We have performed valence-band resonant photoemission spectroscopy across the transition-metal (TM) $L_3$ edge of ${\mathrm{Rh}}_2{\mathrm{O}}_3, {\mathrm{RuO}}_2$, and ${\mathrm{MoO}}_2$. This technique allows us to access important information on the partial contribution of each respective TM. We show that an extended cluster model, coupled with transitions between discrete and continua states, describes well the overall resonance behavior as well as the experimental constant initial-state (CIS) spectra. Moreover, we analyze the resonant enhancement of the main lines and satellite structures to discuss the influence of the different charge regimes, the TM–O $2p$ hybridization, and additional charge screenings to the CIS signal. Finally, we argue that this approach can be extended to the study of other transition-metal oxides.

Intense lasers can easily drive nonadiabatic transitions of excited electron wave packets across the Brillouin zones, thus transition dipole moments (TDM) between energy bands of solids should be continuous, satisfying crystal symmetry, and periodic at zone boundaries. While current ab initio algorithms are powerful in calculating band structures of solids, they all introduced random phases into the eigenfunctions at each crystal momentum $k$ . Here we show how to choose a “smooth-periodic” gauge where TDMs can be smooth versus $k$ , preserving crystal symmetry, as well as maintaining periodic at boundaries. The symmetry properties of TDMs with respect to $k$ ensure the absence of even-order harmonics from MgO with inversion symmetry, while the TDM in the “smooth-periodic” gauge for broken-symmetry ZnO is responsible for even harmonics that were underestimated in previous simulations. These results reveal the importance of correctly treating the complex TDMs that satisfy crystal symmetry and continuous across zone boundaries in nonlinear laser-solid interactions, which has been elusive in most theories so far.

To resolve the discrepancies in the exciton fine structure of aluminum nitride (AlN), polarization- and angle-resolved photoluminescence (PL) spectroscopies are performed. The excitonic PL spectra strongly depend on the optical polarization and detection angle. We propose that both the long-range and short-range electron-hole exchange interaction should be used to interpret the luminescence spectra. The theoretical framework fully explains the present and previous experimental results. The large longitudinal-transverse splitting energy obtained in this study suggests that AlN has strong light-matter coupling without quantum-confined structures.

We investigate self-organized synchronization in a blue-detuned optomechanical cavity that is mechanically coupled to an undriven mechanical resonator. By controlling the strength of the driving field, we engineer a mechanical gain that balances the losses of the undriven resonator. This gain-loss balance corresponds to the threshold where both coupled mechanical resonators enter simultaneously into self-sustained limit cycle oscillations regime. This leads to rich sets of collective dynamics such as in-phase and out-of-phase synchronizations, depending on the mechanical coupling rate, the frequency mismatch between the resonators, and the external driving strength through the mechanical gain and the optical spring effect. Moreover, we show that the introduction of a quadratic coupling, which results from a quadratically coupling of the optical cavity mode to the mechanical displacement, enhances the in-phase synchronization. This work shows how phonon transfer can optomechanically induce synchronization in a coupled mechanical resonator array and opens up new avenues for phonon processing and novel memories concepts.

We calculate the electrical and thermal conductivity of a two-dimensional electron gas with strong spin-orbit coupling in which the scattering is dominated by electron-electron collisions. Despite the apparent absence of Galilean invariance in the system, the two-particle scattering does not affect the electrical conductivity above the band-crossing point where both helicity bands are filled. Below the band-crossing point where one helicity band is empty, switching on the electron-electron scattering leads only to a limited decrease in the electrical conductivity so that its high-temperature value is independent of the scattering intensity. In contrast to this, thermal conductivity is not strongly affected by the spin-orbit coupling and exhibits only a kink as the Fermi level passes through the band-crossing point.

