The support of high-$Q$ resonances in all-dielectric nanostructures are necessary for the enhancement of light-matter interactions. By introducing some perturbations into periodic structures to enable an engineered coupling between the supported bound modes and the exterior environment, quasibound modes can consequently be achieved. However, the nature of those modes is still in need of clear investigation. In this work, we employ two geometrically similar binary gratings and demonstrate with both numerical and experimental results that two fundamentally different bound modes may lie behind the high-$Q$ resonances reported in the literature: the bound state in the continuum (BIC) and the guided modes in subwavelength photonic lattices. These two modes have distinct dependences of the $Q$ factor on the wave vector and when they are converted into quasibound (leaky) modes, the same behaviors are retained. We find that only those high-$Q$ resonances with the predecessor of guided modes have robust $Q$ factors over a large range of wave vectors and frequencies, which suggests the high tunability of resonances with comparable $Q$ values by the incident angle. In contrast, the BIC provides more versatility for dealing with optical resonances with both high-$Q$ factors and exotic polarizations. These findings correct the general misunderstanding of the origin of high-$Q$ resonances in periodic photonic structures and will significantly broaden the landscape of all-dielectric nanophotonics.

The mechanical properties of crystals on curved substrates mix elastic, geometric, and topological degrees of freedom. In order to elucidate the properties of such crystals, we formulate the low-energy effective action that combines metric degrees of freedom with displacement fields and defects. We propose dualities for elasticity coupled to curved geometry formulated in terms of tensor gauge theories. We show that the metric degrees of freedom, evolving akin to linearized gravity, are mapped to tensors with three indices. When coupled to crystals, these degrees of freedom become gapped and, in the presence of dislocations and disclinations, multivalued. The elastic degrees of freedom remain gapless and mapped to symmetric gauge fields with two indices. In analogy with elasticity on flat-space formulation, we assume that the trace of the total quadrupole moment is conserved. In the dual formulation, topological defects, which act as sources for the gauge fields, are fractons or excitations with restricted mobility. This leads to generalized glide constraints that restrict both displacement and gravitational defects.

The traditional approach to studying near-field thermal transfer is based on fluctuational electrodynamics. However, this approach may not be suitable for nonequilibrium states due to dynamic drivings. In our work, we introduce a theoretical framework to describe the phenomenon of near-field heat transfer between two objects when subjected to periodic time modulations. We utilize the machinery of the nonequilibrium Green's function to derive general expressions for the DC energy current in Floquet space. Furthermore, we also obtain the energy current under the condition of small driving amplitude. The external drivings create a nonequilibrium state, which gives rise to various effects such as heat-transfer enhancement, heat-transfer suppression, and cooling. To illustrate these phenomena, we conduct numerical calculations on a system of Coulomb-coupled quantum dots, and specifically investigate the scenario of a periodically driving electronic reservoir. In our calculations, we employ the $G_0{W}_0$ approximation, which does not require self-consistent iteration and is suitable for weak Coulomb interaction. Our theoretical formalism can be applied to study near-field energy transfer between two metallic plates under periodic time modulations.

Interspersing unitary dynamics with local measurements results in measurement-induced phases and transitions in many-body quantum systems. When the evolution is driven by a local Hamiltonian, two types of transitions have been observed, characterized by an abrupt change in the system size scaling of entanglement entropy. The critical point separates the strongly monitored area-law phase from a volume law or a subextensive, typically logarithmiclike, one at low measurement rates. Identifying the key ingredients responsible for the entanglement scaling in the weakly monitored phase is the key purpose of this work. For this purpose, we consider prototypical one-dimensional spin chains with local monitoring featuring the presence/absence of U(1) symmetry, integrability, and interactions. Using exact numerical methods, the system sizes studied reveal that the presence of interaction is always correlated to a volume law weakly monitored phase. In contrast, noninteracting systems present subextensive scaling of entanglement. Other characteristics, namely integrability or U(1) symmetry, do not play a role in the character of the entanglement phase.

