We present a combined experimental and theoretical study of the two-dimensional electron states at the iridium-silicide surface of the antiferromagnet ${\mathrm{GdIr}}_2{\mathrm{Si}}_2$ above and below the Néel temperature. Using angle-resolved photoemission spectroscopy (ARPES) we find a significant spin-orbit splitting of the surface states in the paramagnetic phase. By means of ab initio density-functional-theory (DFT) calculations we establish that the surface electron states that reside in the projected band gap around the $\overline{\mathrm{M}}$ point exhibit very different spin structures which are governed by the conventional and the cubic Rashba effect. The latter is reflected in a triple spin winding, i.e., the surface electron spin reveals three complete rotations upon moving once around the constant energy contours. Below the Néel temperature, our ARPES measurements show an intricate photoemission intensity picture characteristic of a complex magnetic domain structure. The orientation of the domains, however, can be clarified from a comparative analysis of the ARPES data and their DFT modeling. To characterize a single magnetic domain picture, we resort to the calculations and scrutinize the interplay of the Rashba spin-orbit coupling field with the in-plane exchange field, provided by the ferromagnetically ordered $4f$ moments of the near-surface Gd layer.

In intrinsic magnetic semiconductors, the absorption of a single photon can generate a spin polaron, whose magnetic moment reaches many thousands of Bohr magnetons. Here we investigate these huge photoinduced spin polarons, using Monte Carlo simulations. In antiferromagnetic semiconductors, photoinduced spin polarons are most efficiently generated in the whole temperature interval up to the phase transition, whereas in ferromagnetic semiconductors much larger spin polarons can be photoinduced, but only around the phase transition temperature. Because Monte Carlo simulations are computationally expensive, we developed an analytical model, based on Weiss field theory. Although the Weiss model does not provide as much information as a Monte Carlo simulation, such as spin texture and fluctuations, it yields formulas that can be used to estimate instantly the expected photoinduced spin polaron size in many intrinsic magnetic semiconductors.

Non-Hermitian topological phases have gained widespread interest due to their unconventional properties, which have no Hermitian counterparts. In this work, we propose to use machine learning to identify and predict non-Hermitian topological phases, based on their winding number. We consider two examples—non-Hermitian Su-Schrieffer-Heeger model and its generalized version in one dimension and non-Hermitian nodal line semimetal in three dimensions—to demonstrate the use of neural networks to accurately characterize the topological phases. We show that for the one-dimensional model, a fully connected neural network gives an accuracy greater than 99.9% and is robust to the introduction of disorder. For the three-dimensional model, we find that a convolutional neural network accurately predicts the different topological phases.

Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of additional features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and composition-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temperatures, near and off resonance. By combining polarization-resolved Raman data with calculations of the phonon dispersion and using symmetry considerations, we identify the four first-order Raman modes and higher-order two-phonon modes. Moreover, we observe defect-activated phonon processes, which provide a route toward a quantitative assessment of the defect concentration and, thus, the crystal quality of the materials. Our work establishes a solid background for future research on material synthesis, study, and application of Janus TMD monolayers.

We investigate the topological properties, energy spectrum, and persistent currents of a non-Hermitian ring with anti-Hermitian hopping terms. It is demonstrated that the anti-Hermitian hopping can effectively induce a synthetic gauge field. As the magnetic flux of the synthetic gauge field threads through the ring, the non-Hermitian system exhibits an Aharonov-Bohm effect. For the case of a non-Hermitian ring in the topological phase, the system, having an energy spectrum structure with a real gap, supports an imaginary persistent current. For the trivial case, a non-Hermitian system with an imaginary gap supports a real persistent current. Furthermore, we also investigate the transport property of a non-Hermitian Aharonov-Bohm ring connected by two semi-infinite leads. We find that the transmission coefficient shows the Aharonov-Bohm quantum oscillation as a function of the synthetic gauge field.