Single-layer antimony or antimonene is a recently discovered two-dimensional material with high environmental stability and appealing electronic properties. Antimonene is characterized by strong spin-orbit coupling, which suggests unconventional behavior in magnetic fields. In this paper, we report on a computational study of the magnetic response of antimonene focusing on the effect of gate voltage, playing an important role due to the buckled structure of antimonene. We use a tight-binding model and systematically analyze the magnetic susceptibility as well as Landau quantization of the energy spectrum. We show that the magnetic response of undoped antimonene is diamagnetic, similar to its bulk phase. In contrast, for electron-doped antimonene we observe a paramagnetic response, which can be further reversed if the vertical bias voltage is applied. The mechanism of the observed switching is related to a modulation of the orbital susceptibility due to the spin splitting occurring as a result of electric-field-induced inversion symmetry breaking. From the calculated Landau levels, we find a significant difference between the $g$-factors for electrons ($g_e\approx 2.33$) and holes ($g_h\approx 6.48$), which also exhibit a strong dependence on the bias potential.

Several moiré systems created by various twisted bilayers have manifested magnetism under flatband conditions leading to enhanced interaction effects. We theoretically study stability of moiré flatband ferromagnetism against collective excitations, with a focus on the effects of Bloch band quantum geometry. The spin magnon spectrum is calculated using different approaches, including Bethe-Salpeter equation, single mode approximation, and an analytical theory. One of our main results is an analytical expression for the spin stiffness in terms of the Coulomb interaction potential, the Berry curvatures, and the quantum metric tensor, where the last two quantities characterize the quantum geometry of moiré bands. This analytical theory shows that Berry curvatures play an important role in stiffening the spin magnons. Furthermore, we construct an effective field theory for the magnetization fluctuations and show explicitly that skyrmion excitations bind an integer number of electrons that is proportional to the Bloch band Chern number and the skyrmion winding number.

We have analytically studied plasmons in an electron system comprising two spatially separated layers—an infinite two-dimensional electron system (2DES) and a 2D strip. Our analysis reveals the existence of plasmon modes that are localized near and propagate along the strip. These modes are characterized by the wave vector in the direction of the strip, as well as the number of charge density nodes $N$ across the strip. In the long-wavelength limit, the fundamental mode $N=0$ is found to have gapless linear dispersion. When the external perpendicular magnetic field is applied, this mode remains gapless and exhibits peculiar magnetodispersion. We analyze the correlation between our findings and the previously established results on plasmons in gated and partially gated 2DESs.

Single layers of hexagonal two-dimensional nanostructures such as graphene, silicene, and germanene exhibit large carrier Fermi velocities and, consequently, large light-matter coupling strength making these materials promising elements for nano-optoelectronics. Although these materials are centrosymmetric, the spatial dispersion turns out to be quite large allowing the second-order noninear response of such materials to be comparable to the noncentrosymmetric 2D ones. The second-order response of massless Dirac fermions has been extensively studied, however, a general approach correct over the full Brillouin zone is lacking so far. To complete this gap, in the current paper, we develop a general quantum-mechanical theory of the in-plane second-order nonlinear response beyond the Dirac cone approximation and applicable to the full Brillouin zone of the hexagonal tight-binding nanostructures. We present explicit calculation of the nonlinear susceptibility tensor of 2D hexagonal nanostructures applicable to arbitrary three-wave mixing processes.

