Optical isolation enables nonreciprocal manipulations of light with broad applications in optical communications. Optical isolation by rotating structures has drawn considerable attention due to its magnetic-free nature and unprecedented performance. Conventional rotation-based optical isolation relies on the use of bulky cavities hindering applications in subwavelength photonics. Here, we propose a mechanism of optical isolation by integrating the unique dispersion of a hyperbolic metamaterial with the transverse spin-orbit interaction of evanescent waves. We show that rotation of a subwavelength hyperbolic nanoparticle breaks the time-reversal symmetry and yields two resonant chiral modes that selectively couple to the transverse spin of waveguide modes. Remarkably, the transverse spin-orbit interaction can give rise to unidirectional coupling and more than $95\%$ isolation of infrared light at an experimentally feasible rotation speed. Our work fuses the two important fields of optical isolation and photonic spin-orbit interactions, leading to magnetic-free yet compact nonreciprocal devices for applications in optical communications, chiral quantum optics, and topological photonics.

A holistic approach for studying both the nature of atomic order and finite-temperature magnetostrictive behavior in the binary alloy Galfenol (${\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x, 0\le x\le 0.25$) is presented. The phase behavior is studied via atomistic modeling with inputs from ab initio calculations, and the ordered phases of interest at nonstoichiometric concentrations are verified to exhibit $B2$- and ${D0}_3$-like order. The finite-temperature magnetoelasticity of these phases, in particular the magnetoelastic constant $B_1$, is obtained within the same ab initio framework using disordered local moment theory. Our results provide an explanation for the origin of the experimentally observed peak and subsequent fall in the material's magnetostriction at $x\sim 0.19$, which has been disputed. In addition, we show that it is possible to enhance the magnetostriction of ${D0}_3-{\mathrm{Fe}}_3\mathrm{Ga}$ by removing a small fraction of electrons from the system, suggesting that a Fe-Ga-Cu or Fe-Ga-Zn alloy could exhibit greater magnetostrictive properties than Galfenol.

We report the results of the detailed measurements of ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR) spectra, NMR line shift, spin-lattice relaxation rate, bulk magnetization, magnetic susceptibility, and heat capacity on the spin-$\frac{1}{2}$ Heisenberg honeycomb compound ${\mathrm{Li}}_3{\mathrm{Cu}}_2{\mathrm{SbO}}_6$. This system consists of weakly coupled alternating Cu chains and, owing to the site inversion, some of the magnetic Cu ions are substituted by nonmagnetic Li ions. As a result, the effective spin model for ${\mathrm{Li}}_3{\mathrm{Cu}}_2{\mathrm{SbO}}_6$ is a set of Cu alternating chain fragments of different length (the “chain-segments spin model”). Due to this unique structure, various kinds of peculiar magnetic features arise and the combination of local (NMR) and global experimental techniques is especially effective in their study. In order to describe the experimental results theoretically, we perform full diagonalization calculations for the chain-segments spin model taking explicit account of all of the possible dimerized chain fragments of different length. From the fitting of measured susceptibility, we estimate the exchange interactions as $J_{\mathrm{FM}}=-244\mathrm{K}$ and $J_{\mathrm{AF}}=146\mathrm{K}$, and find that about 19% of magnetic Cu ions are replaced by nonmagnetic Li ions. Using those values, we can even quantitatively reproduce the observed NMR line shift and the specific heat as well as field- and temperature-dependent magnetization over a wide temperature range. Although small deviations are seen at very low temperature, they can be corrected by considering the effect of small interchain interactions.

We investigate the role of specific phonon mode symmetries for the room-temperature superconductivity in atomic hydrogen under large pressure. Using anisotropic Migdal-Eliashberg theory with ab initio input from density functional theory, we show that the $E_u$ phonon modes are the dominant driving force for obtaining such high critical temperatures. When going from 400 to 600 GPa, we find an increased transition temperature; however, the total electron-phonon coupling strength is counterintuitively reduced. Our analysis reveals that this is due to an enhanced contribution to the coupling strength by the $E_u$ phonon mode. We furthermore compute the momentum anisotropy of the superconducting gap which we find to be relatively small, about 7% of the mean gap value at 100 K.

