Using a scattering matrix approach we study transport in coherent conductors driven by a time-periodic bias voltage. We investigate the role of electron-like and hole-like excitations created by the driving in the energy current noise and we reconcile previous studies on charge current noise in these kinds of systems. The energy noise reveals additional features due to electron-hole correlations. These features should be observable in power fluctuations. In particular, we show results for the case of a harmonic and biharmonic driving and of Lorentzian pulses applied to a two-terminal conductor, addressing recent experiments [Gabelli and Reulet, Phys. Rev. B 87, 075403 (2013) and Dubois et al., Nature (London) 502, 659 (2013)].

To examine optically excited bound states, excitons and trions, in monolayer ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$, we have formulated and applied a generalized time-dependent density-matrix functional theory approach. Three different types of exchange-correlation (XC) kernels were used and their validity was evaluated through comparison with available experimental data. For excitons, we find that the local kernels, from the local density approximation and its gradient-corrected form, lead to much smaller binding energy than that extracted from experimental data, while those based on long-range (LR) interactions fare much better. The same is the case for the trion binding energy once screening effects are taken into account. Our results suggest that for both excitons and trions, the LR form of the XC kernel is necessary to describe bound states. These results confirm information from experimental data on single-layer dichalcogenides that their exciton and trion binding energies are of the order of hundreds (excitons) and tens (trions) of milli-electron volts, a result that may suggest technological application of these materials at room temperature. The proposed methodology can be straightforwardly extended to bound states with a larger number of electrons and holes than considered here.

We study the dynamical localization transition in a one-dimensional periodically kicked incommensurate lattice, which is created by perturbing a primary optical lattice periodically with a pulsed weaker incommensurate lattice. The diffusion of wave packets in the pulsed optical lattice exhibits either extended or localized behaviors, which can be well characterized by the mean-square displacement and the spatial correlation function. We show that the dynamical localization transition is relevant to both the strength of incommensurate potential and the kicked period, and the transition point can be revealed by the information entropy of eigenfunctions of the Floquet propagator.

Time-resolved wide-field magneto-optical microscopy with picosecond time resolution and with phase-locked harmonic excitation is used to image the fundamental dynamic modes of magnetic domain and domain-wall states in soft-magnetic thick film elements. By analyzing mirrored domain states, the pure in-plane and out-of-plane dynamic magnetic response under a continuous microwave excitation is extracted simultaneously. Domain-wall oscillations and local domain response, including transversely aligned and closure domain states, are visualized below, at, and above the resonance frequency of the magnetic elements. Regions of different high-frequency permeability are distinguished from the laterally resolved measurements. Despite nearly constant precessional frequency, a strong mode amplitude variation is found with varying the magnetic bias field, which is correlated to the exhibited change of the individual domain structure. Inhomogeneous out-of-phase precessional modes within the individual domains occur close to a state of domain instability. The imaging results are compared to existing models of flux response. Magnetic domain knowledge and precise dynamic imaging is needed for the understanding of the dynamic magnetization behavior of even basic domain structures beyond the magnetic material's elementary properties.

We present the effect of yttrium substitution on superconductivity in the ${\mathrm{La}}_{1-\mathit{x}}{\mathrm{Y}}_{\mathit{x}}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ system. Polycrystalline samples with nominal Y concentrations up to 40% were synthesized and characterized via electrical resistivity, magnetic susceptibility, and specific-heat measurements. Y substitution reduces the lattice parameter $a$ and unit-cell volume V, and a correlation between the lattice parameter $c$, the La-O-La bond angle, and the superconducting critical temperature $T_c$ is observed. The chemical pressure induced by Y substitution for La produces neither the high-$T_c$ superconducting phase nor the structural phase transition seen in ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ under externally applied pressure.

