Cerium, in which the $4f$ valence electrons live on the brink between localized and itinerant characters, exhibits varying crystal structures and therefore anomalous physical properties with respect to temperature and pressure. Understanding its electronic structure and related lattice properties is one of the central topics in condensed matter theory. In the present paper, we employed the state-of-the-art first-principles many-body approach (i.e., the density functional theory in combination with the single-site dynamical mean-field theory) to thoroughly study its electronic structure. The momentum-resolved spectral functions, total and $4f$ partial density of states, optical conductivities, self-energy functions, and atomic eigenstate histograms for cerium's four allotropes under ambient pressure were calculated and analyzed carefully. The calculated results demonstrate that the $4f$ electrons in the $\alpha ,\beta ,\gamma$, and $\delta$ phases are all correlated with heavily renormalized electron masses. In the $\alpha$ phase, the $4f$ electrons tend to be itinerant, which causes strong hybridization between the $4f$ and $spd$ bands and a remarkable $4f$ valence state fluctuation, while for the other phases, the $4f$ electrons are close to being localized. Our calculated results support the Kondo volume collapse scenario for the cerium $\alpha \text{−}\gamma$ transition. Finally, we examined the site dependence of the $4f$ electronic structure in the $\beta$ phase. The calculated results suggest that it does not exhibit a site-selective $4f$ localized state.

Interference of transmitted and received signals hinders the simultaneous functionality of a conventional optical antenna as a transmitter and receiver which is required for full-duplex communication. The full-duplex communication schemes enabled by dense wavelength division multiplexed optical networks require distinct transmitter/receiver components operating at different wavelengths which increase the cost, complexity, and footprint of the physical layer. In this work, we demonstrate that an array of nanoantennas with leaky-wave architecture based on spatiotemporal modulation establishes nonreciprocal optical links which can reject the interference of transmitted and received signals by isolating the frequency of transmission and reception modes. For this purpose, we integrate indium-tin oxide into plasmonic nanodipoles which allows for realization of time-modulated nanoantennas in near-infrared frequency regime through electrical modulation of charge carrier density with radio-frequency signals. The radiation characteristics of individual nanoantennas and modal properties of nanoantenna arrays are rigorously studied through linking of charge transport and electromagnetic models. To this end, we extend the formulation of discrete dipole approximation as the standard modeling tool for electromagnetic scattering from nanoantenna arrays to treat realistic time-modulated structures with drastically different timescales between optical and modulation frequencies. The operation of spatiotemporally modulated array antennas in transmission and reception modes is investigated. Moreover, electrical beam-scanning functionality and dependence of antenna characteristics to modulation parameters and wavelength are demonstrated. It is rigorously established that such array antennas can operate as full transceivers by separating the transmitted and received signals propagating along the same direction through down conversion and up conversion of the frequency. Our results provide a route toward realization of optical antenna systems capable of full-duplex communication and real-time beam scanning which can increase the capacity and decrease the complexity of optical networks.

Recently, signatures of high-Tc superconductivity observed in potassium (K)-doped $p$-terphenyl $\left({\mathrm{C}}_{18}{\mathrm{H}}_{14}\right)$ have sparked people's interest in phenyl-based materials. Here we report a low-temperature scanning tunneling microscopy/spectroscopy study on K-doped single-layer $p$-terphenyl films grown on Au (111) substrate. We observe several ordered phases with different morphologies and electronic behavior, in two of which a clear low-energy gap of about 11 meV opens below 50 K. In particular, the gap shows no obvious response to a magnetic field up to 11 T, which excludes the possibility that it is a superconducting gap. Such gapped phases are rarely (if ever) observed in hydrocarbon molecular crystals; several candidates for the origin are discussed. Our work also paves the way for fabricating conducting two-dimensional hydrocarbon materials, which would provide a platform for searching emergent phenomena.

