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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

We theoretically study the superconducting properties of multiband two-dimensional transition metal oxide superconductors by analyzing not only the role played by conventional singlet pairings, but also by the triplet order parameters, favored by the spin-orbit couplings present in these materials. In particular, we focus on the two-dimensional electron gas at the (001) interface between ${\mathrm{LaAlO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ band insulators where the low electron densities and the sizable spin-orbit couplings affect the superconducting features. Our theoretical study is based on an extended superconducting mean-field analysis of the typical multiband tight-binding Hamiltonian, as well as on a parallel analysis of the effective electronic bands in the low-momentum limit, including static on-site and intersite intraband attractive potentials under applied magnetic fields. The presence of triplet pairings is able to strongly reduce the singlet order parameters which, as a result, are no longer a monotonic function of the charge density. The interplay between the singlet and the triplet pairings affects the dispersion of quasiparticle excitations in the Brillouin zone and also induces anisotropy in the superconducting behavior under the action of an in-plane and of an out-of-plane magnetic fields. Finally, nontrivial topological superconducting states become stable as a function of the charge density, as well as of the magnitude and of the orientation of the magnetic field. In addition to the chiral, time-reversal breaking, topological superconducting phase, favored by the linear Rashba couplings and by the on-site attractive potentials in the presence of an out-of-plane magnetic field, we find that a time-reversal invariant topological helical superconducting phase is promoted by nonlinear spin-orbit couplings and by the intersite attractive interactions in the absence of magnetic field.

A new variational approach is proposed at zero temperature for a finite density of charge carriers in order to study ground state features of the Frohlich model including electron-electron and electron-phonon interactions. Within the intermediate electron-phonon coupling regime characteristic of large polarons, the approach takes into account on the same footing polaron formation and polaron-polaron correlations which play a relevant role going from low to high charge densities. Including fluctuations on top of the variational approach, the electronic spectral function is calculated from the weak to the intermediate electron-phonon coupling regime finding a peak-dip-hump line shape. The spectra are characterized by a transfer of spectral weight from the incoherent hump to the coherent peak with decreasing the electron-phonon coupling constant or with increasing the particle density. Three different density regimes stem out: the first, at low densities, where the features of a single large polaron with a substantial incoherent spectral weight are not modified by charge carrier interactions; a second one, at intermediate densities, where the polaronic liquid shows a rapid crossover from incoherent to coherent dynamics; the third one, at high densities, where screening effects are so prominent that the system presents a conventional metallic phase. The results obtained in the low to intermediate density regime turn out to be relevant for the interpretation of recent tunneling and photoemission experiments in SrTiO3-based systems., 16 pages, 6 figures

Linear and non-linear transport properties through an atomic-size point contact based on oxides two-dimensional electron gas is examined using the tight-binding method and the $\mathbf{k\cdot p}$ approach. The ballistic transport is analyzed in contacts realized at the (001) interface between band insulators $LaAlO_3$ and $SrTiO_3$ by using the Landauer-B\"uttiker method for many sub-bands derived from three Ti 3d orbitals ($d_{yz}$, $d_{zx}$ and $d_{xy}$) in the presence of an out-of-plane magnetic field. We focus especially on the role played by the atomic spin-orbit coupling and the inversion symmetry breaking term pointing out three transport regimes: the first, at low energies, involving the first $d_{xy}$-like sub-bands, where the conductance quantization is robust; a second one, at intermediate energies, entailing further $d_{xy}$-like sub-bands, where the sub-band splitting induced by the magnetic field is quenched; the third one, where the mixing between light $d_{xy}$-like, heavy $d_{yz}$-like and $d_{zx}$-like sub-bands is so strong that the conductance plateaus turn out to be very narrow. Very good agreement is found with recent experiments exploring the transport properties at low energies., Comment: 11 pages, 9 figures

