We study the steady-state properties as well as the relaxation dynamics of the nonequilibrium interacting resonant-level model at finite temperatures. It constitutes the prototype model of a correlated charge fluctuating quantum dot. The two reservoirs are held at different chemical potentials—the difference being the bias voltage—and different temperatures; we discuss the transport through as well as the occupancy of the single level dot. First, we show analytically that in the steady state the reservoir temperatures in competition with the other energy scales act as infrared cutoffs. This is rather intuitive but, depending on the parameter regime under consideration, leads to a surprisingly rich variety of power laws in the current as a function of the temperatures and the bias voltage with different interaction dependent exponents. Next we clarify how finite reservoir temperatures affect the dynamics. They allow one to tune the interplay of the two frequencies characterizing the oscillatory part of the time evolution of the model at zero temperature. For the exponentially decaying part we disentangle the contributions of the level-lead hybridization and the temperatures to the decay rates. We identify a coherent-to-incoherent transition in the long-time dynamics as the temperature is raised. It occurs at an interaction dependent critical temperature. Finally, taking different temperatures in the reservoirs we discuss the relaxation dynamics of a temperature gradient driven current.

We have performed soft x-ray angle-resolved photoemission spectroscopy (ARPES) measurements on YbRh${}_2$Si${}_2$ and clarified its three-dimensional bulk valence-band structures. The ARPES spectra have not only Yb${}^{3+}$ multiplet peaks but also a finite contribution of Yb${}^{2+}$ peaks at 15 K, corresponding to the valence fluctuating behavior in this compound. This means that Yb 4$f$ electrons in this compound have itinerant character below the $T_K$. We have found that dispersions of the valence bands except the vicinity of the Yb 4$f$ bands agree better with those of the band-structure calculation of LuRh${}_2$Si${}_2$ than those of YbRh${}_2$Si${}_2$ within a local-density approximation. In addition, the Yb 3$d$-4$f$ resonant photoemission spectra of YbRh${}_2$Si${}_2$ strongly suggest the existence of Yb 5$d$ electrons in the valence band. We conclude that the charge transfer from the Yb 4$f$ state to the Yb 5$d$ state has an important role in the formation of the valence band of YbRh${}_2$Si${}_2$.

The buckminsterfullerene C${}_{60}$ molecules are composed of ∼99$\%$ naturally abundant ${}^{12}$C carbon atoms having spinless nucleus and thus zero hyperfine interaction. Therefore it has been assumed that the spin diffusion length in C${}_{60}$-based spin-valves is large. We fabricated spin-valves based on C${}_{60}$ and studied the magnitude of the obtained giant magnetoresistance (GMR) as a function of bias voltage, temperature, and C${}_{60}$ layer thickness. Surprisingly, we found that GMR first increases as the C${}_{60}$ layer thickness increases, reaching a maximum at ∼35 nm, then exponentially decreases with thickness from which we extracted a small spin diffusion length of ∼12 nm at 10 K. From our data, we obtain two important conclusions. First, the morphology related disorder that originates from the C${}_{60}$ nano-crystalline grains embedded into an amorphous phase of C${}_{60}$ is responsible for an unusual spin diffusion process that results in short spin diffusion length. Second, we identify the main spin relaxation dynamics in the fullerene to be the grain boundaries in which spin-orbit coupling is enhanced by the local electric field.

The optical spin-Hall effect results in the formation of an antisymmetric real-space polarization pattern giving birth to spin currents. In this work, we show that the exciton-polariton parametric scattering allows us to reverse the sign of these currents. We describe the pulsed resonant excitation of a strongly coupled microcavity with a linearly polarized pump at normal incidence. The energy of the pulse is set to be close to the inflexion point of the lower polariton dispersion branch and the focusing in real space populates the reciprocal space on a ring. Above threshold, the parametric scattering towards the idler and the signal state is triggered on the whole elastic circle. The injected particles are scattered toward these states while propagating radially all over the plane, gaining a cross-linear polarization with respect to that of the pump during the nonlinear process. Consequently, the propagation of the polaritons within the effective magnetic field results in the optical spin-Hall effect, with inverted polarization domains.

We show that hot phonons emitted in energy conversion or resistive processes can be converted to electric potential in heterobarrier structures. Using phonon and electron interaction kinetics and self-consistent ensemble Monte Carlo, we find the favorable conditions for unassisted absorption of hot phonons and design graded heterobarriers for their direct conversion into electric energy. Tandem barriers with nearly optical-phonon height allow for substantial potential gain without current loss. We find that 19$\%$ of hot phonons can be harvested with an optimized GaAs/Al${}_x$Ga${}_{1-x}$As barrier structure over a range of current and electron densities, thus enhancing the overall energy conversion efficiency and reducing waste heat.

