As an unconventional magnet, altermagnetism attracts great interest in condensed-matter physics and applies a new research platform for the spintronics. Since the tunneling magnetoresistance (TMR) effect is an important research aspect in spintronics, we theoretically propose a universal altermagnetic sandwich device to achieve the TMR effect and investigate its transport properties. Using the nonequilibrium Green's function method and the Landauer-Büttiker formula, we obtain the conductance and the TMR ratio. By systematically rotating the orientations of the altermagnet and spin, we investigate how the altermagnetic orientations affect the conductance and the TMR ratio and comprehensively demonstrate the dependence of the conductance and the TMR ratio on a range of parameters in the system. By tuning the altermagnetism strength and the Fermi energy, as well as rotating the orientations in the altermagnet, the TMR ratio can reach a value of over 1000%. In addition, we analyze the detailed symmetry relations of the conductance and the TMR ratio in our system. Our approach provides a new design concept for the next-generation information technologies based on the altermagnetic platform, paving the way for the development of spintronics applications.

Antiferromagnets are promising candidates for ultrafast spintronic applications, leveraging current-induced spin-orbit torques. However, experimentally distinguishing between different switching mechanisms of the staggered magnetization (Néel vector) driven by current pulses remains a challenge. In an exemplary study of the collinear antiferromagnetic compound ${\mathrm{Mn}}_2\mathrm{Au}$, we demonstrate that slower thermomagnetoelastic effects predominantly govern switching over a wide parameter range. In the regime of short current pulses in the nanosecond range, however, we observe fully Néel spin-orbit torque driven switching. We show that this ultrafast mechanism enables the complete directional alignment of the Néel vector by current pulses in device structures.

Current studies have revealed that the maximal topological charge of Weyl points (WPs) is four, which gives rise to many charge-number-dependent properties, such as Landau levels, quantized circular photogalvanic effect, and multiple Fermi arcs. However, charge-four Weyl materials (termed C4-WMs) remain scarce. Here, we employ symmetry analysis and first-principles calculations to systematically identify C4-WMs. In the absence of spin-orbit coupling, we find that 13 space groups and 40 candidate materials can host C4-WPs. In ferromagnetic (FM) systems, the two spin channels decouple, allowing each to be treated as a spinless system, and C4-WPs can also emerge. We identify a total of 14 FM C4-WMs. For all 40 candidates (including FM ones), we summarize their crystallographic and magnetic parameters, as well as the positions of C4-WPs in momentum space and energy space. We also take nonmagnetic material ${\mathrm{Bi}}_{12}{\mathrm{PO}}_{20}$ and FM material ${\mathrm{K}}_2{\mathrm{CuPbN}}_6{\mathrm{O}}_{12}$ as examples, analyzing their electronic properties, especially their surface states and topological phase transitions. Our findings establish a platform for experimental exploration and further study of physical properties for C4-WPs.

The T center in silicon has recently drawn a lot of attention for its potential in quantum information science. The sensitivity of the zero-phonon line (ZPL) to electrical field was recently investigated by a combination of different experimental methods but there are still few first principles studies on the Stark shift of the T center. Dealing with the defect-bound exciton nature of the excited state is particularly challenging using density functional theory because of the large spatial delocalization associated with the wavefunction. Here, we tackle this issue by performing a convergence study over the supercell size. We obtain an exciton binding energy of 28.5 meV, in good agreement with experimental results. We then calculate the Stark shift through the dipole moment change of the ZPL transition of the T center using the modern theory of polarization formalism and find a modest linear coefficient of $\mathrm{\Delta }\mu =0.79\mathit{\text{D}}$ along $\mathrm{X}$ and $\mathrm{\Delta }\mu =0.03D$ along $\mathrm{Y}$. We discuss our results in light of the recent experimental measurements of the Stark shift. Our analysis suggests that bound-exciton defects could be particularly sensitive to local field effect as a result of their large spatial extent.

