Nickel-platinum-based synthetic ferrimagnets (SFi's) are highly tunable and rare-earth-free materials that allow ultrafast all optical control of magnetism to be explored. This study considers a SFi composed of a ferromagnetic [Ni/Pt] multilayer and a ferromagnetic Co layer, separated by an Ir layer that mediates an antiferromagnetic coupling. Helicity-independent all optical switching (HI-AOS) between two antiparallel magnetization states is observed. AOS may be realized at increased temperature through reduction of the thickness of the Pt layers. Switching is unidirectional and has a strong dependence on the applied magnetic field history, which suggests the possible presence of a nanoscale magnetic texture that may be important in controlling AOS in SFi systems.

Population annealing, one of the currently state-of-the-art algorithms for solving spin-glass systems, sometimes finds hard disorder instances for which its equilibration quality at each temperature step is severely damaged. In such cases, therefore, one cannot be sure about having reached the true ground state without vastly increasing the computational resources. In this work, we seek to overcome this problem by proposing a quantum-inspired modification of population annealing. Here we focus on the three-dimensional random plaquette gauge model, whose ground-state energy problem seems to be much harder to solve than that of the standard spin-glass Edwards-Anderson model. In analogy to the toric code, by allowing single bond flips we let the system explore nonphysical states, effectively expanding the configurational space by introducing topological defects that are then annealed through an additional field parameter. The dynamics of these defects allow for the effective realization of nonlocal cluster moves, potentially easing the equilibration process. We study the performance of this method in three-dimensional random plaquette gauge model lattices of various sizes, and we compare it against population annealing. With that we conclude that the newly introduced nonlocal moves are able to improve the equilibration of the lattices, in some cases being superior to a normal population annealing algorithm with a computational resource investment that is 16 times higher.

The present study investigated the magnetic nature of a solid solution system consisting of ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ and ${\mathrm{CaCo}}_2{\mathrm{P}}_2$ using a muon spin rotation and relaxation (${\mu }^{+}\mathrm{SR}$) technique, which is sensitive to local magnetic environments. The former compound ${\mathrm{EuCo}}_2{\mathrm{P}}_2$ is known to enter an incommensurate helical antiferromagnetic (AF) phase below 66 K with neutrons, which was confirmed by the present ${\mu }^{+}\mathrm{SR}$. The magnitude of the ordered Eu moments proposed with neutrons was found to be consistent with that estimated by ${\mu }^{+}\mathrm{SR}$. Furthermore, the latter lattice-collapsed tetragonal phase compound ${\mathrm{CaCo}}_2{\mathrm{P}}_2$ is known to enter an $A$-type AF phase below 90 K, and ${\mu }^{+}\mathrm{SR}$ measurements on single crystals revealed the presence of a spin reorientation transition at around 40 K, below which the $A$-type AF order is likely to be completed. Although all ${\mathrm{Eu}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$ compounds were found to enter a magnetic phase at low temperatures regardless of $x$, a static ordered state was formed only at the vicinity of the two end compounds, i.e., $0\le x\le 0.4$ and $0.9\le x\le 1$. Instead, a disordered state, i.e., a random spin-glass state, short-range ordered state, or highly fluctuating state was found in the $x$ range between 0.4 and 0.9, even at the lowest measured temperature (2 K). Together with the magnetization data, our findings clarified the magnetic phase diagram of ${\mathrm{Eu}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$, where a ferromagnetic exchange interaction between Co ions through the ${\mathrm{Eu}}^{2+}$ ion competes with a direct AF interaction among the Co ions, particularly in the $x$ range between 0.57 and 0.9. This competition yielded multiple phases in ${\mathrm{Eu}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$.