We investigate a paradigmatic case of topological superconductivity in a one-dimensional nanowire with $d-$orbitals and a strong interplay of spin-orbital degrees of freedom due to the competition of orbital Rashba interaction, atomic spin-orbit coupling, and structural distortions. We demonstrate that the resulting electronic structure exhibits an orbital dependent magnetic anisotropy which affects the topological phase diagram and the character of the Majorana bound states (MBSs). The inspection of the electronic component of the MBSs reveals that the spin-orbital polarization generally occurs along the direction of the applied Zeeeman magnetic field, and transverse to the magnetic and orbital Rashba fields. The competition of symmetric and antisymmetric spin-orbit coupling remarkably leads to a misalignment of the spin and orbital moments transverse to the orbital Rashba fields, whose manifestation is essentially orbital dependent. The behavior of the spin-orbital polarization along the applied Zeeman field reflects the presence of multiple Fermi points with inequivalent orbital character in the normal state. Additionally, the response to variation of the electronic parameters related with the degree of spin-orbital entanglement leads to distinctive evolution of the spin-orbital polarization of the MBSs. These findings unveil novel paths to single-out hallmarks relevant for the experimental detection of MBSs., Comment: 14 pages, 8 figures

The new frontier for spintronics is the realization of devices in which the spin can be controlled by electric fields. Multiferroics, materials exhibiting strong interplay between spin and orbital degrees of freedom, are candidates for the realization of such a paradigm. In this work, we study the magnetoelectric coupling in epitaxial $$\hbox {BiMnO}_3$$ thin films which exhibit a large saturation magnetization. By combining X-ray absorption spectroscopy data and theoretical modeling, we demonstrate that $$\hbox {BiMnO}_3$$ thin films have an improper magnetoelectric behavior, characterized by competing antiferromagnetic and ferromagnetic correlations. As a consequence, we show that in these materials the Mn-3d orbital and magnetic orders can be tuned via the ferroelectric polarization, opening perspectives for the realization of novel spintronic devices.

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.

We determine the optimal conditions to achieve topological superconducting phases having spin-singlet pairing for a planar nanowire with finite lateral width in the presence of an in-plane external magnetic field. We employ a microscopic description that is based on a three-band electronic model including both the atomic spin-orbit coupling and the inversion asymmetric potential at the interface between oxide band-gap insulators. We consider amplitudes of the pairing gap, spin-orbit interactions and electronic parameters that are directly applicable to nanowires of LaAlO$_3$-SrTiO$_3$. The lateral confinement introduces a splitting of the $d$-orbitals that alters the orbital energy hierarchy and significantly affects the electron filling dependence of the topological phase diagram. Due to the orbital directionality of the $t_{2g}$-states, we find that in the regime of strong confinement the onset of topological phases is pinned at electron filling where the quasi flat heavy bands start to get populated. The increase of the nanowire thickness leads to a changeover from sparse-to-dense distribution of topologically non-trivial domains which occurs at the cross-over associated to the orbital population inversion. These findings are corroborated by a detailed analysis of the most favorable topological superconducting phases in the electron doping-magnetic field plane highlighting the role of orbital selective confinement., Comment: 13 pages, 8 figures, submitted

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

We discuss the formation and post-deposition instability of nanodrop-like structures in thin films of PDIF-CN2 (a perylene derivative) deposited via supersonic molecular beam deposition technique on highly hydrophobic substrates at room temperature. The role of the deposition rate on the characteristic lengths of the organic nanodrops has been investigated by a systematic analysis of atomic force microscope images of the thin films and through the use of the height-height correlation function. The nanodrops appear to be a metastable configuration for the freshly-deposited films. For this reason, post-deposition wetting effect has been examined with unprecedented accuracy throughout a year of experimental observations. The observed time scales, from few hours to months, are related to the growth rate, and characterize the thin films morphological reordering from three-dimensional nanodrops to a well-connected terraced film. While the interplay between adhesion and cohesion energies favors the formation of 3D-mounted structures during the growth, wetting phenomenon following the switching off of the molecular flux is found to be driven by an instability. A slow rate downhill process survives at the molecular flux shutdown and it is accompanied and maybe favored by the formation of a precursor layer composed of more lying molecules. These results are supported by simulations based on a non-linear stochastic model. The instability has been simulated, for both the growth and the post-growth evolution. To better reproduce the experimental data it is needed to introduce a surface equalizer term characterized by a relaxation time taking into account the presence of a local mechanism of molecular correlation.

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].

