We investigate topological phases in a bilayer system composed of an altermagnet and a two-dimensional topological insulator described by the Bernevig-Hughes-Zhang model. A topological phase transition occurs from a first-order topological insulator to a trivial insulator at a certain critical altermagnetization if the Néel vector of altermagnet is along the $x$ or $y$ axis. It is intriguing that valley-protected edge states emerge along the Néel vector in this trivial insulator, which are as stable as the topological edge states. We name it a crystalline valley-edge insulator. On the other hand, the system turns out to be a second-order topological insulator when the Néel vector is along the $z$ axis. The tunneling conductance has a strong dependence on the Néel vector. In addition, the band gap depends on the Néel vector, which is measurable by optical absorption. Hence, it is possible experimentally to detect the Néel vector by measuring tunneling conductance and optical absorption.

ZnO nanowire (NW) lasing driven by mid-infrared (MIR) laser pulses has attracted significant attention owing to its remarkable wavelength-independent lasing threshold and potential applications in diverse situations. However, the properties of MIR laser-driven ZnO microwire (MW) lasing are rarely studied when the wire diameter is increased from nanoscale to microscale, comparable to the wavelength of the driving laser. Here we experimentally measured the ZnO MW lasing driven by MIR laser with different polarizations. The measurements show that the laser polarized along the $c$ axis was more efficient for the lasing in MW with diameter smaller than the driving laser wavelength, while the polarization dependence was ambiguous when the MW diameter was greater than the driving laser wavelength. Through the modeling of the lasing process in ZnO MW, the observed polarization dependence is reproduced and can be attributed to the combined effect of the optical interference of the driving laser in the MW and the varying absorption properties of different MWs. The former is related to the MW diameter and the latter is sensitive to the sample growth condition. Our findings shed light on the feasibility of manipulation of the ZnO MW lasing.

Using the nonequilibrium Green's function formalism, we propose a general microscopic framework to investigate the radiative heat transfer (RHT) between coplanar objects with a square lattice. We employ the obtained formulas to two-dimensional (2D) metal configurations with a tight-binding model and the Drude model. Our results reveal that the RHT between coplanar 2D metals is significantly larger than black-body radiation in both the near and far fields, leading to a global super-Planckian RHT. As the separation distance increases, the heat flux density exhibits a rapid decrease in the near field, followed by a slower decrease and eventual $1/d$ dependence in the far field, while maintaining a much higher magnitude than black-body radiation. Evanescent waves dominate the heat transfer in the near field while propagating waves dominate the far field. Surprisingly, the propagating heat flux remains almost constant over a wide range of distances, resulting in a super-Planckian behavior in the far field. The dispersion relation of the spectrum function reveals distinct contributions from propagating and evanescent waves, with possible origins from surface plasmon resonance. These findings provide insights into the unique characteristics of RHT between coplanar 2D metals and highlight the potential for achieving enhanced heat transfer beyond the black-body limit. Our method is applicable to any coplanar objects with square lattices and paves the way for expanded investigations into various lattice geometries.

We consider the quantum reaction-diffusion dynamics in $d$ spatial dimensions of a Fermi gas subject to binary annihilation reactions $A+A\to \varnothing$. These systems display collective nonequilibrium long-time behavior, which is signalled by an algebraic decay of the particle density. Building on the Keldysh formalism, we devise a field theoretical approach for the reaction-limited regime, where annihilation reactions are scarce. Combining a perturbative expansion of the dissipative interaction with Euler-hydrodynamic scaling limit, we derive a description in terms of a large-scale universal kinetic equation. Our approach shows how the time-dependent generalized Gibbs ensemble assumption, which is often employed for treating low-dimensional nonequilibrium dissipative systems, emerges from systematic diagrammatics. It also allows us to exactly compute—for arbitrary spatial dimension—the decay exponent of the particle density. The latter is based on the large-scale description of the quantum dynamics and it differs from the mean-field prediction even in dimension larger than one. We moreover consider spatially inhomogeneous setups involving an external potential. In confined systems the density decay is accelerated towards the mean-field algebraic behavior, while for deconfined scenarios the power-law decay is replaced by a slower nonalgebraic decay.