New functional materials based on chalcogenides have promising significant applications spanning from field-effect transistors to electronic and optovalleytronic devices, and even to superconductivity. Here we report two chalcogenide materials SbS and $\mathrm{Sb}{\mathrm{S}}_2$, obtained in pressure-induced decomposition of ${\mathrm{Sb}}_2{\mathrm{S}}_3$ at 26.3 GPa. Different from the semiconducting ${\mathrm{Sb}}_2{\mathrm{S}}_3$ precursor, both SbS and $\mathrm{Sb}{\mathrm{S}}_2$ compounds are metallic at high pressure. Superconducting state is observed in the decomposed sample above 42 GPa, signified by a sharp drop of the resistance to zero upon cooling and a decrease of the transition temperature with the external magnetic field. Through diffraction data, SbS is determined as a major component responsible for superconductivity. The measured $T_c$ has a nearly linear increase with applied pressure, i.e., $d{T}_c/dP\sim 0.1\mathrm{K}\mathrm{G}{\mathrm{Pa}}^{–1}$, which reaches 6.6 K at 75.0 GPa. The experimental $T_c$ values are in general agreement with the calculation based on BCS theory, but the trend with pressure is reversed, indicating the abnormal superconducting behavior in SbS. The present results expand the chalcogenide family with two members and also open an avenue in finding the acquisition of superconducting materials in this group.

We investigate the nonequilibrium dynamics of the one-dimension Aubry-André-Harper model with $p$-wave superconductivity by changing the potential strength with slow and sudden quench. Firstly, we study the slow quench dynamics from the localized phase to the critical phase by linearly decreasing the potential strength $V$. The localization length is finite and its scaling obeys the Kibble-Zurek mechanism. The results show that the second-order phase transition line shares the same critical exponent $z\nu$, giving the correlation length $\nu =1$ and dynamical exponent $z=1.373\pm 0.023$, which are different from the Aubry-André model. Secondly, we also study the sudden quench dynamics between three different phases: localized phase, critical phase, and extended phase. In the limit of $V=0$ and $V=\infty$, we analytically study the sudden quench dynamics via the Loschmidt echo. The results suggest that, if the initial state and the post-quench Hamiltonian are in different phases, the Loschmidt echo vanishes at some time intervals. Furthermore, we found that, if the initial value is in the critical phase, the direction of quenching is the same as one of the two limits mentioned before, and similar behaviors will occur.

Metallic nanostructures (nanofilms and nanowires) are widely used in electronic devices, and their thermal transport properties are crucial for heat dissipation. However, there are still gaps in understanding thermal transport in metallic nanostructures, especially regarding the size effect and validity of the Wiedemann-Franz law. In this work, we perform mode-by-mode first-principles calculations combining the Boltzmann transport equation to understand the thermal transport in metallic nanostructures. We take gold (Au) and tungsten (W) nanostructures as prototypes. It is found that when the size of nanostructures is on the order of several tens of nanometers, the electronic/phonon thermal conductivity is smaller than the bulk value and decreases with size. The phonon contribution to the total thermal conductivity increases in nanostructures for those metals with small bulk phonon thermal conductivity (like Au), while it may increase or be suppressed in nanostructures for those metals with large bulk phonon thermal conductivity (like W). By assuming that the grain boundary does not induce inelastic electron-phonon scattering, the Wiedemann-Franz law works well in both Au and W nanostructures if the Lorentz ratio is estimated using electronic thermal conductivity. The Wiedemann-Franz law also works well in Au nanostructures when the Lorentz ratio is estimated by total thermal conductivity.

