We have investigated plasma excitations in the hybrid system with a disk-shaped metallic gate introduced on top of an infinite two-dimensional electron system. The proximity plasma excitations have been identified and explored in this partially-gated system, and the results are compared with the behavior of ordinary gated plasmons excited in a system with the disk geometry. Significantly, with the change of the gate geometry from a disk to a narrow ring, we observed the transformation of proximity plasmons with linear dispersion into those having square-root dependence on the wave vector. Our experimental findings show good agreement with the theory.

Interatomic potentials based on neural-network machine learning (ML) approach to address the long-standing challenge of accuracy versus efficiency in molecular-dynamics simulations have recently attracted a great deal of interest. Here, utilizing Pd-Si system as a prototype, we extend the development of neural-network ML potentials to compounds exhibiting various types of bonding characteristics. The ML potential is trained by fitting to the energies and forces of both liquid and crystal structures first-principles calculations based on density-functional theory (DFT). We show that the generated ML potential captures the structural features and motifs in $\mathrm{P}{\mathrm{d}}_{82}\mathrm{S}{\mathrm{i}}_{18}$ and $\mathrm{P}{\mathrm{d}}_{75}\mathrm{S}{\mathrm{i}}_{25}$ liquids more accurately than the existing interatomic potential based on embedded-atom method (EAM). The ML potential also describes the solid-liquid interface of these systems very well. Moreover, while the existing EAM potential fails to describe the relative energies of various crystalline structures and predict wrong ground-state structures at $\mathrm{P}{\mathrm{d}}_3\mathrm{Si}$ and $\mathrm{P}{\mathrm{d}}_9\mathrm{S}{\mathrm{i}}_2$ composition, the developed ML potential predicts correctly the ground-state structures from genetic algorithm search. The efficient ML potential with DFT accuracy from our study will provide a promising scheme for accurate atomistic simulations of structures and dynamics of complex Pd-Si system.

In spite of recent findings about the pressure-driven insulator-to-metal phase transition, and emerging superconductivity of ${\mathrm{FePS}}_3$ and ${\mathrm{FePSe}}_3$, the knowledge about their atomic structures is still vague. Here, we investigate the pressure-driven structural phase transitions of ${\mathrm{FePS}}_3$ and ${\mathrm{FePSe}}_3$ from 0 to 35 GPa by using ab initio calculations. We find that the ${\mathrm{FePS}}_3-C2/m$ B-I structure transforms to the ${\mathrm{FePS}}_3-C2/m$ B-II phase at about 5 GPa. Then above 17 GPa, the ${\mathrm{FePS}}_3-P\overline{3}1m$ B-III phase becomes energetically favored. For ${\mathrm{FePSe}}_3$, with increasing pressure, the ${\mathrm{FePSe}}_3-R\overline{3}$ T-I phase transforms to the B-II phase at around 6 GPa and further to the B-III phase at about 15 GPa. Our calculation results are consistent with experimentally observed high-pressure induced cell volume collapse, spin crossovers, and insulator-metal transition in ${\mathrm{FePS}}_3$ and ${\mathrm{FePSe}}_3$, which shed light on understanding the high-pressure physics and phase transitions of ${\mathrm{FePS}}_3$ and ${\mathrm{FePSe}}_3$.

We use Monte Carlo simulations to evaluate the feasibility of detecting thermal order by disorder in real antiferromagnetic Ising pyrochlores, frustrated by a magnetic field applied in the [110] direction. Building on an ideal system with only nearest-neighbor exchange interactions and a perfectly oriented field, we consider the effects of dipolar interactions and field misalignment. Our approach is special in that it relies more in the possibility to switch on and off the entropic drive towards order than in the absence of (or immunity to) a particular perturbation. It can then be applied, in principle, to other uncontrolled interactions expected to be naturally present in real magnetic materials. We establish the conditions under which entropic effects can be discerned from an interaction drive towards order, show how to use neutron scattering as a means to unveil this mechanism, and discuss possible materials where to test these ideas.

