We develop a numerically exact scheme for resumming certain classes of Feynman diagrams in the self-consistent perturbative expansion for the electron and magnon self-energies in the nonequilibrium Green function formalism applied to a coupled electron-magnon (e-m) system driven out of equilibrium by the applied finite bias voltage. Our scheme operates with the electronic and magnonic GFs and the corresponding self-energies viewed as matrices in the Keldysh space, rather than conventionally extracting their retarded and lesser components, which greatly simplifies translation of diagrams into compact mathematical expressions and their computational implementation. This is employed to understand the effect of inelastic e-m scattering on charge and spin current vs bias voltage $V_b$ in F/I/F (F-ferromagnet; I-insulating barrier) magnetic tunnel junctions (MTJs), which are modeled on a quasi-one-dimensional (quasi-1D) tight-binding lattice for the electronic subsystem and quasi-1D Heisenberg model for the magnonic subsystem. For this purpose, we evaluate the Fock diagram for the electronic self-energy and the electron-hole polarization bubble diagram for the magnonic self-energy. The respective electronic and magnonic GF lines within these diagrams are the fully interacting ones, thereby requiring to solve the ensuing coupled system of nonlinear integral equations self-consistently. Despite using the quasi-1D model and treating e-m interaction in many-body fashion only within a small active region consisting of few lattice sites around the F/I interface, our analysis captures essential features of the so-called zero-bias anomaly observed [V. Drewello, J. Schmalhorst, A. Thomas, and G. Reiss, Phys. Rev. B 77, 014440 (2008)] in both MgO- and ${\mathrm{AlO}}_x-$based realistic 3D MTJs where the second derivative $d^{2}I/d{V}_b^2$ (i.e., inelastic electron tunneling spectrum) of charge current exhibits sharp peaks of opposite sign on either side $V_b=0$. We show that this is closely related to a substantially modified magnonic density of states (DOS) after the e-m interaction is turned on—the magnonic bandwidth over which DOS is nonzero becomes broadened, thereby making e-m scattering at arbitrary small bias voltage possible, while DOS also acquires peaks (on the top of a continuous background) signifying the formation of quasibound states of magnons dressed by the cloud of electron-hole pair excitations. We also demonstrate that the sum of electronic spin currents in all of the semi-infinite leads attached to the active region quantifies the loss of spin angular momentum carried away from the active region by the magnonic spin current.

${\mathrm{B}}_6\mathrm{O}$ is a member of icosahedral boron-rich solids known for their physical hardness and stability under irradiation bombardment, but it has also recently emerged as a promising high mobility $p-$type transparent conducting oxide. Using a combination of hybrid functional and many-body perturbation theory calculations, we report on the electronic structure and defect properties of this material. Our calculations identify ${\mathrm{B}}_6\mathrm{O}$ has a direct band gap in excess of 3.0 eV and possesses largely isotropic and low effective masses for both holes and electrons. Of the native defects, we identify no intrinsic origin to the reported $p-$type conductivity and confirm that $p-$type doping is not prevented by intrinsic defects such as oxygen vacancies, which we find act exclusively as neutral defects rather than hole-killing donors. We also investigate a number of common impurities and plausible dopants, finding that isolated acceptor candidates tend to yield deep states within the band gap or act instead as donors, and cannot account for $p-$type conductivity. Our calculations identify the only shallow acceptor candidate to be a complex consisting of interstitial H bonded to C substituting on the O site ${\left(\mathrm{CH}\right)}_{\mathrm{O}}$. We therefore attribute the origins of $p-$type conductivity to these complexes formed during growth or more likely via isolated ${\mathrm{C}}_{\mathrm{O}}$ which later binds with H within the crystal. Lastly, we identify Si as a plausible $n-$type dopant, as it favorably acts as a shallow donor and does not suffer from self-compensation as may the C-related defects. Thus, in addition to the observed $p-$type conductivity, ${\mathrm{B}}_6\mathrm{O}$ exhibits promise of $n-$type dopability if the stoichiometry and both native and extrinsic sources of compensation can be sufficiently controlled.

