We describe a simple and robust method using an internal reflection element to measure the spectra of near-field thermal emission. We experimentally demonstrate the spectrally narrow peaks of near-field thermal emission by isotropic media due to the excitation of surface phonon polaritons in quartz and amorphous silica and due to the frustrated total-internal-reflection modes in amorphous silica and polytetrafluoroethylene. Additionally, we demonstrate the broadband near-field thermal emission of hyperbolic modes in hexagonal boron nitride, which is an anisotropic uniaxial medium. We also present a theoretical approach based on the fluctuational electrodynamics and dyadic Green's functions for one-dimensional layered media for accurately modeling the measured spectra.

Weyl semimetals possess unique electrodynamic properties due to a combination of strongly anisotropic and gyrotropic bulk conductivity, surface conductivity, and surface dipole layer. In particular, the gyrotropy caused by Weyl node separation in momentum space gives rise to the optical Hall effect for surface polaritons at the boundaries parallel to the gyrotropic axis. We explore the potential of popular tip-enhanced optical spectroscopy techniques for studies of bulk and surface topological electron states in these materials. Strong anisotropy, anomalous dispersion, and the optical Hall effect for surface polaritons launched by a nanotip provide information about Weyl node position and separation in the Brillouin zone, the value of the Fermi momentum, and the matrix elements of the optical transitions involving both bulk and surface electron states.

It is of theoretical and experimental interest to engineer topological phases with very large topological invariants via periodic driving. As advocated by this paper, such Floquet engineering can be elegantly achieved by the particle swarm optimization (PSO) technique from the swarm intelligence family. With the recognition that conventional gradient-based optimization approaches are not suitable for directly optimizing topological invariants as integers, the highly effective PSO route yields new promises in the search for exotic topological phases, requiring limited physical resources. Our results are especially timely in view of two important insights from literature: low-frequency driving may be beneficial in creating large topological invariants, but an open-ended low-frequency driving often leads to drastic fluctuations in the obtained topological invariants. Indeed, using a simple continuously driven Harper model with three quasienergy bands, we show that the Floquet-band Chern numbers can enjoy a many-fold increase compared with that using a simple harmonic driving of the same period, without demanding more energy cost of the driving field. It is also found that the resulting Floquet insulator bands are still well gapped with the maximized topological invariants in agreement with physical observations from Thouless pumping. The emergence of many edge modes under the open boundary condition is also consistent with the bulk-edge correspondence. Our results are expected to be highly useful towards the optimization of many different types of topological invariants in Floquet topological matter.

Compared with the well-established theory for other graphene Raman bands, the origin of the ${\mathrm{G}}^{*}$ band at $\sim 2450\mathrm{c}{\mathrm{m}}^{-1}$ remains unclear. A major debate lies at whether it should be assigned solely to the combined $q\ne 0$ in-plane (i) transverse-optical (TO) and longitudinal-acoustic (LA) phonon modes, or to such a combination with additional $q=0 i\mathrm{TO}$ phonon overtones. A strain-dependent Raman analysis shows that during uniaxial strain this band exhibits only frequency redshift but no line shape changes. It is well interpreted by first-principles calculations, and thereby unambiguously identifies the origin of the ${\mathrm{G}}^{*}$ band as the $q\ne 0 i\mathrm{TO}$ and LA phonon combination.

We report the magnetoresistance, Hall effect, de Haas–van Alphen (dHvA) oscillations, and electronic structures of single-crystal RhSn, which is a typical material of the CoSi family holding a large Chern number. Large unsaturated magnetoresistance is observed with $B//\left[001\right]$. The Hall resistivity curve indicates that RhSn is a multiband system with high mobility. Evident quantum oscillations have been observed, from which light effective masses are extracted. Ten fundamental frequencies are extracted after the fast Fourier-transform analysis of the dHvA oscillations with $B//\left[001\right]$ configuration. The two low frequencies $F_1$ and $F_2$ do not change obviously and the two high frequencies $F_9$ and $F_{10}$ evolve into four when $B$ rotates from $B//\left[001\right]$ to $B//\left[110\right]$, which is consistent with the band structure in first-principles calculations with spin-orbit coupling (SOC). The extracted Berry phases of the relative pockets show a good agreement with the Chern number $\pm 4$ (with SOC) in the first-principles calculations. Above all, our paper indicates that RhSn is an ideal platform to study the unconventional chiral fermions and the surface states.