In this work, the role of Cu dopants in the development of superconductivity in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ is investigated. A series of $\mathrm{C}{\mathrm{u}}_x\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ ($x=0–0.3$) crystals is grown using the Bridgman method and electrochemical techniques. Based on the observable lattice increases along the $c$ axis, the existence of Cu atoms intercalated in the van der Waals (vdW) gaps in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ is verified by x-ray diffraction, scanning electron microsopy–energy dispersive spectroscopy, and transmission electron microscopy analysis. Furthermore, the chemical state of the Cu is found to be zero valence by characterization of the x-ray photoelectron and Auger electron spectra. The absence of $\mathrm{C}{\mathrm{u}}^{1+}$ and $\mathrm{C}{\mathrm{u}}^{2+}$ in the electron energy-loss spectroscopy near-edge fine structure is confirmed as well. Meanwhile, our Raman data also show the same result: the intercalation of Cu in the vdW gap which weakens the binding of the quintuple layers in the $\mathrm{C}{\mathrm{u}}_{0.1}\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ crystal. The electric resistance and magnetic susceptibility show a superconducting transition near 3–3.4 K in the series of Cu-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. A sharp superconducting transition with the highest value of $T_C=3.4\mathrm{K}$ and the largest magnetic shielding fraction of 84% is observed in the optimized 10% Cu-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. These results imply that the formation of superconducting quasiparticles is not related to the charge transfer of Cu, but is supported by the internal stress of Cu intercalated in the van der Waals gap. The superconducting transition observed in the specific heat implies that the superconductivity in $\mathrm{C}{\mathrm{u}}_x\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ is unconventional.

We introduce a microscopic model aimed at describing the behavior of fermionic excitations in the background of a magnetic texture called a “spin-vortex checkerboard.” This texture was proposed previously as a possible alternative to stripes to interpret the experimental phenomenology of spin and charge modulations in 1/8-doped lanthanum cuprates. The model involves two kinds of interacting fermionic excitations residing in spin-rich and spin-poor regions of the modulated structure. It is a generalization of another model developed earlier for the so-called “grid checkerboard.” The principal terms of our model describe the decay of fermionic pairs belonging to spin-poor regions into single fermions occupying spin-rich regions and vice versa. These terms induce intricate fermionic correlations throughout the system but fall short of inducing superconductivity unless arbitrarily small hopping terms are added to the model Hamiltonian. We present the mean-field solution of the model, including, in particular, the temperature dependence of the energy gap. The latter is found to be in good overall agreement with available experimental data for high-$T_c$ cuprate superconductors.

We report the spin Hall magnetoresistance (SMR) in Pt deposited on a tensile-strained ${\mathrm{LaCoO}}_3$ (LCO) thin film, which is a ferromagnetic insulator with a Curie temperature $T_{\mathrm{c}}=85\mathrm{K}$. The SMR displays a strong magnetic-field dependence below $T_{\mathrm{c}}$, with the SMR amplitude continuing to increase (linearly) with increasing the field far beyond the saturation value of the ferromagnet. The SMR amplitude decreases gradually with raising the temperature across $T_{\mathrm{c}}$ and remains measurable even above $T_{\mathrm{c}}$. Moreover, no hysteresis is observed in the field dependence of the SMR. These unusual behaviors indicate that a low-dimensional magnetic system forms at the surface of LCO and that the LCO/Pt interface decouples magnetically from the rest of the LCO thin film. Transmission electron microscopy analysis of the heterostructure reveals that an ultrathin Co-rich layer forms at the LCO surface upon deposition of Pt, which is separated from the rest of the LCO film by a ∼1-nm La/O-rich layer, thus supporting the presence of a low-dimensional ferromagnetic system. To explain the magnetoresistance measurements, we revisit the derivation of the SMR corrections and relate the spin-mixing conductances to the spin-spin correlation functions and microscopic quantities describing the magnetism at the interface. Comparisons between theory and experiment confirm the existence of a low-dimensional Heisenberg ferromagnet at the interface. Our results pave the way for exploring complex magnetic textures of insulating films by simple transport measurements.

