition. Meanwhile, the ML potential is improved on the fly by training to enhance the sampling, whereby the training data set, which is sampled from the exact ensemble, is created automatically. Using the examples of $\alpha$-quartz crystal ${\mathrm{SiO}}_2^{}$ and phonon-mediated unconventional superconductor ${\mathrm{YNi}}_2^{}{\mathrm{B}}_2^{}\mathrm{C}$ systems, we show that SLHMC with artificial neural networks (ANN) is capable of very efficient sampling, while at the same time enabling the optimization of the ANN potential to within meV/atom accuracy. The ANN potential thus obtained is transferable to ANN molecular dynamics simulations to explore dynamics as well as thermodynamics. This makes the SLHMC approach widely applicable for studies on materials in physics and chemistry.

Recently, many unexpected fine structures in electric, magnetic, and thermoelectric responses at extremely magnetic fields in topological materials have attracted tremendous interest. We propose a mechanism of quantum oscillation beyond the strong-field quantum limit for Dirac fermions. The amplitude of the oscillation is far larger than the usual Shubnikov–de Haas oscillation. The oscillation tends to be periodic in the magnetic field $B$, instead of $1/B$. The period of the oscillation does not depend on the Fermi energy. These behaviors cannot be described by the famous Lifshitz-Kosevich formula. The oscillation arises from a mechanism that we refer to as the inversion of the lowest Landau level, resulting from the competition between the pseudospin Dirac-type Landau levels and real-spin Zeeman spitting beyond the quantum limit. This inversion gives rise to the oscillation of the Fermi energy and conductivity at extremely large magnetic fields. This mechanism will be useful for understanding the unexpected fine structures observed in the strong-field quantum limit in Dirac materials.

In this paper, we study the spin excitation properties of the frustrated triangular-lattice antiferromagnet $\mathrm{Yb}{\left({\mathrm{BaBO}}_3\right)}_3$ with nuclear magnetic resonance. From the spectral analysis, neither magnetic ordering nor spin freezing is observed with temperature down to $T=0.26$ K, far below its Curie-Weiss temperature $|{\theta }_w|\sim 2.3$ K. From the nuclear relaxation measurement, precise temperature-independent spin-lattice relaxation rates are observed at low temperatures under a weak magnetic field, indicating the gapless spin excitations. Further increasing the field intensity, we observe a spin excitation gap with the gap size proportional to the field intensity. These phenomena suggest a very unusual strongly correlated quantum disordered phase, and the implications for the quantum spin liquid state are further discussed.

The chiral tunneling asymmetry for the transmission through a semiconductor interface with spin-orbit coupling (SOC) and antiparallel magnetization is presented from the perspective of the transfer Hamiltonian approach. We explicitly show that the expression for the tunneling rate contains terms which are odd in the momentum along the interface thus contributing to the skew tunneling and the transverse current along the interface. These terms contain the scalar chirality that is the mixed vector product of the effective SOC and exchange fields acting on the tunneling electron or hole. The crucial role of the evanescent states in the tunneling asymmetry is revealed. It is shown that the chiral Hall effect also results in a spin injection across the interface. We finally come up with a more general view on the chiral tunneling as a universal phenomenon expected in various systems.

A quasinormal mode (QNM) coupling theory is proposed for a system of coupled resonators. The theory is generally applicable to systems with energy loss and arbitrary frequency dispersion of permittivity such as plasmonic nanoresonators. The QNM coupling coefficients in the theory are obtained with first-principle calculations without fitting the full-wave numerical results or experimental data. The theory is built up based on the Green's-function formalism, which shows that the coupling between resonators originates from the mutual excitation of scattered field of each resonator. With the theory, the source-excited scattered field or the source-free QNMs of the coupled system can be obtained as an expansion in terms of QNMs of each individual resonator in the system. The theory provides an intuitive physical picture and also shows superior computational efficiency compared with the full-wave numerical method if the scattered field of each resonator in the system can be expanded with a small set of QNMs. The theory is tested against full-wave numerical method and is used to design metallic nanoantennas with a broadband enhancement of the spontaneous emission rate.

The recent discovery of $\mathrm{Fe}{\mathrm{O}}_2$ under high pressures has aroused great interest. With the fully anisotropic density-functional theory+U method, we present a predictive study of structural and magnetic transitions of rutile-type $\mathrm{Fe}{\mathrm{O}}_2$ after reproducing the experimental spiral wave vector in isostructural $\beta \text{−}\mathrm{Mn}{\mathrm{O}}_2$. A second-order structural distortion (tetragonal to orthorhombic) involving octahedral rotation occurs at a critical pressure of $3\sim 4\mathrm{GPa}$. From a global search in the Brillouin zone, the ground-state spin order of rutile-type $\mathrm{Fe}{\mathrm{O}}_2$ is found to be collinear below 22 GPa and transforms to a helix at higher pressures. The phases remain insulating throughout the whole pressure range, with a change from an indirect gap in the high-spin state to a direct gap in the low-spin state. Our work extends the fundamental understanding of iron oxides and provides schemes that treat strongly correlated magnetic systems in a proper and effective way.

Manipulation of magnetic ground states by effective control of competing magnetic interactions has led to the finding of many exotic magnetic states. In this direction, the tetragonal Heusler compounds consisting of multiple magnetic sublattices and crystal symmetry favoring chiral Dzyaloshinskii-Moriya interaction (DMI) provide an ideal base to realize nontrivial magnetic structures. Here we present the observation of a large robust topological Hall effect (THE) in the multisublattice ${\mathrm{Mn}}_{2-x}\mathrm{PtIn}$ Heusler magnets. The topological Hall resistivity, which originates from the nonvanishing real space Berry curvature in the presence of nonzero scalar spin chirality, systematically decreases with decreasing the magnitude of the canting angle of the magnetic moments at different sublattices. With help of first-principle calculations, magnetic and neutron diffraction measurements, we establish that the presence of a tunable noncoplanar magnetic structure arising from the competing Heisenberg exchanges and chiral DMI from the $D_{2d}$ symmetry structure is responsible for the observed THE. The robustness of the THE with respect to the degree of noncollinearity adds up a new degree of freedom for designing THE based spintronic devices.