The neodymium tetraboride ${\mathrm{NdB}}_4$, crystalizing into a tetragonal crystal structure, has the magnetic ions located on a Shastry-Sutherland lattice. The compound undergoes several magnetic phase transitions and exhibits a complex multi-$k$ ground state that has so far been elusive. Here we report the solution and quantitative refinement of the magnetic ground state of ${\mathrm{NdB}}_4$ based on neutron diffraction data. The magnetic structure consists of two separate components; one of them is confined within the $ab$ plane and makes an orthogonal all-in-all-out spin configuration maintaining translation symmetry of the crystal. Another is pointing along the $c$ axis, and this component forms an anharmonic spin density wave consisting of three-up-two-down sequences with fivefold periodicity. The in-plane and out of plane spin structures represent distinct magnetic instabilities which normally would not coexist. Their simultaneous presence in the same phase is an interesting phenomenon associated with anisotropic exchange interactions and coupling to a symmetry breaking nonmagnetic order parameter undetectable by the available diffraction data.

Multiferroic materials consisting of multiple and mutually coupled order parameters including polarization can have an intriguing response to applied electric field, and give rise to the so-called cross-caloric effect, as the nondipolar degrees of freedom can contribute to the electrocaloric effect (ECE) via cross couplings to the electric dipoles. In this study, we use a first-principles-based effective Hamiltonian combined with Monte Carlo simulations to investigate such an effect systematically in the well-known room-temperature multiferroic system ${\mathrm{BiFeO}}_3$ (BFO), in which there exists three coupled order parameters that can respond to electric field, i.e., polarization, octahedral tiltings, and antiferromagnetism. We adopt and compare three methods based on the cumulant formula, Maxwell relation, and a Landau model, respectively. The Landau approach allows the computation of decomposed contribution from each order parameter, and magnetism is found to contribute significantly to the ECE near the Néel temperature. The electrocaloric (EC) adiabatic temperature changes and the EC coefficients are computed with various field directions and magnitudes, with which sizable positive and negative ECE are found.

We investigate spin-current generation in polar collinear antiferromagnets without relying on the relativistic spin-orbit coupling. The symmetry and microscopic model analyses indicate that both linear and nonlinear spin conductivities are induced; the linear spin conductivity is characterized by the non-Hall-type transverse tensor component, while the nonlinear spin conductivity is characterized by the Hall-type tensor component. We show that the emergence of the linear and nonlinear spin currents is caused by the quadrupole and dipole components of the magnetic toroidal multipoles in the collinear antiferromagnetic structure, respectively. We demonstrate the important conditions to enhance and/or suppress the spin conductivity in each component by analyzing a fundamental tight-binding model. The experimental separation between their contributions is also discussed.

Magnetization dynamics in magnetic materials are well described by the modified semiclassical Landau-Lifshitz-Gilbert equation, which includes the magnetic damping $\widehat{\mathit{\alpha }}$ and the magnetic moment of inertia $\widehat{\mathrm{I}}$, both usually being tensors, as key parameters. Both parameters are material specific and physically represent the timescales of damping of precession and nutation in magnetization dynamics. $\widehat{\mathit{\alpha }}$ and $\widehat{\mathrm{I}}$ can be calculated quantum mechanically within the framework of the torque-torque correlation model. The quantities required for the calculation are torque matrix elements, the real and imaginary parts of the Green's function, and its derivatives. Here, we calculate these parameters for the elemental magnets such as Fe, Co, and Ni in an ab initio framework using density functional theory and Wannier functions. We also propose a method to calculate the torque matrix elements within the Wannier framework. We demonstrate the effectiveness of the method by comparing it with the experiments and previous ab initio and empirical studies and show its potential to improve our understanding of spin dynamics and to facilitate the design of spintronic devices.