We report on a combined off-specular and specular x-ray scattering growth study of ultrathin films of the prototypical organic semiconductor diindenoperylene (DIP, ${\mathrm{C}}_{32}{\mathrm{H}}_{16})$. We investigate the evolution of the in-plane correlation length and the growth kinetics of the films including their dependence on the substrate temperature and the growth rate. We observe a temperature-dependent collective rearrangement of DIP molecules from a transient surface induced to the thin-film phase, which can be rationalized by incorporating a thickness-dependent out-of-plane lattice parameter. We further observe that the nucleation behavior of DIP changes from the first to the second monolayer, which we relate to a difference in the diffusion length of the molecules.

Static and dynamic autocorrelations of charge density, composed of positive point ions and instantaneous distribution of electron density, are studied in liquid Li in a pressure range from ambient to 186 GPa using ab initio molecular dynamics simulations. It is shown analytically that the long-wavelength limit of the charge-charge static structure factor $S_{QQ}\left(k\right)$ of liquid metals is proportional to $k^4$. Time-dependent charge-charge correlations in liquid Li at low pressures show identical relaxation as the density-density time correlation functions, in complete agreement with the linear response theory, whereas at extreme pressures we observed different relaxation of the charge and density autocorrelations. The static and dynamic properties of a part of electron density, that corresponds to the nonspherical distribution around ions, are discussed.

Single crystals of ${\mathrm{Sc}}_x$${\mathrm{Ti}}_{1-x}\text{OCl}(x=0.005)$ have been grown by the vapor phase transport technique. Specific heat measurements prove the absence of phase transitions for 4–200 K. Instead, an excess entropy is observed over a range of temperatures that encompasses the incommensurate phase transition at 90 K and the spin-Peierls transition at 67 K of pure TiOCl. Temperature-dependent x-ray diffraction on ${\mathrm{Sc}}_x$${\mathrm{Ti}}_{1-x}$OCl gives broadened diffraction maxima at incommensurate positions between $T_{c1}=61.5\left(3\right)$ and $\sim 90$ K, and at commensurate positions below 61.5 K. These results are interpreted as due to the presence of an incommensurate phase without long-range order at intermediate temperatures, and of a highly disturbed commensurate phase without long-range order at low temperatures. The commensurate phase is attributed to a fluctuating spin-Peierls state on an orthorhombic lattice. The monoclinic symmetry and local structure of the fluctuations are equal to the symmetry and structure of the ordered spin-Peierls state of TiOCl. A novel feature of ${\mathrm{Sc}}_x$${\mathrm{Ti}}_{1-x}\text{OCl}(x=0.005)$ is a transformation from one fluctuating phase (the incommensurate phase at intermediate temperatures) to another fluctuating phase (the spin-Peierls-like phase). This transformation is not a phase transition occurring at a critical temperature, but it proceeds gradually over a temperature range of $\sim 10$ K wide. The destruction of long-range order requires much lower levels of doping in TiOCl than in other low-dimensional electronic crystals, like the canonical spin-Peierls compound ${\mathrm{CuGeO}}_3$. An explanation for the higher sensitivity to doping has not been found, but it is noticed that it may be the result of an increased two-dimensional character of the doped magnetic system. The observed fluctuating states with long correlation lengths are reminiscent of Kosterlitz–Thouless-type phases in two-dimensional systems.

We report alloying-induced and field-induced ferromagnetism in the $\eta$-carbide-type compound ${\mathrm{Fe}}_3{\mathrm{Mo}}_3\mathrm{N}$, which shows a non-Fermi-liquid behavior in the vicinity of a ferromagnetic quantum critical point. Co substitution induces ferromagnetism in ${(\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x)$${}_3{\mathrm{Mo}}_3\mathrm{N}$ in the composition range $0.05\le x\le 0.60$. With increasing $x$, the magnetism varies from the Curie-Weiss-type paramagnetism with a maximum in the temperature dependence of the magnetic susceptibility to another paramagnetism without the maximum via a weak ferromagnetism. An itinerant electron metamagnetic transition is observed for low $x$ at a magnetic field of $\sim$14 T. The magnetic phase diagram, in which the alloying-induced and the field-induced ferromagnetic phases are separated, is different from conventional phase diagrams for weak ferromagnets. Taking account of the fact that a magnetic field induces a Fermi-liquid behavior, the quantum criticality for pure ${\mathrm{Fe}}_3{\mathrm{Mo}}_3\mathrm{N}$ appears to be dominated by the dispersive spin fluctuations observed in the alloying-induced ferromagnetic phase.