We investigate the effect of short-range correlations on the dynamical phase diagram of quantum many-body systems with long-range interactions. Focusing on Ising spin chains with power-law decaying interactions and accounting for short-range correlations by a cluster mean field theory we show that short-range correlations are responsible for the emergence of a chaotic dynamical region. Analyzing the fine details of the phase diagram, we show that the resulting chaotic dynamics bears close analogies with that of a tossed coin.

Zn substitutional lithium (${\mathrm{Li}}_{\text{Zn}}$) and sodium (${\mathrm{Na}}_{\text{Zn}}$) acceptors and their complexes with common donor impurities (${\mathrm{Al}}_{\text{Zn}}, {\mathrm{H}}_i$, and ${\mathrm{H}}_{\text{O}}$) in ZnO have been studied using hybrid functional calculations. The results show that the complexes are not exclusively charge neutral, but rather exhibit a thermodynamic ($+/0$) transition level close to the valence band maximum. The positive charge states are associated with a polaronic defect state, similar to those of the well-studied charge-neutral isolated acceptors. This incomplete passivation has profound consequences for the optical properties of the complexes. Indeed, electron transitions from the conduction band minimum to the ($+/0$) transition level of the complexes result in broad luminescence bands that are blueshifted with respect to those originating from the isolated acceptors. Such complexes are proposed as a potential defect origin of the green luminescence observed at the high-energy side of the orange luminescence band (caused by ${\mathrm{Li}}_{\text{Zn}}$) in hydrothermally grown ZnO. This prediction is supported by experimental photoluminescence and secondary ion mass spectrometry data on a hydrothermally grown ZnO sample. We have also explored how the parameters controlling the fraction and screening of exchange in the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional influence the results by comparing two parametrization approaches: (i) the conventional one where the exchange fraction is adjusted to reproduce the experimental band gap, and (ii) tuning both parameters in order to also comply with the generalized Koopmans theorem (gKT). Interestingly, these approaches were found to yield similar results.

We study the magnetic properties of the surface monolayers with the antiferromagnetic (AF) and ferromagnetic (FM) exchange interactions, where the Dzyaloshinskii-Moriya interaction (DMI) is a result of the mirror symmetry breaking. General expressions are derived for the energy of the DMI helices in the magnetic field. In zero field there are cycloids in both AF and FM cases. Their orientation is determined by the DMI anisotropy with the symmetry of the layer lattice. There are one, two, and three chiral domains in the rectangular, square, and triangular lattices, respectively. This anisotropy may explain the magnetic structure of the $\mathrm{M}\mathrm{n}/\mathrm{W}\left(110\right)$ monolayer. The uniaxial crystal anisotropy may restore a collinear magnetic order. The domains are rotated by the in-plane (along the surface) magnetic field. The spin flops are predicted for some field directions. In the out-of-plane field the magnetization follows to the field and the length of the cycloid wave vector decreases. In the perpendicular field there is the spin flop to the corresponding collinear state.

The influence of temperature, FeCoB layer thickness, and insertion of ultrathin metal spacers (Ta and W) on the magnetic anisotropy of MgO/FeCoB/MgO free layers has been explored by means of ferromagnetic resonance. The second-order contribution to the perpendicular magnetic anisotropy (PMA), stemming from the fluctuations of the first-order term, accounts for the onset of an easy-cone magnetic state in the course of the transition from the in-plane to out-of-plane magnetization. Since the second-order term is small, the easy-cone state is stable only within a narrow range of free-layer thicknesses and temperatures, where the interfacial first-order PMA term gets counterbalanced by the magnetostatic term. We have found that the insertion of metallic spacers in the middle of the FeCoB layer noticeably widens the range of thicknesses and temperatures for obtaining the easy-cone state. We show that the W spacer outperforms its Ta counterpart in this enhancement, albeit increasing the magnetization damping due to a higher degree of alloying with FeCoB. A physical mechanism of the easy-cone stabilization is proposed. It considers a variation of the local saturation magnetization $\left(M_{\mathrm{S}}\right)$ and, notably, Curie temperature ($T_{\mathrm{C}}$) near the MgO/FeCoB interface versus the distance to the metal spacer (or the capping) layer. The larger $M_{\mathrm{S}}$ and $T_{\mathrm{C}}$ values near MgO provide more thermal stability to the first-order PMA ($k_{\mathrm{s}1}$), while the lower $M_{\mathrm{S}}$ and $T_{\mathrm{C}}$ ones near the spacer layer result in a faster drop of the magnetostatic term with temperature. As a result, the effective PMA field exhibits a thermal stabilization effect that can be exploited to stabilize the easy-cone anisotropy. Alongside the improved conditions for setting an easy cone in the MgO/FeCoB/MgO free layers, we demonstrate that an easy-cone configuration with an almost temperature-independent opening angle can be obtained using a MgO/FeCoB(1.6 nm)/Ta free layer.

