Artificial diamagnetics are prominent for achieving extraordinarily strong diamagnetism in a wide frequency range. However, as far as the magnetic fields outside the artificial medium are concerned, bulk conductors show a very similar pattern. The question arises whether the complicated internal structure of artificial diamagnetics can, to this end, be replaced by a simpler object. We show that for an electrically small body, internal structuring is likely to make the effective diamagnetic response weaker than that of a simple conducting object.

We present a comprehensive theoretical framework for the Andreev bound-state population dynamics in superconducting weak links. Contrary to previous works, our approach takes into account the generated nonequilibrium distribution of the continuum quasiparticle states in a self-consistent way. As application of our theory, we show that the coupling of the superconducting contact to environmental phase fluctuations induces a charge imbalance of the continuum quasiparticle population. This imbalance is due to the breaking of the left-right symmetry in the rates connecting continuum quasiparticles and the Andreev bound-state system, and causes a quasiparticle current on top of the Josephson current in a ring geometry. We evaluate the phase dependence of the quasiparticle current for realistic choices of the model parameters. Our theory also allows one to analyze the quantum coherent evolution of the system from an arbitrary initial state.

The organic conductor $\kappa \text{−}\left(\mathrm{BEDT}\text{−}\mathrm{TTF}{)}_2\mathrm{Cu}\right[\mathrm{N}\left(\mathrm{CN}{)}_2\right]\mathrm{Cl}$ has been studied for the examination of the conducting mechanism of the semiconducting behavior at high temperatures by ${}^{13}\mathrm{C}\text{−}\mathrm{NMR}$ spectroscopy. We found that the temperature dependence of the linewidths and the spin-spin relaxation rate $T_2^{-1}$ showed a peak structure characterized by the hyperfine coupling constant in this region, suggesting a connection between ethylene motion and conducting electrons similar to that observed in $\kappa \text{−}(\mathrm{BEDT}\text{−}\mathrm{TTF}{)}_2{\mathrm{Cu}\left(\mathrm{NCS}\right)}_2$. Although $\kappa \text{−}\left(\mathrm{BEDT}\text{−}\mathrm{TTF}{)}_2\mathrm{Cu}\right[\mathrm{N}\left(\mathrm{CN}{)}_2\right]\mathrm{Cl}$ salt was thought to be an insulator at high temperature, our findings suggest that all $\kappa$-type salts at high temperatures could be in the same phase where the scattering caused by ethylene motion is added to bad metal.

We present a subspace projection technique to conduct large-scale Kohn-Sham density functional theory calculations using higher-order spectral finite-element discretization. The proposed method treats both metallic and insulating materials in a single framework and is applicable to both pseudopotential as well as all-electron calculations. The key ideas involved in the development of this method include: (i) employing a higher-order spectral finite-element basis that is amenable to mesh adaption; (ii) using a Chebyshev filter to construct a subspace, which is an approximation to the occupied eigenspace in a given self-consistent field iteration; (iii) using a localization procedure to construct a nonorthogonal localized basis spanning the Chebyshev filtered subspace; and (iv) using a Fermi-operator expansion in terms of the subspace-projected Hamiltonian represented in the nonorthogonal localized basis to compute relevant quantities like the density matrix, electron density, and band energy. We demonstrate the accuracy and efficiency of the proposed approach on benchmark systems involving pseudopotential calculations on aluminum nanoclusters up to 3430 atoms and on alkane chains up to 7052 atoms, as well as all-electron calculations on silicon nanoclusters up to 3920 electrons. The benchmark studies revealed that accuracies commensurate with chemical accuracy can be obtained with the proposed method, and a subquadratic-scaling with system size was observed for the range of materials systems studied. In particular, for the alkane chains—representing an insulating material—close to linear scaling is observed, whereas, for aluminum nanoclusters—representing a metallic material—the scaling is observed to be $\mathcal{O}\left(N^{1.46}\right)$. For all-electron calculations on silicon nanoclusters, the scaling with the number of electrons is computed to be $\mathcal{O}\left(N^{1.75}\right)$. In all the benchmark systems, significant computational savings have been realized with the proposed approach, with approximately tenfold speedups observed for the largest systems with respect to reference calculations.

