Light absorption plays a vital role in governing the performance of optoelectronic devices, such as solar cells and photodetectors. However, it is quite challenging to realize perfect absorption for materials with low intrinsic dissipative losses and requires the careful design of nanoscale structures. This study theoretically proposes a polarization-independent germanium (Ge) metasurface absorber at the telecommunication C band by exploring the quasibound state in the continuum (QBIC). Such a QBIC is excited by breaking the in-plane symmetry of the Ge metasurface in which each unit cell is made of four subwavelength Ge nanodisks. The Q factor of QBIC is mainly governed by the asymmetry parameter, providing a straightforward way of optimizing the light absorption by the critical coupling. We demonstrate two designs that boost the absorption up to 50% in the transmission and up to 100% in the reflection modes.

High-temperature thermopower is interpreted as entropy that a carrier carries. Owing to spin and orbital degrees of freedom, a transition-metal perovskite exhibits large thermopower at high temperatures. In this paper, we revisit the high-temperature thermopower in the perovskites to shed light on the degrees of freedom. Thus, we theoretically derive an expression of thermopower in one-dimensional octahedral-${MX}_6$-cluster chain using linear-response theory and electronic structure calculation of the chain based on the tight-binding approximation. The derived expression of the thermopower is consistent with the extended Heikes formula and well reproduces experimental data of several perovskite oxides at high temperatures. In this expression, a degeneracy of many-electron states in an octahedral ligand field (which is characterized by the multiplet term) appears instead of the spin and orbital degeneracies. Complementarity in between our expression and the extended Heikes formula is discussed.

Fully occupied or unoccupied bands in a solid are often considered inert and irrelevant to a material's low-energy properties. But the discovery of enhanced superconductivity in heavily electron-doped FeSe-derived superconductors poses questions about the possible role of incipient bands (those laying close to but not crossing the Fermi level) in pairing. To answer this question, researchers have studied pairing correlations in the bilayer Hubbard model, which has an incipient band for large interlayer hopping $t_{\perp }$, using many-body perturbation theory and variational methods. They have generally found that superconductivity is enhanced as one of the bands approaches the Lifshitz transition and even when it becomes incipient. Here we address this question using the nonperturbative quantum Monte Carlo (QMC) dynamical cluster approximation (DCA) to study the bilayer Hubbard model's pairing correlations. We find that the model has robust $s^{\pm }$ pairing correlations in the large $t_{\perp }$ limit, which can become stronger as one band is made incipient. While this behavior is linked to changes in the effective interaction, we further find that it is counteracted by a suppression of the intrinsic pair-field susceptibility and does not translate to an increased $T_c$. Our results demonstrate that the highest achievable transition temperatures in the bilayer Hubbard model occur when the system has two bands crossing the Fermi level.

Topological insulators (TIs) are promising materials for future spintronic applications such as emerging spin-to-charge conversion (SCC) devices, possibly working at GHz-THz frequency for ultrafast data processing. These devices will rely on hybrid nanostructures composed, for example, of a ferromagnetic layer deposited on the topological insulator. The efficiency of spin-to-charge conversion will depend on the quality of the interface, including chemical (interfacial chemical reactions) and physical (band bending effect, Fermi pinning) aspects. This paper presents a complete study of electronic structures and photoexcited carrier dynamics in topological insulators capped with iron and iron oxide. We combine static and time-resolved angle-resolved photoemission spectroscopies (ARPES, TR-ARPES) with time-resolved optical methods (transient optical reflectivity and transmission). Both single crystal and thin films of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ are studied. We show that monolayers of iron and iron oxide significantly affect the electronic band structure at the interface by shifting the Fermi level into the conduction band, which we explain by a band bending effect, and is confirmed by in situ XPS measurements. This modified interfacial electronic structure offers a new channel for relaxation of hot carriers, illustrated by a drastic decrease of their characteristic decay time after optical excitation. These results might have a potential impact in the future development of TI-based SCC devices.