The dependence of high-harmonic generation (HHG) on laser ellipticity is investigated using a modified ZnO model. In the driving of a relatively weak field, we reproduce qualitatively the ellipticity dependence as observed in the HHG experiment of wurtzite ZnO. When increasing the field strength, the HHG shows an anomalous ellipticity dependence, similar to that observed experimentally in the single-crystal MgO. With the help of a semiclassical analysis, it is found that the key mechanism inducing the change in ellipticity dependence is the interplay between the dynamical Bloch oscillation and the anisotropic band structure. The dynamical Bloch oscillation contributes additional quantum paths, which are less sensitive to ellipticity. The anisotropic band structure makes the driving pulse with finite ellipticity capable of driving the pairs to the band positions with a larger gap, which extends the harmonic cutoff. The combination of these two effects leads to the anomalous ellipticity dependence. The result reveals the importance of dynamical Bloch oscillations for the ellipticity dependence of HHG from bulk ZnO.

A robust approach to excitation and control of large amplitude standing magnetization waves in an easy axis ferromagnetic by starting from a ground state and passage through resonances with chirped frequency microwave or similar alternative drives (spin torque, additional periodic anisotropy) is proposed. The formation of these waves involves two stages, where in the first stage, a spatially uniform, precessing magnetization is created via passage through a resonance followed by a self-phase-locking (autoresonance) with a constant amplitude drive. In the second stage, the passage through an additional resonance with a spatial modulation of the driving amplitude yields transformation of the uniform solution into a doubly phase-locked standing wave, whose amplitude is controlled by the variation of the driving frequency. The stability of this excitation process is analyzed both numerically and via Whitham's averaged variational principle.

Quantum spin liquids (QSLs) are exotic phases with intrinsic massive entanglements. Instead of microscopic spins, fractionalized particles and gauge fluctuations are emergent, revealing QSLs' exotic natures. Quantum spins with strong spin-orbit coupling on a pyrochlore lattice, for example ${\mathrm{Pr}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$, are suggested to host a U(1) QSL with emergent photons, gapless excitations, as well as emergent monopoles. One of the key issues in QSLs is an interplay between emergent degrees of freedom of QSLs and conventional degrees of freedom, and we investigate the interplay by constructing a general theory of spin-lattice coupling in U(1) QSLs. We find that the coupling induces characteristic interplay between phonons and photons. For example, photons become qualitatively more stable than phonons at low temperature. We also propose mechanisms to detect emergent photons in experiments such as sound attenuation and thermal transport relying on spin-lattice coupling in U(1) QSLs.

In many unconventional superconductors the pairing of electrons is driven by the repulsive interaction, which leads to the sign reversal of superconducting gaps along the Fermi surfaces or between them. However, to measure this sign change is not easy and straightforward. It is known that, in superconductors with sign reversal gaps, nonmagnetic impurities can break Cooper pairs leading to the quasiparticle density of states in the superconducting state. The standing waves of these quasiparticles will interfere with each other leading to the quasiparticle interference (QPI) pattern which carries the phase message reflecting also the superconducting gap structure. Based on the recently proposed defect-bound-state QPI technique, we explore the applicability of this technique to a typical iron-based superconductor ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ with roughly equivalent gap values on the electron and hole pockets connected by the wave vector ${\mathbf{q}}_2=(0,\pi )$. It is found that, on the negative energy side, with the energy slightly below the gap value, the phase reference quantity $\left|g(\mathbf{q},-E)\right|cos({\theta }_{\mathbf{q},+E}-{\theta }_{\mathbf{q},-E})$ becomes negative and the amplitude is strongly enhanced with the scattering vector ${\mathbf{q}}_2$, but that corresponding to the scattering between the electron-electron pockets, namely ${\mathbf{q}}_3=(\pi ,\pi )$, keeps all positive. This is well consistent with the theoretical expectation of the $s^{\pm }$ pairing gap and thus serves as a direct visualization of the sign reversal gaps. This experimental observation is also supported by the theoretical calculations with the Fermi surface structure and $s^{\pm }$ pairing gap.