Possible contributions of electron-magnon scatterings for the ultrafast demagnetization after femtosecond laser irradiation of films of Ni and Fe are investigated by the ab initio spin-density-functional electron theory. The calculations are based on Fermi's golden rule for transition rates, and the nonequilibrium after the action of the laser pulse is modeled by different chemical potentials for electrons which are in a “dominant spin-up” or in a “dominant spin-down” state. First, it is shown that the experimentally observed demagnetization cannot be described by electron-magnon spin-flip scatterings with concomitant changes of the electronic orbital moments which are immediately quenched by the crystal field, a mechanism which has been suggested by Carpene et al. Second, it is argued that the experimentally observed demagnetization possibly can be explained by a combination of individual spin-flip electron-phonon and spin-flip electron-magnon processes. A precondition for this is (among others) that the magnon emission rate is much larger than the magnon absorption rate. It is shown that this precondition is indeed fulfilled for Ni and especially for Fe.

The quasi-one-dimensional (1D) Ising ferromagnet ${\mathrm{CoNb}}_2$${\mathrm{O}}_6$ has recently been driven via applied transverse magnetic fields through a continuous quantum phase transition from spontaneous magnetic order to a quantum paramagnet, and dramatic changes were observed in the spin dynamics, characteristic of weakly perturbed 1D Ising quantum criticality. We report here extensive single-crystal inelastic neutron scattering measurements of the magnetic excitations throughout the three-dimensional (3D) Brillouin zone in the quantum paramagnetic phase just above the critical field to characterize the effects of the finite interchain couplings. In this phase, we observe that excitations have a sharp, resolution-limited line shape at low energies and over most of the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the full bandwidth along the strongly dispersive chain direction and resolve clear modulations of the dispersions in the plane normal to the chains, characteristic of frustrated interchain couplings in an antiferromagnetic isosceles triangular lattice. The dispersions can be well parametrized using a linear spin-wave model that includes interchain couplings and further neighbor exchanges. The observed dispersion bandwidth along the chain direction is smaller than that predicted by a linear spin-wave model using exchange values determined at zero field, and this effect is attributed to quantum renormalization of the dispersion beyond the spin-wave approximation in fields slightly above the critical field, where quantum fluctuations are still significant.

CdSe/CdS heterostructured nanocrystals with quasi-type-II band alignments provide an interesting platform for studying the photoluminescence (PL) blinking associated with their unique morphologies. By using simultaneous measurements of the PL intensity, lifetime, and polarization anisotropy, we reveal the role of the electron delocalization during the blinking of single CdSe/CdS dot-in-rods. We found that a significant change in the PL polarization anisotropy distinguishes between two kinds of charged excitons with different electron delocalizations. We report our observation of unique polarized PL blinking governed by the band alignments and the Coulomb interactions between the charges inside and outside the dot-in-rod.

Understanding the nonmonotonic behavior in the temperature dependent resistance $R$$\left(T\right)$ of strongly correlated two-dimensional (2D) carriers in clean semiconductors has been a central issue in the studies of 2D metallic states and metal-insulator transitions. We have studied the transport of high mobility 2D holes in 20-nm-wide GaAs quantum wells with varying short-range disorder strength by changing the Al fraction $x$ in the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}$As barrier. Via varying the short-range interface roughness and alloy scattering, it is observed that increasing $x$ suppresses both the strength and characteristic temperature scale of the 2D metallicity, pointing to the distinct role of short-range vs long-range disorder in the 2D metallic transport in this correlated 2D hole system with interaction parameter $r_s\sim 20$.