We present experimental and computational findings of the laser-induced nonreciprocal motion of magnetization during ultrafast photomagnetic switching in garnets. We found distinct coherent magnetization precession trajectories and switching times between four magnetization states, depending on both directions of the light linear polarization and initial magnetic state. As a fingerprint of the topological symmetry, the choice of the switching trajectory is governed by an interplay of the photomagnetic torque and magnetic anisotropy. Our results open a plethora of possibilities for designing energy-efficient magnetization switching routes at arbitrary energy landscapes.

Long-range magnetic textures, such as magnetic skyrmions, give rise to rich transport properties in magnetic metals, such as the anomalous Hall effect related to spin chirality, aka topological Hall effect. In addition to the topological Hall effect, recent studies on noncentrosymmetric magnets have found that the spin-orbit interaction of itinerant electrons gives rise to unique contributions related to spin chirality, i.e., the chiral Hall effect. In this paper, we discuss that the spin-orbit interaction has a distinct yet significant effect on the anomalous Hall effect related to spin chirality in centrosymmetric magnets. Using a scattering theory method, we find that the anomalous Hall effect related to scalar spin chirality in a two-dimensional Luttinger model is suppressed by more than one order of magnitude compared to the quadratic dispersion, and the contributions similar to the chiral Hall effect in Rashba models vanishes. At the same time, a unique term related to vector spin chirality occurs, which gives different Hall conductivities for the Bloch and Neel skyrmions, thereby enabling detection of the skyrmion helicity. The striking differences demonstrate the rich effect of crystal symmetry on the chirality-related anomalous Hall effect in materials with strong spin-orbit interactions.

Rectification effects in solid-state devices are a consequence of nonreciprocal transport properties. This phenomenon is usually observed in systems with broken inversion symmetry. In most instances, nonreciprocal transport arises in the presence of an applied magnetic field and the rectified signal has an antisymmetric dependence on the field. We have observed rectification of environmental electromagnetic fluctuations in plain Nb channels without any asymmetry in design, leading to spontaneous voltage peaks at the superconducting transition. The signal is symmetric in the magnetic field and appears even without an applied field at the critical temperature. This is indicative of an unconventional mechanism of nonreciprocal transport resulting from a spontaneous breaking of inversion symmetry.

We investigate the relaxation dynamics in a chiral one-dimensional quantum channel with finite-range interactions, driven out of equilibrium by the injection of high-energy electrons. While the distribution of high-energy electrons, after dissipation of some of their energy, has been examined previously [Phys. Rev. B 108, L081121 (2023)], we study the distribution of charge carriers excited from the channel's Fermi sea during this process. Utilizing a detector to measure the energetic imprint in the Fermi sea downstream of the injection point, we discover an initial symmetry in the distribution of excited electrons and holes relative to the Fermi level. However, this symmetry breaks down with stronger interactions and increased propagation distances, attributed to terms of order four and beyond in the interaction. We interpret these results in terms of interference between states with different numbers of plasmons in the Fermi sea.

Two-dimensional (2D) Dirac semimetals possess intriguing properties due to their low-energy excitations behaving like Dirac fermions. A hallmark of these materials is the unconventional integer quantum Hall effect (IQHE), originating from the quantized Berry phase of Dirac fermions. Herein, using symmetry analysis, tight-binding models, and numerical calculations, we reveal 2D Dirac-Weyl fermions in inversion symmetry breaking systems that exhibit tunable unconventional IQHE. These unique 2D fermions are characterized by a pair of helical edge states related by time-reversal symmetry $\mathcal{T}$, which connect the projections of a Dirac point and two separate Weyl nodes, indicating that the Dirac and Weyl points are interconnected as a whole. We show that these 2D Dirac-Weyl fermions exhibit a tunable unconventional IQHE, featuring a Hall plateau sequence shifted by three units of $\frac{2e^2}{h}$. The distance between adjacent quantized Hall plateaus can be adjusted by strain, which is a unique feature that distinguishes from what is observed in graphene. Through first-principles calculations, we identify an ideal candidate material for hosting 2D Dirac-Weyl fermions, offering a promising avenue for experimental verification. Our findings open up a door to exploring unconventional IQHE in condensed-matter systems beyond graphene.