Compressive mechanical stress exceeding a critical value leads to the formation of periodic surface buckling patterns in film-substrate systems. A comprehensive understanding of this buckling phenomenon is essential in applications where the surface topologies are modulated to achieve multifunctionalities. Here, we reformulate the elastic theory of a film-substrate system by treating the compliant substrate as a nonlinear elastic solid. The resulting elastic free energy functional of the deflection field is shown to be equivalent to a minimal density functional of the phase-field crystal theory plus a Gaussian curvature-related term. The proposed elastic model constructs a phase diagram based on free energy minimization, quantitatively agreeing with the buckling transitions observed in previous experiments. The emerging hexagonal buckling system is shown to be equivalent to a two-dimensional crystal with proper scalings. We further conducted simulations of repeated buckling under cyclic stress to demonstrate a dynamically modulated structural adhesive, which resembles the physical process of repeated crystallization and melting near a critical temperature.

We analyze the properties of flat-band superconductor junctions that behave differently from ordinary junctions containing only metals with Fermi surfaces. In particular, we show how in the tunneling limit the critical Josephson current between flat-band superconductors is inversely proportional to the pair potential, how the quantum geometric contribution to the supercurrent appears even in the normal state of a flat-band weak link, and how Andreev reflection is strongly affected by the presence of bound states. Our results are relevant for analyzing the superconducting properties of junctions involving electronic systems with flat bands.

Halide perovskites have emerged as promising candidates for next-generation photovoltaic applications, attracting significant attention in recent years. Through first-principles calculations combined with group-theoretical analyses, we investigate the structural phase diagram of Pb-free Jahn-Teller active $A_2{\mathrm{Au}}_2{X}_6$ ($A=$ Cs, Rb; $X=$ Cl, Br, I) double perovskites. Our study identifies a previously unreported ferroelectric phase, where the softening of the polar mode, key to ferroelectricity, is driven by an unconventional and indirect coupling with improper strains originating from Jahn-Teller distortions. The proposed strain-mediated pseudotriggered mechanism offers an alternative pathway to enhance the static dielectric constant or even promote (photo)ferroelectricity, addressing challenges such as defects, excitons, and charge scattering that hinder photovoltaic efficiency. More broadly, this unique mechanism could be extended to oxide double perovskites and opens up a new type of ferroelectric phase transition worthy of future investigation.

${\mathrm{RuCl}}_3$ was likely the first ever deliberately synthesized ruthenium compound, following the discovery of the ${}_{44}\mathrm{Ru}$ element in 1844. For a long time it was known as an oxidation catalyst, with its physical properties being discrepant and confusing, until a decade ago when its allotropic form $\alpha \text{−}{\mathrm{RuCl}}_3$ rose to exceptional prominence. This “rediscovery” of $\alpha \text{−}{\mathrm{RuCl}}_3$ has not only reshaped the hunt for a material manifestation of the Kitaev spin liquid, but it has opened the floodgates of theoretical and experimental research in the many unusual phases and excitations that the anisotropic-exchange magnets as a class of compounds have to offer. Given its importance for the field of Kitaev materials, it is astonishing that the low-energy spin model that describes this compound and its possible proximity to the much-desired spin-liquid state is still a subject of significant debate ten years later. In the present study, we argue that the existing key phenomenological observations put strong natural constraints on the effective microscopic spin model of $\alpha \text{−}{\mathrm{RuCl}}_3$, and specifically on its spin-orbit-induced anisotropic-exchange parameters that are responsible for the nontrivial physical properties of this material. These constraints allow one to focus on the relevant region of the multidimensional phase diagram of the $\alpha \text{−}{\mathrm{RuCl}}_3$ model, suggest an intuitive description of it via a different parametrization of the exchange matrix, offer a unifying view on the earlier assessments of its parameters, and bring closer together several approaches to the derivation of anisotropic-exchange models. We explore extended phase diagrams relevant to the $\alpha \text{−}{\mathrm{RuCl}}_3$ parameter space using quasiclassical, Luttinger-Tisza, exact diagonalization, and density-matrix renormalization-group methods, demonstrating a remarkably close quantitative accord between them on the general structure and hierarchy of the phases, with the zigzag, ferromagnetic, and incommensurate phases that are proximate to each other. One of the highlights is the detailed agreement on the nature of the incommensurate phases that realize two distinct counterrotating helical states.