We study transport in the presence of magnetic domain walls (DWs) in a lattice model of ferromagnetic type-I Weyl metals. We compute the diagonal and Hall conductivities in the presence of a DW, using both Kubo and Landauer formalisms, and we uncover the effect of DW scattering. When the Fermi level lies near Weyl points, we find a strong skew scattering at the DW that leads to a significant additional Hall effect. We estimate the average Hall resistivity for multidomain configurations, and we identify the limit where the DW scattering contribution becomes significant. We show that a continuum model obtained by linearizing the lattice dispersion around the Weyl points does not correctly capture this DW physics. Going beyond the linearized theory, and incorporating leading curvature terms, leads to a semiquantitative agreement with our lattice model results. Our results are potentially relevant for the Hall resistivity of spin-orbit coupled ferromagnetic metals, such as ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2, {\mathrm{Co}}_2\mathrm{MnGa}$, and ${\mathrm{SrRuO}}_3$, which can have Weyl points near the Fermi energy.

Skyrmions, spin spirals, and other chiral magnetization structures developing in materials with an intrinsic Dzyaloshinskii-Moriya interaction display unique properties that have been the subject of intense research in thin-film geometries. Here, we study the formation of three-dimensional chiral magnetization structures in FeGe nanospheres by means of micromagnetic finite-element simulations. In spite of the deep submicron particle size, we find a surprisingly large number of distinct equilibrium states, namely, helical, meron, skyrmion, chiral-bobber, and quasisaturation states. The distribution of these states is summarized in a phase diagram displaying the ground state as a function of the external field and particle radius. This unusual multiplicity of possible magnetization states in individual nanoparticles could be a useful feature for multistate memory devices. We also show that the magnetodipolar interaction is almost negligible in these systems, which suggests that the particles could be arranged at high density without experiencing unwanted coupling.

We analyze collective excitations in models of two-dimensional topological insulators using the random phase approximation. In a two-dimensional extension of the Su-Schrieffer-Heeger model, edge plasmonic excitations with induced charge-density distributions localized at the boundaries of the system are found in the topologically nontrivial phase, dispersing similarly as one-dimensional bulk plasmons in the conventional Su-Schrieffer-Heeger chain. For two-dimensional bulk collective modes, we reveal regimes of enhanced interband wave function correlations, leading to characteristic hardening and softening of inter- and intraband bulk plasmonic branches, respectively. In the two-dimensional Haldane Chern insulator model, chiral, unidirectional edge plasmons in nano-ribbon architectures are observed, which can be characterized by an effective Coulomb interaction cross section. Bulk collective excitations in the two-dimensional Haldane model are shown to be originated by single-particle band structure details in different topological phases.

Heavily doped semiconductors have emerged as tunable low-loss plasmonic materials at midinfrared frequencies. In this article we investigate nonlinear optical phenomena associated with high concentration of free electrons. We use a hydrodynamic description to study free electron dynamics in heavily doped semiconductors up to third-order terms, which are usually negligible for noble metals. We find that cascaded third-harmonic generation due to second-harmonic signals can be as strong as direct third-harmonic generation contributions even when the second-harmonic generation efficiency is zero. Moreover, we show that when coupled with plasmonic enhancement free electron nonlinearities could be up to two orders of magnitude larger than conventional semiconductor nonlinearities. Our work might open a new route for nonlinear optical integrated devices at midinfrared frequencies.