Anomalous seismic data on Earth's inner core have challenged our understanding of mineral physics for decades. According to recent ab initio studies, bcc Fe may be a key to solving this long-standing problem. However, simulating the bcc phase requires an enormous supercell, a long simulation time, and a prohibitive computational cost. Consequently, available information about bcc Fe is very limited. Herein, we introduce a more convenient approach for investigating the rheological and mechanical properties of the bcc structure under deep-Earth conditions. Since bcc Fe behaves similarly to a liquid system while remaining within the solid state, we treat it as a hard-sphere glass former in the elastically collective nonlinear Langevin equation theory. On that basis, we can readily evaluate the contribution of local and nonlocal interactions to the motion of atoms at various packing fractions. To consider finite-temperature and hydrostatic-pressure effects, we utilize the statistical moment method. The mentioned strategy enables us to determine the diffusivity, viscosity, and rigidity of bcc Fe without strenuous computational efforts. In addition, we discover a close connection between the glass and superionic transitions. Our theoretical calculations agree quantitatively well with cutting-edge large-scale molecular dynamics simulations. Therefore they would be valuable for unlocking the mysteries of Earth's inner core, such as the low shear resistance, the high Poisson ratio, and the strong seismic-wave attenuation.

Photonic and bosonic systems subject to incoherent, wide-bandwidth driving cannot typically reach stable finite-density phases using only nondissipative Hamiltonian nonlinearities; one instead needs nonlinear losses, or a finite pump bandwidth. We describe here a very general mechanism for circumventing this common limit, whereby Hamiltonian interactions can cut-off heating from a Markovian pump, by effectively breaking a symmetry of the unstable, linearized dynamics. We analyze two concrete examples of this mechanism. The first is a new kind of $\mathcal{PT}$ laser, where Hermitian Hamiltonian interactions can move the dynamics between the $\mathcal{PT}$ broken and unbroken phases and thus induce stability. The second uses on-site Kerr or Hubbard type interactions to break the chiral symmetry in a topological photonic lattice, inducing exotic phenomena from topological lasing to the stabilization of Fock states in a topologically protected edge mode.

We present a detailed investigation of the lattice vibrations and magnetic properties of the spin-$\frac{1}{2}\alpha \text{−}{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ system by means of x-ray diffraction (XRD), magnetic susceptibility, specific heat, x-ray absorption spectroscopy (XAS), x-ray photoelectron spectroscopy (XPS), and Raman scattering measurements along with a phonon structure calculations by density-functional theory (DFT). Thermodynamic measurements show a long-range ordered (LRO) state at Néel temperature $T{}_N\sim 33.4$ K. From the molar susceptibility curves analysis an effective magnetic moment of 1.83 ${\mu }_B$ and an antiferromagnetic intrachain exchange coupling of $\sim 62.13$ K ($\sim 5.35$ meV) is deduced. Despite exchange coupling in 1D chains, the susceptibility, low-temperature heat-capacity, and Raman spectroscopic analysis confirms that the antiferromagnetic order emerges from the mixed dimensionality nature of the exchange couplings and interchain exchange coupling was found to be $\sim 16.91$ K. Temperature-dependent Raman spectra has been performed in the temperature range 3.5 K to 300 K and established the spin-lattice coupling below 50 K, which appears well above $T_N$ and is attributed to short-range magnetic ordering ($T_{SRO}$). The spin-lattice coupling constant has been calculated for various modes. Further, the detailed study of lattice dynamics by first-principle calculations on $\alpha \text{−}{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ is presented.