We investigate transport properties of junctions between two spin-split superconductors linked by a spin-polarized tunneling barrier. The spin-splitting fields in the superconductors (S) are induced by adjacent ferromagnetic insulating (FI) layers with arbitrary magnetization. The aim of this study is twofold. On the one hand, we present a theoretical framework based on the quasiclassical Green's functions to calculate the Josephson and quasiparticle current through the junctions in terms of the different parameters characterizing it. Our theory predicts qualitative new results for the tunneling differential conductance, $dI/dV$, when the spin-splitting fields of the two superconductors are noncollinear. We also discuss how junctions based on FI/S can be used to realize anomalous Josephson junctions with a constant geometric phase shift in the current-phase relation. As a result, they may exhibit spontaneous triplet supercurrents in the absence of a phase difference between the S electrodes. On the other hand, we show results of planar tunneling spectroscopy of a $\mathrm{EuS}/\mathrm{Al}/{\mathrm{AlO}}_x/\mathrm{EuS}/\mathrm{Al}$ junction and use our theoretical model to reproduce the obtained $dI/dV$ curves. Comparison between theory and experiment reveals information about the intrinsic parameters of the junction, such as the size of the superconducting order parameter, spin-splitting fields and spin relaxation, and also about properties of the two EuS films, such as their morphology, domain structure, and magnetic anisotropy.

We have performed a systematic study on the crystal structures and electronic properties of two ternary hydrides, ${\mathrm{YSH}}_6$ and ${\mathrm{LaSH}}_6$, under pressure, using the particle swarm optimization method and first-principles calculations. As a result of extensive structure searches, metallic ${\mathrm{YSH}}_6$ and ${\mathrm{LaSH}}_6$ are thermodynamically stable between 195–237 and 170–300 GPa, respectively. Interestingly, in ${\mathrm{YSH}}_6$ eight neighboring hydrogen atoms form octagons, and the octagons in different layers are connected by four sulfur atoms, forming a cagelike structure with a Y atom at the center, while those octagons in the same layer form polyphenylene-like chains via one shared side. In ${\mathrm{LaSH}}_6$, however, hydrogen atoms form both curved “${\mathrm{H}}_5$” chains or straight chains when bonded to sulfur atoms. Furthermore, electron-phonon coupling calculations indicate that ${\mathrm{YSH}}_6$ and ${\mathrm{LaSH}}_6$ are promising superconductors with estimated $T_c$ values of 91 and 35 K at 210 and 300 GPa, respectively. These results provide guidance for future experimental studies and stimulate more exploration on ternary hydrides.

