We have measured the electric transport, magnetic properties, electronic structure, and ${}^{119}\text{Sn}$ Mössbauer spectra of a series of skutterudite-related ${\mathrm{Ce}}_3{\mathrm{Co}}_{4-x}{\mathrm{Fe}}_x{\mathrm{Sn}}_{13}$ compounds, where $x$ varies from 0 to 0.5. On the basis of these comprehensive studies, we report an observation of the spin-glass phase for samples with $x>0.1$ Fe doping. The effect is, however, not observed for Ni dopants. The critical concentration close to $x=0.1$ separates the magnetically ordered phase with $x>0.1$ from the dense Kondo lattice phase ($x\le 0.1$) with weak magnetic correlations. The Mössbauer spectra consist of three quadrupole doublets, related to covalent bonding among Sn2 and Ce as well as between Sn2 and Co/Fe atoms, which could be a reason for the structural deformation of the cages. These results confirm earlier structural investigations and are in agreement with the band-structure calculations, both of which indicated strong covalent bonding between the atoms $M$, Sn1, or Ce in the center of the cages and the Sn2 atoms forming the respective cages.

We study the behavior of the fidelity and the Uhlmann connection in two-dimensional systems of free fermions that exhibit nontrivial topological behavior. In particular, we use the fidelity and a quantity closely related to the Uhlmann factor in order to detect phase transitions at zero and finite temperature for topological insulators and superconductors. We show that at zero temperature both quantities predict quantum phase transitions: a sudden drop of fidelity indicates an abrupt change of the spectrum of the state, while the behavior of the Uhlmann connection signals equally rapid change in its eigenbasis. At finite temperature, the topological features are gradually smeared out, indicating the absence of finite-temperature phase transitions, which we further confirm by performing a detailed analysis of the edge states. Moreover, we performed both analytical and numerical analysis of the fidelity susceptibility in the thermodynamic limit, providing an explicit quantitative criterion for the existence of phase transitions. The critical behavior at zero temperature is further analyzed through the numerical computation of critical exponents.

A fully relativistic theory of angle-resolved two-photon photoemission for ferromagnetic materials is derived from the general theory of pump-probe photoemission that is based on the Keldysh formalism. Two-photon photoemission spectroscopy is a widely used analytical tool to study nonequilibrium phenomena in nonmagnetic as well as in magnetic solids, as for example ultrafast demagnetization processes. Our theoretical approach aims at a quantitative description of the time-dependent spectroscopic properties of specific solid systems under consideration, and it allows for the inclusion of static correlation effects via the local spin-density and dynamical mean-field theory electronic structure approach. To this end we follow Pendry's one-step theory of the photoemission process as closely as possible and make extensive use of concepts of relativistic multiple-scattering theory within the layer-Korringa-Kohn-Rostoker method in order to guarantee angular resolution in the spectroscopic calculations. As usual the final state of the two-photon photoemission process is represented by a time-reversed low-energy electron diffraction state. The formalism allows for a quantitative calculation of the time-dependent photocurrent for moderately correlated systems like simple ferromagnetic metals. A first application to the Fe(100) surface is discussed in detail.

Conjugate-impedance matched superabsorbers are metamaterial bodies whose effective absorption cross section greatly exceeds their physical dimension. Such objects are able to receive radiation when it is not directly incident on their surface. Here, we develop methods of physical modeling of such structures and investigate interactions of the superabsorbers with passing electromagnetic radiation. The particular superabsorbing structure under study is a wormhole composed of meshes of loaded transmission lines. A theory of electromagnetic wave propagation and absorption in such metamaterial structures is developed. At the frequency of operation, the structure exhibits greatly enhanced absorption as compared to the black-body-type absorber of the same size. Peculiar wave absorption effects such as trapping of nearby passing beams of electromagnetic radiation are demonstrated by numerical simulations. Possible modifications of the wormhole structure under the goal of optimizing absorption while minimizing complexity of the involved metamaterials are discussed. Conjugate-impedance matched superabsorbers may find applications as efficient harvesters of electromagnetic radiation, antennas, and sensors.

Driven many-body quantum systems where some parameter in the Hamiltonian is varied quasiperiodically in time may exhibit nonequilibrium steady states that are qualitatively different from their periodically driven counterparts. Here we consider a prototypical integrable spin system, the spin-$1/2$ transverse field Ising model in one dimension, in a pulsed magnetic field. The time dependence of the field is taken to be quasiperiodic by choosing the pulses to be of two types that alternate according to a Fibonacci sequence. We show that a steady state emerges after an exponentially long time when local properties (or equivalently, reduced density matrices of subsystems with size much smaller than the full system) are considered. We use the temporal evolution of certain coarse-grained quantities in momentum space to understand this nonequilibrium steady state in more detail and show that unlike the previously known cases, this steady state is neither described by a periodic generalized Gibbs ensemble nor by an infinite temperature ensemble. Finally, we study a toy problem with a single two-level system driven by a Fibonacci sequence; this problem shows how sensitive the nature of the final steady state is to the different parameters.

