We study photon-assisted transport in a single-level quantum dot system under a periodically oscillating field. Photon-assisted current noise in the presence of the Coulomb interaction is calculated based on a gauge-invariant formulation of time-dependent transport. We derive the vertex corrections within the self-consistent Hartree-Fock approximation in terms the Floquet-Green's functions (Floquet-GFs) and examine the effects of the Coulomb interaction on the photon-assisted current noise. Moreover, we utilize an effective temperature to characterize nonequilibrium properties under the influence of the ac field. The vertex corrections are suppressed by the rise of the effective temperature, whereas characteristic peak structures appear in the frequency spectra of the vertex corrections. The present result provides a useful viewpoint for understanding photon-assisted transport in interacting electron systems.

We present a detailed investigation of the low-frequency dielectric and conductivity properties of conducting polymer nanowires. Our results, obtained by connecting $\sim {10}^7$ nanowires in parallel, show that these polypyrrole nanowires behave like conventional charge-density wave (CDW) materials, in their nonlinear and dynamic response, together with scaling of relaxation time and conductivity. The observed Arrhenius law for both these quantities gives a CDW gap of 3.5 meV in the regime of temperature $(\sim 40$ K) in which the CDW state survives. We find good agreement with a theory of weakly pinned CDW, screened by thermally excited carriers across the CDW gap. The identification of polymer nanowires as CDW provides us a model system to investigate charge ordering owing to electrostatic interaction, relevant to a variety of systems from dusty plasma to molecular biology.

We investigate the influence of low dimensionality and disorder in phonon transport in ultranarrow armchair graphene nanoribbons (GNRs) using nonequilibrium Green's function (NEGF) simulation techniques. We specifically focus on how different parts of the phonon spectrum are influenced by geometrical confinement and line edge roughness. Under ballistic conditions, phonons throughout the entire phonon energy spectrum contribute to thermal transport. With the introduction of line edge roughness, the phonon transmission is reduced, but in a manner which is significantly nonuniform throughout the spectrum. We identify four distinct behaviors within the phonon spectrum in the presence of disorder: (i) the low-energy, low-wave vector acoustic branches have very long mean-free paths and are affected the least by edge disorder, even in the case of ultranarrow $W=1\mathrm{nm}$ wide GNRs; (ii) energy regions that consist of a dense population of relatively “flat” phonon modes (including the optical branches) are also not significantly affected, except in the case of the ultranarrow $W=1\mathrm{nm}$ GNRs, in which case the transmission is reduced because of band mismatch along the phonon transport path; (iii) “quasiacoustic” bands that lie within the intermediate region of the spectrum are strongly affected by disorder as this part of the spectrum is depleted of propagating phonon modes upon both confinement and disorder [resulting in sparse $E\left(q\right)$ phononic band structure], especially as the channel length increases; and (iv) the strongest reduction in phonon transmission is observed in energy regions that are composed of a small density of phonon modes, in which case roughness can introduce transport gaps that greatly increase with channel length. We show that in GNRs of widths as small as $W=3\mathrm{nm}$, under moderate roughness, both the low-energy acoustic modes and dense regions of optical modes can retain semiballistic transport properties, even for channel lengths up to $L=1\mu \mathrm{m}$. These modes tend to completely dominate thermal transport. Modes in the sparse regions of the spectrum, however, tend to fall into the localization regime, even for channel lengths as short as tens of nanometers, despite their relatively high phonon group velocities.

The strong Coulomb interactions and the small exciton radii in two-dimensional metal dichalcogenides can result in very fast capture of electrons and holes of excitons by mid-gap defects from Auger processes. In the Auger processes considered here, an exciton is annihilated at a defect site with the capture of the electron (or the hole) by the defect and the hole (or the electron) is scattered to a high energy. In the case of excitons, the probability of finding an electron and a hole near each other is enhanced many folds compared to the case of free uncorrelated electrons and holes. Consequently, the rate of carrier capture by defects from Auger scattering for excitons in metal dichalcogenides can be 100–1000 times larger than for uncorrelated electrons and holes for carrier densities in the ${10}^{11}–{10}^{12}{\mathrm{cm}}^{-2}$ range. We calculate the capture times of electrons and holes by defects and show that the capture times can be in the subpicosecond to a few picoseconds range. The capture rates exhibit linear as well as quadratic dependence on the exciton density. These fast time scales agree well with the recent experimental observations and point to the importance of controlling defects in metal dichalcogenides for optoelectronic applications.

