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We compute the spin-Hall conductance in a multiband model describing the two-dimensional electron gas formed at a LaAlO_3/SrTiO_3 interface in the presence of a finite concentration of impurities. Combining linear response theory with a systematic calculation of the impurity contributions to the self-energy, as well as to the vertex corrections of the relevant diagrams, we recover the full spin-Hall vs. sheet conductance dependence of LaAlO_3/SrTiO_3 as reported in [Nano Lett. 20, 395 (2020)], finding a very good agreement with the experimental data below and above the Lifshitz transition. In particular, we demonstrate that the multiband electronic structure leads to only a partial, instead of a complete, screening of the spin-Hall conductance, which decreases with increasing the carrier density. Our method can be generalized to other two-dimensional systems characterized by a broken inversion symmetry and multiband physics., Comment: 21 pages, 11 .eps figures

Double-exchange mechanisms in RE$_{1-x}$AE$_{x}$MnO$_{3}$ manganites (where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion) relies on the strong exchange interaction between two Mn$^{3+}$ and Mn$^{4+}$ ions through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions has ever been considered "silent" with respect to the DE conducting mechanisms. Here we show that a new path for DE-mechanism is indeed possible by partially replacing the RE-AE elements by Mn$^{2+}$-ions, in La-deficient La$_{x}$MnO$_{3-\delta}$ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of Mn$^{2+}$ ions, which is unambiguously proved to be substituted at La-site by Resonant Inelastic X-ray Scattering. Mn$^{2+}$ is proved to be directly correlated to the enhanced magneto-transport properties because of an additional hopping mechanism trough interfiling Mn$^{2+}$-ions, theoretically confirmed by calculations within the effective single band model. The very idea to use Mn$^{2+}$ both as a doping element and an ions electronically involved in the conduction mechanism, has never been foreseen, revealing a new phenomena in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials., Comment: 6 pages, 5 figures

The temperature dependence of the transport properties of (SrMnO(3))(n)/(LaMnO(3))(2n) superlattices has been systematically investigated by changing the number of interfaces while keeping the total thickness of the system fixed. The superlattices undergo a rich variety of transitions among metal, Mott variable range hopping insulator, interaction-induced Efros-Shklovskii insulator, and polaronic insulator. Interfaces play a fundamental role in tuning the metal-insulator transitions controlling the effective doping of the different layers. In the case of a bilayer, as clarified by a theoretical model, the single interface can extend its effects well far from it in the bulk layer.

Dynamical mean-field theory combined with finite-temperature exact diagonalization is shown to be a suitable method to study local Coulomb correlations in realistic multiband materials. By making use of the sparseness of the impurity Hamiltonian, exact eigenstates can be evaluated for significantly larger clusters than in schemes based on full diagonalization. Since finite-size effects are greatly reduced this approach allows the study of three-band systems down to very low temperatures, for strong local Coulomb interactions and full Hund exchange. It is also shown that exact diagonalization yields smooth subband quasiparticle spectra and self-energies at real frequencies. As a first application the correlation induced charge transfer between t2g bands in Na0.3CoO2 is investigated. For both Hund and Ising exchange the small eg Fermi surface hole pockets are found to be slightly enlarged compared to the noninteracting limit, in agreement with previous quantum Monte Carlo dynamical mean-field calculations for Ising exchange, but in conflict with photoemission data.

We discuss the combined effect of Rashba and Dresselhaus spin-orbit interactions in polygonal loops formed by quantum wires, when the electron are injected in a node and collected at the opposite one. The conditions that allow perfect localization zero transmission coefficient are found. Furthermore, we investigate the suppression of the transmission coefficient oscillations that appear, in the presence of a magnetic flux, when the electron is injected and collected at the same node. The transmission suppression appears also in the Aharonov- Bohm configuration where the input and the output node are at opposite sides of the loop. Finally, we point out that a recent experimental realization of a ballistic spin interferometer can be used to obtain a reliable estimate of the magnitude ratio of the two spin-orbit interactions.

The crossover between the large and small polaron is discussed as a function of the electron-phonon coupling constant and the width of the electronic band. We apply the results to discuss some properties of the phase diagram of magnanites and cuprate superconductors in the normal state.

We generalize the modern theory of electric polarization to the case of one-dimensional non-Hermitian systems with line-gapped spectrum. In these systems, the electronic position operator is non-Hermitian even when projected into the subspace of states below the energy gap. However, the associated Wilson-loop operator is biorthogonally unitary in the thermodynamic limit, thereby leading to real-valued electronic positions that allow for a clean definition of polarization. Non-Hermitian polarization can be quantized in the presence of certain symmetries, as for Hermitian insulators. Different from the latter case, though, in this regime polarization quantization depends also on the type of energy gap, which can be either real or imaginary, leading to a richer variety of topological phases. The most counter-intuitive example is the 1D non-Hermitian chain with time-reversal symmetry only, where non-Hermitian polarization is quantized in presence of an imaginary line-gap. We propose two specific models to provide numerical evidence supporting our findings., Comment: 22 pages, 9 figures

Soft nanosystems are electronic nanodevices, such as suspended carbon nanotubes or molecular junctions, whose transport properties are modulated by soft internal degrees of freedom, for example slow vibrational modes. In this review, effects of the electron-vibration coupling on the charge and heat transport of soft nanoscopic systems are theoretically investigated in the presence of time-dependent perturbations, such as a forcing antenna or pumping terms between the leads and the nanosystem. A well established approach valid for non-equilibrium adiabatic regimes is generalized to the case where external time-dependent perturbations are present. Then, a number of relevant applications of the method are reviewed for systems composed by a quantum dot (or molecule) described by a single electronic level coupled to a vibrational mode. Before introducing time-dependent perturbations, the range of validity of the adiabatic approach is discussed showing that a very good agreement with the results of an exact quantum calculation is obtained in the limit of low level occupation. Aim of this review has been to discuss common features of different soft nanosystems under external drive. The most interesting effects induced by time-dependent perturbations are obtained when the external forcing is nearly resonant with the slow vibrational modes. Indeed, not only the external forcing can enhance the electronic response, but it also induces nonlinear regimes where the interplay between electronic and vibrational degrees of freedom plays a major role., 21 pages, 14 figures