In this work, we use Floquet theory to theoretically study the influence of monochromatic circularly and linearly polarized light on the Hofstadter butterfly—induced by a uniform perpendicular magnetic field—for both the kagome and triangular lattices. In the absence of laser light, the butterfly has a fractal structure with inversion symmetry about magnetic flux, $\varphi =1/4,3/4$, and reflection symmetry about $\varphi =1/2$. As the system is exposed to an external laser, we find that circularly polarized light deforms the butterfly by breaking the mirror symmetry at flux $\varphi =1/2$. By contrast, linearly polarized light deforms the original butterfly while preserving the mirror symmetry at flux $\varphi =1/2$. We find the inversion symmetry is always preserved for both linearly and circularly polarized light. For linearly polarized light, the Hofstadter butterfly depends on the polarization direction. Further, we study the effect of the laser on the Chern number of the lowest band in the off-resonance regime (laser frequency is larger than the bandwidth). For circularly polarized light, we find that low laser intensity will not change the Chern number, but beyond a critical intensity the Chern number will change. For linearly polarized light, the Chern number depends on the polarization direction. Our work highlights the generic features expected for the periodically driven Hofstadter problem on different lattices.

The charge disproportionation of Mn $3d$ and O $2p$ electronic states was investigated in two $({\mathrm{LaMnO}}_3{)}_2{\left({\mathrm{SrMnO}}_3\right)}_2$ superlattices using the resonant x-ray scattering technique. The superlattices show significantly different conductivity even with the same composition. The large modulation of the Mn $3d$ electronic state was observed in the insulating sample, however the charge disproportionation and magnetic modulation of O $2p$ were shown in the semiconductive sample. It is suggested that the charge modulation appears in either Mn $3d$ or O $2p$, which is strongly related to strength of the $p\text{−}d$ hybridization, which is the main factor determining the electric conductivity in the superlattices.

We present a comprehensive theoretical description for an irradiation of an ultrashort light pulse normally on thin materials based on first-principles time-dependent density functional theory (TDDFT). As the most elaborate scheme, we develop a microscopic description solving Maxwell equations for electromagnetic fields and the time-dependent Kohn-Sham equation for electron dynamics simultaneously in the time domain using a common spatial grid. We call it the microscopic Maxwell-TDDFT scheme. We test this scheme for silicon thin films of various thicknesses from a few atomic layers to a few tens of nanometers. We show that the microscopic Maxwell-TDDFT scheme provides a satisfactory description incorporating the electronic structure of thin films in the first-principles level, multiple reflections of the electromagnetic fields at the surfaces, and nonlinear light-matter interaction when the incident light pulse is strong. However, the calculation becomes expensive as the thickness increases. We then consider two limiting cases of extremely thin and sufficiently thick films and develop approximate schemes. For the extremely thin case including two-dimensional atomic-layered materials, a two-dimensional macroscopic electromagnetism is developed: a two-dimensional susceptibility is introduced for a weak field, whereas the time-evolution equation is derived for an intense field. For sufficiently thick films, the microscopic Maxwell-TDDFT scheme is expected to coincide with a description utilizing ordinary macroscopic electromagnetism. We numerically confirm it comparing the calculated results: For a weak field, a comparison is made with a description using the bulk dielectric susceptibility. For a strong field, a comparison is made with a multiscale Maxwell-TDDFT scheme which the authors' group developed previously.

$\mathrm{P}{\mathrm{b}}_{1\text{−}x}\mathrm{S}{\mathrm{n}}_x\mathrm{Se}$ hosts three-dimensional (3D) massive Dirac fermions across the entire composition range for which the crystal structure is cubic. In this paper, we present a comprehensive experimental mapping of the 3D band structure parameters of $\mathrm{P}{\mathrm{b}}_{1\text{−}x}\mathrm{S}{\mathrm{n}}_x\mathrm{Se}$ as a function of composition and temperature. We cover a parameter space spanning the band inversion that yields its topological crystalline insulator phase. A nonclosure of the energy gap is evidenced in the vicinity of this phase transition. Using magneto-optical Landau level spectroscopy, we determine the energy gap, Dirac velocity, anisotropy factor, and topological character of $\mathrm{P}{\mathrm{b}}_{1\text{−}x}\mathrm{S}{\mathrm{n}}_x\mathrm{Se}$ epilayers grown by molecular beam epitaxy on $\mathrm{Ba}{\mathrm{F}}_2$ (111). Our results are evidence that $\mathrm{P}{\mathrm{b}}_{1\text{−}x}\mathrm{S}{\mathrm{n}}_x\mathrm{Se}$ is a model system to study topological phases and the nature of the phase transition.