The magnetic properties of Ho-doped langasites ${\left(\mathrm{La}:\mathrm{Ho}\right)}_3\mathrm{G}{\mathrm{a}}_5\mathrm{Si}{\mathrm{O}}_{14}$ are dominated by the Ising-like magnetic moments of the $\mathrm{H}{\mathrm{o}}^{3+}$ ions. In their saturated regime, the induced magnetic state breaks both time and space inversion symmetries, leading to a linear magnetoelectric effect. However, due to distortions induced by a shared Ga/Si occupancy of the $2d$ sites, resolving the microscopic nature of the magnetic configuration remains a difficult task. Here, we combine polarized neutron diffraction and angular-dependent magnetization experiments to determine the local distortions of the $\mathrm{H}{\mathrm{o}}^{3+}$ magnetic moments in doped langasites ${\left(\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{H}{\mathrm{o}}_x\right)}_3\mathrm{G}{\mathrm{a}}_5\mathrm{Si}{\mathrm{O}}_{14}$ with $x\approx 0.015$ and 0.045. We propose a model for a field-induced magnetic configuration with arbitrary orientations of the local Ising axis of $\mathrm{H}{\mathrm{o}}^{3+}$ in distorted positions. The operations of broken local $C_2$ symmetry and rotations around the trigonal $C_3$ axis connect different sites, restoring the global $P321$ symmetry of the crystal and simplifying the description of the magnetic properties. The superposition of two distorted $\mathrm{H}{\mathrm{o}}^{3+}$ positions connected by $C_2$ symmetry determines the local magnetic susceptibility tensor, which no longer appears Ising-like at low fields.

We propose a phase-biased non-Hermitian Josephson junction (NHJJ) composed of two superconductors mediated by a short non-Hermitian link. Such a NHJJ is described by an effective non-Hermitian Hamiltonian derived based on the Lindblad formalism in the weak coupling regime. By solving the Bogoliubov–de Gennes equation, we find that its Andreev spectrum as a function of phase difference exhibits Josephson gaps, i.e., finite phase windows with no Andreev (quasi)bound states. The complex Andreev spectrum and the presence of Josephson gaps constitute particular spectral features of the NHJJ. Moreover, we propose complex supercurrents arising from inelastic Cooper pair tunneling to characterize the anomalous transport in the NHJJ. Additional numerical simulations complement our analytical predictions. We demonstrate that the Josephson effect is strongly affected by non-Hermitian physics.

The coupling between antiferromagnetic spins and infrared-active phonons in solids is responsible for many intriguing phenomena and is a field of intense research with extensive potential applications in the modern devices based on antiferromagnetic spintronics and phononics. Insulating rutile antiferromagnetic crystal ${\mathrm{CoF}}_2$ is one of the model materials for studying nonlinear magnetophononics due to the strong spin-lattice coupling as a result of the orbitally degenerate ground state of ${\mathrm{Co}}^{2+}$ ions manifested in the plethora of static and induced piezomagnetic effects. Here we report results on the complete infrared spectroscopy study of lattice and magnetic dynamics in ${\mathrm{CoF}}_2$ in a wide temperature range and their careful analysis. We observed that infrared-active phonons demonstrate frequency shifts at the antiferromagnetic ordering. Furthermore, using first-principles calculations, we examined the lattice dynamics and disclosed that these frequency shifts are rather due to the spin-phonon coupling than geometrical lattice effects. Next we found that the low-frequency dielectric permittivity demonstrates distinct changes at the antiferromagnetic ordering due to the spontaneous magnetodielectric effect caused by the behavior of infrared-active phonons. In addition, we have observed magnetic excitations in the infrared spectra and identified their magnetodipole origin. To strengthen our conclusions, we analyze the theoretical phonon-magnon coupling overall phonons at the $\mathrm{\Gamma }$ point. We conclude that the largest effect comes from the $A_{1g}$ and $B_{2g}$ Raman-active modes. As such, our results establish a solid basis for further investigations and more deeper understanding of the coupling of phonons with spins and magnetic excitations in antiferromagnets.

