Most earlier work on the effect of chemical substitution in multiferroic MnWO${}_4$ has focused on the 3$d$ transition metal substitution of Mn. In this paper, we investigate the Ru substitution of Mn in polycrystalline Mn${}_{1-x}$Ru${}_x$${}_{/2}$WO${}_4$ in order to unveil the consequence of 4$d$ transition metal substitution in terms of magnetic transitions and ferroelectricity. It is found that the Ru substitution substantially reshuffles the magnetic frustration and stabilizes the incommensurate helical spin-order phase (AF2) by partially suppressing the collinear spin-order phase (AF1 phase). The coexistence of the AF2 and AF1 phases at low temperature is suggested. Consequently, the ferroelectric polarization is remarkably enhanced, accompanied with significant response of the polarization to magnetic field. It is argued that the structural distortion and enhanced spin-orbital coupling associated with the Ru substitution may be responsible for this ferroelectricity enhancement.

The Pippard coherence length ${\xi }_0$ (the size of a Cooper pair) in two extreme type-I superconductors (In and Sn) was determined directly through high-resolution measurement of the nonlocal electrodynamic effect combining low-energy muon spin rotation spectroscopy and polarized neutron reflectometry. The renormalization factor $Z=m_{cp}^{*}/2m$ ($m_{cp}^{*}$ and $m$ are the mass of the Cooper pair and the electron, respectively) resulting from the electron-phonon interaction, and the temperature-dependent London penetration depth ${\lambda }_{\mathrm{L}}\left(T\right)$ were determined as well. An expression linking ${\xi }_0$, $Z$, and ${\lambda }_{\mathrm{L}}\left(0\right)$ is introduced and experimentally verified. This expression allows one to determine experimentally the Pippard coherence length in any superconductor, independent of whether the superconductor is local or nonlocal, conventional or unconventional.

We study superconducting-normal-superconducting (SNS) Josephson junctions in one-dimensional topological superconductors which support more than one Majorana end mode. The variation of the energy spectrum with the superconducting phase is investigated by combining numerical diagonalizations of tight-binding models describing the SNS junction together with an analysis of appropriate low-energy effective Hamiltonians. We show that the $4\pi$-periodicity characteristic of the fractional dc Josephson effect is preserved. Additionally, the ideal conductance of a NS junction with a topological supraconductor, hosting two Majorana modes at its ends, is doubled compared to the single Majorana case. Last, we illustrate how a nonzero superconducting phase gradient can potentially destroy the phases supporting multiple Majorana end states.

We report on the transport properties of clean, epitaxial Fe(Se,Te) thin films prepared on Fe-buffered MgO (001) single crystalline substrates by pulsed laser deposition. Near $T_{\mathrm{c}}$ a steep slope of the upper critical field for $H\left|\right|ab$ was observed (74.1 T/K), leading to a very short out-of-plane coherence length, ${\xi }_{\mathrm{c}}\left(0\right)$, of 0.2 nm, yielding 2${\xi }_{\mathrm{c}}\left(0\right)\approx 0.4\mathrm{nm}$. This value is shorter than the interlayer distance (0.605 nm) between the Fe-Se(Te) planes, indicative of modulation of the superconducting order parameter along the $c$ axis. An inverse correlation between the power law exponent $N$ of the electric field-current density($E$-$J$) curve and the critical current density $J_{\mathrm{c}}$ has been observed at 4 K, when the orientation of $H$ was close to the $ab$ plane. These results prove the presence of intrinsic pinning in Fe(Se,Te). A successful scaling of the angular dependent $J_{\mathrm{c}}$ and the corresponding exponent $N$ can be realized by the anisotropic Ginzburg Landau approach with appropriate $\Gamma$ values $2\sim 3.5$. The temperature dependence of $\Gamma$ behaves almost identically to that of the penetration depth anisotropy.