We analyze the chiral symmetry breaking in a planar four-fermion model with non-null chemical potential, temperature, and including the effect of the tilt of the Dirac cone. The system is modeled with a $(2+1)$-dimensional Gross-Neveu-like interaction model in the context of the generalized Weyl Hamiltonian and its phase structure is studied in the mean-field and large-$N$ approximations. Possible applications of the results obtained, e.g., in connection to graphene, are discussed. We also discuss the effect of an external magnetic field applied to the system, which can give rise to the appearance of the anomalous Hall effect and that is expected to arise in connection with two-dimensional Weyl and Dirac semimetals.

Flat lying physisorbates on surfaces can be expected to be lifted by heating. The details of this lattice expansion depend on the competition between two mechanisms. On the one hand, the interaction potential of each atom with the substrate is anharmonic and constitutes the reason for lifting. On the other hand, the internal bending stiffness of the planar adsorbate reduces its tendency to lift with temperature. Here we show that this competition can be investigated in a simplified molecular-dynamics simulation which considers only the vertical motion of the atoms of the adsorbate in respective model potentials. For two prototypical systems, i.e., graphene and 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on Ag(111), we observe lifting by 0.04 Å (graphene) and 0.06 Å (NTCDA) between zero and room temperature. For the latter system, the impact of the temperature-dependent geometrical and vibrational properties on the occurring interface state (IS) is discussed. By introducing a local electron-phonon interaction model, we combine the indirect dependence of the IS on the average binding distance with direct coupling to the vibrational modes and obtain good agreement with the experimental results.

Recent experiments in exciton-polariton systems have provided high-precision measurements of the value of the polariton-polariton interaction constant, which is a key parameter that governs the nonlinear dynamics of polariton condensates and potentially enables quantum correlated polaritons. Yet, until now, this parameter has only been addressed theoretically using perturbative treatments or approximations that do not include the composite nature of the excitons. Here, we use a recently developed microscopic description of polaritons involving electrons, holes, and photons, where the interactions between charged particles are assumed to be highly screened. Within this model, we perform an exact four-body calculation of the spin-polarized polariton-polariton and polariton-exciton interaction constants. In the limit of weak light-matter coupling relevant to an atomically thin semiconductor in a microcavity, we obtain excellent agreement with a recently proposed universal form of low-energy polariton-polariton scattering [O. Bleu et al., Phys. Rev. Res. 2, 043185 (2020)]. At stronger light-matter coupling, of relevance to multilayer microcavities, we observe that the interaction constant increases towards that predicted by the Born approximation. We show that in all regimes of interest the interaction constant can be accurately obtained from the exciton-exciton scattering phase shift at negative collision energy, and we argue that this has important implications for interactions in other systems featuring strong light-matter coupling.

Magnetically doped graphene has great potential for applications in spintronics, in which local magnetic moments are coupled by the Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction. RKKY interaction in graphene has attracted intensive interest. However, previous studies only show the features of the RKKY interaction dictated by energy dispersion, leaving the effect of the geometric nature of electronic structures unexplored. Here, we focus on the short-range oscillation behaviors of RKKY interaction contributed by intervalley scattering. Due to the unique geometric nature of electronic structures, two wavefront dislocations emerge in the RKKY interaction corresponding to winding number 1 of graphene. We further demonstrate the robustness of wavefront dislocations against the doping, trigonal warping, and gap opening of energy bands. This study reveals a unique feature for the RKKY interaction and implies the importance of the geometric nature of electronic structures for magnetism.

We discuss a physical mechanism which gives rise to a giant magnetoconductivity in noncentrosymmetric superconducting films. This mechanism is caused by a combination of spin-orbit interaction and inversion symmetry breaking in the system, and arises in the presence of an in-plane magnetic field ${\mathbf{H}}_{\parallel }$. It produces a contribution to the conductivity, which displays a strong dependence on the angle between the electric field $\mathbf{E}$ and ${\mathbf{H}}_{\parallel }$, and is proportional to the inelastic relaxation time of quasiparticles. Since in typical situations the latter is much larger than the elastic one, this contribution can be much larger than the conventional Mattis-Bardeen conductivity thus leading to giant microwave absorption.