In this paper, we use path-integral Monte Carlo (pimc) simulations to study how the permutation effect, a specific quantum exchange phenomenon, affects the stability of incommensurate solid ${}^4\mathrm{He}$ with vacancy defects. By comparing simulations within Bose and Boltzmann statistics, we isolate the effects of quantum exchange. We examine the solids with densities from 0.028 to 0.035 ${\AA }^{-3}$ and vacancy concentrations from 0.55 to 5.55% in the temperature range 0.2–2.0 $\mathrm{K}$. The calculations show that permutations reduce the vacancy formation energy by up to 19%, especially at lower densities. Vacancies in the studied concentration range can lead to lattice distortions with a reduction in the local density of 1% to 5% and a compensatory increase in the neighboring regions. The Bose statistics influence the local atomic arrangement, which is reflected in the pair-correlation function, with the peaks and valleys changing by about 2% at larger distances. Permutation effects on the vacancy formation energy show a dependence on density and temperature with “steplike behavior”. Our results demonstrate that quantum statistics play a significant role in shaping the structure and stability of solid ${}^4\mathrm{He}$-containing vacancies.

The XXZ spin-half chain has Heisenberg exchange interactions $J_z$ $\left(J_{\perp }\right)$ in the $z$ $(x,y)$ direction. The model has a transition from the spin-fluid phase for $-J_{\perp }<J_z<J_{\perp }$ to the Néel phase for $J_z>J_{\perp }>0$. When bond alternation $\delta$ is included, the Néel phase transitions to the dimer phase for a finite value of $\delta$. We determine the phase diagram using simple topological indicators related to the polarization of both spins. When a staggered magnetic field $B$ is included, a contour plot of these indicators as a function of $\delta$ and $B$ determine the amount of topological quantized spin pumping around closed circuits in the $(\delta ,B)$ plane.

Extensive efforts are being made to improve the material quality of semiconductor quantum cascade lasers (QCLs) to suppress the carrier scattering from the interface roughness (IFR), which is commonly regarded as the leading cause of the emission linewidth broadening in QCLs. Here, we uncover an intrinsic lower bound to the emission linewidth in the prevailing QCLs by performing atomistic pseudopotential calculations of the electronic and optical properties without ad hoc assumptions as made in standard effective mass approaches. We demonstrate that our atomistic simulation results could reproduce the experimental results well on both emission peaks and emission linewidths for a wide range of temperatures and applied bias voltages, even without considering the effect of the IFR. Specifically, we find that the multiple intersubband optical transitions, which are broadened each by the nonparabolicity of subband energy dispersions, render a very broad emission spectrum and could explain the experimentally measured spectra for a wide external bias. Therefore, we illustrate that the previously ignored multiple intersubband optical transitions give rise to an intrinsic lower bound to the emission linewidth in QCLs. These findings imply that the IFR may play a minor role in broadening the emission linewidth in the state-of-the-art QCLs and shed light on the design of QCLs.

We construct Sierpinski carpets (SCs) based on AA or AB bilayer graphene using atom vacancies, which we denote SC-AA and SC-AB, to investigate the effects of interlayer coupling on the electronic properties of fractals. Compared with monolayer graphene SCs, their densities of states have similar features, such as Van Hove singularities and edge states corresponding to the central peaks near zero energy, but remarkable energy broadening of edge states emerges in SC-AA and SC-AB. The calculated conductance spectrum shows that the conductance fluctuations still retain the Hausdorff fractal dimension behavior even with the interlayer coupling. Thus, the high correlation between quantum conductance and the fractal geometry dimension is not affected by the interlayer coupling in bilayer graphene SCs. We further reveal the quasieigenstates in fractal-like pressure-modulated bilayer graphene, denote SC-pAA and SC-pAB. Numerical results show that the density of states of SC-pAA (SC-pAB) shows an asymptotic behavior compared to that of SC-AA (SC-AB), especially for high-energy quasieigenstates. Within a certain energy range, stronger pressure can lead to stronger localization, forming an efficient fractal space.