The role of local electron&ndash, vibration and electron&ndash, electron interactions on the thermoelectric properties of molecular junctions is theoretically analyzed focusing on devices based on fullerene molecules. A self-consistent adiabatic approach is used in order to obtain a non-perturbative treatment of the electron coupling to low frequency vibrational modes, such as those of the molecule center of mass between metallic leads. The approach also incorporates the effects of strong electron&ndash, electron interactions between molecular degrees of freedom within the Coulomb blockade regime. The analysis is based on a one-level model which takes into account the relevant transport level of fullerene and its alignment to the chemical potential of the leads. We demonstrate that only the combined effect of local electron&ndash, electron interactions is able to predict the correct behavior of both the charge conductance and the Seebeck coefficient in very good agreement with available experimental data.

Spectral and transport properties of small molecule single-crystal organic semiconductors have been theoretically analyzed focusing on oligoacenes, in particular on the series from naphthalene to rubrene and pentacene aiming to show that the inclusion of different electron-phonon couplings is of paramount importance to interpret accurately the properties of prototype organic semiconductors. While, in the case of rubrene, the coupling between charge carriers and low frequency inter-molecular modes is sufficient for a satisfactory description of spectral and transport properties, the inclusion of electron coupling to both low frequency inter-molecular and high frequency intra-molecular vibrational modes is needed to account for the temperature dependence of transport properties in smaller oligoacenes., 18 pages, 5 figures, Review

Theoretical results in the coherent regime are discussed considering in particular a reference simple model with one electronic level and one vibrational mode in order to provide the relevant orders of magnitude for the thermoelectric properties. Moreover, we analyze the effects of molecular many-body interactions, such as electron-vibration couplings, which typically tend to reduce the efficiency [1,2,3]. Indeed, the electron–vibration interaction can enhance both phonon and electron thermal conductance, and it can reduce not only the charge conductance, but also the thermopower. For prototype fullerene junctions, we focus on the results obtained within a non-equilibrium adiabatic approach which includes a strong Coulomb repulsion and applies to the self-consistent calculation of electron and phonon transport properties of massive molecules within the Coulomb blockade regime [3]. In particular, the effect of the strong electron–electron interactions provides a peculiar double-peak structure to the thermopower versus charge conductance curve. Within the regime of weak to intermediate electron–vibration and vibration–lead phonon coupling, the peak values of the thermoelectric figure of merit are slightly less than unity, and the maximal efficiency of the junction can reach values slightly less than half of the Carnot limit for large temperature differences between the leads. Since a fine tuning of many parameters and coupling strengths is required to optimize the thermoelectric conversion in molecular junctions, new theoretically proposed set-ups are mentioned providing the new research directions in the field of molecular thermoelectricity [4].

Focus of the review is on experimental set-ups and theoretical proposals aimed to enhance thermoelectric performances of molecular junctions. In addition to charge conductance, the thermoelectric parameter commonly measured in these systems is the thermopower, which is typically rather low. We review recent experimental outcomes relative to several junction configurations used to optimize the thermopower. On the other hand, theoretical calculations provide estimations of all the thermoelectric parameters in the linear and non-linear regime, in particular of the thermoelectric figure of merit and efficiency, completing our knowledge of molecular thermoelectricity. For this reason, the review will mainly focus on theoretical studies analyzing the role of not only electronic, but also of the vibrational degrees of freedom. Theoretical results about thermoelectric phenomena in the coherent regime are reviewed focusing on interference effects which play a significant role in enhancing the figure of merit. Moreover, we review theoretical studies including the effects of molecular many-body interactions, such as electron-vibration couplings, which typically tend to reduce the efficiency. Since a fine tuning of many parameters and coupling strengths is required to optimize the thermoelectric conversion in molecular junctions, new theoretically proposed set-ups are discussed in the conclusions.

The anomalous properties of the bond stretching (BS) phonon mode are analyzed in the underdoped region of cuprates by studying the t-t′-J-Holstein model and the Hubbard–Holstein model with both numerical and analytical approaches. It is shown that a BS phonon softening occurs at an intermediate wavevector along the horizontal direction of the Brillouin zone (BZ) in agreement with recent INXS experiments as the result of the interaction among strongly correlated holes and antiferromagnetic fluctuations. An explanation of the single vs. double mode scenario is also discussed.

Stimulated by the recent realization of three dimensional topological insulator nanowire interferometers, a theoretical analysis of quantum interference effects on the low energy spectrum of Bi2Se3 nanowires is presented. The electronic properties are analyzed in nanowires with circular, square and rectangular cross-sections starting from a continuum three dimensional model with particular emphasis on magnetic and geometrical effects. The theoretical study is based on numerically exact diagonalizations of the discretized model for all the geometries. In the case of the cylindrical wire, an approximate analytical solution of the continuum model is also discussed. Although a magnetic field corresponding to half quantum flux is expected to close the band gap induced by Berry phase, in all the studied geometries with finite area cross-sections, the gap closes for magnetic fields typically larger than those expected. Furthermore, unexpectedly, due to geometrical quantum interference effects, for a rectangular wire with a sufficiently large aspect ratio and smaller side ranging from 50 ˚A and 100 ˚A, the gap closes for a specific finite area cross-section without the application of a magnetic field.