A number of practical and fundamental applications of graphene requires modification of some of its properties. In this paper we study the effect of polymerization of a hydrogen silsesquioxane film on top of monolayer graphene with the intent to increase the strength of the spin-orbit interaction. The measured nonlocal resistances $R_{NL}$ were found to be up to $700\Omega$, significantly exceeding the expected contribution from conventional Ohmic currents. The $R_{NL}$ dependence on the channel length resembles exponential decay with a characteristic length of $\lambda \simeq 500\mathrm{nm}$ that is close to the spin-relaxation length in graphene reported elsewhere. The sensitivity of the measured effect to the electron-beam exposure was shown to decrease with an increased level of the surface contamination. However, no modulation of the effect is observed when an in-plane magnetic field is applied. This implies that a spin Hall model fails to explain the observed phenomenon and an alternative interpretation is required.

The interacting bosons in one-dimensional inversion-symmetric superlattices are investigated from the topological aspect. The complete phase diagram is obtained by an atomic-limit analysis and quantum Monte Carlo simulations and comprises three kinds of phases: superfluid, persisted charge-density-wave and Mott insulators, and emergent insulators in the presence of nearest-neighbor hoppings. We find that all emergent insulators are topological, which are characterized by the Berry phase $\pi$ and a pair of degenerate in-gap boundary states. The mechanism of the topological bosonic insulators is qualitatively discussed and the ones with higher fillings can be understood as a $\frac{1}{3}$-filling topological phase on a background of trivial charge-density-wave or Mott insulators.

Metamaterial effective parameters may exhibit freedom from typical dispersion constraints. For instance, the emergence of a magnetic response in arrays of split ring resonators for long wavelengths cannot be attained in a passive continuous system obeying the Kramers-Kronig relations. We characterize such freedom by identifying the three possible asymptotes which effective parameters can approach when analytically continued. Apart from their dispersion freedom, we also demonstrate that the effective parameters may be redefined in such a way that they have a certain physical meaning for all frequencies. There exist several possible definitions for the effective permittivity and permeability whereby this is achieved, thereby giving several possible frequency variations for high frequencies, while nevertheless converging to the same dispersion for long wavelengths.

Electron magnetic chiral dichroism (EMCD) in a transmission electron microscope is an element-specific magnetic characterization technique and is extremely powerful for understanding magnetism of materials at the nanoscale. However, quantitative EMCD remains a challenge. In the present paper, we have highlighted and overcome major difficulties associated with the technique. For example, the experimentally observed low dichroic signal and imbalance between the $L_3$ and $L_2$ edge have been explained based on the oscillatory nature of electron propagation through the crystal thickness and specific momentum resolved signal detection, respectively. With this advancement in understanding, site-specific quantitative EMCD has been accomplished in epitaxial thin films of two important ferrimagnetic spinel oxides, ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ (NFO) and ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ (CFO), with varying degree of cation mixing and $A$ site cation defects. A simple model based on phenomenological absorption has been developed for different site-specific signal contributions for the inverse spinel structure. The experimental moment values for NFO and CFO obtained using EMCD are in good agreement with first principle based theoretical calculations and the results strengthen the promise of utilizing EMCD as a routine nanoscale magnetic characterization technique.

The effect of orthogonal exchange coupling between antiferromagnetic CoO and ferromagnetic Ni/Pd(001) on the perpendicular anisotropy of Ni films is studied. The thickness range in which Ni films show a perpendicular easy magnetization axis is extended by growing CoO on top of them, however, only at temperatures below $T_N$ of CoO. The perpendicular orientation of Ni spins and the in-plane orientation of CoO spins are confirmed by the magneto-optic Kerr effect/x-ray magnetic circular dichroism and x-ray magnetic linear dichroism, respectively. Additionally, a perpendicular exchange bias shows up at low temperature.

We study quantum phase slips (QPS) in ultrathin superconducting wires. Starting from an effective one-dimensional microscopic model, which includes electromagnetic fluctuations, we map the problem to a (1+1)-dimensional gas of interacting instantons. We introduce a method to calculate the tunneling amplitude of quantum phase slips directly from Monte Carlo simulations. This allows us to go beyond the dilute instanton gas approximation and study the problem without any limitations of the density of QPS. We find that the tunneling amplitude shows a characteristic scaling behavior near the superconductor-insulator transition. We also calculate the voltage-charge relation of the insulating state, which is the dual of the Josephson current-phase relation in ordinary superconducting weak links. This evolves from a sinusoidal form in the regime of dilute QPS to more exotic shapes for higher QPS densities, where interactions are important.