We present a theoretical study of the directionality effects in spontaneous emission and resonance fluorescence of a quantum two-level dipole emitter near an ultrathin closely packed periodically aligned single-wall carbon nanotube film. Such films present an example of highly anisotropic flexible metasurfaces that are now available experimentally. The nanotube alignment is shown to provide an extra measure for quantum control of dipolar spontaneous emission and resonance fluorescence in such systems, in addition to film thickness and composition parameters such as tube diameter, chirality, and translational period. The processes studied are shown to be highly anisotropic, being enhanced by orders of magnitude in the direction perpendicular to the alignment and metasurface plane, contrasting with the commonly believed viewpoint of their uncontrollably random directionality.

We give an explicit operator representation (via a sequential circuit and projection to symmetry subspaces) of Kramers-Wannier duality transformation in higher-dimensional subsystem symmetric models, generalizing the construction in the 1D transverse-field Ising model. Using the Kramers-Wannier duality operator, we also construct the Kennedy-Tasaki transformation that maps subsystem symmetry-protected topological phases to spontaneous subsystem symmetry-breaking phases, where the symmetry group for the former is either ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ or ${\mathbb{Z}}_2$. This also generalizes the recently proposed picture of the one-dimensional Kennedy-Tasaki transformation as a composition of manipulations involving gauging and stacking symmetry-protected topological phases to higher dimensions.

Intense ultrashort laser pulse irradiation of solid targets was systematically investigated at the first-principles level, both theoretically and computationally. In the method, the propagation of a pulsed light through a thin film is described by a one-dimensional Maxwell's equation, and the microscopic electronic motion at different positions in the film is described by employing first-principles time-dependent density functional theory (TDDFT). The method uses a coarse-graining approximation to couple light propagation and electronic motion, and is termed the multiscale Maxwell-TDDFT method. The reflectance, transmittance, and absorbance of pulsed light incident normally on thin films of 50–200 nm thickness were calculated for materials with different optical properties, such as aluminum (simple metal), graphite (semimetal), silicon (small-gap dielectric), and quartz (wide-gap dielectric). Optical response transitions were explored as the light intensity shifted from the linear regime, represented by the dielectric function for weak light, to the extremely nonlinear regime, represented by plasma reflection under intense light conditions. Numerous mechanisms that depend on the laser pulse intensity and material type were found to contribute to these changes. These include multiphoton absorption, saturable absorption, sign change of the effective dielectric constant, and transition from quantum occupation to classical Boltzmann distribution. Thus, the calculations provide a unified understanding of the interaction of intense pulsed light with solids, occurring on an extremely short timescale.

The electronic and vibrational properties and electron-phonon couplings of one-dimensional materials will be key to many prospective applications in nanotechnology. Dimensionality strongly affects these properties and has to be correctly accounted for in first-principles calculations. Here we develop and implement a formulation of density functional and density functional perturbation theory that is tailored for one-dimensional systems. A key ingredient is the inclusion of a Coulomb cutoff, a reciprocal-space technique designed to correct for the spurious interactions between periodic images in periodic-boundary conditions. This restores the proper one-dimensional open-boundary conditions, letting the true response of the isolated one-dimensional system emerge. In addition to total energies, forces and stress tensors, phonons and electron-phonon interactions are also properly accounted for. We demonstrate the relevance of the present method on a portfolio of realistic systems: BN atomic chains, BN armchair nanotubes, and GaAs nanowires. Notably, we highlight the critical role of the Coulomb cutoff by studying previously inaccessible polar-optical phonons and Fröhlich electron-phonon couplings. We also develop and apply analytical models to support the physical insights derived from the calculations and we discuss their consequences on electronic lifetimes. The present study unlocks the possibility to accurately simulate the linear-response properties of one-dimensional systems, sheds light on the transition between dimensionalities, and paves the way for further studies in several fields, including charge transport, optical coupling, and polaritronics.