Accurate electronic structure descriptions of layered oxides are highly relevant yet complex to obtain by ab initio methods. The functionalities of layered transition metal oxides are usually heavily influenced by their electronic structure and the hybridization/delocalization of $d$ electrons of the transition metals. Therefore understanding the electronic structure can enable a more rational design of this highly important material class, which is widely utilized in practical applications such as cathodes in secondary batteries of portable electronic devices and electric vehicles. However, it is well known that standard quantum-mechanic approximations to the many-body problem overlocalize (Hartree-Fock) or overdelocalize (density functional theory) electrons in these systems. A mixture of different methods (hybrid functionals) can partially resolve the problem but introduces at least one additional parameter (the mixing parameter) that has to be determined by e.g., higher levels of theory ($GW$). In this study, we focus on the electronic structure of lithiated layered transition-metal oxides based on Co, Ni, Mn, and their binary systems that form the foundation of state-of-the-art lithium-ion batteries. The influence of Hartree-Fock mixing in the PBE0 hybrid functional on the electronic structure is compared as well as using different Hartree-Fock mixings as starting points for $GW$ calculations. Two nonempirical $GW$-based fitting approaches to determine the optimal Hartree-Fock mixing are considered. We show that one of the fitting approaches suggests small mixing parameters and is in satisfactory agreement with experimental results, while the other approach has a stronger theoretical foundation and indicates higher mixing parameters, which are close to the value obtained by perturbation theory by Perdew, Ernzerhof, and Burke. Finally, it is shown that larger mixing parameters are required when screening is introduced to the Hartree-Fock term.

Crystalline topological insulators have recently become a powerful platform for realizing photonic topological states from microwaves to the visible. Appropriate geometric symmetries of the lattice are at the core of their functionality. Here we put forward an alternative approach to craft those systems by designing the internal symmetries of the Hamiltonian via accidental mode degeneracies. We illustrate our approach constructing an analog of breathing honeycomb lattice using simpler lattice geometry and six times less meta-atoms, revealing edge and corner states, and calculating the relevant topological invariants.

We present a theory for thin films of the Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ in the presence of magnetic fields. We show that, above a critical thickness, specific subbands $n$ of thin film ${\mathrm{Cd}}_3{\mathrm{As}}_2$ are in a quantum spin Hall insulator regime and study their response to in- and out-of-plane magnetic fields. We find that sufficiently large in-plane Zeeman fields drive the system toward a 2D Dirac semimetal regime, provided the field is directed perpendicular to a high-symmetry mirror plane. For other directions, we find the Dirac points to be weakly gapped. We further investigate how the system responds to finite out-of-plane field components, both starting from the quantum spin Hall regime at small in-plane fields and from the 2D Dirac semimetal regimes at larger in-plane fields, addressing recent experimental observations in A. C. Lygo et al. [Phys. Rev. Lett. 130, 046201 (2023)] and B. Guo et al. [Phys. Rev. Lett. 131, 046601 (2023)].

The bulk-boundary correspondence (BBC) relates in-gap boundary modes to bulk topological invariants. In certain non-Hermitian topological systems, conventional BBC becomes invalid in the presence of the non-Hermitian skin effect (NHSE), which manifests as distinct energy spectra under the periodic and open boundary conditions and massive eigenstate localization at boundaries. In this paper, we introduce a scheme to induce NHSE without breaking conventional BBC, dubbed as the topologically compatible NHSE (TC-NHSE). In a general one-dimensional two-band model, we unveil two types of TC-NHSE that do not alter topological phase transition points under any circumstance or only in a certain parameter regime, respectively. Extending our model into two dimension, we find that TC-NHSE can be selectively compatible to different sets of Weyl points between different bands of the resultant semimetallic system, transforming some of them exceptional points connected by bulk Fermi arcs while keeping the rest unchanged. Our paper hence helps clarify the intricate interplay between topology and NHSE in non-Hermitian systems, and provides a versatile approach for designing non-Hermitian topological systems where topological properties and NHSE do not interfere each other.

The exploration of various topological nodal-line phonons with novel topological properties and unique geometrical configurations has long been considered one of the central topics in topological physics. In this work we find that Weyl nodal lines are composed of two basic categories, i.e., linear and quadratic. According to the dispersion relations of degeneracy points in them, both linear and quadratic nodal lines can be divided into three types, i.e., types I, II, and III. Based on these basic types, two unusual nodal lines, i.e., anisotropic and hybrid, may be defined here. Unfortunately, real materials which may simultaneously generate these basic and unusual nodal-line phonons with different types and categories have still rarely been reported, owing to the highly required symmetry conditions. By using symmetry analysis and first-principles calculations, we uncover that a material family $\mathrm{Ba}{X\mathrm{N}}_2(X=\mathrm{Ti},\mathrm{Zr},\mathrm{Hf})$ in space group 129 may generate novel butterfly-shape nodal-birdcage phonons that are composed of three hourglasslike type-II nodal rings and four straight nodal lines. More importantly, multiple unusual nodal lines, including anisotropic and hybrid ones, also exist in these materials. The topologically nontrivial features of these various nodal-line phonons are confirmed by the nonzero Berry phases and the drumheadlike surface states. Our theoretical results provide an ideal material platform to study the topological properties of various nodal-line phonons, including three unusual types.