Two-dimensional van der Waals (vdW) heterostructures have recently emerged as attractive candidates to work as spintronic and optoelectronic devices. Here, two types of magnetic ${\mathrm{CrI}}_3/{\mathrm{NiCl}}_2$ vdW heterostructures are constructed to design spin caloritronic devices. The first-principles calculations uncover that the magnetic configurations of ${\mathrm{CrI}}_3/{\mathrm{NiCl}}_2$ vdW heterostructures can be converted easily to a ferromagnetic, an antiferromagnetic, and even a bipolar magnetic semiconducting state by an external electric field. More interestingly, two thermal spin-dependent currents with opposite spin orientations can be driven by a temperature gradient to flow in opposite transport directions independently in the different layers of vdW heterostructures, demonstrating that the ${\mathrm{CrI}}_3/{\mathrm{NiCl}}_2$ vdW heterostructures can exhibit a nearly perfect thermal spin-filtering effect in each layer while generating a well-defined spin-Seebeck effect in the whole system. Our work puts forward a class of material candidates to design spin caloritronic devices characterized by multiple inspiring thermal-spin transport behaviors.

Carbon nanomembranes made from aromatic precursor molecules are freestanding nanometer-thin materials of macroscopic lateral dimensions. Although produced in various versions for about two decades, not much is known about their internal structure. Here we present a systematic theoretical attempt to model the formation, structure, and mechanical properties of carbon nanomembranes using classical molecular dynamics simulations. We find theoretical production scenarios under which stable membranes form. They possess pores as experimentally observed. Their Young's modulus, however, is systematically larger than experimentally determined.

Manipulation of the valley degree of freedom provides a novel paradigm in quantum information technology. In this work, through first principles calculations, we demonstrate that monolayer $\mathrm{Gd}{\mathrm{I}}_2$ is a promising candidate material for valleytronic applications. Monolayer $\mathrm{Gd}{\mathrm{I}}_2$ can be easily exfoliated from bulk, and it is spontaneously valley polarized with a giant splitting of 149 meV due to its intrinsic ferromagnetism and large spin orbital coupling. The anomalous valley Hall effect could be realized in monolayer $\mathrm{Gd}{\mathrm{I}}_2$ by an appropriate external electric field. Furthermore, the valley polarization feature is stable against the biaxial in-plane strain. Our findings provide an extraordinary and potential material platform for experimental studies and practical applications in the emergent field of valleytronics.

Recently, the topological phase in non-Hermitian systems has been a rapidly expanding field. The iconic features of non-Hermitian systems are exceptional points at which the eigenmodes coalesce and the non-Hermitian skin effect. We study the non-Hermitian Su-Schrieffer-Heeger model with long-range nonreciprocal hopping and find the model exhibiting topologically nontrivial phases which can be characterized by the non-Bloch winding number. With specific parameter values, the skin effect can be eliminated. As long-range nonreciprocal hopping is not easy for experimental implementations, we furthermore propose a feasible electrical-circuit simulation with operational amplifiers to implement the non-Hermitian term to realize these interesting states.

Understanding of the energy exchange between electrons and phonons in metals is important for micro- and nanomanufacturing and system design. The electron-phonon ($e\text{−}ph$) coupling constant describes such exchange strength, yet its variation remains still unclear at micro- and nanoscale where the nonequilibrium effects are significant. In this work, an $e\text{−}ph$ coupling model is proposed by transforming the full scattering terms into relaxation time approximation forms in the coupled electron and phonon Boltzmann transport equations. Consequently, the nonequilibrium effects are included in the calculation of the $e\text{−}ph$ coupling constant. The coupling model is verified by modeling the ultrafast dynamics in femtosecond pump-probe experiments on a metal surface, which shows consistent results with the full integral treatment of scattering terms. The $e\text{−}ph$ coupling constant is strongly reduced due to both the temporal nonequilibrium between different phonon branches and the spatial nonequilibrium of electrons in confined space. The present work will promote not only a fundamental understanding of the $e\text{−}ph$ coupling constant but also the theoretical description of coupled electron and phonon transport at micro- and nanoscale.

We develop a unified framework for understanding the sign of fermion-mediated interactions by exploiting the symmetry classification of Green's functions. In particular, we establish a theorem regarding the sign of fermion-mediated interactions in systems with chiral symmetry. The strength of the theorem is demonstrated within multiple examples with an emphasis on electron-mediated interactions in materials.