We present a theoretical analysis of Dirac magneto-plasmons in topological insulator nanowires. We discuss a cylindrical geometry where Berry phase effects induce the opening of a gap at the neutrality point. By taking into account surface electron wave functions introduced in previous papers and within the random phase approximation, we provide an analytical form of the dynamic structure factor. Dispersions and spectral weights of Dirac plasmons are studied with varying the radius of the cylinder, the surface doping, and the strength of an external magnetic field. We show that, at zero surface doping, inter-band damped plasmon-like excitations form at the surface and survive at low electron surface dopings ($\sim 10^{10} cm^{-2} $). Then, we point out that the plasmon excitations are sensitive to the Berry phase gap closure when an external magnetic field close to half quantum flux is introduced. Indeed, a well-defined magneto-plasmon peak is observed at lower energies upon the application of the magnetic field. Finally, the increase of the surface doping induces a crossover from damped inter-band to sharp intra-band magneto-plasmons which, as expected for large radii and dopings ($\sim 10^{12} cm^{-2}$), approach the proper limit of a two-dimensional surface., Comment: 18 pages, 11 figures, 2 Appendices

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

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.

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.

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.

Polaron formation is investigated in a one-dimensional chain by taking into account both the local Holstein and the non-local SSH electron-phonon interactions. The study of the adiabatic regime points out that the combined effects of the two interactions are important mainly in the weak coupling regime. Thus, using the weak-coupling perturbation theory, spectral weights, effective masses, polaronic phase-diagram, and band structures are discussed. Contrarily to what happens in the Frohlich and Holstein models, we find that the ratio between the coherent spectral weight and the mass renormalization ratio is greater than 1. Moreover, we show that the non-local electron-phonon interaction is responsible for the largest deviations of the band structure from the cosine shape of the free energy band.

We calculate the spectral function of the one dimensional Hubbard-Holstein model using the time dependent Density Matrix Renormalization Group (tDMRG), focusing on the regime of large local Coulomb repulsion, and away from electronic half-filling. We argue that, from weak to intermediate electron-phonon coupling, phonons interact only with the electronic charge, and not with the spin degrees of freedom. For strong electron-phonon interaction, spinon and holon bands are not discernible anymore and the system is well described by a spinless polaronic liquid. In this regime, we observe multiple peaks in the spectrum with an energy separation corresponding to the energy of the lattice vibrations (i.e., phonons). We support the numerical results by introducing a well controlled analytical approach based on Ogata-Shiba's factorized wave-function, showing that the spectrum can be understood as a convolution of three contributions, originating from charge, spin, and lattice sectors. We recognize and interpret these signatures in the spectral properties and discuss the experimental implications., 8 pages, 7 figures

The thermoelectric properties of a molecular junction model, appropriate for large molecules such as fullerenes, are studied within a non-equilibrium adiabatic approach in the linear regime at room temperature. A self-consistent calculation is implemented for electron and phonon thermal conductance showing that both increase with the inclusion of the electron-vibration coupling. Moreover, we show that the deviations from the Wiedemann-Franz law are progressively reduced upon increasing the interaction between electronic and vibrational degrees of freedom. Consequently, the junction thermoelectric efficiency is substantially reduced by the electron-vibration coupling. Even so, for realistic parameters values, the thermoelectric figure of merit can still have peaks of the order of unity. Finally, in the off-resonant electronic regime, our results are compared with those of an approach which is exact for low molecular electron densities. We give evidence that in this case additional quantum effects, not included in the first part of this work, do not affect significantly the junction thermoelectric properties in any temperature regime., 15 pages, 11 figures, 2 Appendices

The linear thermoelectric properties of molecular junctions are theoretically studied close to room temperature within a model including electron-electron and electron-vibration interactions on the molecule. A nonequilibrium adiabatic approach is generalized to include large Coulomb repulsion through a self-consistent procedure and applied to the investigation of large molecules, such as fullerenes, within the Coulomb blockade regime. The focus is on the phonon thermal conductance which is quite sensitive to the effects of strong electron-electron interactions within the intermediate electron-vibration coupling regime. The electron-vibration interaction enhances the phonon and electron thermal conductance, and it reduces the charge conductance and the thermopower inducing a decrease of the thermoelectric figure of merit. For realistic values of junction parameters, the peak values of the thermoelectric figure of merit are still of the order of unity since the phonon thermal conductance can be even smaller than the electron counterpart., Comment: 8 pages, 1 Appendix, 12 pages. arXiv admin note: substantial text overlap with arXiv:1406.3771