Spin waves excited in periodically modulated magnetic nanomaterials, known as magnonic crystals, exhibit characteristic band structures. These bands can be tuned by material engineering and have been attractive for potential spin-based applications. When periodic nanomaterials with handedness are introduced, spin waves inherit the chiral feature in their behavior and manifest an exciting range of novel physics, including asymmetric and unidirectional propagation, low-frequency magnonic flat bands, and indirect band gaps. This study investigates the properties of these chiral magnonic excitations. The analysis is performed in ferromagnetic films patterned with nanowires of two different materials that produce periodically modulated perpendicular magnetic anisotropy and interfacial antisymmetric exchange (Dzyaloshinskii-Moriya interaction). The low-frequency flat modes are studied using a magnonic localization diagram that distinguishes the spatial confinement degree in zones with and without antisymmetric exchange. An analytical expression is derived for the transition region in the localization diagram that outlines the zones where magnonic confinement occurs. The findings reveal the presence of flat modes with nonreciprocal magnetization oscillation amplitudes between waves with opposite propagation directions when the spin-wave localization occurs in regions with Dzyaloshinskii-Moriya interaction. Conversely, reciprocal oscillation amplitudes are observed when modes localize in the nanowires with perpendicular anisotropy. Micromagnetic simulations demonstrate the amplitude asymmetry of the flat modes, yielding perfect agreement with the theoretical predictions. This paper provides a deeper understanding of the behavior of spin-wave modes in chiral magnonic crystals and establishes a method to control their associated magnonic bands for designing spin-wave-based nanodevices.

We propose and analyze a laser-driven protocol to generate long-range ordered patterns in paramagnets, based on nonadiabatically driven aggregation dynamics. We derive the optimal driving parameters that maximize, respectively, the correlation length or the size of defect-free antiferromagnetic clusters in a one-dimensional chain. We show that one can reach correlation lengths that are exponentially larger than those achieved by adiabatic pumping schemes. The resonantly driven dynamics of cluster fronts are shown to map to an exactly solvable model of free fermions.

For ${\mathrm{Y}}_2{\mathrm{C}}_3$ with a superconducting critical temperature ($T_c$) $\sim 18\mathrm{K}$, zone-center imaginary optical phonon modes have been found for the high-symmetry $I\text{−}43d$ structure due to C dimer wobbling motion and electronic instability from a flat band near Fermi energy. After lattice distortion to the more stable lowest-symmetry $P1$ structure, these stabilized low-energy phonon modes with a mixed C and Y character carry a strong electron-phonon coupling to give rise to the observed sizable $T_c$. Our work shows that compounds with the calculated dynamical instability should not be simply excluded in the high-throughput search for new phonon-mediated superconductors. Moreover, we have studied the phase stability of the $I\text{−}43d$ structure by calculating the enthalpy of different structural motifs of binary compounds containing group-IV elements at the 2:3 composition and also exploring the energy landscapes via $abinitio$ molecular dynamics near and out of the $I\text{−}43d$ structure. Our results show that the $I\text{−}43d$-type structures with C dimers are preferred in the low to medium pressure range. Because of the wobbling motion of the C dimers, there are many local energy minimums with degenerated energies. Thus, the ensemble average of many $I\text{−}43d$-distorted structures with C dimer wobbling motion at finite temperature still gives an overall $I\text{−}43d$ structure.

Nitrogen represents an archetypal example of material exhibiting a pressure-driven transformation from molecular to polymeric state. Detailed investigations of such transformations are challenging because of a large kinetic barrier between molecular and polymeric structures, making the transformation largely dependent on kinetic stimuli. In the case of nitrogen, additional complications occur due to the rich polymorphism in the vicinity of the transition. Here, we report the observation of both molecular $(\theta )$ and black phosphorus (bp) polymeric phases, crystallized upon temperature quenching of fluid nitrogen to room temperature at 97–114 GPa. Synchrotron single-crystal x-ray diffraction, Raman spectroscopy, and first-principles theoretical calculations have been used for diagnostics of the phases and determination of their structure and stability. The structure of $\theta -{\mathrm{N}}_2$ is determined as tetragonal, space group $P{4}_2{2}_{1}2$—one of the phases previously predicted theoretically above 9.5 GPa but never recognized experimentally. Molecular $\theta -{\mathrm{N}}_2$ is the most stable among molecular phases bordering the stability field of polymeric phases, partially settling a previously noted discrepancy between theory and experiment concerning the thermodynamic stability limit of molecular phases.