We investigate the impact of topology on the existence of impurity subgap states in a time-reversal-invariant superconductor with an extended $s$-wave pairing and strong spin-orbit coupling. By simply tuning the chemical potential, we access three distinct phases: topologically trivial $s$-wave, topologically nontrivial $s_{\pm }$-wave, and nodal superconducting phase. For a single potential impurity, we find subgap impurity bound states in the topological phase, but notably no subgap states in the trivial phase. This is in sharp contrast with the expectation that there would be no subgap state in the presence of potential impurities in $s$-wave superconductors. These subgap impurity states have always finite energies for any strength of the potential scattering and, subsequently, the superconducting gap in the topological $s_{\pm }$-wave phase survives but is attenuated in the presence of finite disorder. By creating islands of potential impurities, we smoothly connect the single impurity results to topological edge states of impurity islands. On the other hand, magnetic impurities lead to the formation of Yu-Shiba-Rusinov states in both the trivial and topological phases, which even reach zero energy at certain scattering strengths. We thus propose that potential impurities can be a very valuable tool to detect time-reversal-invariant topological superconductivity.

We study the spin transport theoretically in heterostructures consisting of a ferromagnetic metallic thin film sandwiched between heavy-metal and oxide layers. The spin current in the heavy-metal layer is generated via the spin Hall effect, whereas the oxide layer induces at the interface with the ferromagnetic layer a spin-orbital coupling of the Rashba type. Impact of the spin-Hall effect and Rashba spin-orbit coupling on the spin-Seebeck current is explored with a particular emphasis on nonlinear effects. Technically, we employ the Fokker-Planck approach and contrast the analytical expressions with full numerical micromagnetic simulations. We show that, when an external magnetic field $H_0$ is aligned parallel (antiparallel) to the Rashba field, the spin-orbit coupling enhances (reduces) the spin pumping current. In turn, the spin-Hall effect and the Dzyaloshinskii-Moriya interaction are shown to increase the spin pumping current.

Hysteresis loops and the associated avalanche statistics of spin systems, such as the random-field Ising and Edwards-Anderson spin-glass models, have been extensively studied. A particular focus has been on self-organized criticality, manifest in power-law distributions of avalanche sizes. Considerably less work has been done on the statistics of the times between avalanches. This paper considers this issue, generalizing the work of Nampoothiri et al. [Phys. Rev. E 96, 032107 (2017)] in one space dimension to higher space dimensions. In addition to the interevent statistics of all avalanches, we also consider what happens when events are restricted to those exceeding a certain threshold size. Doing so raises the possibility of altering the definition of time to count the number of small events between the large ones, which provides for an analog to the concept of natural time introduced by the geophysics community with the goal of predicting patterns in seismic events. We analyze the distribution of time and natural time intervals both in the case of models that include only nearest-neighbor interactions, as well as models with (sparse) long-range couplings.

We report Mn and Co substitution at the Fe site in the tetragonal checkerboard Mott insulator ${\mathrm{Sr}}_2{\mathrm{F}}_2{\mathrm{Fe}}_2{\mathrm{OS}}_2$, which develops an antiferromagnetic two-$\mathbf{k}$ spin vortex crystal at $T_N=110$ K. While Co substitution increases $T_N$ to 127 K $\left(x=0.3\right)$ in ${\mathrm{Sr}}_2{\mathrm{F}}_2{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{OS}}_2$, Mn substitution suppresses $T_N$ for $x$ up to 0.2 in ${\mathrm{Sr}}_2{\mathrm{F}}_2{\left({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\right)}_2{\mathrm{OS}}_2$ replacing the vortex crystal with a spin glass for $0.2<x<0.5$. The activation gap determined from resistivity has a local minimum of $E_g=0.18$ eV for $x=0.5$ where a new phase appears at $T_N=45$ K. Neutron diffraction data along with corroborating magnetic evidence shows this is a $G$-type collinear antiferromagnet, with a transition temperature increasing for $0.5<x<0.8$. Density functional theory calculations point to the correlated nature of the transition metal $d$ bands: A modest Hubbard energy $U=2$ eV is sufficient to open a Mott gap around the Fermi level. The calculations also correctly reproduce the ordered spin structure of ${\mathrm{Sr}}_2{\mathrm{F}}_2{\mathrm{Fe}}_2{\mathrm{OS}}_2$ and ${\mathrm{Sr}}_2{\mathrm{F}}_2{\mathrm{FeMnOS}}_2$. A deeper understanding of the nature of the magnetic ordering is obtained within an effective theoretical model of local moments with doping-dependent competing exchange interactions $J_1\text{−}{J}_2\text{−}{J}_2^{\prime }$.