The coupled valence and spin state transition (VSST) taking place in (Pr${}_{0.7}$Sm${}_{0.3}$)${}_{0.7}$Ca${}_{0.3}$CoO${}_3$ was investigated by soft x-ray absorption spectroscopy (XAS) experiments carried out at the Pr-$M_{4,5}$, Co-$L_{2,3}$, and O-1s edges. This VSST is found to be composed of a sharp Pr/Co valence and Co spin state transition centered at $T^{*}$ ∼ 89.3 K, followed by a smoother Co spin-state evolution at higher temperatures. At $T<T^{*}$, we found that the praseodymium displays a mixed valence Pr${}^{3+}$/Pr${}^{4+}$ with about 0.13 Pr${}^{4+}$/f.u., while all the Co${}^{3+}$ is in the low-spin (LS) state. At $T\sim T^{*}$, the sharp valence transition converts all the Pr${}^{4+}$ to Pr${}^{3+}$ with a corresponding Co${}^{3+}$ to Co${}^{4+}$ compensation. This is accompanied by an equally sharp spin state transition of the Co${}^{3+}$ from the low to an incoherent mixture of low and high-spin (HS) states. An involvement of the intermediate-spin (IS) state can be discarded for the Co${}^{3+}$. While above $T^{*}$ and at high temperatures the system shares rather similar properties as Sr-doped LaCoO${}_3$, at low temperatures, it behaves much more like EuCoO${}_3$ with its highly stable LS configuration for the Co${}^{3+}$. Apparently, the mechanism responsible for the formation of Pr${}^{4+}$ at low temperatures also helps to stabilize the Co${}^{3+}$ in the LS configuration despite the presence of Co${}^{4+}$ ions. We also found out that that the Co${}^{4+}$ is in an IS state over the entire temperature range investigated in this study (10–290 K). The presence of Co${}^{3+}$ HS and Co${}^{4+}$ IS at elevated temperatures facilitates the conductivity of the material.

A hybrid approach to nonequilibrium dynamics of quantum impurity systems is presented. The numerical renormalization group serves as a means to generate a suitable low-energy Hamiltonian, allowing for an accurate evaluation of the real-time dynamics of the problem up to exponentially long times using primarily the time-adaptive density-matrix renormalization group. In particular, by constructing a suitable hybrid chain, discretization errors are essentially eliminated on all time scales of interest. We extract the decay time of the interaction-enhanced oscillations in the interacting resonant-level model and show their quadratic divergence with the interaction strength $U$. Our numerical analysis is in excellent agreement with analytic predictions based on an expansion in $1/U$.

The Kerr effect can arise in a time-reversal invariant dissipative medium that is “gyrotropic,” i.e., one that breaks inversion ($\mathcal{I}$) and all mirror symmetries. Examples of such systems include electron analogs of cholesteric liquid crystals and their descendants, such as systems with chiral charge ordering. We present arguments that the striking Kerr onset seen in the pseudogap phase of a large number of cuprate high-temperature superconductors is an evidence of chiral charge ordering. We discuss additional experimental consequences of a phase transition to a gyrotropic state, including the appearance of a zero-field Nernst effect.

We investigate the static and dynamical behavior of one-dimensional interacting fermions in disordered Hubbard chains contacted to semi-infinite leads. The chains are described via the repulsive Anderson-Hubbard Hamiltonian, using static and time-dependent lattice density-functional theory. The dynamical behavior of our quantum transport system is studied using an integration scheme available in the literature, which we modify via the recursive Lanczos method to increase its efficiency. To quantify the degree of localization due to disorder and interactions, we adapt the definition of the inverse participation ratio to obtain an indicator which is suitable for quantum transport geometries and can be obtained within density-functional theory. Lattice density-functional theories are reviewed and, for contacted chains, we analyze the merits and limits of the coherent-potential approximation in describing the spectral properties, with interactions included via lattice density-functional theory. Our approach appears to be able to capture complex features due to the competition between disorder and interactions. Specifically, we find a dynamical enhancement of delocalization in the presence of a finite bias and an increase of the steady-state current induced by interparticle interactions. This behavior is corroborated by results for the time-dependent densities and for the inverse participation ratio. Using short isolated chains with interaction and disorder, a brief comparative analysis between time-dependent density-functional theory and exact results is then given, followed by general concluding remarks.