Accurately predicting the electronic energy deposition of ions in materials is an important challenge in both fundamental and applied research. While employing ab initio simulations to investigate electronic stopping of ions in matter holds promise, its combined use with experimental measurements paves the way for obtaining reliable data. In this paper, we present a collaborative study using real-time time-dependent density functional theory and experimental methods to determine the electronic stopping power of hydrogen and helium ions in tungsten, a primary candidate material for future nuclear fusion devices. While calculated stopping powers in hyperchanneling trajectories are significantly lower than the experimental data, off-center and random geometries demonstrate a better agreement. We show that the deviation from velocity proportionality for both projectiles traversing the hyperchanneling directions can be explained through the existence of a threshold velocity leading to the activation of semicore states. Additionally, we analyze the pseudopotential and the trajectory dependence of computed electronic energy losses. It is demonstrated that the role of including semicore electrons varies depending on the velocity range. While these states play a crucial role at high projectile velocities by introducing additional dissipation channels, their impact diminishes in the low-velocity range. Finally, we introduce a simple expression that links electronic energy losses in different trajectories to local electron density, and we show that utilizing this formula allows for quite accurate predictions of stopping powers around the Bragg peak.

Iron ions play significant roles as recombination centers and activators in insulators. However, their identification and mechanisms through which they operate are often controversial. In this study, we utilized first-principles calculations to investigate the site occupancy, valence states, excited states, and optical properties of iron ions acting as luminescent centers. We also explored potential quenching processes when these ions serve as recombination centers in various crystals. We found that iron ions can occupy sites with tetrahedral, octahedral and dodecahedral coordinations. Specifically, tetrahedrally coordinated ${\text{Fe}}^{3+}$ in oxide insulators exhibit near-infrared emission in the range of 670–830 nm, with the specific wavelength dependent on the degree of structure distortion. In contrast, octahedrally coordinated ${\text{Fe}}^{3+}$ ions in spinels and garnets exhibit considerably lower transition energies due to stronger ligand fields and significant excited-state structural relaxation. This makes them more susceptible to nonradiative decay and quenching. We also successfully elucidated the luminescence of those six-coordinated ${\text{Fe}}^{3+}$ in perovskites, where the weaker ligand fields and smaller excited-state relaxation than those in spinels and garnets contribute to the observed emissions. Furthermore, we have confirmed the existence of dodecahedrally coordinated ${\text{Fe}}^{3+}$ in hosts with a zircon structure, which results in a large transition energy due to the combined effect of the small nephelauxetic effect in phosphates and the small ligand field strengths associated with a subspherical coordination distribution. Additionally, we revisited and reinterpreted some experimental results based on our calculations. This study offers consistent and reliable interpretations of optical transitions of iron impurities in solids, which can be beneficial for the design and optimization of luminescent materials.

The ubiquitous phenomenon of synchronization is inherently characteristic of dynamical dissipative nonlinear systems. In particular, synchronization has been theoretically and experimentally demonstrated for exciton-polariton condensates with time-independent coupling. Unlike that case, we theoretically investigate the effects of a fixed mechanical harmonic driving, i.e., a coherent phonon population, that induces time modulation in the coupling of two coupled condensates. Our model applies both to electrically generated modulation, through coherent bulk acoustic waves, and to self-induced optomechanical vibrations. We consider linear as well as quadratic phonon-displacement-induced modulations of the polariton coupling. Peculiar asynchronously locked phases are found and analyzed in the context of synchronization and Josephson-type oscillations phenomena that appear in the nondriven case. Notably, Arnold tongues corresponding to the asynchronously locked phases emerge at condensate detunings that correspond to integer numbers of the mechanical frequency and also to rational fractions of it. Unlocked quasiperiodic and chaotic regimes can also be reached. In particular, the phonon-induced fractional locking frequencies arrange in a Farey sequence that produces a devil's staircase of the steady-state dressed detuning between the condensates. Importantly, both polariton-polariton and reservoir-polariton interactions facilitate the realization of coherent phonon-induced asynchronously locked phases. The relation to recent experiments on optomechanically driven condensates in arrays of polariton traps is discussed.