Low-energy electron scattering with nanoflakes is investigated using a time-dependent density functional theory (TDDFT) simulation in real time and real space. By representing the incident electron as a finite-sized wave packet, we obtain diffraction patterns that show not only the regular features of conventional low-energy electron diffraction (LEED) for periodic structures but also special features resulting from the local atomic inhomogeneity. We have also found a signature of $\pi$ plasmon excitation upon electron impact on a graphene flake. The present study shows the remarkable potential of TDDFT for simulating the electron scattering process, which is important for clarifying the local and periodic atomic geometries as well as the electronic excitations in nanostructures.

Coherence in electronic interferometers is typically believed to be restored fully in the limit of small voltages, frequencies, and temperatures. However, it is crucial to check this essentially perturbative argument by nonperturbative methods. Here we use the numerical renormalization group to study ac transport and decoherence in an experimentally realizable model interferometer, a parallel double quantum dot coupled to a phonon mode. The model allows us to clearly distinguish renormalization effects from decoherence. We discuss finite-frequency transport and confirm the restoration of coherence in the dc limit.

The appearance of topologically protected states at the surface of an ordinary insulator is a rare occurrence and to date only a handful of materials are known for having this property. An intriguing question concerns the possibility of forming topologically protected interfaces between different materials. Here we propose that a topological phase can be transferred to graphene by proximity with the three-dimensional topological insulator ${\mathrm{Bi}}_2$${\mathrm{Se}}_3$. By using density functional and transport theory, we prove that, at the verge of the chemical bond formation, a hybrid state forms at the graphene/${\mathrm{Bi}}_2$${\mathrm{Se}}_3$ interface. The state has Dirac-cone-like dispersion at the $\Gamma$ point and a well defined helical spin texture, indicating its topologically protected nature. This demonstrates that proximity can transfer the topological phase from ${\mathrm{Bi}}_2$${\mathrm{Se}}_3$ to graphene.

We study lattice thermal transport in large-area polycrystalline graphene, such as the samples grown by chemical vapor deposition (CVD) of carbon on Cu. These systems are composed of single-crystalline grains with a broad range of sizes and crystal orientations, separated by atomically rough grain boundaries. We solve the phonon Boltzmann transport equation and calculate the thermal conductivity in each grain, including scattering from the grain boundary roughness. Thermal transport in the large-area sample is considered in the Corbino-membrane geometry, with heat flowing through a network of thermal resistors and away from a pointlike heat source. The thermal transport in polycrystalline graphene is shown to be highly anisotropic, depending on the individual properties of the grains (their size and boundary roughness), as well as on grain connectivity. Strongest heat conduction occurs along large-grain filaments, while the heat flow is blocked through regions containing predominantly small grains. We discuss how thermal transport in CVD graphene can be tailored by controlling grain disorder.

We analyze the collective magnetoroton excitations of bosonic Laughlin $\nu =\frac{1}{2}$ fractional quantum Hall (FQH) states on the torus and of their analog on the lattice, the fractional Chern insulators (FCIs). We show that, by applying the appropriate mapping of momentum quantum numbers between the two systems, the magnetoroton mode can be identified in FCIs and that it contains the same number of states as in the FQH case. Further, we numerically test the single-mode approximation to the magnetoroton mode for both the FQH and FCI cases. This proves particularly challenging for the FCI because its eigenstates have a lower translational symmetry than the FQH states. In spite of this, we construct the FCI single-mode approximation such that it carries the same momenta as the FQH states, allowing for a direct comparison between the two systems. We show that the single-mode approximation captures well a dispersive subset of the magnetoroton excitations both for the FQH and the FCI cases. We find remarkable quantitative agreement between the two systems. For example, the many-body excitation gap extrapolates to almost the same value in the thermodynamic limit.