We investigate ramifications of the persistent spin helix symmetry in two-dimensional hole gases in the conductance of disordered mesoscopic systems. To this end we extend previous models by going beyond the axial approximation for III-V semiconductors. For heavy-hole subbands we identify an exact spin-preserving symmetry analogous to the electronic case by analyzing the crossover from weak antilocalization to weak localization and spin transmission as a function of extrinsic spin-orbit interaction strength.

We present a high-resolution angle-resolved photoemission spectroscopy study in conjunction with first-principles calculations to investigate how the interaction of electrons with phonons in graphene is modified by the presence of Yb. We find that the charges transferred from Yb to the graphene layer hybridize with the graphene $\pi$ bands, leading to a strong enhancement of the electron-phonon interaction. Specifically, the electron-phonon coupling constant is increased by as much as a factor of 10 upon the introduction of Yb with respect to as-grown graphene ($\le 0.05$). The observed coupling constant constitutes the highest value ever measured for graphene and suggests that the hybridization between graphene and the adatoms might be a critical parameter in realizing superconducting graphene.

Common wisdom asserts that bound excitons cannot form in high-dimensional ($d>1$) metallic structures because of their overwhelming screening and the unavoidable resonance with nearby continuous bands. Strikingly we illustrate that this prevalent assumption is not quite true. A key ingredient has been overlooked: Destructive coherent effects are capable of thwarting the formation of resonance. As an example of this general mechanism, we focus on an experimentally relevant material and predict bound excitons in twisted bilayer graphene, which is a two-dimensional gapless structure that exhibits metallic screening. The binding energies calculated by first-principles simulations are surprisingly large. The low-energy effective model reveals that these bound states are produced by a unique destructive coherence between two alike subband resonant excitons. In particular, this coherent effect is not sensitive to the screening and dimensionality, and hence may persist as a general mechanism for creating bound excitons in various metallic structures, opening the door for excitonic applications based on metallic structures.

We study theoretically the low-energy phonons and the static strain in cylindrical core/shell nanowires (NWs). Assuming pseudomorphic growth, isotropic media, and a force-free wire surface, we derive algebraic expressions for the dispersion relations, the displacement fields, and the stress and strain components from linear elasticity theory. Our results apply to NWs with arbitrary radii and arbitrary elastic constants for both core and shell. The expressions for the static strain are consistent with experiments, simulations, and previous analytical investigations; those for phonons are consistent with known results for homogeneous NWs. Among other things, we show that the dispersion relations of the torsional, longitudinal, and flexural modes change differently with the relative shell thickness, and we identify new terms in the corresponding strain tensors that are absent for uncapped NWs. We illustrate our results via the example of Ge/Si core/shell NWs and demonstrate that shell-induced strain has large effects on the hole spectrum of these systems.

The key process in organic photovoltaic cells is the charge separation at organic donor-acceptor interfaces. However, exactly how the charges separate into free charge carriers still remains a puzzle. We present here simulations of the electron dynamics of this process using a nonadiabatic Ehrenfest method. From these simulations, we give a direct illustration of the charge separation process. The results show that the delocalization of the electronic states involved plays a critical role in order to overcome the Coulomb attraction of the charge transfer (CT) exciton. Charge separation only occurs for sufficiently strong intermolecular interactions. Alternatively, the CT exciton relaxes into a bound polaron pair. The results also show that the “excess energy” of the hot CT exciton facilitates the charge separation process to a certain degree.