It has been well established that both in bulk at ambient pressure and for films under modest strains, cubic ${\mathrm{SrCoO}}_{3-\delta }$ ($\delta <0.2$) is a ferromagnetic metal. Recent theoretical work, however, indicates that a magnetic phase transition to an antiferromagnetic structure could occur under large strain accompanied by a metal-insulator transition. We have observed a strain-induced ferromagnetic-to-antiferromagnetic phase transition in ${\mathrm{SrCoO}}_{3-\delta }$ films grown on ${\mathrm{DyScO}}_3$ substrates, which provide a large tensile epitaxial strain, as compared to ferromagnetic films under lower tensile strain on ${\mathrm{SrTiO}}_3$ substrates. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a $G$-type antiferromagnetic structure with Neél temperatures between $T_N\sim 135\pm 10\text{K}$ and $\sim 325\pm 10\text{K}$, depending on the oxygen content of the samples. Therefore, our data experimentally confirm the predicted strain-induced magnetic phase transition to an antiferromagnetic state for ${\mathrm{SrCoO}}_{3-\delta }$ thin films under large epitaxial strain.

We establish the phase diagram of the strongly interacting Bose-Hubbard model defined on a two-leg ladder geometry in the presence of a homogeneous flux. Our work is motivated by a recent experiment [M. Atala et al., Nat. Phys. 10, 588 (2014)], which studied the same system, in the complementary regime of weak interactions. Based on extensive density matrix renormalization group simulations and a bosonization analysis, we fully explore the parameter space spanned by filling, interleg tunneling, and flux. As a main result, we demonstrate the existence of gapless and gapped Meissner and vortex phases, with the gapped states emerging in Mott-insulating regimes. We calculate experimentally accessible observables such as chiral currents and vortex patterns.

Electric fields are generally known to favor the ferroelectric polar state over the antiferroelectric nonpolar state for their Coulomb interactions with dipoles in the crystal. In this paper, we directly image an electric-field-assisted ferroelectric-to-antiferroelectric phase transition during polarization reversal of the ferroelectric phase in polycrystalline $\mathrm{P}{\mathrm{b}}_{0.99}\left\{\mathrm{N}{\mathrm{b}}_{0.02}{\left[{\left(\mathrm{Z}{\mathrm{r}}_{0.57}\mathrm{S}{\mathrm{n}}_{0.43}\right)}_{0.92}\mathrm{T}{\mathrm{i}}_{0.08}\right]}_{0.98}\right\}{\mathrm{O}}_3$. With the electric-field in situ transmission electron microscopy technique, such an unlikely phenomenon is verified to occur by both domain morphology change and electron-diffraction analysis. The slower kinetics of the phase transition, compared with ferroelectric polarization reversal, is suggested to contribute to this unusual behavior.

The detailed optical properties of the multiband iron-chalcogenide superconductor ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ have been reexamined for a large number of temperatures above and below the critical temperature $T_c=14\mathrm{K}$ for light polarized in the $a\text{−}b$ planes. Instead of the simple Drude model that assumes a single band, above $T_c$ the normal-state optical properties are best described by the two-Drude model that considers two separate electronic subsystems; we observe a weak response $({\omega }_{p,D;1}\simeq 3000{\mathrm{cm}}^{-1})$ where the scattering rate has a strong temperature dependence ($1/{\tau }_{D,1}\simeq 32{\mathrm{cm}}^{-1}$ for $T\gtrsim T_c$), and a strong response $({\omega }_{p,D;2}\simeq 14500{\mathrm{cm}}^{-1})$ with a large scattering rate $(1/{\tau }_{D,2}\simeq 1720{\mathrm{cm}}^{-1})$ that is essentially temperature independent. The multiband nature of this material precludes the use of the popular generalized-Drude approach commonly applied to single-band materials, implying that any structure observed in the frequency-dependent scattering rate $1/\tau \left(\omega \right)$ is spurious and it cannot be used as the foundation for optical inversion techniques to determine an electron-boson spectral function ${\alpha }^{2}F\left(\omega \right)$. Below $T_c$ the optical conductivity is best described using two superconducting optical gaps of $2{\Delta }_1\simeq 45$ and $2{\Delta }_2\simeq 90{\mathrm{cm}}^{-1}$ applied to the strong and weak responses, respectively. The scattering rates for these two bands are vastly different at low temperature, placing this material simultaneously in both clean and dirty limit. Interestingly, this material falls on the universal scaling line initially observed for the cuprate superconductors.