Effects of pressure on the electronic structure, electron-phonon interaction, and superconductivity of the high entropy alloy ${(\mathrm{TaNb})}_{0.67}{\left(\mathrm{HfZrTi}\right)}_{0.33}$ are studied in the pressure range 0–100 GPa. The electronic structure is calculated using the Korringa-Kohn-Rostoker method with the coherent potential approximation. Effects of pressure on the lattice dynamics are simulated using the Debye-Grüneisen model and the Grüneisen parameter at ambient conditions. In addition, the Debye temperature and Sommerfeld electronic heat capacity coefficient were experimentally determined. The electron-phonon coupling parameter $\lambda$ is calculated using the McMillan-Hopfield parameters and computed within the rigid muffin-tin approximation. We find that the system undergoes the Lifshitz transition, as one of the bands crosses the Fermi level at elevated pressures. The electron-phonon coupling parameter $\lambda$ decreases above 10 GPa. The calculated superconducting $T_c$ increases up to 40–50 GPa and, later, is stabilized at the larger value than for the ambient conditions, in agreement with the experimental findings. Our results show that the experimentally observed evolution of $T_c$ with pressure in ${\left(\mathrm{TaNb}\right)}_{0.67}{\left(\mathrm{HfZrTi}\right)}_{0.33}$ can be well explained by the classical electron-phonon mechanism.

Valley, as a degree of freedom, raises the valleytronics and classical analogs in fundamental and applied science. Here, by designing asymmetrical elastic metamaterials made of hard and soft materials, we present the valley anisotropy that has not been explored in conventional symmetric systems. We demonstrate the creation of valley topological insulators by tuning an anisotropic Berry curvature and show topological waves relying on operating frequencies. The proposed topological properties can contribute to better understanding the valley topology and to creating a different type of topological insulator.

Magnetic properties of $\mathrm{MnS}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_4$ were examined through magnetic susceptibility, specific-heat, and neutron-diffraction measurements. As opposed to isostructural $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ with the antiferromagnetic ground state, $\mathrm{MnS}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_4$ develops a spontaneous magnetization below 25 K. From our first-principles calculations on the material in a ferromagnetic state, the state could be interpreted as a type-II Weyl semimetal state with broken time-reversal symmetry. Detailed structural refinements using x-ray-diffraction and neutron-diffraction data reveal the presence of site mixing between Mn and Sb sites, leading to the ferrimagnetic ground state. With theoretical calculations, we found that the presence of site mixing plays an important role for the interlayer Mn-Mn ferromagnetic interactions.

A reformulation of site-occupation embedding theory (SOET) in terms of Green's functions is presented. Referred to as site-occupation Green's-function embedding theory (SOGET), this extension of density-functional theory for model Hamiltonians shares many features with dynamical mean-field theory but is formally exact (in any dimension). In SOGET, the impurity-interacting correlation potential becomes a density-functional self-energy which is frequency dependent and in principle nonlocal. A simple local-density-functional approximation (LDA) combining the Bethe ansatz LDA with the self-energy of the two-level Anderson model is constructed and successfully applied to the one-dimensional Hubbard model. Unlike in previous implementations of SOET, no many-body wave function is needed, thus reducing drastically the computational cost of the method.

We study properties of localized effective spins induced in gapped quantum spin chains by local inhomogeneities of the lattice. As a prototype, we study effective spins induced in an impunity sites doped AKLT model by constructing the exact ground state in a matrix product state (MPS) form. We characterize their responses to external fields by studying an extended Zeeman interaction. We also study the antiferromagnetic bond-alternating Heisenberg chain with defect structures. For this model, an MPS representation similar to that for the AKLT model, “a uniform MPS with windows,” is constructed, and it gives a good approximation of the ground state. We discuss the trade-off relation between the window length and the precision of the MPS ansatz. The effective exchange interaction between the induced spins is also investigated by using this representation.

We develop a many-body theory for the properties of exciton-polaritons interacting strongly with a Bose-Einstein condensate (BEC) of exciton-polaritons in another spin state. Interactions lead to the presence of a two-body bound state, the biexciton, giving rise to a Feshbach resonance in the polariton spectrum when its energy is equal to that of two free polaritons. Using the minimal set of terms to describe this resonance, our theory recovers the main findings of two experiments probing interaction effects for upper and lower polaritons in a BEC. This strongly supports that Feshbach physics has, indeed, been realized, and we furthermore extract the energy and decay of the biexciton from the experimental data. The decay rate is predicted to be much larger than that coming from its dissociation into two free polaritons, indicating that other decay channels are important.