We study a Thouless pump realized with an elastically \textit{deformable quantum dot} whose center of mass follows a non-linear stochastic dynamics. The interplay of noise, non-linear effects, dissipation and interaction with an external time-dependent driving on the pumped charge is fully analyzed. The results show that the quantum pumping mechanism not only is not destroyed by the force fluctuations, but it becomes stronger when the forcing signal frequency is tuned close to the resonance of the vibrational mode. The robustness of the quantum pump with temperature is also investigated and an exponential decay of the pumped charge is found when the coupling to the vibrational mode is present. Implications of our results for nano-electromechanical systems are also discussed., 2 Appendices and figures added

A variational procedure is developed to study the properties of the bipolaron in the Fröhlich model including explicitly the electron band structure and taking into account the long-range repulsive electron-electron interaction. Adopting two different ansatze for the longitudinal optical phonon distribution function, the large-bipolaron and small-bipolaron limits are obtained. The evolution of the bipolaron ground state as a function of the electron-phonon coupling constant and of the bare-electronic bandwidth is discussed and the bipolaron phase diagram is presented. A clear crossover from the large-bipolaron to the small-bipolaron regime takes place.

We propose a microscopic model for a nanoelectromechanical system made by a radio-frequency driven suspended carbon nanotube (CNT) in the presence of an external magnetic field perpendicular to the current. As a main result, we show that, when the device is driven far from equilibrium, one can tune the CNT mechanical properties by varying the external magnetic field. Indeed, the magnetic field affects the CNT bending mode dynamics inducing an enhanced damping as well as a noise term due to the electronic phase fluctuations. The quality factor, as observed experimentally, exhibits a quadratic dependence on external magnetic field strength. Finally, CNT resonance frequencies as a function of gate voltage acquire, increasing the magnetic field strength, a peculiar dip-peak structure that should be experimentally observed., 15 pages, 9 figures, submitted to Phys. Rev. B

We study a general model describing a self-detecting single electron transistor realized by a suspended carbon nanotube actuated by a nearby antenna. The main features of the device, recently observed in a number of experiments, are accurately reproduced. When the device is in a low current-carrying state, a peak in the current signals a mechanical resonance. On the contrary, a dip in the current is found in high current-carrying states. In the nonlinear vibration regime of the resonator, we are able to reproduce quantitatively the characteristic asymmetric shape of the current-frequency curves. We show that the nonlinear effects coming out at high values of the antenna amplitude are related to the effective nonlinear force induced by the electronic flow. The interplay between electronic and mechanical degrees of freedom is understood in terms of an unifying model including in an intrinsic way the nonlinear effects driven by the external probe., 7 pages, 3 figures, submitted to Physical Review B

The formation of the electron-phonon induced bipolaron is shown to be feasible in organic semiconductors at the interface with dielectric gates due to the coupling of the carriers with interface vibrational modes and to the weak to intermediate strength of bulk electron-electron interaction. The polaronic bound states are found to be quite robust in a model with realistic strengths of electron coupling to both bulk and interface phonons. The crossover to nearly on-site bipolarons occurs for coupling values much smaller than those for nearly on-site polarons, but, on the other hand, it gives rise to an activated behavior of mobility with much larger activation energies. The results are discussed in connection with rubrene field-effect transistors., Comment: 5 pages, 3 figures, ccepted for publication in EPL

Spectral and transport properties of the quasi two-dimensional adiabatic Su-Schrieffer-Heeger model are studied adjusting the parameters in order to model rubrene single-crystal field effect transistors with small but finite density of injected charge carriers. We show that, with increasing temperature $T$, the chemical potential moves into the tail of the density of states corresponding to localized states, but this is not enough to drive the system into an insulating state. The mobility along different crystallographic directions is calculated including vertex corrections which give rise to a transport lifetime one order of magnitude smaller than spectral lifetime of the states involved in the transport mechanism. With increasing temperature, the transport properties reach the Ioffe-Regel limit which is ascribed to less and less appreciable contribution of itinerant states to the conduction process. The model provides features of the mobility in close agreement with experiments: right order of magnitude, scaling as a power law $T^{-\gamma}$, with $\gamma$ close or larger than two, and correct anisotropy ratio between different in-plane directions. Due to a realistic high dimensional model, the results are not biased by uncontrolled approximations., Comment: 10 pages, 9 figures, Submitted