The superfluid ${}^3\mathrm{He}$ formed by spin-triplet $p$-wave Cooper pairs is a typical topological superfluid. In the superfluid ${}^3\mathrm{He}\text{−}B$ phase, several kinds of vortices classified by spatial symmetries $P_1$, $P_2$, and $P_3$ are produced, where $P_1$ is inversion symmetry, $P_2$ is magnetic reflection symmetry, and $P_3$ is magnetic $\pi$-rotation symmetry. We have calculated the vortex bound states by the Bogoliubov–de Gennes theory and the quasiclassical Eilenberger theory, and also clarified symmetry protection of the low-energy excitations by the spatial symmetries. On the symmetry protection, $P_3$ symmetry plays a key role which gives twofold-degenerate Majorana zero modes. Then, the bound states in the most symmetric $o$ vortex with $P_1$, $P_2$, and $P_3$ symmetries and in $w$ vortex with $P_3$ symmetry have the symmetry-protected degenerate Majorana zero modes. On the other hand, zero-energy modes in the $v$ vortex believed to be realized in the actual $B$ phase are not protected, and in consequence become gapped by breaking axial symmetry. The excitation gap may have been observed as the variation of critical velocity. We have also suggested an experimental setup to create the $o$ vortex with Majorana zero modes by a confinement and a magnetic field.

We investigate the excitation spectrum of strongly correlated sodium cobaltate within a realistic many-body description beyond dynamical mean-field theory (DMFT). At lower doping around $x=0.3$, rather close to Mott-critical half-filling, the single-particle spectral function of ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ displays an upper Hubbard band which is captured within DMFT. Momentum-dependent self-energy effects beyond DMFT become dominant at higher doping. Around a doping level of $x\sim 0.67$, the incoherent excitations give way to finite-energy spin-polaron excitations in close agreement with optics experiments. These excitations are a direct consequence of the formation of bound states between quasiparticles and paramagnons in the proximity of in-plane ferromagnetic ordering.

We introduce a strictly single-site DMRG algorithm based on the subspace expansion of the alternating minimal energy (AMEn) method. The proposed new MPS basis enrichment method is sufficient to avoid local minima during the optimization, similar to the density matrix perturbation method, but computationally cheaper. Each application of $\widehat{H}$ to $|\Psi \rangle$ in the central eigensolver is reduced in cost for a speed-up of $\approx (d+1)/2$, with $d$ the physical site dimension. Further speed-ups result from cheaper auxiliary calculations and an often greatly improved convergence behavior. Runtime to convergence improves by up to a factor of 2.5 on the Fermi-Hubbard model compared to the previous single-site method and by up to a factor of 3.9 compared to two-site DMRG. The method is compatible with real-space parallelization and non-Abelian symmetries.

Electronic transport through disordered organic materials is relevant in many applications, including organic light-emitting diodes and organic photovoltaics. The charge-carrier mobility is one of the most important material characteristics that must be optimized to make organic devices competitive. Here we introduce a general effective-medium model for the analytic calculation of zero-field mobilities on the basis of material-specific parameters that are obtained from extensive ab initio simulations. By means of kinetic Monte Carlo simulations, we generalize the model to also include the strong disorder limit. As a proof of concept the model is applied to two different disordered organic materials exhibiting medium and strong disorder, respectively. Surprisingly, even at strong disorder the hole mobilities computed with the effective-medium model in its original form are found to agree best with the experimental data. Seeking a possible explanation for this result, we investigate the strong dependence of the mobility on the connectivity of the model topology and show that the distribution of hopping matrix elements in the material is indeed much broader than assumed in simple lattice models. As the input parameters of the model can be computed on the basis of relatively small samples, this model may be used for materials’ screening without adjustable parameters.

We report selective injection of both spin-up and spin-down single electrons into a quantum dot (QD) from spin-polarized nonequilibrium quantum Hall edge channels (ECs) generated by selective transmission of spin-resolved ECs using a surface gate placed at a distance from the QD. We change the spin polarization of nonequilibrium ECs by changing the bias voltages applied to different source Ohmic contacts. The efficiency of spin-up electron injection reaches 0.5, which is approximately 0.2 higher than that induced by spin-dependent tunnel coupling between QD and ECs. On the other hand, the efficiency of spin-down electron injection reaches 0.4. In addition, we rectify the underestimation of the efficiency of spin filtering for equilibrium ECs by numerically subtracting the contribution of the excited states in the QD. The obtained spin-filtering efficiency is higher than that evaluated from the raw experimental data and increases with magnetic field as expected with the increase in the spatial separation between ECs.

Using the tight-binding approach, we calculate the electronic structure of triangular and hexagonal ${\mathrm{MoS}}_2$ quantum dots. Due to the orbital asymmetry we show that it is possible to form quantum dots with the same shape but having different electronic properties. The electronic states of triangular and hexagonal quantum dots are explored, as well as the local and total density of states and the convergence towards the bulk spectrum with dot size is investigated. Our calculations show that: (1) edge states appear in the band gap, (2) that there are a larger number of electronic states in the conduction band as compared to the valence band, and (3) the relative number of edge states decreases with increasing dot size.