Raman scattering with regard to circularly polarized incident and scattered lights is closely related to the circular activity of a given system. We investigate the symmetry of its activity, called the cross-circular and parallel-circular Raman optical activity. The analysis is systematically performed with the magnetic point groups, and it indicates that the response allows for a useful diagnosis of the symmetry of materials such as chirality and (magneto)axiality. It is also shown that the Stokes and anti-Stokes processes are related to each other by the conserved antiunitary symmetry, which can be either the time-reversal operation or the combination of time-reversal and mirror reflection.

We use high-resolution angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) to investigate the electronic structure of trigonal phase ${\mathrm{PtBi}}_2$ ($t-{\mathrm{PtBi}}_2$), a proposed Weyl semimetal that is expected to exhibit topological Fermi arcs. Our ARPES data elucidates the topography of these objects and confirms their Fermi arc character. The arcs are formed by surface bands that have fairly flat bottoms located very close to the chemical potential $\sim 6–8\mathrm{meV}$, before they merge with bulk bands at higher binding energy. Comparison of the ARPES data with DFT calculations shows good agreement about their location and topography. Data acquired at low temperatures does not show any signatures of superconductivity down to 3 K in terms of expected changes in dispersion due to formation of Bogoliubov quasiparticles or superconducting gap in the form of reliable shifts of peaks in energy distribution curves.

The anomalous Hall effect due to an intrinsic magnetic ordering combined with topological electronic states is at the forefront of recent study in condensed matter physics. The seminal compound ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has fueled an enormous amount of interest in the study of various exotic quantum phenomena such as the axion insulator state, the Chern insulator state, and the quantum anomalous Hall effect on this material as well as its heterostructural cousins (${\mathrm{MnBi}}_2{\mathrm{Te}}_4){\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$. Despite the theoretically expected gapped Dirac surface states in these time-reversal symmetry broken topological materials, the electronic band structures are variously reported with regard to the presence of the band gap in the Dirac cone. Here we report on Sb-doped higher-order Mn-Bi-Te magnetic topological materials in our combined study of magnetotransport and angle-resolved photoemission spectroscopy. We found multiple magnetic ground states in the Sb-doped antiferromagnetic topological insulator ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$, driven by a delicate balance of magnetic interactions along the van der Waals interlayer direction. Additionally, we detected a gradual shift of the Fermi level from the conduction band to the valence band, accompanied by charge compensation at approximately 33% Sb doping. Interestingly, we observed the gapless Dirac states in pristine ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ and the gapped states in Sb-doped ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$. We explored this unconventional scenario in the light of recent studies from various research groups.

Phase-field modeling, which is based on the Landau-Ginzburg-Devonshire theory, has achieved great success in explaining and predicting the behavior of ferroelectrics. However, such phase-field modeling is based on continuum mechanics, and thus cannot be applied directly to atomic systems, even though interest in ferroelectrics moves to smaller and smaller scales. Here, we have developed phase-field modeling of ferroelectrics from the atomic scale; that is, we derived the free energy of ferroelectrics by combining the bond-valence molecular dynamics potential and the variational Gaussian theory. By solving the phase-field equation based on this free energy, we successfully reproduced the temperature dependence of electric polarization in ${\mathrm{PbTiO}}_3$. Furthermore, local thermodynamic analysis based on our modeling suggests that this phase transition occurs because the heavy atoms such as Pb do not have enough space for vibration in the ferroelectric state, and this increases free energy locally at high temperatures. Our study expands the application of phase-field modeling to the atomic scale and expands our understanding of phase transitions.