The tunneling processes between independently contacted graphene sheets separated by thin insulator are restricted by the momentum and energy conservation laws. Because of this, both dissipative tunneling transitions, with momentum transfer due to disorder scattering, and nondissipative regime of tunneling, which appears due to intersection of electron and hole branches of energy spectrum, must be taken into account. The tunneling current density is calculated for the graphene-boron nitride-graphene layers, which is described by the tight-binding approach, and for the predominant momentum scattering by static disorder. Dependencies of current on concentrations in top and bottom graphene layers, which are governed by the voltages applied through independent contacts and gates, are considered for the back- and double-gated structures. The current-voltage characteristics of the back-gated structure are in agreement with the recent experiment [Science 335, 947 (2012)]. For the double-gated structures, the resonant dissipative tunneling causes a 10-fold enhancement of response which is important for transistor applications.

We present detailed low-temperature scanning tunneling spectroscopy (STS) investigations of the local electronic density of states (DOS) on the fivefold and twofold $i$-Al${}_{70}$Pd${}_{21}$Mn${}_9$; pseudo tenfold ξ′-Al${}_{72}$Pd${}_{25}$Mn${}_3$; tenfold $d$-Al${}_{77}$Ni${}_{17}$Co${}_6$; twofold-(12110) $d$-Al${}_{72.9}$Ni${}_{10.4}$Co${}_{16.7}$; (100) Y-Al${}_{75.8}$Ni${}_{2.1}$Co${}_{22.1}$; and (111) Al surface in the range of +/$-$ 1 eV around the Fermi energy. All these quasicrystals and approximants exhibit the theoretically predicted electronic pseudogap. We observe no evidence for a correlation between the depth of this pseudogap, as measured by STS, and the remarkably high electrical resistivity of the investigated quasicrystals and approximants. A parametrization of the spatial variability of STS spectra is introduced as a measure of the magnitude of the electron localization for a given system. The resulting $\widehat{\mathrm{S}}$ parameter is found to be proportional to the square root of the electrical resistivity. This finding supports the validity of interpreting spatial variations of fine-structure features in the STS spectra of Al-based quasicrystals and approximants in terms of electron localization.

The ground-state magnetic properties of free-standing Fe wires having linear, zigzag, and rectangular geometries are investigated in the framework of density-functional theory. The stability of various wires geometries and types of magnetic orders is analyzed as a function of the spin-density-wave (SDW) number $q$. In particular, we consider ferromagnetic (FM) and antiferromagnetic (AF) collinear states as well as periodic and alternating noncollinear (NC) states. The relaxed interatomic bond lengths $d$, frozen-magnon dispersion relations $\Delta E\left(q\right)$, local magnetic moments ${\mu }_i$, spin-polarized electronic densities of states ${\rho }_{d\sigma }\left(\varepsilon \right)$, and effective exchange couplings $J_{ij}$ are determined. Iron zigzag chains show FM order ($q=0$) at the equilibrium lattice parameter $a$ with $\Delta E\left(q\right)$ increasing monotonously with $q$. In contrast, biatomic rectangular ladders (RLs) develop spontaneously a spiral SDWs along the wire, keeping FM dimer couplings in the perpendicular direction. This NC order is further stabilized by compressing the wire. However, stretched RLs show FM order. In all cases, strong ${\mu }_i$ are found within the Wigner-Seitz spheres, which decrease moderately with increasing $q$. The electronic structures of collinear and NC states are analyzed by comparing the density of states ${\rho }_{d\sigma }\left(\varepsilon \right)$ for different representative values of $q$. Effective exchange-interaction parameters $J_{ij}$ between the local moments ${\mu }_i$ and ${\mu }_j$ are derived by fitting the ab initio magnon dispersion relation $\Delta E\left(q\right)$ to a classical Heisenberg model. The interplay between the various competing $J_{ij}$ is shown to depend decisively on the lattice parameter and wire geometry. Comparing the ab initio results within a model phase diagram provides useful insights on the magnetic order of Fe wires from a local perspective.