This work presents a spectroscopic study of the thermally enhanced photoinduced electron emission from nitrogen-doped diamond films prepared on $p$-type silicon substrates. It has been shown that photon-enhanced thermionic emission (PETE) can substantially enhance thermionic emission intensity from a $p$-type semiconductor. An $n$-type diamond/$p$-type silicon structure was illuminated with $400–450$ nm light, and the spectra of the emitted electrons showed a work function less than 2 eV and nearly an order of magnitude increase in emission intensity as the temperature was increased from ambient to $\sim 400 °\mathrm{C}$. Thermionic emission was negligible in this temperature range. The results are modeled in terms of contributions from PETE and direct photoelectron emission, and the large increase is consistent with a PETE component. The results indicate possible application in combined solar/thermal energy conversion devices.

We have grown high quality $p$-type δ-doped InAs quantum dots and have demonstrated coherent spin pumping and repumping of a hole spin in a positively charged quantum dot by means of a single-laser driving scheme under a high magnetic field in the Voigt configuration. Modeling of our system shows excellent qualitative agreement with the experimental findings and further explores the performance of the single-laser scheme for spin pumping and repumping.

We present a theory of magnetic and magnetotransport phenomena at ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ interfaces, which as a central ingredient includes coupling between the conduction bands and local magnetic moments originating from charge traps at the interface. Tuning the itinerant electron density in the model drives transitions between a heavy Fermi liquid phase with screened moments and various magnetic states. The dependence of the magnetic phenomena on the electron density or gate voltage stems from competing magnetic interactions between the local moments and the different conduction bands. At low densities only the lowest conduction band, composed of the $d_{xy}$ orbitals of Ti, is occupied. Its antiferromagnetic interaction with the local moments leads to screening of the moments at a Kondo scale that increases with density. However, above a critical density, measured in experiments to be $n_c\approx 1.7\times {10}^{13}{\mathrm{cm}}^{-2}$, the $d_{xz}$ and $d_{yz}$ bands begin to populate. Their ferromagnetic interaction with the local moments competes with the antiferromagnetic interaction of the $d_{xy}$ band leading to eventual reduction of the Kondo scale with density. We explain the distinct magnetotransport regimes seen in experiments as manifestations of the magnetic phase diagram computed from the model. We present data showing a relation between the anomalous Hall effect and the resistivity in the system. The data strongly suggest that the concentration of local magnetic moments affecting the transport in the system is much lower than the carrier density, in accord with the theoretical model.

The existence and the identification of only one or several coparticipating luminescent ${\mathrm{Yb}}^{3+}$ centers in the heavily doped $\mathrm{Yb}:{\mathrm{CaF}}_2$ laser crystals which are considered in the development of several high intensity laser chains have been examined first by using two complementary and original experimental approaches, i.e., registration of low temperature site-selective laser excitation spectra related to near-infrared and visible cooperative emission processes, on the one hand, and direct imaging at the atomic scale of isolated ions and clusters using a high-resolution scanning transmission electron microscope in the high angle annular dark-field mode, on the other hand, and then correlating the data with simple crystal field calculations. As a consequence, and although all the experimental details could not be accounted for quantitatively, a good overall correlation was found between the experimental and the theoretical data. The results show that at the investigated dopant concentrations, $\mathrm{Yb}:{\mathrm{CaF}}_2$ should be considered as a multisite system whose luminescent and lasing properties are dominated by a series of ${\mathrm{Yb}}^{3+}$ clusters ranging from dimers to tetramers. Hexameric luminescent centers may be dominant at really high dopant concentrations (likely above 20 at. %), as was originally proposed, but certainly not at the intermediate dopant concentrations which are considered for the laser application, i.e., between about 0.5 and 10 at. %.

The thermoelectric properties of a molecular junction model, appropriate for large molecules such as fullerenes, are studied within a nonequilibrium adiabatic approach in the linear regime at room temperature. A self-consistent calculation is implemented for electron and phonon thermal conductance showing that both increase with the inclusion of the electron-vibration coupling. Moreover, we show that the deviations from the Wiedemann-Franz law are progressively reduced upon increasing the interaction between electronic and vibrational degrees of freedom. Consequently, the junction thermoelectric efficiency is substantially reduced by the electron-vibration coupling. Even so, we find that, for realistic parameters values, the thermoelectric figure of merit can still have peaks of the order of unity.

