Using advanced three-dimensional simulations, we show that an Abrikosov vortex, trapped inside a cavity perpendicular to an artificial Josephson junction, can serve as a very efficient source for generation of Josephson vortex-antivortex pairs in the presence of the applied electric current. In such a case, the nucleation rate of the pairs can be tuned in a broad range by an out-of-plane ac magnetic field in a broad range of frequencies. This parametrically amplified vortex-antivortex nucleation can be considered as a macroscopic analog of the dynamic Casimir effect, where fluxon pairs mimic the photons and the ac magnetic field plays the role of the oscillating mirrors. The emerging vortex pairs in our system can be detected by the pronounced features in the measured voltage characteristics, or through the emitted electromagnetic radiation, and exhibit resonant dynamics with respect to the frequency of the applied magnetic field. Reported tunability of the Josephson oscillations can be useful for developing high-frequency emission devices.

We observed that electric field induces phase transition from tetragonal to rhombohedral in polycrystalline morphotropic lead zirconate titanate (PZT) films, as reported in 2011 for bulk PZT. Moreover, we evidenced that this field-induced phase transition is strongly correlated with PZT film piezoelectric properties, that is to say the larger the phase transition, the larger the longitudinal piezoelectric coefficient $d_{33,\mathrm{eff}}$. Although $d_{33,\mathrm{eff}}$ already reaches $130-150\mathrm{pm}/\mathrm{V}$, our observation suggests that one could obtain larger $d_{33,\mathrm{eff}}$ values, namely $250\mathrm{pm}/\mathrm{V}$, by optimizing the field-induced phase transition thanks to composition fine tuning as close as possible to the morphotropic phase boundary.

We predict a candidate high-temperature, high-pressure structure of ${\mathrm{FeSiO}}_3$ with space-group symmetry Cmmm by applying an evolutionary algorithm within density functional theory (DFT)+U that we call post-perovskite II (PPv-II). An exhaustive search found no other competitive candidate structures with ${AB\text{O}}_3$ composition. We compared the x-ray diffraction pattern of ${\mathrm{FeSiO}}_3$ PPv-II with experimental results of the recently reported “H phase” of (Fe,Mg)${\mathrm{SiO}}_3$. The intensities and positions of two main x-ray diffraction peaks of PPv-II ${\mathrm{FeSiO}}_3$ compare well with those of the H phase. We also calculated the static equation of state, the enthalpy, and the bulk modulus of the PPv-II phase and compared it with those of the perovskite (Pv) and post-perovskite (PPv) phases of ${\mathrm{FeSiO}}_3$. According to the static DFT+U computations, the PPv-II phase of ${\mathrm{FeSiO}}_3$ is less stable than the Pv and PPv phases under lower mantle pressure conditions at $T=0$ K and has a higher volume. PPv-II may be entropically stabilized, and may be a stable phase in Earth's lower mantle, coexisting with -${\mathrm{PbO}}_2$ (columbite-structure) silica and perovskite, or with magnesiowustite and/or ferropericlase, depending on the bulk composition.

Internal friction (IF) describes the ability of materials to damp out mechanical oscillations. It is a crucial engineering parameter and also conveys unique microscopic information about structural defects, transport phenomena, and phase transformations in solids. While IF predominately results from lattice defects in crystalline materials, the origin of IF remains unclear in disordered materials, like metallic glasses. In this paper, we study the atomic rearrangements that govern IF in a model metallic glass, via numerical simulations of dynamical mechanical spectroscopy together with structural analysis. We identify cooperative and avalanchelike thermal-driven excitations as an underlying mechanism and demonstrate a linearlike relation between the concentrations of these excitations and the values of IF. Structurally, these excitations can be hindered, and thus suppress IF, by slow atoms that usually associate with full icosahedral symmetry. Our results also provide practical guides in tuning IF in metallic glasses from atomistic perspectives.

Long-range magnetic ordering in the quasi-one-dimensional random-bond antiferromagnet ${\mathrm{BaCu}}_2{\left({\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x\right)}_2{\mathrm{O}}_7$ is studied in muon spin rotation experiments as a function of disorder strength. Compared to the disorder-free parent materials, the saturation-ordered moment is found to be considerably reduced. Moreover, even in weakly disordered species, the magnetically ordered state is shown to be highly inhomogeneous. The results are interpreted in terms of weakly coupled random spin chains, governed by the “infinite randomness” fixed point.

We demonstrate enhancement of the spin precession of orthoferrite ${\mathrm{ErFeO}}_3$ using the magnetic near-field produced by a split-ring resonator (SRR), using the terahertz pump-optical Faraday probe measurement. The precession amplitude was enhanced by ∼8 times when the resonance frequency of spin precession was close to the magnetic resonance of SRR. The time evolution of spin precession was successfully reproduced by a coupled spin- and SRR-resonance model mediated by the magnetic near-field. It is suggested that optimization of the metamaterial structure would further increase the enhancement factor, leading to the nonlinear control of spin dynamics using terahertz radiation.