Employing ab initio density functional theory (DFT), we performed a systematic investigation of the electronic structure and the magnetic properties of atomic bilayers composed of a $4d$ transition-metal layer (Rh, Pd, and Ru) and a Co layer on the Re(0001) surface. Our DFT calculations reveal the influence of the bilayer composition and their stacking sequence on the magnetic ground state and magnetic interactions. We obtain the magnetic interactions by mapping the total energies onto an effective spin Hamiltonian which contains the exchange interaction and the Dzyaloshinskii-Moriya interaction (DMI), as well as the magnetocrystalline anisotropy energy (MAE). We observe noticeable changes in bilayer hybridization due to variation in bilayer composition and overlayer symmetry, leading to significant variation in magnetic interactions. In all considered systems, the effective exchange interaction is ferromagnetic, however, the value varies by up to a factor of 5. The effective DMI constant exhibits variation in sign over the films considered, favoring either right- (clockwise) or left-handed (counterclockwise) cycloidal spin spirals. The value of the DMI changes by up to a factor of 27 among the films. For most of the systems, the MAE favors an out-of-plane easy magnetization axis; however, for hcp-Co/Rh and hcp-Co/Ru bilayers on Re(0001), it prefers an in-plane magnetization axis. The magnitude of the MAE varies from a small value of about 0.1 meV/Co atom up to about 2 meV/Co atom for Co/Pd bilayers. The spin spiral energy dispersion curve rises quite quickly close to the ferromagnetic state for films in which the Co layer is adjacent to the vacuum indicating a large effective exchange constant which stabilizes a ferromagnetic ground state in $\mathrm{Co}/4d$ bilayers on Re(0001). The energy dispersion curve becomes flatter for films with a Co layer that is sandwiched between a $4d$ overlayer and the Re(0001) surface. In this case, the exchange constant is much reduced and the ground state is determined by the competition among the exchange interaction, favoring the FM state, the DMI, which favors cycloidal spin spirals, and the MAE, which disfavors spin spirals over the FM state. As a result, hcp-Rh/Co/Re(0001) shows a spin spiral ground state driven by DMI with a period of 13 nm, while the other films exhibit a ferromagnetic ground state. The spin spiral energy dispersion of hcp-Rh/Co/Re(0001) indicates that isolated skyrmions can be stabilized in the ferromagnetic background with an applied magnetic field. Our results further suggest that isolated skyrmions could be realized even in the absence of an external field in fcc-Rh/Co/Re(0001), hcp-Pd/Co/Re(0001), fcc-Pd/Co/Re(0001), and fcc-Ru/Co/Re(0001). A total energy comparison reveals that these five promising films are energetically strongly preferred over films with a Co overlayer. This makes ultrathin films composed of a $4d$ transition-metal overlayer on Co/Re(0001) promising candidates for the search of isolated skyrmions.

