In this paper, research has been carried out on the electronic properties of nanostructured graphene. We focus our attention on trapped states of the proposed systems such as spherical and toroidal graphene quantum dots (GQDs). Using a continuum model, by solving the Dirac-Weyl equation, and applying periodic boundary conditions of two types, i.e., either with zigzag edges only or with both armchair and zigzag edges, we obtain analytical results for energy levels yielding self-similar energy bands located subsequently one after another on the energy scale. Only for the toroidal quantum dot (owing to the lack of curvature) the distribution of electron density is like Bohr atomic orbitals. However, although the quasi-zero-energy band exists for both spherical and toroidal quantum dots, no electron density is present on this band for the toroidal quantum dot. This causes the formation of a pseudogap between the hole and electron bands because of the absence of the electron density at the quantum dot center, like in the case of an ordinary atom. Conversely, the confinement of the charge-carrier density is observed for both geometries of GQDs.

Lattice gauge theories are a fascinating and rich class of theories relating to the most fundamental models of particle physics, and as experimental control on the quantum level increases there is a growing interest in nonequilibrium effects such as dynamical quantum phase transitions. To demonstrate how these physical theories can be accessed in near-term quantum devices, we study the dynamics of a $(1+1)\mathrm{D}$ U(1) quantum link model following quenches of its mass term. We find that the system undergoes dynamical quantum phase transitions for all system sizes considered, even the smallest where the dynamics can be solved analytically. We devise a gauge-invariant string order parameter whose zeros correlates with the structure of the Loschmidt amplitude, making the order parameter useful for experimental study in near-term devices. The zeros of the Loschmidt amplitude as well as the zeros of our order parameter are revealed by vortices in their phases, which can be counted by a topologically invariant winding number. With noisy intermediate-scale quantum devices in mind, we propose a class of superconducting circuits for the general implementation of U(1) quantum link models. The principles of these circuits can be generalized to implement other, more complicated gauge symmetries. Furthermore, the circuit can be modularly scaled to any lattice configuration. Simulating the circuit dynamics with realistic circuit parameters, we find that it implements the target dynamics with a steady average fidelity of $99.5\%$ or higher. Finally, we consider readout of the circuit using a method that yields information about all the degrees of freedom with resonators coupled dispersively to only a subset of them. This constitutes a direct and relatively straightforward protocol to access both Loschmidt amplitudes and the order parameter.

The Holstein Hamiltonian describes fermions hopping on a lattice and interacting locally with dispersionless phonon degrees of freedom. In the low-density limit, dressed quasiparticles, polarons and bipolarons, propagate with an effective mass. At higher densities, pairs can condense into a low-temperature superconducting phase and, at or near commensurate filling on a bipartite lattice, to charge density wave (CDW) order. CDW formation breaks a discrete symmetry and hence occurs via a second-order (Ising) transition and therefore at a finite $T_{\mathrm{cdw}}$ in two dimensions. Quantum Monte Carlo calculations have determined $T_{\mathrm{cdw}}$ for a variety of geometries, including square, honeycomb, and Lieb lattices. The superconducting transition, on the other hand, in $d=2$ is in the Kosterlitz-Thouless universality class and is much less well characterized. In this paper we determine $T_{\mathrm{sc}}$ for the square lattice for several values of the density $\rho$ and phonon frequency ${\omega }_0$. We find that quasilong-range order sets in at $T_{\mathrm{sc}}\lesssim t/20$, where $t$ is the near-neighbor hopping amplitude, consistent with previous rough estimates from simulations which extrapolated to only the temperatures we reach from considerably higher $T$. We also show evidence of a discontinuous evolution of the density as the CDW transition is approached at half filling.

By means of high-pressure resistivity measurements on single crystals, we investigate the charge transport properties of ${\mathrm{Cu}}_x{\mathrm{PdTe}}_2$, notable for the combination of topological type-II Dirac semimetallic properties with superconductivity up to $T_c=2.5\mathrm{K}$. In both pristine $(x=0)$ and intercalated $(x=0.05)$ samples, we find an unconventional $T^4$ power law behavior of the low-temperature resistivity visible up to $\sim 40$ K and remarkably stable under pressure up to 8.2 GPa. This observation is explained by the low carrier density $n$, which strongly reduces the $k$ region available for electron-phonon scattering, as previously reported in other low-$n$ two-dimensional systems, such as multilayer graphene and semiconductor heterostructures. Our data analysis complemented by specific heat measurements and supported by previous quantum oscillation studies and ab initio calculations suggests a scenario of one-band charge transport. Within this scenario, our analysis yields a large value of the transport electron-phonon coupling constant ${\lambda }_{tr}=1.2$ at ambient pressure that appears to be strongly enhanced by pressure assuming a constant effective mass.