The combined effect of bulk and interface electron-phonon couplings on the transport properties is investigated in a model for organic semiconductors gated with polarizable dielectrics. While the bulk electron-phonon interaction affects the behavior of mobility in the coherent regime below room temperature, the interface coupling is dominant for the activated high $T$ contribution of localized polarons. In order to improve the description of the transport properties, the presence of disorder is needed in addition to electron-phonon couplings. The effects of a weak disorder largely enhance the activation energies of mobility and induce the small polaron formation at lower values of electron-phonon couplings in the experimentally relevant window $150 K, Comment: 4 pages, 3 figures

The stability of striped magnetic phases in films of La(1-x)A(x)MnO(3) perovskites is investigated. A variational analysis is developed for different film thicknesses at fixed hole density (x = 0.3) and the competition among magnetic phases as a function of the transfer integral and the temperature is analyzed. The stabilization of an in-plane striped magnetic phase is observed with reducing the film thickness at low temperatures below the metal-insulator transition temperature. Within the adopted variational scheme, treating perturbatively the residual electron-phonon interaction, the dependence of the in-plane resistivity on temperature for different thicknesses is calculated. At low temperatures, due to the striped magnetic phase, the resistivity shows an important in-plane anisotropy. The obtained results are found to be consistent with experiments.

Spectral, optical and transport properties of an anisotropic three-dimensional Holstein model are studied within the adiabatic approximation. The parameter regime is appropriate for organic semiconductors used in single crystal based field effect transistors. Different approaches have been used to solve the model: self-consistent Born approximation valid for weak electron-phonon coupling, coherent potential approximation exact for infinite dimensions, and numerical diagonalization for finite lattices. With increasing temperature, the width of the spectral functions gets larger and larger making the approximation of quasi-particle less accurate. On the contrary, their peak positions are never strongly renormalized in comparison with the bare ones. As expected, the density of states is characterized by an exponential tail corresponding to localized states at low temperature. For weak electron-lattice coupling, the optical conductivity follows a Drude behavior, while, for intermediate electron-lattice coupling, a temperature dependent peak is present at low frequency. For high temperatures and low particle densities, the mobility always exhibits a power-law behavior as function of temperature. With decreasing the particle density, at low temperature, the mobility shows a transition from metallic to insulating behavior. Results are discussed in connection with available experimental data., Comment: 9 pages, 7 figures, submitted to Phys. Rev. B

Electron coupling to intra- and inter-molecular vibrational modes is investigated in models appropriate to single crystal organic semiconductors, such as oligoacenes. Focus is on spectral and transport properties of these systems beyond perturbative approaches. The interplay between different couplings strongly affects the temperature band renormalization that is the result of a subtle equilibrium between opposite tendencies: band narrowing due to interaction with local modes, band widening due to electron coupling to non local modes. The model provides an accurate description of the mobility as function of temperature: indeed, it has the correct order of magnitude, at low temperatures, it scales as a power-law $T^{-\delta}$ with the exponent $\delta$ larger than unity, and, at high temperatures, shows an hopping behavior with a small activation energy., Comment: 3 Figures, Submitted

A correlated inhomogeneous mean-field approach is proposed in order to study a tight-binding model of the manganite heterostructures (LaMnO3)2n/(SrMnO3)n with average hole doping x = 1/3. Phase diagrams, spectral and optical properties of large heterostructures (up to 48 sites along the growth direction) with a single interface are discussed analyzing the effects of electron-lattice anti-adiabatic fluctuations and strain. The formation of a metallic ferromagnetic interface is quite robust with varying the strength of electron-lattice coupling and strain, though the size of the interface region is strongly dependent on these interactions. The density of states never vanishes at the chemical potential due to the formation of the interface, but it shows a rapid suppression with increasing the electron-lattice coupling. The in-plane and out-of-plane optical conductivities show sharp differences since the in-plane response has metallic features, while the out-of-plane one is characterized by a transfer of spectral weight to high frequency. The in-plane response mainly comes from the region between the two insulating blocks, so that it provides a clear signature of the formation of the metallic ferromagnetic interface., 11 pages, 7 figures, submitted to Phys. Rev. B

Double-exchange (DE) mechanisms in $R{E}_{1\ensuremath{-}x}A{E}_{x}{\text{MnO}}_{3}$ 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 ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$ 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 $RE\text{\ensuremath{-}}AE$ elements by ${\text{Mn}}^{2+}$ ions, in La-deficient ${\text{La}}_{x}{\text{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of ${\text{Mn}}^{2+}$ ions, which is unambiguously proved to be substituted at La site by resonant inelastic x-ray scattering. ${\text{Mn}}^{2+}$ is proved to be directly correlated with the enhanced magnetotransport properties because of an additional hopping mechanism trough interfiling ${\text{Mn}}^{2+}$ ions. Such a scenario has been theoretically confirmed by calculations within the effective single-band model. The use of ${\text{Mn}}^{2+}$ both as a doping element and an ion electronically involved in the conduction mechanism reveals an unconventional phenomenon in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials.