Polycrystalline samples of ${\mathrm{Lu}}_{1-x}{\mathrm{Sc}}_x\mathrm{MnSi}$ ($x=0$, 0.25, 0.5) are studied using powder x-ray diffraction, heat capacity $C_{\mathrm{p}}$, magnetization, magnetic susceptibility $\chi$, and electrical resistivity $\rho$ measurements versus temperature $T$ and magnetic field $H$. This system crystallizes in the primitive orthorhombic TiNiSi-type structure (space group Pnma) as previously reported. The $\rho \left(T\right)$ data indicate metallic behavior. The $C_{\mathrm{p}}\left(T\right)$, $\chi \left(T\right)$, and $\rho \left(T\right)$ measurements consistently indicate long-range antiferromagnetic (AF) transitions with AF ordering temperatures $T_{\mathrm{N}}=246$, 215, and 188 K for $x=0$, 0.25, and 0.5, respectively. A second transition is observed at somewhat lower $T$ for each sample from the $\chi \left(T\right)$ and $\rho \left(T\right)$ measurements, which we speculate are due to spin reorientation transitions; these second transitions are completely suppressed in $H=5.5$ T. The $C_{\mathrm{p}}$ data below 10 K for each composition indicate an enhanced Sommerfeld electronic heat capacity coefficient for the series in the range $\gamma =24$–29 mJ/mol ${\mathrm{K}}^2$. The $\chi \left(T\right)$ measurements up to 1000 K were fitted by local-moment Curie-Weiss behaviors which indicate a low Mn spin $S\sim 1$. The $\chi$ data below $T_{\mathrm{N}}$ are analyzed using the Weiss molecular field theory for a planar noncollinear cycloidal AF structure with a composition-dependent pitch, following the previous neutron diffraction work of Venturini et al. [J. Alloys Compd. 256, 65 (1997)]. Within this model, the fits indicate a turn angle between Mn ordered moments along the cycloid axis of $\sim {100}^{\circ }$ or $\sim {145}^{\circ }$, either of which indicate dominant AF interactions between the Mn spins in the ${\mathrm{Lu}}_{1-x}{\mathrm{Sc}}_x\mathrm{MnSi}$ series of compounds.

The strong composition dependence of the cell parameters in (more than 700) compounds with the ${{\mathrm{ThCr}}_2\mathrm{Si}}_2$-type structure has been the subject of discussion for several decades. Recent findings of ferromagnetism in ${\mathrm{SrCo}}_2{\left[{\mathrm{Ge}}_{1-x}{\mathrm{P}}_x\right]}_2$ with a quantum critical point at $x\approx 0.325$ have revived interest in the composition-property relationship in such systems. In this article, we present a theoretical insight on these phenomena, supported by electronic structure calculations. We discuss the differences between ${\mathrm{SrCo}}_2{\left[{\mathrm{Ge}}_{1-x}{\mathrm{P}}_x\right]}_2$ and related compounds, and we conclude that previous analyses based on the filling of “relevant” molecular orbitals are case specific and do not describe the general mechanism behind the changes in the cell parameters. We propose a more physical reasoning, based on electronic kinetic energy and electron-core interactions, which gives a unified picture of all cases. With respect to the magnetism in ${\mathrm{SrCo}}_2{\left[{\mathrm{Ge}}_{1-x}{\mathrm{P}}_x\right]}_2$, we find that the density of states at the Fermi level and the Curie temperature display very similar dependences on the composition. Our calculations support the classification of the magnetic dome as a weak itinerant ferromagnetism, and they are consistent with the Stoner-enhanced paramagnetism observed in samples with $x>0.75$. We argue that the interlayer bonding is metallic (instead of covalent) from $x=0$ up to the value at which the magnetic remanence disappears.

The superconducting properties of ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ $(x=0.38–0.45)$ have been studied using magnetization and muon-spin rotation or relaxation $(\mu \mathrm{SR}$) measurements. These measurements show that the superconducting critical temperature $T_c$ of ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ increases with increasing $x$, reaching a maximum at around 4.8 K for $x=0.45$. Zero-field $\mu \mathrm{SR}$ results indicate that time-reversal symmetry is preserved in this material. Transverse-field muon-spin rotation has been used to study the temperature dependence of the magnetic penetration depth $\lambda \left(T\right)$ in the mixed state. For all the compositions studied, $\lambda \left(T\right)$ can be well described using a single-gap $s$-wave BCS model. The magnetic penetration depth at zero temperature $\lambda \left(0\right)$ ranges from 500 to 580 nm. Both the superconducting gap $\Delta \left(0\right)$ at 0 K and the gap ratio $\Delta \left(0\right)/k_B{T}_c$ indicate that ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ $(x=0.38–0.45)$ should be considered as a superconductor with intermediate to strong coupling.

We report field- and temperature-dependent measurements of the thermoelectric power (TEP) across the pseudometamagnetic transition (MMT) in ${\mathrm{CeRu}}_2{\mathrm{Si}}_2$. We applied the thermoelectric gradient parallel and perpendicular to the field along the $c$ axis of the tetragonal crystal which is the easy magnetization axis. At the MMT at $H_m=7.8$ T, a strong anomaly in the TEP is observed for both configurations with opposite signs. The anomaly at $H_m$ becomes a cascade of anomalies at very low temperature which seems to be a generic feature of the TEP at a change in the topology of the Fermi surface (FS) in heavy Fermion multiband systems. Simultaneously, quantum oscillations in the magnetic field dependence of the TEP are observed for both configurations below and above the MMT.