We report experiments on proton radiation-enhanced self- and boron (B) diffusion in germanium (Ge) for temperatures between 515 ${}^{\circ }$C and 720 ${}^{\circ }$C. Modeling of the experimental diffusion profiles measured by means of secondary ion mass spectrometry is achieved on the basis of the Frenkel pair reaction and the interstitialcy and dissociative diffusion mechanisms. The numerical simulations ascertain concentrations of Ge interstitials and B-interstitial pairs that deviate by several orders of magnitude from their thermal equilibrium values. The dominance of self-interstitial related defects under irradiation leads to an enhanced self- and B diffusion in Ge. Analysis of the experimental profiles yields data for the diffusion of self-interstitials ($I$) and the thermal equilibrium concentration of B$I$ pairs in Ge. The temperature dependence of these quantities provides the migration enthalpy of $I$ and formation enthalpy of B$I$ that are compared with recent results of atomistic calculations. The behavior of self- and B diffusion in Ge under concurrent annealing and irradiation is strongly affected by the property of the Ge surface to hinder the annihilation of self-interstitials. The limited annihilation efficiency of the Ge surface can be caused by donor-type surface states favored under vacuum annealing, but the physical origin remains unsolved.

We present a comprehensive theory of spin-dependent optical transitions and their dependence on strain in Si and Ge. Symmetries of wave functions and interactions are used to derive concise ratios between intensities of the right and left circularly polarized luminescence for each of the dominant phonon-assisted optical transitions. These ratios are then used to explain the circular polarization degrees of the luminescence peaks in the spectra of biaxially strained Si and Ge, and of relaxed ${\mathrm{Si}}_{1\text{−}x}{\mathrm{Ge}}_x$ alloys. The spectra are numerically calculated by a combination of an empirical pseudopotential method, an adiabatic bond-charge model, and a rigid-ion model.

We study the transport properties of setups with one and two mesoscopic rings threaded by ac magnetic fluxes of the form $\Phi \left(t\right)={\Phi }^{\mathrm{dc}}+{\Phi }^{\mathrm{ac}}\cos ({\Omega }_{0}t+\delta )$ and connected to two different particle reservoirs. We analyze the conditions to generate a pumped dc current in the adiabatic regime. We also study the symmetry properties of the induced dc current as a function of the static component of the flux ${\Phi }^{\mathrm{dc}}$ with and without a dc bias voltage applied at the reservoirs. We analyze, in particular, the validity of the Onsager-Casimir relations for different configurations of the setups.

We investigate the relationship between the Curie temperature $T_C$ and the carrier density $p$ in the ferromagnetic semiconductor (Ga,Mn)As. Carrier densities are extracted from analysis of the Hall resistance at low temperatures and high magnetic fields. Results are found to be consistent with ion channeling measurements when performed on the same samples. We find that both $T_C$ and the electrical conductivity increase monotonically with increasing $p$, and take their largest values when $p$ is comparable to the concentration of substitutional Mn acceptors. This is inconsistent with models in which the Fermi level is located within a narrow isolated impurity band.

We calculate the finite-frequency conductivity of bilayer graphene with a relative twist between the layers. The low-frequency response at zero doping shows a flat conductivity with value twice that of the monolayer case and at higher frequency a strong absorption peak occurs. For finite doping, the low-frequency flat absorption is modified into a peak centered at zero frequency (the Drude response) accompanied by an interband edge which results from the transfer of spectral weight from interband to intraband absorption due to Pauli blocking. If the system is doped sufficiently such that the chemical potential reaches beyond the low-energy saddle point in the twisted bilayer band structure, a strong low-frequency absorption peak appears at an energy related to an effective interlayer hopping energy, which may be used to identify this parameter and confirm the existence of the saddle point which gives rise to a low-energy van Hove singularity in the electronic density of states.

We propose an out-of-equilibrium impurity model for the dynamical mean-field description of the Hubbard model driven by a finite electric field. The out-of-equilibrium impurity environment is represented by a collection of equilibrium reservoirs at different chemical potentials. We discuss the validity of the impurity model and propose a non-perturbative method, based on a quantum Monte Carlo solver, which provides the steady-state solutions of the impurity and original lattice problems. We discuss the relevance of this approach to other non-equilibrium steady-state contexts.