Optical pulling, a counterintuitive phenomenon in optical manipulation, has garnered significant attention for its applications in fields such as biomedicine, nanotechnology, and microfluidic systems. However, achieving robust optical pulling in free space remains challenging due to numerous scattering channels and environmental interference. Topological photonics, with its definite light propagation modes, offers promising solutions. In this study, we demonstrate robust long-range optical pulling using a single-mode topological chiral edge state in both the magnetic photonic crystal-air configuration and the line defect magnetic photonic crystal configuration. When the introduction of the manipulated particle creates a local gap, evanescent modes emerge in the system. This means that when the single-mode topological chiral edge state encounters this particle, it cannot bypass it. Instead, electromagnetic waves enter the particle and decay rapidly within it, generating a pulling force through dominant negative electric field intensity gradients. This pulling force remains robust to the particle's shape or length. The topological protection ensures consistent optical pulling with minimal interference from impurities. Unlike multimode states, a single-mode topological chiral edge state can pull multiple particles simultaneously within a single channel, simplifying the structure and increasing integration potential. By adjusting the excitation frequency, we can control both the pulling force magnitude and the number of simultaneously pulled particles. This work provides a new approach to particle manipulation using topological chiral edge states.

Transport and local spectroscopy measurements have revealed that monolayers of tungsten ditelluride ($1T^{\prime }{\mathrm{WTe}}_2$) display a quantum spin Hall effect and an excitonic gap at neutrality, besides becoming superconducting at low electron concentrations. With the aim of studying the competition between different broken-symmetry phases upon electron doping, we perform extensive Hartree-Fock calculations as a function of electron density and Coulomb interaction strength. At charge neutrality, we reproduce the emergence of a spin density wave and a spin spiral state surrounding a quantum spin Hall insulator at intermediate interaction strengths. For stronger interactions, the spin spiral is disrupted by a state breaking both inversion and time-reversal symmetries (but not their product) before the system becomes a trivial band insulator. With electron doping, the quantum spin Hall insulator evolves into an easy-plane ferromagnet due to a Stoner-like instability of the conduction band. This phase competes energetically with the spin spiral state. We discuss how our results may help to interpret past and future measurements.

A thorough understanding of lattice dynamics, particularly how they couple with spin through spin-phonon interactions, is crucial for pioneering new spin-caloritronic applications. Despite extensive studies on ${\mathrm{Cr}}_2{\mathrm{O}}_3$, the origin of nonlinear thermal expansions and the unusual stiffening of optical phonon modes is still elusive. Through inelastic neutron scattering and atomistic calculations, we found that these behaviors can be attributed to the renormalization of electron states owing to the magnetic transition, and we ruled out the effects from thermal expansion, phonon anharmonicity, magnetostriction, or electron-phonon interactions. Importantly, our quantitative modeling suggests that the common belief that dynamic spin-phonon interactions are the origin of anomalous phonon energy stiffening in magnetic compounds does not apply in ${\mathrm{Cr}}_2{\mathrm{O}}_3$.

Recent experiments have established a strong case for ${\mathrm{Ce}}_2$(Zr, Sn, ${\mathrm{Hf})}_2{\mathrm{O}}_7$ to host $\pi$-flux quantum spin ice (QSI). However, an irrefutable conclusion still requires strong, multifaceted evidence. In dipolar-octupolar (DO) compounds, external magnetic fields only strongly couple with the dipolar component ${\tau }_z$ along its local $z$-axis in contrast to octupolar components ${\tau }^{x,y}$. This gives rise to the unique ways magnetic fields interact with the system and, in turn, provides us with a variety of tuning knobs to generate comprehensive experimental results. In this work, we focus on magnetic fields along the (110), (111), and (001) directions and present a plethora of remarkable experimental signatures to probe the underlying physics of $\pi$-flux QSI using gauge mean field theory (GMFT) and Monte Carlo simulations. In particular, we present unique signatures in magnetic field-dependent phase diagrams, equal-time and dynamical structure factors, and magnetostriction signatures.