Different types of ordering phenomena may occur during phase transitions, described within the universal framework of the Landau theory through the evolution of one, or several, symmetry-breaking order parameters η. In addition, many systems undergo phase transitions related to an electronic instability, in the absence of symmetry breaking and eventually described through the evolution of a totally symmetric order parameter $q$ linearly coupled to volume change. Analyzing the coupling of a non-symmetry-breaking electronic instability, responsible for volume strain, to symmetry-breaking phenomena is of importance for many systems in nature and here we show that the symmetry-allowed $q{\eta }^2$ coupling plays a central role. We use as a case study the rubidium manganese hexacyanoferrate Prussian blue analog, exhibiting phase transitions with hysteresis that may exceed 100 K, and based on intermetallic charge transfer (CT). During the phase transition, the intermetallic CT described through the evolution of $q$ is coupled to cubic-tetragonal ferroelastic symmetry breaking described through the evolution of η. In this system, the symmetry-breaking and non-symmetry-breaking deformations have similar amplitudes but the large volume strain is mainly due to CT. We analyze both the ferroelastic and the CT features of the phase transition within the frame of the Landau theory, taking into account the $q{\eta }^2$ coupling, stabilizing concomitant CT, and Jahn-Teller distortion. The results show that the phase transition and its wide thermal hysteresis originate from the coupling between both processes and that the elastic coupling of each order parameter with the volume strain is responsible for the $q{\eta }^2$ coupling. The phase diagrams obtained with this model are in good qualitative agreement with various experimental findings and apply to diverse families of materials undergoing Mott transition, spin-crossover, neutral-ionic transition, etc., for which isostructural electronic instability driving volume strain can couple to symmetry-breaking or not, create phase transition lines, and drive cooperative phenomena.

We describe a scheme for finding quantum critical points based on studies of a nonequilibrium susceptibility during finite-rate quenches taking the system from one phase to another. We assume that two such quenches are performed in opposite directions and argue that they lead to the formation of peaks of a nonequilibrium susceptibility on opposite sides of a critical point. Its position is then narrowed to the interval marked off by these values of the parameter driving the transition, at which the peaks are observed. Universal scaling with the quench time of precision of such an estimation is derived and verified in two exactly solvable models. The experimental relevance of these results is expected.

Magnetic skyrmions have been the focus of intense research due to their unique qualities which result from their topological protections. Previous work on ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, the only known insulating multiferroic skyrmion material, has shown that chemical substitution alters the skyrmion phase. We chemically substitute Zn, Ag, and S into powdered ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ to study the effect on the magnetic phase diagram. In both the Ag and the S substitutions, we find that the skyrmion phase is stabilized over a larger temperature range, as determined via magnetometry and small-angle neutron scattering (SANS). Meanwhile, while previous magnetometry characterization suggests two high temperature skyrmion phases in the Zn-substituted sample, SANS reveals the high temperature phase to be skyrmionic while we are unable to distinguish the other from helical order. Overall, chemical substitution weakens helical and skyrmion order as inferred from neutron scattering of the $\left|q\right|\approx 0.01{\text{Å}}^{-1}$ magnetic peak.

The physical properties of URhSn with quasi-kagome structure are studied using single-crystalline samples via electrical resistivity, magnetic susceptibility, heat capacity, thermal expansion, and high-field magnetization measurements. Remarkable magnetic anisotropy is found in the ferromagnetic (FM) state below $T_{\mathrm{C}}=16\mathrm{K}$ as well as in the ordered state between $T_{\mathrm{C}}$ and $T_{\mathrm{O}}=54\mathrm{K}$, where the easy and hard magnetization directions are the hexagonal [0001] and $\left[10\overline{1}0\right]$ axes. In the paramagnetic state, the magnetic susceptibility shows a Curie-Weiss behavior; the Weiss temperatures are positive and negative for [0001] and $\left[10\overline{1}0\right]$, respectively, indicating the presence of both FM and antiferromagnetic (AFM) correlations. The entropy release for $5f$ electrons is approximately $R\ln 3$ at $T_{\mathrm{O}}$. The thermal expansion coefficient is strongly anisotropic around $T_{\mathrm{O}}$ between the hexagonal basal plane and the [0001] axis, indicating its remarkable anisotropic magnetoelastic response and uniaxial stress dependences. Interestingly, the magnetic field response of the higher-temperature ordered state is unusual: $T_{\mathrm{O}}\left(H\right)$ increases and the heat-capacity jump is enhanced with the magnetic field for $H\left|\right|\left[0001\right]$. Based on the established thermodynamic evidence for the second-order transition at $T_{\mathrm{O}}\left(H\right)$, a plausible scenario is the occurrence of a canted AFM ordering or a conical state under magnetic fields, which is stabilized when coupled with field-induced magnetic moments along the [0001] axis. Another possibility is the occurrence of quadrupole ordering at $T_{\mathrm{O}}\left(H\right)$.