Traditional understanding of the thermal boundary resistance (TBR) across solid-solid interfaces posits that the vibrational densities of states overlap between materials dictates interfacial energy transport, with phonon scattering occurring at the interface. Using atomistic simulations, we show a mechanism for control of TBR; point defects near an interface can lead to both short- and midrange disorder, accelerating the conversion of vibrational energy between bulk and interfacial modes, ultimately reducing the TBR. We experimentally demonstrate this reduction through ion irradiation of gallium nitride and subsequently measuring the TBR across Al/GaN interfaces.

A deconfined quantum phase transition (DQPT) provides an extraordinary possibility of the quantum phase transition beyond the Ginzburg-Landau paradigm, which is interwoven with numerous exotic phenomena of the strongly correlated quantum many-body system, e.g., fractional excitation, emergent symmetries, and gauge field. However, various candidates of DQPT have been demonstrated to be weakly first order, and the conformal field theory has to be altered into a nonunitary one. Here we numerically found two linear dispersions with different velocities in one of the few survivors of DQPT—the extended hard-core Bose-Hubbard model on the kagome lattice. Such counterintuitive results directly lead to the negation of possible emergent Lorentz symmetry and the breakdown of conventional theory of DQPT. Furthermore, the snapshots of boson configuration hint that these two velocities may correspond to the dynamics of the fractional excitations and quantum strings, respectively. Our work will inspire researchers to revisit the theory of DQPT and benefit the field of quantum materials and quantum simulations.

The control of magnetization using electric fields has been extensively studied in magnetoelectric multiferroic insulating materials while changes in magnetization in bulk metals caused by electric currents have attracted less attention. The recently discovered metallic magnet ${\mathrm{Ce}}_3{\mathrm{TiBi}}_5$ has been reported to exhibit current-induced magnetization. Here we determined the magnetic structure of ${\mathrm{Ce}}_3{\mathrm{TiBi}}_5$ using neutron diffraction, aiming to understand the microscopic origin of this magnetoelectric phenomenon in a metal. We established that the antiferromagnetic order emerging below $T_N=5\mathrm{K}$ is a cycloid order described by $P{6}_3/mcm.1^{\prime }(0,0,g)00sss$ with small moment sizes of $0.50\left(2\right){\mu }_B$ and propagation vector $\mathbf{k}=(0,0,0.386)$. Surprisingly, the symmetry of this magnetic structure is inconsistent with the presence of current-induced magnetization and potential origins of this inconsistency with previous results are discussed. Additionally, our results suggest that moments order along their hard magnetic direction in ${\mathrm{Ce}}_3{\mathrm{TiBi}}_5$, a phenomenon which has been observed in other Kondo systems.

In superconductivity, a surge of interests in enhancing $T_{\mathrm{c}}$ is ever mounting, where a recent focus is toward multiband superconductivity. In $T_{\mathrm{c}}$ enhancements specific to two-band cases, especially around the Bardeen-Cooper-Schrieffer to Bose-Einstein condensate crossover considered here, we have to be careful about how quantum fluctuations affect the many-body states, i.e., particle-hole fluctuations suppressing the pairing for attractive interactions. Here, we explore how to circumvent the suppression by examining multichannel pairing interactions in two-band systems. With the Gor'kov-Melik-Barkhudarov (GMB) formalism for particle-hole fluctuations in a continuous space, we look into the case of a deep dispersive band accompanied by an incipient heavy-mass (i.e., quasiflat) band. We find that, while the GMB corrections usually suppress $T_{\mathrm{c}}$ significantly, this in fact competes with the enhanced pairing arising from the heavy band, with the trade-off leading to a peaked structure in $T_{\mathrm{c}}$ against the band-mass ratio when the heavy band is incipient. The system then plunges into a strong-coupling regime with the GMB screening vastly suppressed. This occurs prominently when the chemical potential approaches the bound state lurking just below the heavy band, which can be viewed as a Fano-Feshbach resonance, with its width governed by the pair-exchange interaction. The diagrammatic structure comprising particle-particle and particle-hole channels is heavily entangled, so that the emergent Fano-Feshbach resonance dominates all the channels, suggesting a universal feature in multiband superconductivity and superfluidity.