Here, we study the effect of external pressure on martensitic transition, magnetic, and magnetocaloric properties of Mn-rich ${\mathrm{Mn}}_{50}{\mathrm{Ni}}_{\text{41-}x}{\mathrm{Sn}}_{\text{9}+x}$ $(x=\mathrm{0} \mathrm{and} 2)$ Heusler alloys by using a combined experimental and first principles simulation. The $x=0$ alloy exhibits martensitic transition around room temperature (RT), which increases appreciably under external pressure for both the alloys. External pressure and magnetic field show opposite effects on martensitic transition $\left({\mathrm{T}}_M\right)$. The $x=0$ alloy shows a maximum isothermal magnetic entropy change $\left(\mathrm{\Delta }{S}_M\right)$ of 6.5 J/kg $\mathrm{K}$ under ambient pressure at RT, which is comparatively larger than that reported in many other Heusler systems at RT. Interestingly, $\mathrm{\Delta }{S}_M$ decreases with pressure for $x=0$, while it shows an increasing trend for $x=2$. A maximum refrigeration capacity of around $79\mathrm{J}/\mathrm{kg}$ is observed for $x=0$. Similar to the magnetic entropy change, the net magnetization for $x=0$ and $x=2$ show opposite trend under external pressure. This is explained by our ab initio simulation by closely inspecting the consequence of nonuniform strain along three crystallographic directions on the net magnetization. This actually arises due to considerable magnetocrystalline anisotropy in these alloys. The unconventional mechanism behind the influence of pressure on magnetic properties is also discussed in the light of varying bond lengths between different magnetic species, and hence on the antiferromagnetic/ferromagnetic exchange coupling strengths under pressure.

Interlayer coupling can significantly influence the physical properties of layered transition-metal compounds. The superconductivity in layered ${\mathrm{Mo}}_2\mathrm{C}$ systems, belonging to the emergent family of MXene, has garnered considerable attention. However, the impact of interlayer coupling on superconductivity, and the anisotropic superconducting properties in these systems are not yet clear. By performing first-principles calculations of electron-phonon coupling and anisotropic superconducting properties, we show that the interlayer coupling in bilayer $1T\text{−}{\mathrm{Mo}}_2\mathrm{C}$ suppresses superconductivity, resulting in a significant drop in superconducting transition temperature $\left(T_{\mathrm{c}}\right)$ from 4.2 K in its monolayer form to nearly 0 K. By introducing lithium atoms into the interlayer space of the bilayer, the interlayer coupling can be effectively weakened, transforming the system into a two-gap superconductor with a $T_{\mathrm{c}}$ above 10 K. A 3% tensile strain can further transform the system into a three-gap superconductor with a significantly enhanced $T_{\mathrm{c}}$ of approximately 24.7 K, which is very high in the ${\mathrm{Mo}}_2\mathrm{C}$ related systems. The enhancement of the superconductivity induced by the strain is mainly due to the downshift of an energy band with a flat dispersion to the energy near the Fermi level. The in-plane vibrations of Mo atoms and the $d$-orbital electrons of Mo atoms are most important for the formation of the superconductivity. Our method can also be applied to multilayer ${\mathrm{Mo}}_2\mathrm{C}$ systems. Given the successful synthesis of layered ${\mathrm{Mo}}_2\mathrm{C}$ systems and the experimental realization of alkaline-metal atom depositions, our work presents a practically feasible strategy for achieving high-$T_{\mathrm{c}}$ and multigap superconductivity in layered ${\mathrm{Mo}}_2\mathrm{C}$.

Due to the linear scaling of the computational cost with system size, orbital-free density functional theory (OF-DFT) offers a promising approach for large-scale materials simulations. However, the lack of high transferability local pseudopotentials in OF-DFT has impeded its wide use for materials simulations. Recently, the nonlocal pseudopotential energy density functional (NLPPF) method [Nat. Commun. 13, 1385 (2022)] has been proposed to enable OF-DFT to directly use nonlocal pseudopotentials and successfully applied to simple metallic systems formed by $sp$-block metallic elements. Here, we extend the NLPPF scheme for applications to the semiconducting systems by employment of the revised Huang-Carter kinetic-energy density functional. Applications of typical semiconductors including Si, Ge, and GaAs have systematically benchmarked the scheme. The results demonstrate that the NLPPF scheme combined with the revised Huang-Carter kinetic-energy density functional can significantly improve the accuracy of OF-DFT for simulating the semiconductors without extensive requirements of computational budget, which opens up new opportunities for OF-DFT applications to large-scale semiconducting systems.