A time-dependent formulation for electron-hole excitations in extended finite systems, based on the Bethe-Salpeter equation (BSE), is developed using a stochastic wave function approach. The time-dependent formulation builds on the connection between time-dependent Hartree-Fock (TDHF) theory and the configuration-interaction with single substitution (CIS) method. This results in a time-dependent Schrödinger-like equation for the quasiparticle orbital dynamics based on an effective Hamiltonian containing direct Hartree and screened exchange terms, where screening is described within the random-phase approximation (RPA). To solve for the optical-absorption spectrum, we develop a stochastic formulation in which the quasiparticle orbitals are replaced by stochastic orbitals to evaluate the direct and exchange terms in the Hamiltonian as well as the RPA screening. This leads to an overall quadratic scaling, a significant improvement over the equivalent symplectic eigenvalue representation of the BSE. Application of the time-dependent stochastic BSE (TDsBSE) approach to silicon and CdSe nanocrystals up to size of $\approx 3000$ electrons is presented and discussed.

Cadmium telluride (CdTe) is a direct band-gap semiconducting material with broad applications in optoelectronic devices. Here we report on a high-resolution angle-resolved photoemission (ARPES) study of $\mathrm{CdTe}(111)$ surfaces prepared by sputtering and annealing that show a $(2\times 2)$ reconstruction as observed by electron diffraction. The ARPES maps along high-symmetry directions show prominent features with their intensities modulated by varying the incident photon energy, thus suggesting important matrix element effects associated with photoemission. The results are in excellent agreement with first-principles calculations of the bulk band structure and one-dimensional density of states. A prominent surface state is observed that exhibits a $(2\times 2)$ periodicity in agreement with the symmetry of the surface reconstruction.

The magnetic-field-containing relativistic tight-binding approximation (MFRTB) method [Phys. Rev. B 91, 075122 (2015)] is the first-principles calculation method for electronic structures of materials immersed in the magnetic field. In this paper, the MFRTB method is applied to the simple cubic lattice immersed in the magnetic field. The total energy and magnetization oscillate with the inverse of the magnitude of the magnetic field, which means that the de Haas–van Alphen oscillation is revisited directly through the MFRTB method. It is shown that the conventional Lifshitz-Kosevich (LK) formula is a good approximation to the results of the MFRTB method in the experimentally available magnetic field. Furthermore, the additional oscillation peaks of the magnetization are found especially in the high magnetic field, which cannot be explained by the LK formula.

We study a pair of quantum dot exciton qubits interacting with a number of fluctuating charges that can induce a Stark shift of both exciton transition energies. We do this by solving the optical master equation using a numerical transfer matrix method. We find that the collective influence of the charge environment on the dots can be detected by measuring the correlation between the photons emitted when each dot is driven independently. Qubits in a common charge environment display photon bunching, if both dots are driven on resonance or if the driving laser detunings have the same sense for both qubits, and antibunching if the laser detunings have opposite signs. We also show that it is possible to detect several charges fluctuating at different rates using this technique. Our findings expand the possibility of measuring qubit dynamics in order to investigate the fundamental physics of the environmental noise that causes decoherence.

We analyze the segregation behavior of bimetallic nanoparticles (NPs) with moderate miscibility gap by applying a combined experimental/simulation approach to Cu/Ni as a model system. Using a hybrid molecular dynamics/Metropolis Monte Carlo algorithm (MD/MMC) based on the embedded atom method (EAM), we derive the equilibrium distribution of atomic species in the clusters for varying concentrations and particle structures. To cross-check the predictive power of our approach, we compare these results with modified EAM (MEAM) and density functional theory based ab initio calculations. This permits to identify possible shortcomings of the EAM when used to describe bimetallic NPs. Additionally, we isolate vibrational entropy contributions to the free energy of the NPs and special focus is put on the possibility of entropic stabilization of segregated versus solid solution NPs. Finally, we complement our simulations by experimental results, which are obtained by studying the behavior of plasma gas condensation synthesized Ni@Cu core-shell (CS) particles upon annealing.

The electronic-transport properties of single-layer graphene that has a dilute coating of indium adatoms have been investigated. Our studies establish that isolated indium atoms donate electrons to graphene and become a source of charged impurity scattering, affecting the conductivity as well as magnetotransport properties of the pristine graphene. Notably, a positive magnetoresistance is observed over a wide density range after In doping. The low-field magnetoresistance carries signatures of quantum interference effects which are significantly altered by the adatoms.

We present a universal relation between the flow of a Renyi entropy and the full counting statistics of energy transfers. We prove the exact relation for a flow to a system in thermal equilibrium that is weakly coupled to an arbitrary time-dependent and nonequilibrium system. The exact correspondence, given by this relation, provides a simple protocol to quantify the flows of Shannon and Renyi entropies from the measurements of energy transfer statistics.