Taking the competition and the mutual screening of various bosonic fluctuations in correlated electron systems into account requires an unbiased approach to the many-body problem. One such approach is the self-consistent solution of the parquet equations, whose numerical treatment in lattice systems is however prohibitively expensive. In a recent article it was shown that there exists an alternative to the parquet decomposition of the four-point vertex function, which classifies the vertex diagrams according to the principle of single-boson exchange (SBE) [Krien, Valli, and Capone, Phys. Rev. B 100, 155149 (2019)]. Here we show that the SBE decomposition leads to a closed set of equations for the Hedin three-leg vertex, the polarization, and the electronic self-energy, which sums self-consistently the diagrams of the Maki-Thompson type. This circumvents the calculation of four-point vertex functions and the inversion of the Bethe-Salpeter equations, which are the two major bottlenecks of the parquet equations. The convergence of the calculation scheme starting from a fully irreducible vertex is demonstrated for the Anderson impurity model.

We report an above-room-temperature ferromagnetic state realized in a proximitized Dirac semimetal, which is fabricated by growing typical Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ films on a ferromagnetic garnet with strong perpendicular magnetization. Observed anomalous Hall conductivity with substantially large Hall angles is found to be almost proportional to magnetization and opposite in sign to it. Theoretical calculations based on first-principles electronic structure also demonstrate that the Fermi-level dependent anomalous Hall conductivity reflects the Berry curvature originating in the split Weyl nodes. The present Dirac-semimetal/ferromagnetic-insulator heterostructure will provide a platform for exploring Weyl-node transport phenomena and spintronic functions lately proposed for topological semimetals.

Photonic and polaritonic lattices were recently theoretically proposed and experimentally realized as many-body simulators due to the rich behaviors exhibited by such systems at the macroscale. We show that the networks of polariton condensates encapsulate a large variety of behaviors of systems of coupled oscillators. By eliminating spatial degrees of freedom in a nonresonantly pumped polariton network, we establish that depending on the values of experimentally tunable parameters the networks of polariton condensates may represent Kuramoto, Sakaguchi-Kuramoto, Stuart-Landau, or Lang-Kobayashi oscillators and beyond. The networks of polariton condensates are therefore capable of implementing various regimes acting as analog spin Hamiltonian minimizers, producing complete and cluster synchronization, exotic spin glasses, and large-scale secondary synchronization of oscillations. We suggest that the recently implemented control of the system parameters for individual sites in polariton lattices allows addressing the interaction of sublattices that belong to different oscillatory classes.

Carbon monoxide on copper surfaces continues to be a fascinating, rich microlab for many questions evolving in surface science. Recently, hot-electron mediated, femtosecond-laser pulse induced dynamics of CO molecules on Cu(100) were the focus of experiments [Inoue et al., Phys. Rev. Lett. 117, 186101 (2016)] and theory [Novko et al., Phys. Rev. Lett. 122, 016806 (2019)], unraveling details of the vibrational nonequilibrium dynamics on ultrashort (subpicoseconds) timescales. In the present work, full-dimensional time-resolved hot-electron driven dynamics are studied by molecular dynamics with electronic friction (MDEF). Dissipation is included by a friction term in a Langevin equation which describes the coupling of molecular degrees of freedom to electron-hole pairs in the copper surface, calculated from gradient-corrected density functional theory (DFT) via a local density friction approximation (LDFA). Relaxation due to surface phonons is included by a generalized Langevin oscillator model. The hot-electron induced excitation is described via a time-dependent electronic temperature, the latter derived from an improved two-temperature model. Our parameter-free simulations on a precomputed potential energy surface allow for excellent statistics, and the observed trends are confirmed by on-the-fly ab initio molecular dynamics with electronic friction (AIMDEF) calculations. By computing time-resolved frequency maps for selected molecular vibrations, instantaneous frequencies, probability distributions, and correlation functions, we gain microscopic insight into hot-electron driven dynamics and we can relate the time evolution of vibrational internal CO stretch-mode frequencies to measured data, notably an observed redshift. Quantitatively, the latter is found to be larger in MDEF than in experiment and possible reasons are discussed for this observation. In our model, in addition we observe the excitation and time evolution of large-amplitude low-frequency modes, lateral CO surface diffusion, and molecular desorption. Effects of surface atom motion and of the laser fluence are also discussed.

We present a first-principles study of stoichiometric $\mathrm{LiNb}{\mathrm{O}}_3$ (0001) clean positive and negative surface and with ${\mathrm{H}}_2\mathrm{O}$, OH and H adsorption. While we exclude internal compensation mechanisms such as enrichment or depletion of certain atomic species, in the frame of surface relaxation with constant stoichiometry and considering the foreign adsorbates, the structures with lowest energy states are obtained. We find that the positive stoichiometric surface compensates the surface charges by abnormal ion displacements. We also suggest that foreign adsorbates are in significantly different bonds with opposite polar surfaces, leading to diverse mechanisms of surface charge compensation. Water molecules adsorbed on negative surface may even alter the surface polarity. We refer to five mechanisms for the charge compensation and the stabilization of stoichiometric polar surfaces, that is, the bonding interaction between adsorbed atoms and surface ions, the dipole moment of adsorbates, the (abnormal) ion displacements-induced change of surface bound charge, the variation of surface spontaneous polarization, and the mobile carriers, with the priority lowering in turn.