Although the direct or indirect nature of the band-gap transition is an essential parameter of semiconductors for optoelectronic applications, the reasons for why some of the conventional semiconductors have direct or indirect band gaps remains ambiguous. In this paper, we reveal that the existence of the occupied cation $d$ bands is a prime element in determining the directness of the band gap of semiconductors through the $s\text{−}d$ and $p\text{−}d$ couplings, which push the conduction band energy levels at the $X$ and $L$ valley up, but leave the Γ-valley conduction state unchanged. This unified theory unambiguously explains why diamond, Si, Ge, and Al-containing group III-V semiconductors, which do not have active occupied $d$ bands, have indirect band gaps, and the remaining common semiconductors, except GaP, have direct band gaps. Besides $s\text{−}d$ and $p\text{−}d$ couplings, bond length and electronegativity of anions are two remaining factors regulating the energy ordering of the Γ, $X$, and $L$ valleys of the conduction band, and are responsible for the anomalous band-gap behaviors in GaN, GaP, and GaAs that have direct, indirect, and direct band gaps, respectively, despite the fact that N, P, and As are in ascending order of the atomic number. This understanding will shed light on the design of direct band-gap light-emitting materials.

We have generalized a quasiclassical model for Weyl semimetals with a tilted band in the presence of an externally applied magnetic field. This model is applicable to ballistic, moderately disordered, and samples containing a high density of nonmagnetic impurities. We employ this formalism and show that a self-biased supercurrent, creating a ${\phi }_0$ junction, can flow through a triplet channel in Weyl semimetals. Furthermore, our results demonstrate that multiple supercurrent reversals are accessible through varying junction thickness and parameters characterizing Weyl semimetals. We discuss the influence of different parameters on the Fraunhofer response of charge supercurrent, and how these parameters are capable of shifting the locations of proximity-induced vortices in the triplet channel.

Based on the Lorenz-Mie theory, we derive analytical expressions of radiative and nonradiative transition rates for different orientations of a point dipole emitter in the vicinity of an infinitely long circular cylinder of arbitrary radius. Special attention is devoted to the spontaneous decay rate of a dipole emitter near a subwavelength-diameter nanowire coated with a graphene monolayer. We show that plasmonic Fano resonances associated with light scattering by graphene-coated nanowires appear in the Purcell factor as a function of transition wavelength. Furthermore, the Fano line shape of transition rates can be tailored and electrically tuned by varying the distance between emitter and cylinder and by modulating the graphene chemical potential, where the Fano asymmetry parameter is proportional to the square root of the chemical potential. This gate-voltage-tunable Fano resonance leads to a resonant enhancement and suppression of light emission in the far-infrared range of frequencies. This result could be explored in applications involving ultrahigh-contrast switching for spontaneous emission in specifically designed tunable plasmonic nanostructures.

Wave effects of phonons play an important role in modulating thermal transport in nanostructures. In this paper, we calculated the thermal conductivity of a species of graphene nanoribbon phononic metamaterials (GNPM), pillared graphene nanoribbon, through nonequilibrium molecular dynamics. Interestingly, it is found that the thermal conductivity of GNPM is enhanced by isotopic resonance at pillars, which is in contrast to the reduction by isotopic doping. Furthermore, mode-analysis and atomic Green's function calculations reveal that the mechanism of enhancement originates from the broken hybridization. Additionally, we calculated the effects of the ribbon's width and pillars’ height on the thermal conductivity of GNPM. The isotopic resonance provides a mechanism to manipulate phonon transport in nanostructures.

The $k$-projection method provides an approach to separate the contributions from different constituents in heterostructure systems, and can act as an aid to connect the results of experiments and calculations. We show that the technique can be used to “unfold” the calculated electronic bands of interfaces and supercells, and provide local band structure by integrating the projected states over specified regions of space, a step that can be implemented efficiently using fast Fourier transforms. We apply the method to investigate the effects of interfaces in heterostructures consisting of a graphene bilayer on H-saturated SiC(0001), BAs monolayer on the ferromagnetic semiconductor ${\mathrm{CrI}}_3$, silicene on Ag(111), and to the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ surface. Our results reveal that the band structure of the graphene bilayer around the Dirac point is strongly dependent on the termination of SiC(0001): on the C face, the graphene is $n$ doped and a gap of $\sim 0.13$ eV is opened, whereas on the Si face, the graphene is essential unchanged and neutral. We show that for BAs/${\mathrm{CrI}}_3$, the magnetic proximity effect can effectively induce a spin splitting up to about 50 meV in BAs. For silicene/Ag(111), our calculations reproduce the angle-resolved photoemission spectroscopy results, including linearly dispersing bands at the edge of the first Brillouin zone of Ag(111); although these states result from the interaction between the silicene overlayer and the substrate, we demonstrate that they are not Dirac states.