We report the magnetic and calorimetric measurements in single-crystal samples of the square lattice $J_1\text{−}{J}_2$ quantum antiferromagnet $\mathrm{BaCdVO}{\left({\mathrm{PO}}_4\right)}_2$. An investigation of the scaling of magnetization reveals a “dimensionality reduction” indicative of a strong degree of geometric frustration. Below a characteristic temperature of $T^{*}\simeq 150$ mK we observe the emergence of an additional strongly fluctuating quantum phase close to full magnetic saturation. It is separated from the magnetically ordered state by first- and second-order phase transitions, depending on the orientation of the applied magnetic field. We suggest that this phase may indeed be related to the theoretically predicted spin-nematic state.

Unconventional superconductivity frequently emerges as the transition temperature of a magnetic phase, typically antiferromagnetic (AFM), is suppressed continuously toward zero temperature. Here, we report contrary behavior in pressurized $\mathrm{CeRhG}{\mathrm{e}}_3$, a noncentrosymmetric heavy-fermion compound. We find that its pressure-tuned AFM transition temperature ($T_N$) appears to avoid a continuous decrease to zero temperature by terminating abruptly above a dome of pressure-induced superconductivity. Near 21.5 GPa, evidence for $T_N$ suddenly vanishes, the electrical resistance becomes linear in temperature, and the superconducting transition reaches a maximum. Analysis of high-pressure resistance and x-ray-absorption spectroscopy measurements suggest that the anomalous connection between antiferromagnetic and superconducting phases in pressurized $\mathrm{CeRhG}{\mathrm{e}}_3$ is associated with proximity to a critical valence instability.

We consider an atomic chain of magnetic impurities on the surface of a spin-orbit coupled superconductor with a dominating $d$-wave and subdominating $s$-wave order parameters. In particular, we investigate the properties of the Majorana bound states (MBSs) emerging at the chain end points in the topological phase and how MBSs are affected by the $d$-wave order parameter. We provide a comprehensive picture by both studying time-reversal invariant and breaking superconducting substrates as well as chains oriented in different directions relative to the $d$-wave rotation. We show that increasing the $d$-wave order parameter significantly enhances the localization of MBSs and their protective minigap, as long as the direction along which the impurity chain is oriented does not cross any nodal lines of the gap function. Moreover, we find an extra gap-closing for a specific condensate and chain orientation within the topological phase, which we are able to attribute to simple geometrical effects in the corresponding two-dimensional limit. These results show how high-temperature $d$-wave superconductors can be used to significantly enhance the properties and stability of MBSs.

Using the mean-field approximation, we study the $k$-space spin textures and local spin currents emerged in the spin-triplet excitonic insulator states of the two-band Hubbard model defined on the square and triangular lattices. We assume a noninteracting band structure with a direct band gap and introduce $s$-, $p$-, $d$-, and $f$-type cross-hopping integrals, i.e., the hopping of electrons between different orbitals on adjacent sites with four different symmetries. First, we calculate the ground-state phase diagrams in the parameter space of the band filling and interaction strengths, whereby we present the filling dependence of the amplitude and phase of the excitonic order parameters. Then, we demonstrate that the spin textures (or asymmetric band structures) are emerged in the Fermi surfaces by the excitonic symmetry breaking when particular phases of the order parameter are stabilized. Moreover, in the case of the $p$-type cross-hopping integrals, we find that the local spin current can be induced spontaneously in the system, which does not contradict the Bloch theorem for the absence of the global spin current. The proofs of the absence of the global spin current and the possible presence of the local spin currents are given on the basis of the Bloch theorem and symmetry arguments.