Surface plasmon polaritons (SPPs) due to their subwavelength nature could significantly modify electronic transition behaviors in various optoelectronic systems. Here, using a model system with a spherical quantum dot (QD) close to a flat metal surface, we show that the conventional forbidden optical transitions in a QD could be largely enabled by the spontaneous SPP decay. The electronic states of the QD are approximated by a Bloch state combined with wave functions in a spherical potential well, which provides multiple hole states with mixed electronic multipoles. Moreover, the SPP is quantized by using a canonical quantization scheme followed by a Green's function approach to introduce its dissipation. In particular, we find that when the SPP dissipation is included, the spontaneous decay of the corresponding QD exciton is dominant by the transition into the off-resonance mode of SPPs with large momenta. Also, we have studied the dependence of spontaneous decay rates on the size and crystal orientation of a QD, the distance between the QD and metal surface, and the linewidth of SPPs. Some useful scaling relations have been revealed, and the multipole transitions are found to be comparable with the dipole transition under specific system parameters. These findings have important implications for our understanding of the electronic transition at a metal near field and might prove instrumental for the future design of plasmonic and QD devices.

The spectra and role in the spin dynamical properties of bound states of elementary magnetic excitations named Bethe strings that occur in some integrable spin and electronic one-dimensional models have recently been identified and realized in several materials by experiments. Corresponding theoretical studies have usually relied on the one-dimensional spin-$1/2$ Heisenberg antiferromagnet in a magnetic field. At the isotropic point, it describes the large onsite repulsion $U$ limit of the spin degrees of freedom of the one-dimensional fermionic Hubbard model with one electron per site in a magnetic field $h$. In this paper we consider the thermodynamic limit and study the effects of lowering the latter quantum problem ratio $u=U/4t$, where $t$ is the first-neighbor transfer integral, on the line-shape singularities in $(k,\omega )$-plane regions at and just above the lower thresholds of the transverse and longitudinal spin dynamical structure factors. The most significant spectral weight contribution from Bethe strings leads to a gapped continuum in the spectrum of the spin dynamical structure factor $S^{+-}(k,\omega )$. Our study focuses on the line shape singularities at and just above the gapped lower threshold of that continuum, which have been identified in experiments. Our results are consistent with the contribution of Bethe strings to $S^{zz}(k,\omega )$ being small at low spin densities and becoming negligible upon increasing that density. Our results provide physically important information about how electron itinerancy affects the spin dynamics.

Phonon Anderson localization has been receiving increasing interest in creating unique thermal transport properties. However, due to challenges to resolve and track the mode-specific transmission, the existence of phonon Anderson localization has only been inferred from the decay of overall thermal conductivity versus device length. In this work, we present direct evidence of phonon Anderson localization using the exponential decay of their mode-resolved transmissions. We clearly capture localized modes in different structures, even with a monotonically upward thermal conductivity trend.

We present calculations of the two-photon absorption cross section ${\sigma }^{\left(2\right)}\left(\omega \right)$ for nanocrystals (NCs) of the inorganic perovskite ${\mathrm{CsPbBr}}_3$ for photon energies $\omega$ ranging from the absorption threshold at $2\omega \approx 2.3\mathrm{eV}$ up to $2\omega =3.1\mathrm{eV}$. The calculations employ a $4\times 4\mathbf{k}·\mathbf{p}$ envelope-function model, with final-state excitons described in a self-consistent Hartree-Fock approximation. The $\mathbf{k}·\mathbf{p}$ corrections to ${\sigma }^{\left(2\right)}\left(\omega \right)$ are found to be rather large, giving a reduction of about 30% in the cross section at the largest energies considered. The cross section is shown to be independent of polarization in the effective-mass approximation (EMA), but including $\mathbf{k}·\mathbf{p}$ corrections leads to a small difference in ${\sigma }^{\left(2\right)}\left(\omega \right)$ between circular and linear polarization, which rises to about 16% at $2\omega =3.1\mathrm{eV}$. The theoretical cross section follows closely a power-law dependence on NC size, ${\sigma }^{\left(2\right)}\left(\omega \right)\propto L^{\alpha }$, with theoretical exponents $\alpha =3.4$ (EMA) or $\alpha =3.2$ ($4\times 4\mathbf{k}·\mathbf{p}$ model), in excellent agreement with experiment. The dominant contribution to the exponent $\alpha$ is shown to be the number of final-state excitons per unit energy. Measured values of the absolute (normalized) cross section ${\sigma }^{\left(2\right)}\left(\omega \right)$ show a large spread of values, differing by as much as a factor of 25 for some NC sizes. Our calculations strongly favor a group of measurements at the lower end of the reported range of ${\sigma }^{\left(2\right)}\left(\omega \right)$.