Comprehensive muon-spin-rotation/relaxation (μSR) and neutron powder-diffraction (NPD) studies supported via bulk measurements have been performed on the ordered double-perovskite ${\mathrm{Sr}}_2\mathrm{YbRu}{\mathrm{O}}_6$ to investigate the nature of the magnetic ground state. Two sharp transitions at $T_{\mathrm{N}1}\sim 42\mathrm{K}$ and $T_{\mathrm{N}2}\sim 36\mathrm{K}$ have been observed in the static and dynamic magnetization measurements, coinciding with the heat-capacity data. In order to confirm the origin of the observed phase transitions and the magnetic ground state, microscopic evidences are presented here. An initial indication of long-range magnetic ordering comes from a sharp drop in the muon initial asymmetry and a peak in the relaxation rate near $T_{\mathrm{N}1}$. NPD confirms that the magnetic ground state of ${\mathrm{Sr}}_2\mathrm{YbRu}{\mathrm{O}}_6$ consists of an antiferromagnetic (AFM) structure with interpenetrating lattices of parallel $\mathrm{Y}{\mathrm{b}}^{3+}$ and $\mathrm{R}{\mathrm{u}}^{5+}$ moments lying in the $ab$ plane and adopting an A-type AFM structure. Intriguingly, a small but remarkable change is observed in the long-range ordering parameters at $T_{\mathrm{N}2}$ confirming the presence of a weak spin reorientation (i.e., change in spin configuration) transition of Ru and Yb moments, as well as a change in the magnetic moment evolution of the $\mathrm{Y}{\mathrm{b}}^{3+}$ spins at $T_{\mathrm{N}2}$. The temperature-dependent behavior of the $\mathrm{Y}{\mathrm{b}}^{3+}$ and $\mathrm{R}{\mathrm{u}}^{5+}$ moments suggests that the $4d$ electrons of $\mathrm{R}{\mathrm{u}}^{5+}$ play a dominating role in stabilizing the long-range-ordered magnetic ground state in the double-perovskite ${\mathrm{Sr}}_2\mathrm{YbRu}{\mathrm{O}}_6$ whereas only the $\mathrm{Y}{\mathrm{b}}^{3+}$ moments show an arrest at $T_{\mathrm{N}2}$. The observed magnetic structure and the presence of a ferromagnetic interaction between Ru and Yb ions are explained with use of the Goodenough-Kanamori-Anderson rules. Possible reasons for the presence of the second magnetic phase transition and of a compensation point in the magnetization data are linked to competing mechanisms of magnetic anisotropy.

Lattice and magnetic degrees of freedom are strongly coupled in magnetic materials. We propose a consistent first-principles framework to explore the joint configurational space. For this, we define atomic spin moments from the projector augmented-wave formalism of density-functional theory and control them via Lagrangian constraints. We demonstrate our approach for vacancy formation and migration in collinear paramagnetic bcc iron by implementing a relaxation scheme based on spin-space averaged forces (SSA relaxation). Based on these results we discuss the impact of the magnetic state on vacancy formation energies, migration barriers, and relaxations.

Time-resolved x-ray absorption experiments have been performed on shocked tantalum. Using a single 100 ps synchrotron x-ray pulse, nanosecond-lived equilibrium states of shocked solid Ta have been measured by $L_3$-edge absorption spectroscopy. The energy shift and intensity change of the white line were measured up to 130 GPa and 2000 K. The experimental results are discussed in light of density functional theory calculations and FEFF simulations. The absence of occurrence of the hcp-$\omega$ phase is suggested from the analysis of the shape of the white line.