Here, we present a detailed account of the fundamental problems one encounters in projection theory when nonorthogonal basis sets are used for representation of the operators. In particular, we reexamine the use of projection operators in connection with the calculation of projected (or reduced) Green's functions and associated physical quantities such as the local density of states (LDOS), local charge, and conductance. The unavoidable ambiguity in the evaluation of the LDOS and charge is made explicit with the help of simple examples of metallic nanocontacts while the conductance, within certain obvious limits, remains invariant against the type of projection. We also examine the procedure to obtain effective Hamiltonians from reduced Green's functions. For completeness, we include a comparison with results obtained with block-orthogonal basis sets where both direct and dual spaces are used.

Quantum-dot-in-nanowire systems constitute building blocks for advanced photonics and sensing applications. The electronic symmetry of the emitters impacts their function capabilities. Here we study the fine structure of gallium-rich quantum dots nested in the shell of $\mathrm{GaAs}\text{−}{\mathrm{Al}}_{0.51} {\mathrm{Ga}}_{0.49}$ As core-shell nanowires. We used optical spectroscopy to resolve the splitting resulting from the exchange terms and extract the main parameters of the emitters. Our results indicate that the quantum dots can host neutral as well as charged excitonic complexes and that the excitons exhibit a slightly elongated footprint, with the main axis tilted with respect to the long axis of the host nanowire. $\mathrm{GaAs}\text{−}{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ emitters in a nanowire are particularly promising for overcoming the limitations set by strain in other systems, with the benefit of being integrated in a versatile photonic structure.

Using molecular dynamics simulations, we study the role of finite size effects on the determination of interfacial thermal resistance between two solids characterized by high phonon mean free paths. In particular, we will show that a direct, heat source-sink method leads to strong size effect, associated with ballistic phonon transport to and from, and specular reflections at the simulation domain boundary. Lack of proper account for these effects can lead to incorrect predictions about the role of interfacial bonding and structure on interfacial thermal resistance. We also show that the finite size effect can be dramatically reduced by introduction of rough external boundaries leading to diffuse phonon scattering, as explicitly demonstrated by phonon wave-packet simulations. Finally, we demonstrate that when careful considerations are given to the effects associated with the finite heat capacity of the simulation domains and phonon scattering from the external surfaces, a size-independent interfacial resistance can be properly extracted from the time integral of the correlation function of heat power across the interface. Our work demonstrates that reliable and consistent values of the interfacial thermal resistance can be obtained by equilibrium and nonequilibrium methods with a relatively small computational cost.

Electron transport in inorganic semiconductors and metals occurs through delocalized bands formed by overlapping electron orbitals. Strong correlation of electronic wave functions with the ionic cores couples the electron and lattice motions, leading to efficient interaction and scattering that degrades coherent charge transport. By contrast, unoccupied electronic states at energies near the vacuum level with diffuse molecular orbitals may form nearly-free-electron bands with density maxima in non-nuclear interstitial voids, which are subject to weaker electron-phonon interaction. The position of such bands typically above the frontier orbitals, however, renders them unstable with respect to electronic interband relaxation and therefore unsuitable for charge transport. Through electronic-structure calculations, we engineer stable, non-nuclear, nearly-free-electron conduction channels in low-dimensional molecular materials by tailoring their electrostatic and polarization potentials. We propose quantum structures of graphane-derived Janus molecular sheets with spatially isolated conducting and insulating regions that potentially exhibit emergent electronic properties, as a paradigm for molecular-scale non-nuclear charge conductors; we also describe tuning of their electronic properties by application of external fields and calculate their electron–acoustic-phonon interaction.

It has been shown that a slab of a negative index material can behave as a superlens enhancing the imaging resolution beyond the wavelength limit. We show here that if such a slab possesses in addition some magneto-optical activity, it could act as an ideal optical filter and exhibit an extraordinary transverse magneto-optical Kerr effect. Moreover, we show that losses, which spoil the imaging resolution of these lenses, are a necessary ingredient to observe this effect.

Raman spectroscopic measurements of sodium bismuth titanate $({\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3$ or NBT) were carried out upon cooling from 1000 to 80 K to study its two phase transitions. Full spectral deconvolution was performed, comparing the results from several fitting models, with a particular emphasis on the evolution of the central peak and low-frequency phonon modes. The central intensity profile is found to be composed of two well-defined Lorentzian peaks, the temperature dependence of which suggests the presence of fluctuations in $M$-point and $R$-point rotations of the oxygen octahedra as well as cation displacements related to polar nanodomains. The temperature behavior of both central peaks is discussed in relation to the (tilting or shifting) transition behavior in similar perovskite systems. The four observed low-frequency phonon modes indicate a different symmetry assignment than the commonly assigned $R3c$ for the low-temperature phase.