Boron vacancies at the Si(111):B-$\sqrt{3}$ surface are model systems in the comprehension of strongly correlated semiconductor surfaces. By using scanning tunneling spectroscopy, the origin of the single-vacancy electronic structure is addressed. It is shown to originate from the localization of a well-identified dangling-bond surface state with significant B character. The bivacancy defect, which is characterized by energy-split bonding and antibonding states, is interpreted within the textbook diatomic molecule picture. From the hopping parameter, we determine the bandwidth of the surface state from which the impurity state derives and evaluate the strength of many-body effects. Our analysis supports the realization of the Mott-Hubbard insulator state in half-filled dangling-bond surface states on $\sqrt{3}$-reconstructed surfaces, as proposed recently for SiC(0001), Sn/Ge(111), and Sn/Si(111).

Electronic and magnetic properties of a single molecular layer on the GaAs(110) surface have been investigated using first-principles calculations based on the spin-density-functional theory. An anisotropic spin interaction has been confirmed in the MnAs layer on GaAs(110): a ferromagnetic alignment along the $\langle 110\rangle$ direction, while a vanishingly small interaction along the $\langle 001\rangle$ direction. Effects of extrinsic charges on the magnetic interaction in the MnAs layer have also been explored. The MnAs layer on the GaAs(110) surface exhibits less anisotropic magnetic interaction as holes are injected, taking on the two-dimensional ferromagnetism mediated by the surface states of GaAs(110). If the MnAs(110) single layer is embedded in the bulk GaAs matrix, the injection of holes suppresses the in-plane ferromagnetism.

We report ultrafast surface pump and interface probe experiments on photoexcited carrier transport across single crystal bismuth films on sapphire. The film thickness is sufficient to separate carrier dynamics from lattice heating and strain, allowing us to investigate the time scales of momentum relaxation, heat transfer to the lattice, and electron-hole recombination. The measured electron-hole ($e-h$) recombination time is 12–26 ps and ambipolar diffusivity is 18–40 cm${}^2$/s for carrier excitation up to $\sim {10}^{19}{\text{cm}}^{-3}$. By comparing the heating of the front and back sides of the film, we put lower limits on the rate of heat transfer to the lattice, and by observing the decay of the plasma at the back of the film, we estimate the time scale of electron-hole recombination. We interpret each of these time scales within a common framework of electron-phonon scattering and find qualitative agreement between the various relaxation times observed. We find that the carrier density is not determined by the $e-h$ plasma temperature after a few picoseconds. The diffusion and recombination become nonlinear with initial excitation $\gtrsim {10}^{20}{\text{cm}}^{-3}$.

We investigate the scattering off dilute magnetic impurities placed on the surface of three-dimensional topological insulators. In the low-temperature limit, the impurity moments are Kondo-screened by the surface-state electrons, despite their exotic locking of spin and momentum. We determine signatures of the Kondo effect appearing in quasiparticle interference (QPI) patterns as recorded by scanning tunneling spectroscopy, taking into account the full energy dependence of the T matrix as well as the hexagonal warping of the surface Dirac cones. We identify a universal energy dependence of the QPI signal at low scanning energies as the fingerprint of Kondo physics, markedly different from the signal due to nonmagnetic or static magnetic impurities. Finally, we discuss our results in the context of recent experimental data.

An anomalous Hall insulator without magnetic long-range ordering is theoretically reported in the absence of the relativistic spin-orbit coupling. It is realized in itinerant electrons coupled with the Ising spins on a $\langle 111\rangle$ kagome plane of pyrochlore spin ice in applied magnetic field. We find that the kagome-ice type local spin correlation in the magnetization plateau state opens a charge gap without magnetic ordering, which results in quantization of the Hall conductivity. By Monte Carlo simulation, we identify the anomalous Hall insulating region in the magnetic phase diagram, in addition to another anomalous Hall insulator in a fully saturated state.

We study the fundamental limit on single-photon indistinguishability imposed by decoherence due to phonon interactions in semiconductor quantum dot-cavity quantum electrodynamics systems. Employing an exact diagonalization approach we find large differences compared to standard methods. An important finding is that short-time non-Markovian effects limit the maximal attainable indistinguishability. The results are explained using a polariton picture that yields valuable insight into the phonon-induced dephasing dynamics.

Using a first-principles method we show that graphene based materials, functionalized with hydroxyl groups, constitute a class of multifunctional, lightweight, and nontoxic organic materials with functional properties such as ferroelectricity, multiferroicity, and can be used as proton battery cathode materials. For example, the polarizations of semihydroxylized graphane and graphone, as well as fully hydroxylized graphane, are much higher than any organic ferroelectric materials known to date. Further, hydroxylized graphene nanoribbons with proton vacancies at the end can have much larger dipole moments. They may also be applied as high-capacity cathode materials with a specific capacity that is six times larger than lead-acid batteries and five times that of lithium-ion batteries.