Bound states in the continuum (BICs) have been observed in a variety of systems. A plasmonic BIC offers interesting opportunities, since a surface plasmon is known to confine light to the nanometer scale. However, the observation and manipulation of plasmonic BICs is a challenge due to the intrinsic loss of metals. Here, we study plasmonic BICs in the visible range in a one-dimensional all-metallic grating. First, by tuning the resonances of localized and propagating surface plasmon modes to resonance, we successfully observe symmetry-protected plasmonic BICs in an all-metallic system. Next, by continuously tuning the localized mode, we demonstrate topological band inversion characterized by a Zak phase transition. In addition, we engineer off-Γ-point BICs and confirm their formation mechanism. Finally, we experimentally determine that the quality ($Q$) factor of a 10-groove structure can exceed 60, about one order of magnitude greater than conventional metallic structures. The simulations reveal that, with more grooves, the $Q$ factor can be over 200. The plasmonic BICs in the visible range demonstrated in this paper pave the way to promising applications in lasers, sensors, light-matter interactions, nonlinear optics, and quantum optics.

Two-dimensional heterostructures and superlattices consisting of transition metal dichalcogenides offer huge potential in the next generation of optoelectronic devices. For such transition metal dichalcogenides, a predictive theory of their properties, based upon in-depth understanding of the interlayer interactions, is desirable due to the huge potential number of combinations. These weakly interacting heterobilayers provide an excellent case study to understand how interlayer interactions interfere with the Anderson/Schottky picture of band alignment at interfaces. We demonstrate here, using a combination of first principles and tight-binding methods, how the band alignment can be predicted in terms of a modified form of Anderson's rule. We explain how two physically based corrections to Anderson's rule, $\mathrm{\Delta }{E}_{\mathrm{\Gamma }}$ and $\mathrm{\Delta }{E}_{\text{IF}}$, are necessary for accurate band alignment prediction. We identify the interlayer interactions that affect band alignment prediction and show how these take the form of long range interactions between the $d_{z^2}$ and/or $p_z$ orbitals and induced fields between layers. Finally, we apply this theorem to Moiré and strained structures to predict these structures band alignment and provide a comprehensive guide to accurately predict the resultant band gap of the various transition metal dichalcogenide heterostructures.

In light of the race towards macroscale superlubricity of graphitic contacts, the effect of grain boundaries on their frictional properties becomes of central importance. Here, we elucidate the unique frictional mechanisms characterizing topological defects along typical graphene grain boundaries that can vary from being nearly flat to highly corrugated, depending on the boundary misfit angle. We find that frictional energy dissipation over grain boundaries can originate from variations of compressibility along the surface, heat produced during defect (un)buckling events, and elastic energy storage in irreversible buckling processes. These may lead to atypical nonmonotonic dependence of the average friction on the normal load. The knowledge gained in the present study is an important step towards the understanding of frictional effects of extended grain boundaries.

We present a theory of Anderson localization on a one-dimensional lattice with translation-invariant hopping. We find by analytical calculation the localization length for arbitrary finite-range hopping in the single propagating channel regime. Then by examining the convergence of the localization length, in the limit of infinite hopping range, we revisit the problem of localization criteria in this model and investigate the conditions under which it can be violated. Our results reveal possibilities of having delocalized states by tuning the long-range hopping.