The recent discovery of superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ at a critical temperature above 80 K points to a nonconventional pairing mechanism in nickelates as in cuprates, possibly due to electronic correlations. We have calculated from first principles the electronic structure of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under the effect of pressure and epitaxial strain including correlations by the $GW$ approximation to the many-body self-energy. We find that the Fermi surface is composed of a characteristic cuprate-shape sheet $\beta$ plus a nickelate-specific cylinder $\alpha$, both from Ni $e_g$ orbitals, with a nonnegligible drop in the quasiparticle weight and an effective 1D character. This topology results from a delicate balance between the Ni-$3d_{z^2}$ hole pocket $\gamma$, which is suppressed by correlations, and an emerging La-$5d_{x^2-y^2}$ electron pocket induced by both correlation and pressure/strain effects and whose role at low energy has been neglected so far. Unlike cuprates, the electronic structure of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ is already correctly described from ab initio and in agreement with the experiment without the need to introduce adjustable parameters.

We explore, through a combination of x-ray diffraction and elastoresistivity measurements, the effect of disorder on the strain-tuned charge density wave (CDW) and associated multicriticality in ${\mathrm{Pd}}_x{\mathrm{ErTe}}_3$ ($x=0$, 0.01, 0.02 and 0.026). We focus particularly on the behavior near the strain-tuned bicritical point that occurs in pristine ${\mathrm{ErTe}}_3$ ($x=0$). Our study reveals that while Pd intercalation somewhat broadens the signatures of the CDW phase transitions, the line of first-order transitions at which the CDW reorients as a function of applied strain persists in the presence of disorder and still seemingly terminates at a critical point. The critical point occurs at a lower temperature and a lower strain compared to pristine ${\mathrm{ErTe}}_3$. Similarly, the nematic elastoresistance of ${\mathrm{Pd}}_x{\mathrm{ErTe}}_3$, though suppressed in magnitude and broadened relative to that of ${\mathrm{ErTe}}_3$, has a markedly more symmetric response around the critical point. These observations point to disorder driving a reduction in the system's electronic orthorhombicity even while the material remains irrevocably orthorhombic due to the presence of a glide plane in the crystal structure. Disorder, it would appear, reinforces the emergence of a pseudotetragonal electronic response in this fundamentally orthorhombic material.

Photonic cluster states are highly entangled states that allow for photonic quantum computing and memory-less quantum repeaters. Their generation has been recently demonstrated using semiconductor quantum dots emitting at the 900 nm wavelength range. However, a similar demonstration at the communication-optimal telecom range has remained elusive. A key ingredient that is still missing is an appropriate optical excitation method. A central requirement of such a method is to allow an arbitrary spin initialization of quantum-dot excitonic complexes. In this work, we report on developing such a method based on a quasiresonant $p$-shell excitation for an InAs/InGaAs quantum dot emitting at 1535 nm. We show qubit writing of a neutral exciton and spin-preserving excitation of a negative trion. Using the Larmor precession of the negative trion under an externally applied magnetic field, we determine the in-plane $g$ factors of both the electron and the hole in the investigated quantum dot. In addition, we measure a lower bound on the hole coherence time, $T_2^{*}>6.4\mathrm{ns}$, boosting its candidacy as a sound photon entangler for more advanced quantum photonic schemes.

We combine ferromagnetic resonance absorption measurements (FMR) with spin pumping measurements to ascertain the full magnetization dynamics of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG)/${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (GdIG)/Pt heterostructures. This trilayer system offers the unique possibility to individually investigate the spin dynamics of the ferrimagnetic GdIG close to its compensation temperature. We show that this trilayer acts as a highly tunable spin current source and our experimental results are corroborated by micromagnetic simulations. The detected spin current in the top Pt layer is governed by spin dynamics in the GdIG layer, while the broadband FMR absorption spectrum of the full heterostructure comprises contributions from spin dynamics in both layers. Thus, combining the measurements of FMR absorption and spin current generation from the spin pumping and spin Seebeck effect allows us to understand the spin dynamics contributions of both constituents.