In this work, we study the Anderson-Falicov-Kimball model within the dynamical mean field theory for the Bethe lattice, restricting our analysis to the nonmagnetic case. The one-particle density of states is obtained by both arithmetic and geometric averages over disorder, since only the latter can detect localization in the absence of an energy gap. Varying the strengths of Coulomb interaction and disorder at zero temperature, we construct phase diagrams for this model, where we distinguish spectral regions with localized states, with extended states, or with a correlation-induced gap. With this, we identify metal-insulator transitions driven by correlation and disorder, as well as the competition between these effects. This is done for various band fillings, since our main interest here is to study how the variation of the electron density affects the phase diagrams previously obtained for half-filling. The picture revealed by the density of states is further checked by evaluating the static and dynamic conductivities, including temperature effects.

Accurate valence-band dispersions $E\left(k_{\perp }\right)$ have been determined for the technologically relevant Γ-Δ-$X$ symmetry direction of hydrogen-terminated diamond (100) using normal emission angle-resolved photoemission spectroscopy for photon energies between 30 and 206 eV. By analyzing the data using a free-electron final-state model, the emission features can be well understood by assuming primary cone transitions. Critical point energies in the valence band are in good agreement with self-consistent quasiparticle $GW$ calculations. Substantial modulations in the valence-band dispersion occurring in specific regions of the Brillouin zone have been traced to band crossings in the unoccupied free-electron final state. A one-band effective mass of (0.39 ± 0.30)$m_0$ is determined from the band dispersion close to the Γ point and compared with values in the literature.

The energy of magnetic and orbital excitations of copper-oxide systems have been assessed in bulk materials, but little is known about how they change in thin films or heterostructures. Although in these samples the epitaxial strain is known to alter important physical properties, such as $T_c$, the actual relation to fundamental excitations remains to be established. Here we determine how the magnetic interactions and orbital splitting scale with lattice constant under strain in the prototypical insulating cuprate CaCuO${}_2$ . We extract the scaling power laws both experimentally, using Cu $L_3$ resonant inelastic x-ray scattering, and theoretically, with ab initio quantum chemical calculations. This combined approach quantifies the large impact of small lattice variations on the magnetic and orbital energy scales and opens the way to the production of strain-engineered samples.

The molecular orientation–dependent electronic properties of monolayer dipolar molecule chloroaluminum phthalocyanine (ClAlPc) on Au(111) are investigated by ultraviolet photoemission spectroscopy and scanning tunneling microscopy. The relation between geometrical and electronic structures has been revealed in the binding energies of the highest occupied molecular orbital states and vacuum level (VL) shifts. Two molecular orientations, Cl-up- and Cl-down-oriented molecules, coexist in the as-grown monolayer ClAlPc films on Au(111) without the formation of staggered molecular pairs to cancel the dipoles and phase separation, as is the case on graphite. After annealing, only the Cl-up-oriented molecules remain on Au(111), as on graphite. Interestingly, an extraordinarily large VL shift of −0.89 eV is observed in the annealed monolayer ClAlPc film on Au(111), which is opposite to that of +0.46 eV on graphite even though the molecular dipoles are oriented similarly.

The adsorption characteristics of alkali, alkaline-earth, and transition metal adatoms on silicene, a graphene-like monolayer structure of silicon are analyzed by means of first-principles calculations. In contrast to graphene, interaction between the metal atoms and the silicene surface is quite strong due to its highly reactive buckled hexagonal structure. In addition to structural properties, we also calculate the electronic band dispersion, net magnetic moment, charge transfer, work function, and dipole moment of the metal adsorbed silicene sheets. Alkali metals, Li, Na, and K, adsorb to hollow sites without any lattice distortion. As a consequence of the significant charge transfer from alkalis to silicene, metalization of silicene takes place. Trends directly related to atomic size, adsorption height, work function, and dipole moment of the silicene/alkali adatom system are also revealed. We found that the adsorption of alkaline-earth metals on silicene is entirely different from their adsorption on graphene. The adsorption of Be, Mg, and Ca turns silicene into a narrow gap semiconductor. Adsorption characteristics of eight transition metals Ti, V, Cr, Mn, Fe, Co, Mo, and W are also investigated. As a result of their partially occupied $d$ orbital, transition metals show diverse structural, electronic, and magnetic properties. Upon the adsorption of transition metals, depending on the adatom type and atomic radius, the system can exhibit metal, half-metal, and semiconducting behavior. For all metal adsorbates, the direction of the charge transfer is from adsorbate to silicene, because of its high surface reactivity. Our results indicate that the reactive crystal structure of silicene provides a rich playground for functionalization at nanoscale.