We investigate the interplay between strong electron correlations and charge-lattice interaction in cuprates. The coupling between half breathing bond stretching phonons and doped holes in the t-t'-J model is studied by limited phonon basis exact diagonalization method. Nonadiabatic electron-phonon interaction leads to the splitting of the phonon spectral function at half-way to the zone boundary at $\vec{q}_s=\{(\pm \pi / 2, 0), (0, \pm \pi / 2) \}$ and to low energy kink feature in the electron dispersion, in agreement with experimental observations. Another kink due to strong electron correlation effects is observed at higher energy, depending on the strength of the charge-lattice coupling., Comment: 4 pages, 3 figures

We propose a very accurate computational scheme for the dynamics of a classical oscillator coupled to a molecular junction driven by a finite bias, including the finite mass effect. We focus on two minimal models for the molecular junction: Anderson-Holstein (AH) and two-site Su-Schrieffer-Heeger (SSH) models. As concerns the oscillator dynamics, we are able to recover a Langevin equation confirming what found by other authors with different approaches and assessing that quantum effects come from the electronic subsystem only. Solving numerically the stochastic equation, we study the position and velocity distribution probabilities of the oscillator and the electronic transport properties at arbitrary values of electron-oscillator interaction, gate and bias voltages. The range of validity of the adiabatic approximation is established in a systematic way by analyzing the behaviour of the kinetic energy of the oscillator. Due to the dynamical fluctuations, at intermediate bias voltages, the velocity distributions deviate from a gaussian shape and the average kinetic energy shows a non monotonic behaviour. In this same regime of parameters, the dynamical effects favour the conduction far from electronic resonances where small currents are observed in the infinite mass approximation. These effects are enhanced in the two-site SSH model due to the presence of the intermolecular hopping t. Remarkably, for sufficiently large hopping with respect to tunneling on the molecule, small interaction strengths and at intermediate bias (non gaussian regime), we point out a correspondence between the minima of the kinetic energy and the maxima of the dynamical conductance., Comment: 19 pages, 16 figures, submitted to Physical Review B

Quantum entropies and state distances are analyzed in polaronic systems with short range (Holstein model) and long range (Fr$\ddot{o}$hlich model) electron-phonon coupling. These quantities are extracted by a variational wave function which describes very accurately polaron systems with arbitrary size in all the relevant parameter regimes. With the use of quantum information tools, the crossover region from weak to strong coupling regime can be characterized with high precision. Then, the linear entropy is found to be very sensitive to the range of the electron-phonon coupling and the adiabatic ratio. Finally, the entanglement entropy is studied as a function of the system size pointing out that it not bounded, but scales as the logarithm of the size either for weak electron-phonon coupling or for short range interaction. This behavior is ascribed to the peculiar coupling induced by the single electron itinerant dynamics on the phonon subsystem., Comment: 4 figures, to be published in Phys. Rev. B

The magnetic and electronic modifications induced at the interfaces in ${({\text{SrMnO}}_{3})}_{n}/{({\text{LaMnO}}_{3})}_{2n}$ superlattices have been investigated by linear and circular magnetic dichroism in the $\text{Mn}\text{ }{L}_{2,3}$ x-ray absorption spectra. Together with theoretical calculations, our data demonstrate that the charge redistribution across interfaces favors in-plane ferromagnetic (FM) order and ${e}_{g}({x}^{2}\ensuremath{-}{y}^{2})$ orbital occupation, in agreement with the average strain. Far from interfaces, inside ${\text{LaMnO}}_{3}$, electron localization and local strain favor antiferromagnetism (AFM) and ${e}_{g}(3{z}^{2}\ensuremath{-}{r}^{2})$ orbital occupation. For $n=1$ the high density of interfacial planes ultimately leads to dominant FM order forcing the residual AFM phase to be in-plane too, while for $n\ensuremath{\ge}5$ the FM layers are separated by AFM regions having out-of-plane spin orientation.