We theoretically investigate the effects of Coulomb interaction, at the level of the unscreened Hartree-Fock approximation in an equation of motion framework, on third harmonic generation from undoped graphene. The unperturbed electronic states are described by a widely used two-band tight-binding model and the Coulomb interaction is described by the Ohno potential. The ground state is renormalized by taking into account the Hartree-Fock term and the optical conductivities are obtained by numerically solving the equations of motion. The absolute values of conductivity for third-harmonic generation depend on the photon frequency $\mathrm{\Omega }$ as ${\mathrm{\Omega }}^{-n}$ for $\hslash \mathrm{\Omega }<1$, and then show a peak as $3\hslash \mathrm{\Omega }$ approaches the renormalized energy of the $M$ point. Taking into account the Coulomb interaction, $n$ is found to be 5.5, which is significantly greater than the value of 4 found with the neglect of the Coulomb interaction. Therefore, the Coulomb interaction enhances third-harmonic generation at low photon energies—for our parameters $\hslash \mathrm{\Omega }<0.8$ eV—and then reduces it until the photon energy reaches about 2.1 eV. The effect of the background dielectric constant is also considered.

High harmonic generation (HHG) in solids provides us compact and coherent extreme ultraviolet sources. However, the low yield of HHG is the bottleneck restricting its wide applications. We propose a pump-probe scheme to selectively enhance the yield of HHG. Our simulations illustrate abnormal harmonic yield dependence on the intensity of driving or preexcitation pulses. A multielectron interference model is proposed to reveal the mechanism of the extreme nonlinear optical phenomena. We find that the $k$-resolved intraband harmonic demonstrates different symmetry, phase, and amplitude, which can be observed by using a resonant preexcitation pulse. The position of the interference minimum obtained by solving the semiconductor Bloch equations agrees well with that obtained by our $k$-resolved semiclassical model. This interference model can help us probe the $k$-resolved band structure and optimize the ultrafast electron-hole dynamics for solid harmonic processes.

We study the $t\text{−}V$ disordered spinless fermionic chain in the strong-coupling regime, $t/V\to 0$. Strong interactions highly hinder the dynamics of the model, fragmenting its Hilbert space into exponentially many blocks in system size. Macroscopically, these blocks can be characterized by the number of new degrees of freedom, which we refer to as movers. We focus on two limiting cases: blocks with only one mover and ones with a finite density of movers. The former many-particle block can be exactly mapped to a single-particle Anderson model with correlated disorder in one dimension. As a result, these eigenstates are always localized for any finite amount of disorder. The blocks with a finite density of movers, on the other side, show a many-body localized (MBL) transition that is tuned by the disorder strength. Moreover, we provide numerical evidence that its ergodic phase is diffusive at weak disorder. Approaching the MBL transition, we observe subdiffusive dynamics at finite timescales and find indications that this might be only a transient behavior before crossing over to diffusion.

We present bulk tensor networks that exactly represent the ground states of a continuous family of one-dimensional frustration-free Hamiltonians. These states, which are known as area-deformed Motzkin and Fredkin states, exhibit a novel quantum phase transition. By tuning a single parameter, they go from a phase obeying an area law to a highly entangled “rainbow” phase, where the half-chain entropy scales with the volume. Using the representation of these ground states as superpositions of random walks, we introduce tensor networks for these ground states where local and global rules of the walker are baked into bulk tensors, thereby providing an efficient description of the ground states (some of which satisfy a volume law scaling of entanglement entropy).

Chiral magnetic textures in ultrathin perpendicularly magnetized multilayer film stacks with an interfacial Dzyaloshinskii-Moriya interaction have been the focus of much research recently. The chirality associated with the broken inversion symmetry at the interface between an ultrathin ferromagnetic layer and a heavy metal with large spin-orbit coupling supports homochiral Néel domain walls and hedgehog (Néel) skyrmions. Under spin-orbit torques these Néel-type magnetic structures are predicted, and have been measured, to move at high velocities. However recent studies have indicated that some multilayered systems may possess a more complex hybrid domain wall configuration, due to the competition between interfacial DMI and interlayer dipolar fields. These twisted textures are expected to have thickness dependent Néel and Bloch contributions to the domain or skyrmion walls. In this work, we use the methods of Lorentz microscopy to determine quantitatively experimentally both (i) the contributions of the Néel and Bloch components and (ii) their spatial spin variation at high resolution. These are compared with modeled and simulated structures that are in excellent agreement with our experimental results. Our quantitative analysis provides powerful direct evidence of the Bloch wall component which exists in these hybrid walls and will be significant when exploiting chirality in spintronic applications.