We study the spin-1 model on a triangular lattice in the presence of a uniaxial anisotropy field using a cluster mean-field (CMF) approach. The interplay among antiferromagnetic exchange, lattice geometry, and anisotropy forces Gutzwiller mean-field approaches to fail in a certain region of the phase diagram. There, the CMF method yields two supersolid phases compatible with those present in the $\text{spin}-1/2$ XXZ model onto which the spin-1 system maps. Between these two supersolid phases, the three-sublattice order is broken and the results of the CMF approach depend heavily on the geometry and size of the cluster. We discuss the possible presence of a spin liquid in this region.

${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$ is a recently discovered superconductor that has been suggested to be an unconventional superconductor near magnetism. We report electronic structure calculations showing that despite the layered crystal structure the material is an anisotropic three-dimensional (3D) metal. The Fermi surface contains prominent one-dimensional (1D) and two-dimensional (2D) features, including nested 1D sheets, a 2D cylindrical section, and a 3D sheet. The electronic states that make up the Fermi surface are mostly derived from Te $p$ states with small Ta $d$ and Pd $d$ contributions. This places the compound far from magnetic instabilities. The results are discussed in terms of multiband superconductivity.

We report on polarized infrared reflectivity measurements of multiferroic, monoclinic ${\mathrm{MnWO}}_4$ between 10  and 295 K. All five nonvanishing components of the dielectric tensor have been determined in the frequency range of the phonons. All infrared-active phonon modes (7 $A_u$ modes and 8 $B_u$ modes) are unambiguously identified. In particular, the strongest $B_u$ modes have been overlooked in previous studies, in which the monoclinic symmetry was neglected in the analysis. The combined analysis of reflectance data measured in different experimental geometries ($R_{ac}$ and $R_p$) is particularly helpful for a proper identification of the $B_u$ modes. Using a generalized Drude-Lorentz model, we determine the temperature dependence of the phonon parameters, including the orientation of the $B_u$ modes within the $ac$ plane. The phonon parameters and their temperature dependence were discussed controversially in previous studies, which did not include a full polarization analysis. Our data do not confirm any of the anomalies reported above 20 K. However, in the paramagnetic phase we find a drastic reduction of the spectral weights of the weakest $A_u$ mode and of the weakest $B_u$ mode with increasing temperature. Below 20 K, the parameters of the $A_u$ phonon modes for $E\parallel b$ show only subtle changes, which demonstrate a finite but weak coupling between lattice dynamics and magnetism in ${\mathrm{MnWO}}_4$. A quantitative comparison of our infrared data with the quasistatic dielectric constant ${\varepsilon }_b$ yields a rough estimate for the oscillator strength $\Delta {\varepsilon }_{\mathrm{em}}⪅0.02$ of a possible electromagnon for $E\parallel b$. Furthermore, we report on a Kramers-Kronig-consistent model which is able to describe non-Lorentzian line shapes in compounds with monoclinic symmetry.

The position of the band edges of a material plays a key role in determining the properties for a range of applications, but fundamental band bending is an interface-dependent property that cannot be quantified without knowledge of bulk electron energy levels. We present a method for calculating the bulk position of the valence band maximum, and therefore the bulk ionization potential, from periodic plane wave calculations as shown for a range of rocksalt ionic oxides. We demonstrate that, for the popular “slab alignment” technique, explicit consideration of any surface induced electronic polarization is necessary to calculate accurate bulk ionization potentials. Our proposed method to quantify these surface effects, using polarizable-shell based interatomic potentials, is very computationally affordable, and our updated slab alignment method yields much improved agreement with the available experimental data.

We investigate Ge mixing at the Si(001) surface and characterize the $2\times N$ Si(001) reconstruction by means of hybrid quantum and molecular mechanics calculations (QM/MM). Avoiding fake elastic dampening, this scheme allows to correctly take into account long-range deformation induced by reconstructed and defective surfaces. We focus in particular on the dimer vacancy line (DVL) and its interaction with Ge adatoms. We first show that calculated formation energies for these defects are highly dependent on the choice of chemical potential and that the latter must be chosen carefully. Characterizing the effect of the DVL on the deformation field, we also find that the DVL favors Ge segregation in the fourth layer close to the DVL. Using the activation-relaxation technique (ART nouveau) and QM/MM, we show that a complex diffusion path permits the substitution of the Ge atom in the fourth layer, with barriers compatible with mixing observed at intermediate temperature. We also show that the use of QM/MM results in much more significant corrections at the saddle points (up to 0.5 eV) that at minima, demonstrating its importance for describing kinetics correctly.

We demonstrate a single-photon collection efficiency of $(44.3\pm 2.1)\%$ from a quantum dot in a low-Q mode of a photonic-crystal cavity with a single-photon purity of $g^{\left(2\right)}\left(0\right)=(4\pm 5)\%$ recorded above the saturation power. The high efficiency is directly confirmed by detecting up to $962\pm 46$ kilocounts per second on a single-photon detector on another quantum dot coupled to the cavity mode. The high collection efficiency is found to be broadband, as is explained by detailed numerical simulations. Cavity-enhanced efficient excitation of quantum dots is obtained through phonon-mediated excitation and under these conditions, single-photon indistinguishability measurements reveal long coherence times reaching $0.77\pm 0.19$ ns in a weak-excitation regime. Our work demonstrates that photonic crystals provide a very promising platform for highly integrated generation of coherent single photons including the efficient out-coupling of the photons from the photonic chip.