We demonstrate how broadband angular selectivity can be achieved with stacks of one-dimensionally periodic photonic crystals, each consisting of alternating isotropic layers and effective anisotropic layers, where each effective anisotropic layer is constructed from a multilayered metamaterial. We show that by simply changing the structure of the metamaterials, the selective angle can be tuned to a broad range of angles; and, by increasing the number of stacks, the angular transmission window can be made as narrow as desired. As a proof of principle, we realize the idea experimentally in the microwave regime. The angular selectivity and tunability we report here can have various applications such as in directional control of electromagnetic emitters and detectors.

Adjusting the free-electron concentration, the surface plasmon frequency of the semiconductor ZnOGa is tuned into resonance with the molecular vibrations of the $n$-alkane tetracontane. Closed molecular films deposited on the semiconductor's surface in the monolayer regime generate distinct signatures in total-attenuated-reflection spectra at the frequencies of the symmetric and asymmetric stretching vibrations of the ${\mathrm{CH}}_2$ group. Their line shape undergoes profound changes from absorptive to dispersive and even antiresonance behavior when moving along the surface-plasmon dispersion by the angle of incidence. We demonstrate that this line-shape diversity results from a phase-sensitive perturbation of the surface-plasmon-polariton generation at the molecule-metal interface.

The influence of acoustic phonons on the emission spectra of quantum dashes (QDashes), that are quasi-zero-dimensional epitaxial nanostructures with significant shape anisotropy, is investigated both experimentally and theoretically. Photoluminescence (PL) spectra of single InAs/InGaAlAs/InP (001) QDashes exhibit sidebands of the main emission peak, clearly indicating the contribution of phonon-assisted emission to the exciton luminescence, which dominates the PL line shape at higher temperatures (between 50 and 100 K, usually). By utilizing the independent boson model we perform systematic and comprehensive studies of the influence of the overall geometry of quantum confinement on this spectral feature in an uncommon quantum system. A comparison of the experimental data and the results of modeling have confirmed the existence of two types of states differing in the spatial confinement and symmetry within one sample, i.e., typical for large elongated objects or characteristic for smaller and more symmetric structures. The latter are supposed to correspond to local widenings or zigzag bends present in some of the dashes and acting as additional localization centers, which confine excitons in a much smaller volume and decrease effectively the resulting in-plane anisotropy. Those observations evidence a nontrivial spatial character of the quantum confinement in these structures. They are consistent with our previous polarization-resolved study on the QDash ensemble and correlate well with the exciton decay times, and the spectral-diffusion-dominated line broadenings at low temperatures reflecting the effect of electric field fluctuations on the excitons of a different spatial extension. Finally, we demonstrate a pronounced suppression of phonon-induced decoherence for such strongly elongated nanostructures.

Valence fluctuations of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ were studied in a solid solution of ${\mathrm{Li}}_x{\mathrm{FePO}}_4$ by nuclear resonant forward scattering of synchrotron x rays while the sample was heated in a diamond-anvil pressure cell. The spectra acquired at different temperatures and pressures were analyzed for the frequencies of valence changes using the Blume-Tjon model of a system with a fluctuating Hamiltonian. These frequencies were analyzed to obtain activation enthalpies and an activation volume for polaron hopping. There was a large suppression of hopping frequency with pressure, giving an activation volume for polaron hopping of $5.8\pm 0.7 {\AA }^3$. This large, positive value is typical of ion diffusion, which indicates correlated motions of polarons and ${\mathrm{Li}}^{+}$ ions that alter the dynamics of both. Monte Carlo simulations were used to estimate the strength of the polaron-ion interaction energy.

We study the effects of nonlinear dynamics and damping by phonons on a system of interacting electronically pumped magnons in a ferromagnet. The nonlinear effects are crucial for constructing the dynamic phase diagram, which describes how “swasing” and Bose-Einstein condensation emerge out of the quasiequilibrated thermal cloud of magnons. We analyze the system in the presence of magnon damping and interactions, demonstrating the continuous onset of stable condensates as well as hysteretic transitions.