The interface state (IS) between a noble metal and an organic layer shifts to lower energy by $\sim 100$ meV when heated from zero to room temperature. Here, we show that this shift can only be explained including the energy renormalization caused by a quantum-mechanical electron-phonon interaction (EPI) between molecular vibrations and the IS. We investigate the EPI by a simplified model on the bases of ab initio data. By combining EPI with a classical thermal lifting effect of the molecular layer, we explain the recently published temperature-dependent experimental data for the NTCDA/Ag(111) system.

Control of the spin wave dynamics in nanomagnetic elements is very important for the realization of a broad range of novel magnonic devices. Here we study experimentally the spin wave resonance in thick ferromagnetic rings (100 nm) using perpendicular ferromagnetic resonance spectroscopy. Different from what was observed for the continuous film of the same thickness, or from rings with similar lateral dimensions but with lower thicknesses, the spectra of thick patterned rings show a nonmonotonic dependence of the mode intensity on the resonance field for a fixed frequency. To explain this effect, the theoretical approach by considering the dependence of the mode profiles on both the radial and axial coordinates was developed. It was demonstrated that such unusual behavior is a result of the competition between exchange and dipolar fields acting at the spin excitations in the structure under study. The calculations are in a good agreement with the experimental results.

An ultrafast spin current can be induced by femtosecond laser excitation in a ferromagnetic (FM) thin film in contact with a nonmagnetic (NM) metal. The propagation of an ultrafast spin current into an NM metal has recently been found in experiments to generate transient spin accumulation. Unlike spin accumulation in equilibrium NM metals that occurs due to spin transport at the Fermi energy, transient spin accumulation involves highly nonequilibrium hot electrons well above the Fermi level. To date, the diffusion and dissipation of this transient spin accumulation has not been well studied. Using the superdiffusive spin transport model, we demonstrate how spin accumulation is generated in NM metals after laser excitation in an $\mathrm{FM}/\mathrm{NM}$ bilayer. The spin accumulation shows an exponential decay from the $\mathrm{FM}/\mathrm{NM}$ interface, with the decay length increasing to the maximum value and then decreasing until saturation. By analyzing the ultrafast dynamics of laser-excited hot electrons, the “effective mean free path,” which can be characterized by the averaged product of the group velocity and lifetime of hot electrons, is found to play a key role. The interface reflectivity modulates the spin accumulation near the $\mathrm{FM}/\mathrm{NM}$ interface and varies the spin decay length quantitatively.

In this work we investigate the stability of an algebraically localized phase subject to periodic driving. First, we focus on a noninteracting model exhibiting algebraically localized single-particle modes. For this model we find numerically that the algebraically localized phase is stable under driving, meaning that the system remains localized at arbitrary frequencies. We support this result with analytical considerations using simple renormalization group arguments. Second, we inspect the case in which short-range interactions are added. By studying both the eigenstate properties of the Floquet Hamiltonian and the out-of-equilibrium dynamics in the interacting model, we provide evidence that ergodicity is restored at any driving frequencies. In particular, we observe that for the accessible system sizes localization sets in at driving frequency that is comparable with the many-body bandwidth and thus it might be only transient, suggesting that the system might thermalize in the thermodynamic limit.

The newly discovered superconductor $\mathrm{U}{\mathrm{Te}}_2$ is a strong contender for a topological spin-triplet state wherein a multicomponent order parameter arises from two nearly degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that $\mathrm{U}{\mathrm{Te}}_2$ at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature ($T_c$). Notably, samples with higher $T_c$ only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line (constant pressure) in the pressure-temperature phase diagram but also provide a consistent scenario for the sample dependence of $\mathrm{U}{\mathrm{Te}}_2$.