The Floquet topological superconducting state is a nonequilibrium time-periodic state hosting Majorana fermions. We study its transport properties by using the Kitaev model with time-periodic incommensurate potentials, which experiences phase transition from the Floquet topological superconducting phase to the Anderson localized phase with increasing driving strength. We study both the real time dynamics of the current and the nonanalytic behavior of the tunneling conductance at the transition. Especially, we find that the tunneling conductance changes continuously at the transition, being a finite value in the presence of Floquet-Majorana fermions, but drops to zero as the Majorana fermions vanish. For a special choice of parameters, the Majorana fermions revive at a larger driving strength, accompanied by the revival of conductances.

We study the transport properties of a four-terminal corner junction made by etching a two-dimensional topological insulator to form a quantum point contact (QPC). The QPC geometry enables interboundary tunneling processes allowing for the coupling among states with different helicity, while the tight confinement in the QPC region activates charging effects leading to the Coulomb-blockade physics. Peculiar signatures of these effects are theoretically investigated using a scattering field theory modified to take into account the electron-electron interaction within a self-consistent mean field approach. The current-voltage characteristics and the current fluctuations (noise) are derived beyond the linear response regime. Universal aspects of the thermal noise of the corner junction made of helical matter are also discussed.

Collective charge-density modes (plasmons) of the clean two-dimensional unpolarized electron gas are stable, for momentum conservation prevents them from decaying into single-particle excitations. Collective spin-density modes (spin plasmons) possess no similar protection and rapidly decay by production of electron-hole pairs. Nevertheless, if the electron gas has a sufficiently high degree of spin polarization ($P>1/7$, where $P$ is the ratio of the equilibrium spin density and the total electron density, for a parabolic single-particle spectrum) we find that a long-lived spin plasmon—a collective mode in which the densities of up and down spins oscillate with opposite phases—can exist within a “pseudogap” of the single-particle excitation spectrum. The ensuing collectivization of the spin excitation spectrum is quite remarkable and should be directly visible in Raman-scattering experiments. The predicted mode could dramatically improve the efficiency of coupling between spin-wave-generating devices, such as spin-torque oscillators.

The performance of dye-sensitized solar cells is tightly linked to the relative energy level alignment of its constituents. In this paper the electronic properties of a model of dye-sensitized solar cell are studied by accurate first-principle calculations taking into account many-body effects beyond density-functional theory. The cell model includes one layer of co-adsorbed solvent (water or acetonitrile) molecules. Solvent molecules induce an upwards energy shift in the ${\mathrm{TiO}}_2$ bands; such a shift is larger in the case of acetonitrile. The accurate determination of the energy levels allows the theoretical estimation of the maximum attainable open circuit voltage $\left(V_{\mathrm{oc}}\right)$.

We study the energy transport between two interacting spin chains which are initially separated, held at different temperatures, and subsequently put in contact. We consider the spin-1/2 XXZ model in the gapless regime and exploit its integrability properties to formulate an analytical ansatz for the nonequilibrium steady state even at temperatures where the low-energy Luttinger liquid description is not accurate. We apply our method to compute the steady energy current and benchmark it both with the known low-energy limit and at higher temperatures with numerical simulations. We find an excellent agreement even at high temperatures, where the Luttinger liquid prediction is shown to fail.

We have experimentally studied the pump-probe Kerr rotation dynamics in ${\mathrm{WSe}}_2$ monolayers. This yields a direct measurement of the exciton valley depolarization time ${\tau }_v$. At $T=4\mathrm{K}$, we find ${\tau }_v\approx 6$ ps, a fast relaxation time resulting from the strong electron-hole Coulomb exchange interaction in bright excitons. The exciton valley depolarization time decreases significantly when the lattice temperature increases, with ${\tau }_v$ being as short as 1.5 ps at 125 K. The temperature dependence is well explained by the developed theory, taking into account the exchange interaction and fast exciton scattering time on the short-range potential.

We report on the first experimental observation of an apparent metal-insulator transition in a two-dimensional electron gas confined in an InAs quantum well. At high densities we find that the carrier mobility is limited by background charged impurities and the temperature dependence of the resistivity shows a metallic behavior with resistivity increasing with increasing temperature. At low densities we find an insulating behavior below a critical density of $n_c=5\times {10}^{10}{\mathrm{cm}}^{-2}$ with the resistivity decreasing with increasing temperature. We analyze this transition using a percolation model arising from the failure of screening in random background charged impurities. We also examine the percolation transition experimentally by introducing remote ionized impurities at the surface. Using a bias during cooldown, we modify the screening charge at the surface, which strongly affects the critical density. Our study shows that transition from a metallic to an insulating phase in our system is due to percolation transition.