Polaritons have often been proposed as promising candidates for the realization of all-optical devices due to the easy manipulation and control of their density and spin. In this paper we present a relaxation mechanism for confined polaritons and its application in a device based on polariton lasing in which the incoming monochromatic beam can be channeled into several polariton lasers at different wavelengths. Polaritons are injected in a 3 $\mu$m diameter trap by an incoming beam slightly detuned with respect to a confined state, and the resulting optical bistability is then imprinted on the lower confined states through an efficient relaxation mechanism which combines phonon interaction and bosonic stimulation. Above threshold, all the confined states behave like lasers and the onset of coherence is demonstrated by spectral narrowing. Moreover, the initial polarization of the laser is conserved during the relaxation process allowing for spin logic operations. In this paper we demonstrate that, due to the nonlinear behavior, it is possible to switch ON and OFF the lasing from the confined states of the trap by tailoring the input beam. All the experimental findings are validated and very well reproduced in the framework of the generalized Gross-Pitaevskii equation.

Using first-principles simulation, the correlation between structure, phonon spectra, thermal expansion, and thermomechanics of single-layer ${\mathrm{MoS}}_2$ is established. The laminar structure results in the low-dimension ZA mode with a parabolic dispersion and negative Grüneisen constants ($\gamma$), while the nonorthogonal covalent Mo–S bonds (or intralayer thickness) result in the interatom and interdirection vibrational hybridizations, which tend to increase $\gamma$. There is a negative-positive crossover in thermal expansion coefficient at 20 K, because of the competition between the modes with negative and positive $\gamma$. Although the phononic activation at finite temperatures has a stiffening effect on the bulk modulus, the dominant effect from thermal expansion softens the lattice upon heating. The intralayer thickness results in the similarity between the thermal expansions of SL and bulk ${\mathrm{MoS}}_2$. Our numerical results explicitly support that the experimentally measured thermal shifts of the Raman modes are dominated by multiphonon scattering, but not thermal expansion.

The microstructural origin of the exceptionally high piezoelectric response of polycrystalline $\mathrm{0.5Ba}\left({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}\right)O_3-0.5({\mathrm{Ba}}_{0.7}$${\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_3$ is investigated using in situ transmission electron microscopy, in addition to a wide variety of bulk measurements and first-principles calculations. A direct correlation is established relating a domain wall-free state to the ultrahigh piezoelectric $d_{33}$ coefficient in this ${\mathrm{BaTiO}}_3$-based composition. The results suggest that the unique single-domain state formed during electrical poling is a result of a structural transition from coexistent rhombohedral and tetragonal phases to an orthorhombic phase that has an anomalously low elastic modulus. First-principles calculations indicate that incorporating ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Zr}}^{4+}$ into ${\mathrm{BaTiO}}_3$ reduces the differences in structure and energy of the variant perovskite phases, and $\mathrm{0.5Ba}({\mathrm{Zr}}_{0.2}$${\mathrm{Ti}}_{0.8})O_3-0.5({\mathrm{Ba}}_{0.7}$${\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_3$ is identified as unique because the variant phases become almost indistinguishable. The structural instability and elastic softening observed here are responsible for the excellent piezoelectric properties of this lead-free ceramic.

Nanogranular trititanium pentaoxide $\left({\mathrm{Ti}}_3{\mathrm{O}}_5\right)$, an oxide showing a photoreversible phase transition at room temperature, has recently attracted considerable attention because of its applicability in optical data storage. We report the dynamics of the photoinduced semiconductor-to-metal transition (β phase to λ phase) at room temperature. With time-resolved diffuse reflection spectroscopy conducted over a wide range of time scales, from femtoseconds to microseconds, the overall relaxation behavior was characterized. We found that a transient phase transition occurs within a few hundreds of femtoseconds and that 40% of the converted λ-phase fractions revert to β phase within 1 ps. Carriers are generated instantaneously and disappear within 20 ns. The λ phase increases from 1 to 10 ps, indicating growth of the metallic domain. A slow recovery process, with a duration ranging from ${10}^{-9}$ to ${10}^{-4}$ s, was found and is ascribed to the shrinking and annihilation of the size-distributed metastable λ-phase domains. The fast onset, with a duration within a few hundreds of femtoseconds and the subsequent stabilization process of less than 10 ps, signal the applicability of this material to ultrafast photoswitching.