The magnetism, magnetotransport, and Seebeck coefficients $\left(S\right)$ for three ruthenates ${\mathrm{Ba}}_{1-\delta }{M}_{2+x}{\mathrm{Ru}}_{4-x}{\mathrm{O}}_{11}$ $(\delta =0.06;$ $M=\mathrm{Mn}, \mathrm{Co}; x=0.4)$ and ${\mathrm{Sr}}_{1-\delta }{M}_{2+x}{\mathrm{Ru}}_{4-x}{\mathrm{O}}_{11}$ $(\delta =0.02; M=\mathrm{Fe}; x=0.7)$ compositions have been studied. Their crystallographic structures contain three metal sites, edge-sharing octahedra forming kagome lattices, face-shared octahedra with the shortest $\mathrm{Ru}\left(M\right)\text{−}\mathrm{Ru}\left(M\right)$ distance, and ${M\mathrm{O}}_5$ trigonal bipyramids. These three compositions have been selected for their transport behavior exhibiting small resistivity values (∼mΩ cm) together with a complex ferrimagnetic behavior, with localization increasing from $M=\mathrm{Co}$ to $M=\mathrm{Fe}$. This enabled the thermopower to be measured in hexagonal ruthenates in which the conducting kagome layers are more or less diluted by three different magnetic cations substituted for Ru. The positive Seebeck coefficient of the three compounds is found to increase up to 750 K to values in the range of 22 to $35\mu \mathrm{V}{\mathrm{K}}^{\text{–}1}$. Such values, similar to those of perovskite ruthenates, reveal a Seebeck coefficient dominated by the Ru network at high temperature whatever the foreign magnetic cation is. In addition, below about 50 K, the values of $S$ are very small for $M=\mathrm{Mn}$ and Co, and the $S\left(T\right)$ curves of the ${\mathrm{Ba}}_{1-\delta }{M}_{2.4}{\mathrm{Ru}}_{3.6}{\mathrm{O}}_{11}$ compounds exhibit similarities with that of ruthenium metal. This is interpreted by shorter Ru-Ru distances as compared with perovskite ruthenates allowing a metallic direct exchange. The ferrimagnetism associated with the $M$ cation does not seem to play a major role in transport, as there is almost no impact of the magnetic ordering on thermopower and electrical resistivity and the values of magnetoresistance remain very small, reaching at most −1% in 9 T at 5 K for $M=\mathrm{Mn}$, and −0.4% at $T_{\mathrm{C}}$ for $M=\mathrm{Co}$. The present results obtained in these phases containing hexagonal Ru networks show that Hund's metal model developed to describe the thermopower of perovskite ruthenates with a Ru square lattice can have a broader range of validity.

We study the transmission of narrow electromagnetic beams associated to different $K$ valleys, from both the armchair and zigzag edges, in a graphenelike triangular metallic photonic crystal. Due to the metallic nature of the structure, the propagation of an electromagnetic wave, with the wave vector close to the quasitriangular isofrequency contours of the Dirac cones resulting from the band trigonal warping, can be considered as being based on a series of bonding or antibonding states, formed by the local resonance modes in the unit cells. The transmission of a narrow incident beam, of the width of the crystal period size, depends on the local structure configurations in the beam impact zone that determine the distribution of the excited local resonance modes as well as the coupling between the latter and the incident field, thus the beam propagation in different directions. This investigation allows us to understand the interaction between an incident field and the resonance states associated to the Dirac cones at elementary structure scales reaching the size of these states. It can find application in valley photonics for transport tuning and optimization.

The interplay of optical driving and hyperfine interactions between an electron confined in a quantum dot and its surrounding nuclear spin environment produces a range of interesting physics stemming from the creation of dynamic nuclear polarization. In this paper, we go beyond the ubiquitous spin-1/2 approximation for nuclear spins and present a comprehensive theoretical framework for an optically driven electron spin in a self-assembled quantum dot coupled to a nuclear spin bath of arbitrary spin. Using a dynamical mean-field approach, we compute the nuclear spin polarization distribution with and without the quadrupolar coupling. We find that while hyperfine interactions drive dynamic nuclear polarization in such a way that the electron spin precession becomes synchronized with the driving, quadrupolar couplings counteract this phenomenon. The tension between these mechanisms is imprinted on the steady-state electron spin evolution, providing a way to measure the importance of quadrupolar interactions in a quantum dot. Our results show that higher-spin effects such as quadrupolar interactions can have a significant impact on the generation of dynamic nuclear polarization and how it influences the electron spin evolution.