The propagation of the acoustic and the electromagnetic signals is studied in materials with tetragonal and hexagonal symmetries that are piezoelectric, piezomagnetic, or magnetoelectric. Three magnetic spatial symmetries are considered: 622 for hexagonal crystals and 422 and 4(')22(') for tetragonal crystals. The equations for the five modes (three mainly acoustic and two mainly electromagnetic) are solved both in the general case and in specific cases in which the material is only piezoelectric, piezomagnetic, or magnetoelectric. It is found that the piezomagnetic and the magnetoelectric coefficients and the magnetic permeability renormalize the elastic constants, the piezoelectric coefficients, and the dielectric tensor. The acoustic frequencies depend on the angle theta that the component of the wave vector in the a-b plane forms with the a axis. The main contribution to the electromagnetic modes derives only from the dielectric tensor and the magnetoelectric coefficients and it is independent of theta and of the piezoelectric and the piezomagnetic coefficients. Starting from the general equations, a method has been devised to study separately the acoustic and the electromagnetic solutions. It is found that a number of relations must be verified in order to have stability of the electromagnetic and the acoustic modes.

The optical conductivity (OC) of cuprates is studied theoretically in the low-density limit of the $t\text{\ensuremath{-}}{t}^{\ensuremath{'}}\text{\ensuremath{-}}J$ model taking into account the hole-lattice coupling. By developing a limited phonon basis exact diagonalization method capable of treating the lattice of largest size ever considered $(4\ifmmode\times\else\texttimes\fi{}4)$, we are able to discern the fine features of the mid-infrared (MIR) part of the OC revealing a three-peak structure. The two lowest peaks are observed in experiments and the highest one is tacitly resolved in moderately doped cuprates. Comparison of OC with the results of semianalytic approaches and detailed analysis of the calculated isotope effect indicate that the middle-energy MIR peak is of mostly magnetic origin while the lowest MIR band originates from the scattering of holes by phonons.

The temperature dependence of the transport properties of ${({\text{SrMnO}}_{3})}_{n}/{({\text{LaMnO}}_{3})}_{2n}$ 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 ${t}_{2g}$ bands in ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_{2}$ is investigated. For both Hund and Ising exchange the small ${e}_{g}^{\ensuremath{'}}$ 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.

The resistivity of thin La0.7Sr0.3MnO3 films was first investigated in a wide temperature (T) range (10-750 K). Films grown by different techniques and on several substrates enabled to analyze samples with different amounts of disorder. The aim of this work was to elucidate the nature of the metal-insulator (M-I) transition occur-ring at T= T-p in these films and its relation with the different kinds of inhomogeneities they could present like intrinsic electric disorder and co-existence of two different electrical and/or magnetic phases. The low-temperature resistivity state was described mostly by a law which scales as T-alpha with alpha approximate to 2.5. This supports the theoretical proposal of single magnon scattering in presence of minority spin states localized by the disorder. In the whole range of temperatures the experimental data are found to be consistent with a phase separation (PS) scenario. In order to go through the origin of the characteristic length scale of inhomogeneity found, preliminarily low frequency noise measurements as a function of T in a range of temperature around the M-I transition were made. The samples used were patterned using photolithography into bridges with various widths and lengths. No clear sign of separation phase dynamic has been observed in our noise measurements. Unexpectedly the normalized Hooge parameter alpha(H)/n was found not to be volume (Q) independent. The LSMO electrical properties may strongly be driven by disorder and new design for magnetoresistance sensors may have to take into account their intrinsic PS. (C) 2007 Published by Elsevier Ltd.

The effect of Rashba spin-orbit interaction in quantum wires with hard-wall boundaries is discussed. The exact wavefunction and eigenvalue equation are worked out, pointing out the mixing between the spin and spatial parts. The spectral properties are also studied within perturbation theory with respect to the strength of the spin - orbit interaction and diagonalization procedure. A comparison is made with the results of a simple model, the two-band model, that takes account only of the first two sub-bands of the wire. Finally, the transport properties within the ballistic regime are analytically calculated for the twoband model and through a tight-binding Green function for the entire system. Single and double interfaces separating regions with different strengths of spin orbit interaction are analysed by injecting carriers into the first and the second sub-band. It is shown that in the case of a single interface the spin polarization in the Rashba region is different from zero, and in the case of two interfaces the spin polarization shows oscillations due to spin-selective bound states.