We study the effects of Rashba spin-orbit coupling on the spin torque induced by spin waves, which are the plane-wave dynamics of magnetization. The spin torque is derived from linear-response theory, and we calculate the dynamic spin torque by considering the impurity-ladder-sum vertex corrections. This dynamic spin torque is divided into three terms: a damping term, a distortion term, and a correction term for the equation of motion. The distorting torque describes a phenomenon unique to the Rashba spin-orbit coupling system, where the distorted motion of magnetization precession is subjected to the anisotropic force from the Rashba coupling. The oscillation mode of the precession exhibits an elliptical trajectory, and the ellipticity depends on the strength of the nesting effects, which could be reduced by decreasing the electron lifetime.

Superfluid ${}^4\mathrm{He}$ atoms are injected (withdrawn) asymmetrically into (from) a chamber filled with solid ${}^4\mathrm{He}$. Two in situ capacitance pressure gauges are located at the ends of the solid helium sample at different distances from where the superfluid meets the solid ${}^4\mathrm{He}$. The pressure change kinetics is slower at the more distant gauge. This demonstrates the presence of a mass flux bottleneck inside the solid helium sample. The temperature dependence of the maximum flux reveals a crossover and this is discussed in the context of phase slips on quasi-1D pathways.

The phenomenon of stable persistent currents is central to the studies of superfluidity in a range of physical systems. While most of the previous theoretical studies of superfluid flows in annular geometries concentrated on conservative systems, here we extend the dynamical stability analysis of persistent currents to open dissipative exciton-polariton superfluids. By considering an exciton-polariton condensate in an optically induced annular trap, we determine dynamical stability conditions for an initially imposed flow with a nonzero orbital angular momentum. We show, theoretically and numerically, that the system can sustain metastable persistent currents in a large parameter region, and describe scenarios of the supercurrent decay due to the dynamical instability.

A comparative density functional plus dynamical mean field theory study of the pseudocubic ruthenate materials ${\text{CaRuO}}_3$ and ${\text{SrRuO}}_3$ is presented. Phase diagrams are determined for both materials as a function of Hubbard repulsion $U$ and Hund's rule coupling $J$. Metallic and insulating phases are found, as are ferromagnetic and paramagnetic states. The locations of the relevant phase boundaries are determined. Based on the computed phase diagrams, Mott dominated and Hund's dominated regimes of strong correlation are distinguished. Comparison of calculated properties to experiments indicates that the actual materials are in the Hund's coupling dominated region of the phase diagram so can be characterized as Hund's metals, in common with other members of the ruthenate family. Comparison of the phase diagrams for the two materials reveals the role played by rotational and tilt $({\text{GdFeO}}_3$-type) distortions of the ideal perovskite structure. The presence of magnetism in ${\text{SrRuO}}_3$ and its absence in ${\text{CaRuO}}_3$ despite the larger mass and larger tilt/rotational distortion amplitude of ${\text{CaRuO}}_3$ can be understood in terms of density of states effects in the presence of strong Hund's coupling. Comparison of the calculated low-$T$ properties of ${\text{CaRuO}}_3$ to those of ${\text{SrRuO}}_3$ provides insight into the effects of magnetic order on the properties of a Hund's metal. The study provides a simultaneous description of magnetism and correlations and explicates the roles played by band theory and Hubbard and Hund's interactions.

Small differential conductance oscillations as a function of source-drain bias are observed and systematically studied in an asymmetric quantum point contact. These oscillations become significantly suppressed in a small in-plane magnetic field ($\sim 0.7$ T) or at higher temperatures ($\sim 800$ mK). Qualitatively, their temperature evolution can be simulated numerically based on smearing of the Fermi distribution, whereas features near zero bias cannot. Single-particle scenarios are unsatisfactory in accounting for the oscillations, suggesting that they are likely caused by electron- and spin-correlation effects.

Determination of properties of quasiparticle excitations is an important task in the experimental investigation of the fractional quantum Hall effect (FQHE). We propose a model-independent method for finding the scaling dimension of FQHE quasiparticles from measurements of the electric current tunneling between two FQHE edges and its noise. In comparison to the commonly used method based on measuring the tunneling current only, the proposed method is less prone to the errors due to nonuniversal physics of tunnel junctions.