We obtain an analytical expression for the linewidth of the $1s$ exciton as a function of temperature in transition metal dichalcogenides. The total linewidth, as a function of temperature, is dominated by three contributions: (i) the radiative decay (essentially temperature independent), (ii) the phonon-induced intravalley scattering, and (iii) the phonon-induced intervalley scattering. Our approach uses a variational Ansatz to solve the Wannier equation, allowing for an analytical treatment of the excitonic problem, including rates of the decay dynamics. Our results are in good agreement with experimental data already present in the literature and can be used to readily predict the value of the total linewidth at any temperature in the broad class of excitonic two-dimensional materials.

Time-dependent orbital-free density functional theory (DFT) is an efficient method for calculating the dynamic properties of large-scale quantum systems due to the low computational cost compared to standard time-dependent DFT. We formalize this method by mapping the real system of interacting fermions onto a fictitious system of noninteracting bosons. The dynamic Pauli potential and associated kernel emerge as key ingredients of time-dependent orbital-free DFT. Using the uniform electron gas as a model system, we derive an approximate frequency-dependent Pauli kernel. Pilot calculations suggest that space nonlocality is a key feature for this kernel. Nonlocal terms arise already in the second-order expansion with respect to unitless frequency and reciprocal space variable ($\frac{\omega }{q{k}_F}$ and $\frac{q}{2k_F}$, respectively). Given the encouraging performance of the proposed kernel, we expect it will lead to more accurate orbital-free DFT simulations of nanoscale systems out of equilibrium. Additionally, the proposed path to formulate nonadiabatic Pauli kernels presents several avenues for further improvements which can be exploited in future work to improve the results.

We calculate the exact time-resolved ARPES spectrum of a two-band model semiconductor driven out of equilibrium by resonant and nonresonant laser pulses, highlighting the effects of phonon-induced decoherence and relaxation. Resonant excitations initially yield a replica of the valence band shifted upward by the energy of the exciton peak in photoabsorption. This phase is eventually destroyed by phonon-induced decoherence: The valence-band replica lowers in energy by the Stokes shift, locating at the energy of the exciton peak in photoluminescence, and its width grows due to phonon dressing. Nonresonant excitations initially yield a map of the conduction band. Then electrons transfer their excess energy to the lattice and bind with the holes left behind to form excitons. In this relaxed regime a replica of the conduction band appears inside the gap. At fixed momentum the lineshape of the conduction-band replica versus the photoelectron energy is proportional to the exciton wave function in “energy space” and it is highly asymmetric. Although the two-band model represents an oversimplified description of real materials, the highlighted features are qualitative in nature; hence they provide useful insights into time-resolved ARPES spectra and their physical interpretation.

We have carried out detailed high-resolution angle-resolved photoemission spectroscopy measurements and band structure calculations to study the electronic structure of $\mathrm{Ca}\mathrm{Mn}{\mathrm{Sb}}_2$. The observed Fermi surface mainly consists of one hole pocket around the $\mathrm{\Gamma }$ point and one tiny hole pocket at the $Y$ point. Strong spectral weight accumulation along the $\mathrm{\Gamma }-X$ direction is observed on the holelike Fermi surface around the $\mathrm{\Gamma }$ point, suggesting strong anisotropy of the density of states along the Fermi surface. The tiny hole pocket at the $Y$ point originates from an anisotropic Dirac-like band with the crossing point of the linear bands lying $\sim 10$ meV above the Fermi level. These observations are in good agreement with the band structure calculations. In addition, we observe additional features along the $\mathrm{\Gamma }-Y$ line that cannot be accounted for by the band structure calculations. Our results provide important information in the understanding and exploration of novel properties in $\mathrm{Ca}\mathrm{Mn}{\mathrm{Sb}}_2$ and related materials.

The nonequilibrium properties of strongly correlated materials present a target in the search for new phases of matter. It is important to observe the types of excitations that exist in these materials and their associated relaxation dynamics. We have studied the photoexcitations in a spin-orbit assisted Mott insulator $\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$ using time-resolved two-photon photoemission spectroscopy and transient reflection spectroscopy. We find that photoexcited carriers (doublons) in the upper Hubbard band rapidly relax to Mott-Hubbard excitons on a timescale of less than 200 fs. Subsequently, further relaxation of these lower-energy quasiparticles occurs with an energy-dependent time constant of that ranges from 370 to 600 fs due to exciton cooling. The population of Mott-Hubbard excitons persists for timescales up to several microseconds.