Ferroelectric behavior on the meso- and macroscopic scale depends on the formation and dynamics of domains and controlling the domain patterns is imperative to device performance. While density functional theory (DFT) calculations have successfully described the basic properties of ferroelectric domain walls, the necessarily small cell sizes used for the calculations hampers DFT studies of complex domain patterns. Here, we simulate large-scale complex ferroelectric domain patterns in ferroelectric ${\mathrm{YGaO}}_3$ using multisite local orbitals as implemented in the DFT code conquest. The multisite local orbital basis set gives similar bulk structural and electronic properties, and atomic domain wall structures and energetics as those obtained with conventional plane-wave DFT. With this basis set model, 3600-atom cells are used to simulate topologically protected vortices. The local atomic structure at the vortex cores is subtly different from the domain walls, with a lower electronic band gap suggesting enhanced local conductance at these cores.

We study ergodicity breaking in the clean Bose-Hubbard chain for small hopping strength. We see the existence of a nonergodic regime by means of indicators as the half-chain entanglement entropy of the eigenstates, the average level spacing ratio, the properties of the eigenstate-expectation distribution of the correlation and the scaling of the inverse participation ratio averages. We find that this ergodicity breaking is different from many-body localization because the average half-chain entanglement entropy of the eigenstates obeys volume law. This ergodicity breaking appears unrelated to the spectrum being organized in quasidegenerate multiplets at small hopping and finite system sizes, so in principle, it can survive also for larger system sizes. We find that some imbalance oscillations in time which could mark the existence of glassy behavior in space are well described by the dynamics of a single symmetry-breaking doublet and quantitatively captured by a perturbative effective XXZ model. We show that the amplitude of these oscillations vanishes in the large-size limit. Our findings are numerically obtained for systems for $L<12$. Extrapolations of our scalings to larger system sizes should be taken with care, as discussed in the paper.

We report large magnetoresistance (MR) and Shubnikov-de Haas (SdH) oscillations in single crystals of magnetically (M= Ni and Fe) doped ${\mathrm{M}}_x{\mathrm{Bi}}_{0.97-x}{\mathrm{Sb}}_{0.03}$ $(x=0, 0.02)$ topological insulators. The $R\overline{3}m$ symmetry and phase have been confirmed by the Rietveld refinement of x-ray diffraction data. Interestingly, a magnetic field induced phase transition from semimetallic to semiconducting type is found with the energy gap around 80 meV at 15 T in the $x=0$ sample. Moreover, we observe linear behavior of MR up to 15 T in transverse mode and SdH oscillations in longitudinal mode where the field direction is parallel with respect to the current and crystal plane. For the parent sample, we found the coherence length $L_{\varphi }=12.7\mathrm{nm}$ through the fitting of MR data in transverse mode with modified H-L-N equation. The extracted frequencies of SdH oscillations using the fast Fourier transform method and Landau level (LL) fan diagram are found to be consistent for the parent and Ni doped samples. The determined Fermi surface area is found to be slightly larger in Ni doped as compared to the parent sample possibly due to change in the Fermi energy. The Kohler's plots indicate a single scattering mechanism below 100 K. More importantly, the analysis with the help of the LL fan diagram reveals the nonzero Berry phase ${\varphi }_{\mathrm{B}}=-(1\pm 0.1)\pi$, which demonstrates the nontrivial topological states near the Dirac point in the parent and Ni doped samples.

The control of emergent magnetic monopoles for the generation of monopole currents in artificial spin ice is essential for their use in nanomagnet-based device applications. Here we present a scheme to inject monopole currents into an artificial square ice at specific locations, which provides a means to control the propagation of the generated emergent monopole currents. Specifically, we modify an artificial square ice by populating two of its edges with different vertex configurations consisting of two, three, and four nanomagnets meeting at a common point. After setting an initial state with a global magnetic field, injection of monopoles occurs at one of the edges where the vertices have higher switching probability. We experimentally observe this vertex-specific nucleation of emergent magnetic monopoles using x-ray photoemission electron microscopy. Additionally, we demonstrate that a lateral shift in the reversal of the magnets, leading to the formation of large domains, is consistent with theoretical simulations incorporating higher-order contributions in the magnetic Hamiltonian. Finally, we find a strong correlation between the location of emergent monopole injection and the film thickness, which is a result of the switching probability associated with the different vertex configurations.