We have derived a series of analytical formulas describing the motion of a nanotip elastically driven across a Gaussian potential well of depth $U_1$ and half width $\sigma$. The tip gets pinned if the impact parameter $y_0$ is below a well-defined threshold value. Together with $U_1$ and $\sigma$, this value determines the pinning force. The scenario is slightly modified by superimposing a periodic potential with amplitude $U_0$ in the background, provided that $U_0\ll U_1$. The formulas presented here can be applied to estimate the pinning energy and the linear extension of the perturbation created by a point defect such as an atomic vacancy on a crystal surface from atomic-scale friction force measurements performed by scanning probe microscopy.

The critical behavior of the order-disorder phase transition in the buckled dimer structure of the Si(001) surface is investigated both theoretically by means of first-principles calculations and experimentally by spot profile analysis low-energy electron diffraction (SPA-LEED). We use density functional theory (DFT) with three different functionals commonly used for Si to determine the coupling constants of an effective lattice Hamiltonian describing the dimer interactions. Experimentally, the phase transition from the low-temperature $c(4\times 2)$- to the high-temperature $p(2\times 1)$-reconstructed surface is followed through the intensity and width of the superstructure spots within the temperature range $78–400\mathrm{K}$. Near the critical temperature $T_{\mathrm{c}}=190.6\mathrm{K}$, we observe universal critical behavior of spot intensities and correlation lengths, which falls into the universality class of the two-dimensional (2D) Ising model. From the ratio of correlation lengths along and across the dimer rows we determine effective nearest-neighbor couplings of an anisotropic 2D Ising model, $J_{\parallel }=(-24.9\pm 0.9_{\mathrm{stat}}\pm 1.3_{\mathrm{sys}})\mathrm{meV}$ and $J_{\perp }=(-0.8\pm 0.1_{\mathrm{stat}})\mathrm{meV}$. We find that the experimentally determined coupling constants of the Ising model can be reconciled with those of the more complex lattice Hamiltonian from DFT when the critical behavior is of primary interest. The anisotropy of the interactions derived from the experimental data via the 2D Ising model is best matched by DFT calculations using the PBEsol functional. The trends in the calculated anisotropy are consistent with the surface stress anisotropy predicted by the DFT functionals, pointing towards the role of surface stress reduction as a driving force for establishing the $c(4\times 2)$-reconstructed ground state.

Elliptical skyrmions within epitaxial bulk materials have been intensively studied due to their intriguing dynamics behaviors. In two-dimensional materials, however, elliptical spin textures are rarely explored. In this paper, we investigate various properties of ${\text{V}X}_2$ via first-principles calculations and Monte Carlo simulations. At zero external field, the $\text{sub}\text{−}10\mathrm{nm}$ circular antiskyrmions are obtained in the monolayer ${\mathrm{VTe}}_2$. Interestingly, these antiskyrmions can be transformed into bimerons and elliptical antiskyrmions under biaxial and uniaxial strains. More importantly, even without strains and external fields, the intrinsic elliptical antiskyrmions and bimerons are discovered in monolayer $\text{V}XY$ ($X,Y=$ S, Se, Te), which present rather high Curie temperatures. These spin textures exhibit stability over a wide temperature range. Our results provide a very feasible approach to realizing elliptical spin textures in two-dimensional materials.

A fundamental brick of light-matter interaction at large optical intensities is the generation of a plasma. The optically induced plasma in turn plays a fundamental role in determining the optical propagation. The plasma generation is a result of the interplay between multiphoton, tunnel, and avalanche ionization. Here we use the basic rate equations to discuss an analytical model for the interaction between these physical effects. After defining a nonlinear impulse response for the system, we describe how the interplay depends on the features of the optical pulses. Our approach strongly simplifies the modeling of the propagation of ultrashort pulses, paving the way to a much easier and faster interpretation of experimental observations, with potential impact on the broad fields of ultrafast light-matter interaction and laser micromachining.