A polycrystalline sample of superconducting $\mathrm{ThI}{\mathrm{r}}_3$ was obtained by arc-melting Th and Ir metals. Powder x-ray diffraction revealed that the compound crystalizes in a rhombohedral crystal structure (R-3m, s.g. #166) with the lattice parameters: $a=5.3394\left(1\right)\AA$ and $c=26.4228\left(8\right)\AA$. Normal and superconducting states were studied by magnetic susceptibility, electrical resistivity, and heat capacity measurements. The results showed that $\mathrm{ThI}{\mathrm{r}}_3$ is a type-II superconductor (Ginzburg-Landau parameter $\kappa =38)$ with the critical temperature $T_{\mathrm{c}}=4.41\mathrm{K}$. The heat capacity data yielded the Sommerfeld coefficient $\gamma =17.6\mathrm{mJ}/\left(\mathrm{mo}\mathrm{l}{\mathrm{K}}^2\right)$ and the Debye temperature ${\mathrm{\Theta }}_{\mathrm{D}}=169\mathrm{K}$. The ratio $\mathrm{\Delta }C/\left(\gamma T_{\mathrm{c}}\right)=1.6$, where $\mathrm{\Delta }C$ stands for the specific heat jump at $T_{\mathrm{c}}$, and the electron–phonon coupling constant ${\lambda }_{e-p}=0.74$ suggest that $\mathrm{ThI}{\mathrm{r}}_3$ is a moderate-strength superconductor. The experimental studies were supplemented by band structure calculations, which indicated that the superconductivity in $\mathrm{ThI}{\mathrm{r}}_3$ is governed mainly by $5d$ states of iridium. The significantly smaller band-structure value of Sommerfeld coefficient as well as the experimentally observed quadratic temperature dependence of resistivity and enhanced magnetic susceptibility suggest the presence of electronic interactions in the system, which compete with superconductivity.

The zero-field domain-mode ferromagnetic resonances (DM-FMRs) in the weakly nonlinear regime have been investigated for garnet films with a perpendicular anisotropy supporting parallel stripe domains by means of broadband microwave measurements, and analytical and numerical approaches. First, the perpendicular ferromagnetic resonance (FMR) in the saturated state is considered as a reference case. A redshift of the resonance frequency and a broadening of the resonance line are experimentally observed for increasing the input microwave power. These power-dependent characteristics are nicely reproduced by an analytical model taking into account the linear polarization of the pumping field and a nonlinear Gilbert-type damping parameter. Then, the effect of the stripe domain pattern is analyzed. A redshift and a line broadening are also experimentally revealed both for the acoustic and the optical modes with an anisotropic character between the two modes. Numerical simulations based on the DM-FMR model extended in the nonlinear regime and including a nonlinear Gilbert-type damping parameter account quantitatively for the power dependences of the DM resonance frequency shifts and the DM resonance linewidths. These results highlight the requirement of a nonlinear damping term for explaining the FMR and DM-FMR spectra in the weakly nonlinear regime within the framework of the phenomenological approach of magnetization dynamics.

Based on the Bogoliubov–de Gennes equations, we provide an exact numerical solution for the critical current of Josephson junctions with a composite ferromagnetic bilayer. We demonstrate that for the antiparallel orientation of the magnetic moments of the bilayer, the presence of a potential barrier at the bilayer interface results in large oscillations of the critical current as a function of ferromagnet thickness and/or exchange field. Because of this, and remarkably, in the range of small exchange field and thicknesses, the magnetism leads to the increase of the critical current. This effect is well pronounced at low temperature but disappears near $T_c$. If the potential barrier is replaced by a spin-active barrier at the bilayer interface the conventional 0-$\pi$ transition, similar to the case of an uniform ferromagnetic Josephson junction, is observed. Strikingly, for a parallel orientation of the magnetic moments of the bilayer, the presence of the spin-active barrier restores the anomalous behavior—potential barrier in the antiparallel case. These behaviors result from the resonant tunneling of Cooper pairs across the composite barrier—an effect related to the spin-dependent Fermi vector in the presence of the ferromagnets' exchange field.

We study the superfluid density of hard-core bosons on quasi-one-dimensional lattices using the quantum Monte Carlo method. Because of phase slippage, the superfluid density drops quickly to zero at finite temperatures with increasing the system length $\ell$ and the superfluid transition temperature is zero in one spatial dimension and also in quasi-one dimension in the limit of $\ell \to \infty$. We calculate the superfluid density of a model where no phase slippage is allowed and show that the superfluid density remains finite at finite temperatures even in the one-dimensional limit. We also discuss how finite superfluid density can be observed in a quasi-one-dimensional system using a torsional oscillator.