We propose a discrete model—the twisted quantum double model—of 2D topological phases based on a finite group $G$ and a 3-cocycle $\alpha$ over $G$. The detailed properties of the ground states are studied, and we find that the ground-state subspace can be characterized in terms of the twisted quantum double $D^{\alpha }\left(G\right)$ of $G$. When $\alpha$ is the trivial 3-cocycle, the model becomes Kitaev's quantum double model based on the finite group $G$, in which the elementary excitations are known to be classified by the quantum double $D\left(G\right)$ of $G$. Our model can be viewed as a Hamiltonian extension of the Dijkgraaf-Witten topological gauge theories to the discrete graph case with gauge group being a finite group. We also demonstrate a duality between a large class of Levin-Wen string-net models and certain twisted quantum double models, by mapping the string-net $6j$ symbols to the corresponding 3-cocycles.

Carrier relaxation processes have been studied in low indium content self-assembled (In,Ga)As/GaAs quantum dots (QDs). Temperature-dependent photoluminescence of the wetting layer (WL) and QDs, and QD photoluminescence rise time elongation from $\sim$100 to $\sim$200 ps in the range of 10–45 K, indicated a complex carrier relaxation scheme. It involves localization of carriers/excitons in the WL, their temperature-mediated release, and subsequent transfer between the states of the WL and QD ensemble. These observations are explained by a thermal hopping model, in which electron-hole pairs are redistributed within two separate sets of zero-dimensional states of considerably different densities connected by a two-dimensional mobility channel.

We derive a general formalism to model the polariton states resulting from the radiation-matter interaction between an arbitrary number of excitonic transitions in semiconductor quantum dots and photon modes in a photonic crystal structure in which the quantum dots are embedded. The Maxwell equations, including the linear nonlocal susceptibility of the exciton transitions in the quantum dots, are cast into an eigenvalue problem, which can be applied to any structure whose photon modes can be computed with reliable accuracy, and in addition naturally allows for disorder effects to be taken into account. We compute realistic photon modes using Bloch-mode expansion. As example systems, we study typical InGaAs quantum dots in a GaAs photonic crystal structures—an $Ln$ cavity or a $\mathit{W}\mathit{1}$ waveguide. For a single dot, we reproduce known analytical results, while for the two-dot case we study the radiative excitation transfer mechanism and characterize its strength, the dependence on the detuning between quantum dot and photon modes, and the dependence on interdot distance. We find in particular that the interdot radiative coupling strength can reach $100\mu$eV in a short cavity, and its decay with distance in longer cavities and waveguides is determined by the group velocity of the exchanged photons and their radiative lifetime. We also show that, for an $Ln$ cavity of increasing length, the radiative excitation transfer mechanism is subject to a crossover from a regime where a single photon mode is dominating, to a multimode regime—occurring around $\mathit{n}$ $=$ 150 for the system under study.

Using Keldysh-Green function formalism we theoretically analyzed the dynamics of multimode exciton-polariton Josephson junctions. We took into account the spinor nature of polaritons and considered in detail the role of coupling of the fundamental modes with excited states. We demonstrate that the coupling to the reservoir results in a change of the oscillation pattern. In particular, it can lead to renormalization of the oscillation frequency, appearance of higher order harmonics, and induce transition between the regimes of free Josephson oscillations and macroscopic quantum self-trapping.

To clarify lattice distortions induced by adding Ga atoms in the InSb crystal, Ga $K\alpha$ x-ray fluorescence holography (XFH) experiments were carried out on an In${}_{0.995}$Ga${}_{0.005}$Sb diluted mixed single crystal, and three-dimensional atomic images around the Ga atoms were reconstructed. Although the atomic images are located almost at ideal positions of the InSb crystal, some differences can be observed for only the first- and second-neighboring atoms. By combining them with x-ray absorption fine structure data, large spatial fluctuations of the first-neighboring atoms appear in the angular direction, which can be clarified from the present XFH results. From the XFH results, it is concluded that lattice distortions are limited within the second neighbors in this diluted mixed crystal, in contrast to five chemical bonds in a heavily doped mixed crystal of Zn${}_{0.4}$Mn${}_{0.6}$Te reported previously.