Based on the refined crystal structure comprised of columns of $3\times 4$ planar blocks of ${\mathrm{NbO}}_6$ octahedra and first principles electronic structure methods, we find that orthorhombic $\left(o\right)\text{−}{\mathrm{Nb}}_{12}{\mathrm{O}}_{29}$ introduces a new class of transition metal oxide. The electronic system consists of a large Nb dimer-based localized orbital comparable in size to those in organometallic compounds, yet is tightly bound and weakly interacting with itinerant electronic bands. These local moments—a rare occurrence for Nb—form one-dimensional spin chains that criss-cross perpendicularly oriented conducting “nanowires.” The local moment bandwidth is comparable to what is seen in rare earth compounds with extremely localized orbitals. The microscopic origin is traced to the local structure of the ${\mathrm{NbO}}_6$ octahedra and associated orbital+spin ordering. The resulting $1{\mathrm{D}}_s\times 1{\mathrm{D}}_c$ anisotropic two dimensional Heisenberg-Kondo lattice model $(s=\text{spin}$; $c=\text{charge})$ provides a strongly anisotropic spin-fermion lattice system for further study.

A topological crystalline insulator has an even number of Dirac cones (i.e., multiple valleys) in its surface band structure, thus potentially leading to valleytronic applications such as graphene. Using the density-functional-theory method, we systematically investigate the strain-induced evolution of topological surface states on the SnTe(111) surface. Our results show that compressive strain can shift the Dirac cones at the $\overline{\mathrm{\Gamma }}$ and $\overline{M}$ valleys to different extents (even oppositely) in energy, while the tensile strain can induce different band gaps at the valleys due to the enhanced penetration depths of surface states. Exploiting a strain-induced nanostructure with well-defined edges on the (111) surface, we demonstrate strong valley-selective filtering for massless Dirac fermions by dynamically applying local external pressure. Our findings may pave the way for strain-engineered valley-resolved manipulation of Dirac fermions with high tunability and scalability.

Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature range. Here we report a series of indium-doped ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ compounds that manifest huge bulk resistivities together with evidence consistent with the topological character of the surface states for $x\gtrsim 0.35$, based on thickness-dependent transport studies and magnetoresistance measurements. For these bulk-insulating materials, the surface states determine the resistivity for temperatures beyond 20 K.

In this work we present the concept and design of an ultrathin (λ/22) terahertz (THz) unidirectional carpet cloak based on the local phase compensation approach enabled by gradient metasurfaces. A triangular surface bump with center height of 4.1 mm (1.1λ) and tilt angle of 20° is covered with a metasurface composed of an array of suitably designed closed ring resonators with a transverse gradient of surface impedance. The ring resonators provide a wide range of control for the reflection phase with small absorption losses, enabling efficient phase manipulation along the edge of the bump. Our numerical results demonstrate a good performance of the designed cloak in both near field and far field, and the cloaked object mimics a flat ground plane within a broad range of incidence angles, over 35° angular spectrum centered at 45°. The presented cloak design can be applied in radar and antenna systems as a thin, lightweight, and easy to fabricate solution for radio and THz frequencies.

We propose a model for planar nanoscale dielectric capacitors consisting of a single layer, insulating hexagonal boron nitride (BN) stripe placed between two metallic graphene stripes, all forming commensurately a single atomic plane. First-principles density functional calculations on these nanoscale capacitors for different levels of charging and different widths of graphene-BN stripes mark high gravimetric capacitance values, which are comparable to those of supercapacitors made from other carbon-based materials. Present nanocapacitor models allow the fabrication of series, parallel, and mixed combinations which offer potential applications in two-dimensional flexible nanoelectronics, energy storage, and heat-pressure sensing systems.

We find a 5 eV satellite in the $\mathrm{Ti}1s$ photoelectron spectrum of the transition-metal oxide $\mathrm{SrTi}{\mathrm{O}}_3$. This satellite appears in addition to the well-studied 13 eV structure that is typically associated with the $\mathrm{Ti}2p$ core line. We give direct experimental evidence that the presence of two satellites is due to the crystal-field splitting of the metal $3d$ orbitals. They originate from ligand $2p{t}_{2g}\to \mathrm{metal}3d{t}_{2g}$ and ligand $2p{e}_g\to \mathrm{metal}3d{e}_g$ monopole charge-transfer excitations within the sudden approximation of quantum mechanics. This assignment is made by the energetics of the resonant and high-energy threshold behaviors of the $\mathrm{Ti}K\text{−}{L}_2{L}_3$ Auger decay that follows $\mathrm{Ti}1s$ photoionization.

We use magnetoconductance measurements in dual-gated InSb nanowire devices, together with a theoretical analysis of weak antilocalization, to accurately extract spin-orbit strength. In particular, we show that magnetoconductance in our three-dimensional wires is very different compared to wires in two-dimensional electron gases. We obtain a large Rashba spin-orbit strength of $0.5–1\text{eV}\text{Å}$ corresponding to a spin-orbit energy of $0.25–1\text{meV}$. These values underline the potential of InSb nanowires in the study of Majorana fermions in hybrid semiconductor-superconductor devices.