In the physics of the fractional quantum Hall (FQH) effect, a zoo of Abelian topological phases can be obtained by varying the magnetic field. Aiming to reach the same phenomenology in spin like systems, we propose a family of $\mathrm{SU}\left(N\right)$-symmetric models in the fundamental representation, on the square lattice with short-range interactions restricted to triangular units, a natural generalization for arbitrary $N$ of an SU(3) model studied previously where time-reversal symmetry is broken explicitly. Guided by the recent discovery of ${\mathrm{SU}\left(2\right)}_1$ and ${\mathrm{SU}\left(3\right)}_1$ chiral spin liquids (CSL) on similar models we search for topological $\mathrm{SU}({N)}_1$ CSL in some range of the Hamiltonian parameters via a combination of complementary numerical methods such as exact diagonalizations (ED), infinite density matrix renormalization group (iDMRG) and infinite Projected Entangled Pair State (iPEPS). Extensive ED on small (periodic and open) clusters up to $N=10$ and an innovative $\mathrm{SU}\left(N\right)$-symmetric version of iDMRG to compute entanglement spectra on (infinitely long) cylinders in all topological sectors provide unambiguous signatures of the $\mathrm{SU}({N)}_1$ character of the chiral liquids. An SU(4)-symmetric chiral PEPS, constructed in a manner similar to its SU(2) and SU(3) analogs, is shown to give a good variational ansatz of the $N=4$ ground state, with chiral edge modes originating from the PEPS holographic bulk-edge correspondence. Finally, we discuss the possible observation of such Abelian CSL in ultracold atom setups where the possibility of varying $N$ provides a tuning parameter similar to the magnetic field in the physics of the FQH effect.

The recent discovery and studies of 1144-phase compounds, e.g., $\mathrm{Ca}\mathrm{K}{\mathrm{Fe}}_4{\mathrm{As}}_4$, have attracted significant research interest. In this paper, based on the first-principles density functional calculations, we present a systematic study on the electronic structure of the recently synthetized 1144-type quaternary compound $\mathrm{Ca}\mathrm{K}{\mathrm{Ru}}_4{\mathrm{P}}_4$. We find that the Ru-based 1144-type compound possesses a different electronic structure from that of iron-based superconductors, even though they share very similar crystallographic structures. In $\mathrm{Ca}\mathrm{K}{\mathrm{Ru}}_4{\mathrm{P}}_4$, there is no hole-type carrier if spin-orbit interaction is not considered. And a long-range magnetic order is absent in its ground state. With the application of pressure, the electronic structure of $\mathrm{Ca}\mathrm{K}{\mathrm{Ru}}_4{\mathrm{P}}_4$ becomes similar to those of the ternary 122-type compounds $\mathrm{La}{\mathrm{Ru}}_2{\mathrm{P}}_2$ and $\mathrm{La}{\mathrm{Ru}}_2{\mathrm{As}}_2$. $\mathrm{Ca}\mathrm{K}{\mathrm{Ru}}_4{\mathrm{P}}_4$ is very likely to be a phonon-mediated medium coupled BCS superconductor. Furthermore, type-I and type-II Dirac fermions can be created and regulated in this system with pressure. The quaternary compound $\mathrm{Ca}\mathrm{K}{\mathrm{Ru}}_4{\mathrm{P}}_4$ therefore has a potential to be an attractive platform for the study of topological physics and superconductivity.

Four crystal classes are optically active yet not chiral (nonenantiomorphic). Corresponding Bragg diffraction patterns calculated for circularly polarized (helical) x-rays tuned to an atomic resonance display angular anisotropy in the distribution of electrons at spots not indexed on the chemical structure (defined by a space group derived from Thomson scattering by perfect spheres of charge). Templeton-Templeton scattering, as it is usually called, of helical x-rays is quantified in terms of a chiral signature defined as the partial diffracted intensity hallmarked by x-ray helicity. Our electric dipole-electric dipole (E1-E1) chiral signature for a space group in the optically active crystal class $\overline{4}2m$ $\left(D_{2\mathrm{d}}\right)$ affords a complete interpretation in terms of copper quadrupoles of diffraction data collected on copper metaborate [Ovchinnikova et al., J. Synchrotron Radiat. 28, 1455 (2021)]. An example of a chiral signature derived from a parity-odd resonance event (E1-E2) is included in our study. It is firmly established that tuning the energy of x-rays to an atomic resonance enhances the intensity of inherently weak Templeton-Templeton scattering and renders space-group forbidden Bragg spots element specific. Our chiral signature as a function of rotation about the reflection vector (azimuthal-angle scan) is specific to position multiplicity, Wyckoff letter and symmetry in a favorable space group.