We study the Hall response of topologically trivial mobile impurities (Fermi polarons) interacting weakly with majority fermions forming a Chern-insulator background. This setting involves a rich interplay between the genuine many-body character of the polaron problem and the topological nature of the surrounding cloud. When the majority fermions are accelerated by an external field, a transverse impurity current can be induced. To quantify this polaronic Hall effect, we compute the drag transconductivity, employing controlled diagrammatic perturbation theory in the impurity-fermion interaction. We show that the impurity Hall drag is not simply proportional to the Chern number characterizing the topological transport of the insulator on its own—it also depends continuously on particle-hole breaking terms, to which the Chern number is insensitive. However, when the insulator is tuned across a topological phase transition, a sharp jump of the impurity Hall drag results, for which we derive an analytical expression. We describe how to experimentally detect the polaronic Hall drag and its characteristic jump, setting the emphasis on the circular dichroism displayed by the impurity's absorption rate.

The interplay of interactions and disorder in low-dimensional superconductors supports the formation of multiple quantum phases as possible instabilities of the superconductor-insulator transition (SIT) at a singular quantum critical point. We explore a one-dimensional model which exhibits such a variety of phases in the strongly quantum fluctuations regime. Specifically, we study the effect of weak disorder on a two-leg Josephson ladder with comparable Josephson and charging energies $(E_J\sim E_C)$. An additional key feature of our model is the requirement of perfect ${\mathbb{Z}}_2$ symmetry, respected by all parameters including the disorder. Using a perturbative renormalization-group (RG) analysis, we derive the phase diagram and identify at least one intermediate phase between a full-fledged superconductor and a disorder-dominated insulator. Most prominently, for repulsive interactions on the rungs we identify two distinct mixed phases: In both of them the longitudinal charge mode is a gapless superconductor, however one phase exhibits a dipolar charge density order on the rungs, while the other is disordered. This latter phase is characterized by coexisting superconducting (phase-locked) and charge-ordered rungs, and encompasses the potential of evolving into a Griffith's phase characteristic of the random-field Ising model in the strong disorder limit.

The heterojunction between different materials often exhibits a rectifying effect; e.g., pn junction is used for diode. On the other hand, the Josephson junction between two different superconductors is assumed to show symmetric response between two directions of the current, i.e., the voltage drop $V$ is antisymmetric with respect to the sign change of the current $I$. However, there should be an asymmetry between the states of charge accumulation on the right and left sides of the Josephson junction, which can lead to the nonreciprocal responses. Here we demonstrate theoretically that nonreciprocal $I\text{−}V$ characteristic appears due to this charging energy difference both in the classical and quantum regimes.

We investigate the quantum entangled state of two protons terminating on a silicon surface. The entangled states were detected using the surface vibrational dynamics of nanocrystalline silicon with inelastic neutron scattering spectroscopy. The protons are identical, therefore the harmonic oscillator parity constrains the spin degrees of freedom, forming strongly entangled states for all the energy levels of surface vibrations. Compared to the proton entanglement previously observed in hydrogen molecules, this entanglement is characterized by an enormous energy difference of 113 meV between the spin singlet ground state and the spin triplet excited state. We theoretically demonstrate the cascade transition of terahertz entangled photon pairs utilizing proton entanglement. A combination of proton qubits and a modern silicon technology can result in a natural unification of computing platforms, thereby achieving unprecedented levels of massive parallelism processing.

We develop a theory of optical transitions in Coulomb impurity-doped two-dimensional transition metal dichalcogenide monolayers and study the transitions from the spin-resolved valence band to the (Coulomb) donor and acceptor impurities under the influence of a synthetic valley-selective magnetic field produced by a mechanical strain. It is shown that the optical properties of the system are determined by the strength of the synthetic magnetic field, which uncovers an experimental tool which can be used to manipulate the properties of two-dimensional materials in valley magneto-optoelectronics.

In a recent paper, Ibañez-Azpiroz et al. [Phys. Rev. B 97, 245143 (2018)] derive a band-truncation-error-free formula for calculating the generalized derivative of the interband dipole matrix using Wannier interpolation. In practice, the denominators involving intermediate states are regularized by introducing a finite broadening parameter. In this Comment, I show that when a finite broadening parameter is used, a correction term must be added to the generalized derivative to obtain results that are independent of the phase convention for the Bloch sums.