Efficient electrical generation of torque is desired to develop innovative magnetic nanodevices. The torque can be generated by charge to spin conversion of heavy-metal layers through their strong spin-orbit interaction followed by the injection of the converted spin into adjacent ferromagnetic layers. However heavy atomic elements indispensable for this torque generation scheme are often incompatible with device mass production processes. Here we demonstrate efficient torque generation without heavy elements in ferromagnetic metal/$\mathrm{Cu}/{\mathrm{Al}}_2{\mathrm{O}}_3$ trilayers. Despite the absence of heavy elements, their effective spin Hall conductivity can be one order of magnitude larger than those of heavy-metal based multilayers. Properties of the measured torque deviate from those of the spin-injection induced torque and are consistent instead with a recently proposed torque mechanism based on orbital angular momentum injection. Our results demonstrate a direction for magnetic nanodevices based on the orbital angular momentum injection.

Motivated by the poor understanding of the applicability of new exchange-correlation (XC) functionals to warm dense matter (WDM), we designed and performed multiple-shock reverberation compression experiments on dense krypton to evaluate explicitly the implications of recently derived XC functionals. The equation of states of krypton up to 155 GPa and 45 000 K, which ranges from an initial dense gaseous state up to the insulator-metal transition regime, were determined accurately. It is found that the experimental data are better reproduced by the strongly constrained and appropriately normed (SCAN) XC functional compared to the conventional Perdew-Burke-Ernzerhof and Van der Waals (vdW) DF1 functionals, elucidating that the introduction of the kinetic energy density and the intermediate-range vdW interaction is decisive. However, the incorporation of long-range interactions into the SCAN ($\mathrm{SCAN}+\mathrm{rVV}10$ XC functional) results in a noticeably stiffer prediction due to an overestimation of the density and internal energy of the system at low densities and temperatures. Our evaluation of the Karasiev-Sjostrom-Dufty-Trickey free-energy functional experimentally validates the XC thermal effect in the WDM regime, verifies the previous predictions, and sheds light on a direction for future theoretical efforts. Finally, a phase diagram of krypton is given, which provides a clear picture for understanding the thermophysical behavior of krypton in a wider temperature-pressure range.

Lattice dynamics of low-dimensional ${\mathrm{BaVS}}_3$ is reported across the metal-insulator Peierls transition occurring at $T_P=69$ K using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density-functional theory calculations. The nondetection of a Kohn anomaly points to a unique situation of an order-disorder Peierls instability with a quasielastic critical scattering which has been fully characterized. These observations are discussed in the scope of a Peierls instability dominated by strong electron-phonon coupling and/or nonadiabatic effects.

We investigate the magnetoresistance of ferrimagnetic GdFeCo across the magnetization compensation temperature $T_{\mathrm{M}}$. The magnetic field dependence of longitudinal resistivity (${\rho }_{\mathrm{xx}}$) shows opposite trends below and above $T_{\mathrm{M}}$, and the variation of ${\rho }_{\mathrm{xx}}$ with $B$ becomes more significant as the temperature decreases. The observed unconventional magnetoresistance is attributed to the sperimagnetism of GdFeCo. Further investigations on the transverse resistivity (${\rho }_{\mathrm{xy}}$) of GdFeCo unveils that, contrary to the recent reports that the transition metal dominates transport of rare-earth transition-metal ferrimagnets, the Gd contribution to magnetoresistance is comparable to the FeCo contribution, showing that the transport of GdFeCo is antiferromagnetic. Our results therefore show that ferrimagnets are a convenient platform for studying antiferromagnetic spin transport and also are potential materials that can enable antiferromagnetic spin devices.

We report temperature dependent magnetization and neutron diffraction measurements on the corundum related magnetoelectric antiferromagnet ${\mathrm{Mn}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$. Measurements performed on a single crystalline specimen reveal that the magnetization is anisotropic, and a weak ferromagnetic component emerges well within the antiferromagnetically ordered state. Powder neutron diffraction measurements indicate that the magnetic structure comprises of antiferromagnetically coupled ferromagnetic chains of ${\mathrm{Mn}}^{2+}$ spins aligned along the trigonal $c$ axis, in contrast to that reported in other isostructural members of this family. Magnetic measurements performed under a periodic electric field indicate that the magnetoelectric response is also anisotropic, with this coupling along the trigonal $c$ axis and that perpendicular to it having different signs.