The incorporation of cubic $\beta /{\beta }_0$ phase improves significantly the ductility and workability of TiAl-based alloys. However, the ductility deteriorates with the precipitation of the ordered hexagonal ${\omega }_0$ phase in the $\beta /{\beta }_0$ phase. To avoid the formation of the ${\omega }_0$ phase in alloy design, fundamentals about the $\beta /{\beta }_0$ to ${\omega }_0$ phase transition and its alloying element dependence have to be known. In the present work, we investigated the phase transition and phase stability in ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Nb}$ and ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Mo}$ alloys using a first-principles method based on density functional theory. Our calculations indicate that for the ${\beta }_0$ to ${\omega }_0$ transition, the “collapse-diffusion” pathway is easier than the “diffusion-collapse” one; however, both pathways are quite difficult due to the high-energy barriers. The ${\beta }_0$ to ${\omega }^{\prime \prime }$ transition occurs through the “diffusion-collapse” pathway without an energy barrier (except for the activation energy for the short-range diffusion needed for atomic rearrangement). Comparing the total energies of different phases, we find that the ${\omega }_0$ phase of ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Nb}$ is more stable than ${\beta }_0$ and vice versa for ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Mo}$ whereas the relative stability of ${\omega }^{\prime \prime }$ to ${\beta }_0$ for ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Nb}$ is stronger than that for ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Mo}$, indicating that the addition of Mo inhibits the transition from the ${\beta }_0$- to the $\omega$-related phases in ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Nb}$. Our calculations demonstrate that ${\omega }^{\prime \prime }$ is more stable than ${\omega }_0$ for both ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Nb}$ and ${\mathrm{Ti}}_4{\mathrm{Al}}_3\mathrm{Mo}$, in contrast to the previously proposed picture based on some experimental observations, the possible origins of which are discussed in detail.

We report a density functional theory (DFT) study of ${\mathrm{Ag}}_3\mathrm{Co}\left(\mathrm{CN}\right){}_6$, a material noted for its colossal positive and negative thermal expansion, and its giant negative linear compressibility. Here, we explicitly include the dispersive interaction within the DFT calculation, and find that it is essential to reproduce the ground state, the high-pressure phase, and the phonons of this material, and hence essential to understand this material's remarkable physical properties. New exotic properties are predicted. These include heat enhancement of the negative linear compressibility, a large reduction in the coefficient of thermal expansion on compression with change of sign of the mode Grüneisen parameters under pressure, and large softening of the material on heating. Our results suggest that these are associated with the weak Ag–Ag dispersive interactions acting with an efficient hinging mechanism in the framework structure.

It is important to know the decoherence mechanism of a qubit based on Josephson junctions. At low temperatures, as quasiparticle concentration becomes exponentially small, one needs to consider energy transfer from tunneling electrons to other degrees of freedom to find dissipation in Josephson junctions and decoherence in qubits. Here we discuss the energy transfer to two-level systems, i.e., the transitions between two different configurations of ions inside an insulating layer separated by a potential barrier. We derive a general equation of motion for the phase difference between two superconducting electrodes and we find a retarded dissipation term due to electromagnetic mechanism and also contribution due to electron tunneling mechanism. Using the equation of motion we calculate the decay of Rabi oscillations and frequency shift in qubits due to the presence of the two-level systems. In the long-time limit our results coincide with those obtained by Martinis et al. [Martinis et al., Phys. Rev. Lett. 95, 210503 (2005)] within the Fermi's Golden Rule approach up to a numerical factor.

Motivated by the findings of logarithmic spreading of entanglement in a many-body localized system, we identify and untangle two factors contributing to the spreading of entanglement in the fully many-body localized phase, where all many-body eigenstates are localized. Performing full diagonalizations of an $XXZ$ spin model with random longitudinal fields, we demonstrate a linear dependence of the spreading rate on the decay length $\left(\xi \right)$ of the effective interaction between localized pseudospins (l-bits), which depends on the disorder strength, and on the final value of entanglement per spin $\left(s_{\infty }\right)$, which primarily depends on the initial state. The entanglement entropy thus grows with time as $\sim \xi \times s_{\infty }logt$, providing support for the phenomenology of many-body localized systems proposed by Huse and Oganesyan.