$1H$-phase monolayer transition-metal dichalcogenides have spin-split electronic band structures near the $K\left(K^{\prime }\right)$ point due to strong spin-orbit coupling and intrinsic inversion symmetry breaking. In the case of the monolayer ${\mathrm{Mo}X}_2$ ($X=\text{S}$, Se, Te), the spin-split conduction bands cross near the $K\left(K^{\prime }\right)$ point. We show that the band crossing occurs not only along the high-symmetry directions, but also along arbitrary directions due to a mirror reflection symmetry of the monolayer. As a result, $n$-doped monolayer ${\mathrm{Mo}X}_2$ is a two-dimensional type-II nodal-line material.

Exchange-coupled hard-soft biphase magnets are technologically relevant systems in that they enable tailoring the magnetization reversal process. Here, exchange-spring behavior is observed in $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$/FeCo bilayers for soft thicknesses as thin as 2 nm, at least four times below the exchange length of the system. This result is in contrast with the accepted theory for spring magnets that states that the exchange length defines the critical thickness below which both magnetic phases should be rigidly coupled. In combination with micromagnetic calculations, this surprising observation is understood as a consequence of the dominance of domain-wall propagation in the soft phase during the reversal process, so far unaccounted for in theoretical descriptions. Our results emphasize the need to expand the existing spring theory from coherent rotation to domain-wall related processes in multidomain configurations in order to accurately design magnetic heterostructures with controllable reversal.

Carbon, a basic versatile element in our universe, exhibits rich varieties of allotropic phases, most of which possess promising nontrivial topological fermions. In this work, we identify a distinct topological phonon phase in a realistic carbon allotrope with a body-centered cubic structure, termed bcc ${\mathrm{C}}_8$. We show by symmetry arguments and effective model analysis that there are three intersecting phonon nodal rings perpendicular to each other in different planes. The intersecting phonon nodal rings are protected by time-reversal and inversion symmetries, which quantize the corresponding Berry phase into integer multiples of $\pi$. Unlike the electron systems, the phonon nodal rings in bcc ${\mathrm{C}}_8$ are guaranteed to remain gapless due to the lack of spin-orbital coupling. The nearly flat drumhead surface states projected on semi-infinite (001) and (110) surfaces of bcc ${\mathrm{C}}_8$ are clearly visible. Our findings not only discover promising nodal ring phonons in a carbon allotrope but also provide emergent avenues for exploring topological phonons beyond fermionic electrons in carbon-allotropic structures with attractive features.

Time-quasiperiodic Majoranas are generalizations of Floquet Majoranas in time-quasiperiodic superconducting systems. We show that in a system driven at $d$ mutually irrational frequencies, there are up to $2^d$ types of such Majoranas, coexisting despite spatial overlap and lack of time-translational invariance. Although the quasienergy spectrum is dense in such systems, the time-quasiperiodic Majoranas can be stable and robust against resonances due to localization in periodic-drive-induced synthetic dimensions. This is demonstrated in a time-quasiperiodic Kitaev chain driven at two frequencies. We further relate the existence of multiple Majoranas in a time-quasiperiodic system to the time-quasicrystal phase introduced recently. These time-quasiperiodic Majoranas open a possibility for braiding which will be pursued in the future.

Recent experiments on ${\mathrm{K}}_3{\mathrm{C}}_{60}$ and layered copper-oxide materials have reported substantial changes in the optical response following application of an intense THz pulse. These data have been interpreted as the stimulation of a transient superconducting state even at temperatures well above the equilibrium transition temperature. We propose an alternative phenomenology based on the assumption that the pulse creates a nonsuperconducting, though nonequilibrium, situation in which the linear response conductivity is negative. The negative conductivity implies that the spatially uniform prepulse state is unstable and evolves to a new state with a spontaneous electric polarization. This state exhibits coupled oscillations of entropy and electric charge whose coupling to incident probe radiation modifies the reflectivity, leading to an apparently superconductinglike response that resembles the data. Dependencies of the reflectivity on polarization and angle of incidence of the probe are predicted and other experimental consequences are discussed.