We present an algorithm to evaluate Matsubara sums for Feynman diagrams composed of bare Green's functions with single-band dispersions and local $U$ Hubbard interaction vertices. The algorithm provides an exact construction of the analytic result for the frequency integrals of a diagram that can then be evaluated for all parameters $U$, temperature $T$, chemical potential $\mu$, external frequencies, and internal/external momenta. This method allows for symbolic analytic continuation of results to the real frequency axis, avoiding any ill-posed numerical procedure. This method can also be used to simultaneously evaluate diagrams throughout the entire $T\text{−}U\text{−}\mu$ phase space of Hubbard-like models even in the $T=0$ limit at minimal computational expense.

The charge carrier dynamics of doped electronic correlated systems on ladders and chains, subject to ultrafast photoirradiation, is investigated using the time-dependent Lanczos method. The time-resolved optical conductivity and the temporal profiles of other relevant quantities, including the doublon number, the kinetic energy, and the interaction energy, are calculated. Two competitive factors that can influence the transient charge carrier dynamics are identified as the thermal effect and the charge effect. We demonstrate that the analysis of their interplay can provide an intuitive way to understand the numerical results and the recent optical pump-probe experiment on a two-leg ladder cuprate.

We demonstrate an effective epitaxial route for the manipulation and further enrichment of the intriguing spin-dependent phenomena boasted by germanium. We show optical initialization and readout of spins in Ge-rich germanium-tin alloys and report on spin quantum beats between Zeeman-split levels under an external magnetic field. While heavy Sn atoms can be readily utilized to strengthen the spin-orbit coupling, our experiments reveal robust spin orientation in a wide temperature range and a persistent spin lifetime that noticeably approaches the nanosecond regime at room temperature. In addition, time decay photoluminescence experiments evidence a temperature-induced monotonic decrease of the carrier lifetime, eventually providing crucial insights also into nonradiative recombination mechanisms.

In this paper, we have studied two-dimensional (2D) electron systems described by the effective Hamiltonians containing spin-orbit coupling (SOC) terms up to an arbitrary odd order in wave vector k. The general condition for realizing a SOC-induced effective magnetic field (SOF) in such systems, formulated only in terms of the SOC parameters, is derived. When this condition is satisfied, the projection of the electron spin on the direction of the collinear SOF is a conserved quantity. The complete set of the k-linear and k-cubic Dresselhaus SOC contributions to the effective 2D Hamiltonian of an arbitrarily oriented zinc-blende quantum well is computed by a proper averaging of the corresponding tight-binding bulk SOC Hamiltonian. We investigate possibilities for realization of the collinear SOF in zinc-blende quantum wells of different orientation and obtain some interesting findings, which supplement the results of earlier works. Application of the developed formalism to wurtzite semiconductor 2D systems shows that the collinear SOF can be also realized in a wide class of such quantum wells.

Using density-functional theory calculations, the atomic and electronic structure of single-layer ${\mathrm{WS}}_2$ attached to Zr and Co contacts are determined. Both metals form stable interfaces that are promising as contacts for injection of $n$-type carriers into the conduction band of ${\mathrm{WS}}_2$ with Schottky barriers of 0.45 eV and 0.62 eV for Zr and Co, respectively. With the help of quantum transport calculations, we address the conductive properties of a freestanding ${\mathrm{WS}}_2$ sheet suspended between two Zr contacts. It is found that such a device behaves like a diode with steep I–V characteristics. Spin-polarized transport is calculated for such a device with a floating-gate Co electrode added. Depending on the geometrical shape of the Co gate and the energy of the carriers in ${\mathrm{WS}}_2$, the transmission of spin majority and minority electrons may differ by up to an order of magnitude. Thus the steep I–V characteristics of the nanoscale device makes it possible to realize a spin filter.