Dedicated to Professor Jozef T. Devreese on the occasion of his 65th birthday PACS 31.15.Ew, 47.3... more Dedicated to Professor Jozef T. Devreese on the occasion of his 65th birthday PACS 31.15.Ew, 47.37.+q, 71.15.Mb The occurrence of polaronic behaviour in real solids is often accompanied by strong exchange and correlation effects in the electron gas besides being strongly entangled to the phononic degrees of freedom. Such effects can be relevant for the instability towards Wigner crystallization or charge density waves, or yet for the polaron role in high-T c superconductors and in manganites. The inclusion of exchange and correlation in the coupled electron-phonon system is thus highly desirable. In this work, we exploit the density functional theory and its time-dependent extension to construct an appropriate effective potential useful for studying the properties of interacting polarons. The validity and the possible developments of this approach are discussed, and future applications are proposed.
We introduce a variational wave-function to study the polaron formation when the electronic trans... more We introduce a variational wave-function to study the polaron formation when the electronic transfer integral depends on the relative displacement between nearest-neighbor sites giving rise to a non-local electron-phonon coupling with optical phonon modes. We analyze the ground state properties such as the energy, the electron-lattice correlation function, the phonon number and the spectral weight. Variational results are found in good agreement with analytic weak-coupling perturbative calculations and exact numerical diagonalization of small clusters. We determine the polaronic phase diagram and we find that the tendency towards strong localization is hindered from the pathological sign change of the effective next-nearest-neighbor hopping.
Single-parameter adiabatic charge pumping, induced by a nearby radio-frequency antenna, is achiev... more Single-parameter adiabatic charge pumping, induced by a nearby radio-frequency antenna, is achieved in suspended carbon nanotubes close to the mechanical resonance. The charge pumping is due to an important dynamic adjustment of the oscillating motion to the antenna signal and it is different from the mechanism active in the two-parameter pumping. Finally, the second harmonic oscillator response shows an interesting relationship with the first harmonic that should be experimentally observed.
We study a Thouless pump realized with an elastically deformable quantum dot whose center of mass... more We study a Thouless pump realized with an elastically 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.
Interfaces can drastically modify the electronic properties of correlated electron materials. Rec... more Interfaces can drastically modify the electronic properties of correlated electron materials. Recent examples include the formation of a thin metallic layer at the interface between band and Mott insulators as, for example, between SrTiO3 and LaTiO3 Refs. 1 and 2 or between the band insulators LaAlO3 and SrTiO3. 3 It has been also shown that metallicity and ferromagnetism can be induced at the interface between the antiferromagnetic insulating manganites LaMnO3 LMO Mn3+, t2g 3eg and SrMnO3 SMO Mn4+, t2g 3. 4 In this ...
Journal of Superconductivity and Novel Magnetism, 2012
The anomalous properties of the bond stretching (BS) phonon mode are analyzed in the underdoped r... more 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.
Semiconductors II: Surfaces, interfaces, microstructures, and related topics-Ballistic transport in one-dimensional loops with Rashba and Dresselhaus spin-orbit coupling
An optimized phonon approach for the numerical diagonalization of interacting electron-phonon sys... more An optimized phonon approach for the numerical diagonalization of interacting electron-phonon systems is proposed. The variational method is based on an expansion in coherent states that leads to a dramatic truncation in the phonon space. The reliability of the approach is demonstrated for the extended Holstein model showing that different types of lattice distortions are present at intermediate electron-phonon couplings as observed in strongly correlated systems. The connection with the density matrix renormalization group is discussed.
The European Physical Journal B - Condensed Matter, 2003
We study the scattering of an electron of a 2DEG through a large point contact separating a regio... more We study the scattering of an electron of a 2DEG through a large point contact separating a region where the electrons are free and a region where the Rashba spin-orbit coupling is present. The scattering depends dramatically on the electron incidence angle showing double refraction within the Rashba region. For incidence not normal to the interface the electron spin state is not conserved. The calculated conductance exhibits an oscillating behavior as a function of spin state of the incident electrons with different spin down and spin up currents. Our model describes both a ferromagnetic semimetallic source and a simple metallic injection electrode. In the first case the electrons are injected in a pure spin state and in the second one they are unpolarized, that is in a statistical mixture of spin up and down states. In both the cases the passage through the large point contact produces spin polarized currents.
First principles calculations were performed to study the interface electronic structure and the ... more First principles calculations were performed to study the interface electronic structure and the Schottky barrier heights (SBHs) of ZnO–metal interfaces. Different kinds of metals were considered with different chemistries on the polar (0 0 0 1) and (0 0 0 $\bar1$ ) ZnO surfaces. The projection of the density of states on the atomic orbitals of the interface atoms reveals that two kinds of interface electronic states appear: states due to the chemical bonding which appear at well defined energies and conventional metal-induced gap states associated with a smooth density of states in the bulk ZnO band gap region. The relative weight and distribution of the two classes of states depend on both the ZnO substrate termination and on the metal species. SBHs are found to be very sensitive to the specific interface chemical bonding. In particular, it is possible to note the occurrence of either Schottky barriers or Ohmic contacts. Our results have been compared with experiments and with available phenomenological theories, which estimate the SBH from few characteristic material parameters. Finally, the electronic and structural contributions to the SBH have been singled out and related to the different charge transfers occurring at the different interfaces.
Electronic structure: Wide-band, narrow-band, and strongly correlated systems-Effects of electron-phonon coupling near and within the insulating Mott phase
We calculate the spectral function of the one dimensional Hubbard-Holstein model using the time d... more 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. arXiv:1406.6340v1 [cond-mat.str-el]
The so-called proximity effect is the manifestation, across an interface, of the systematic compe... more The so-called proximity effect is the manifestation, across an interface, of the systematic competition between magnetic order and superconductivity. This phenomenon has been well documented and understood for conventional superconductors coupled with metallic ferromagnets; however it is still less known for oxide materials, where much higher critical temperatures are offered by copper oxide-based superconductors. Here we show that, even in the absence of direct Cu-O-Mn covalent bonding, the interfacial CuO2 planes of superconducting La(1.85)Sr(0.15)CuO(4) thin films develop weak ferromagnetism associated to the charge transfer of spin-polarised electrons from the La(0.66)Sr(0.33)MnO(3) ferromagnet. Theoretical modelling confirms that this effect is general to all cuprate/manganite heterostructures and the presence of direct bonding only affects the strength of the coupling. The Dzyaloshinskii-Moriya interaction, also at the origin of the weak ferromagnetism of bulk cuprates, propag...
The linear thermoelectric properties of molecular junctions are theoretically studied close to ro... more 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.
Spectral and transport properties of small molecule single-crystal organic semiconductors have be... more 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. For rubrene, a very accurate analysis in the relevant experimental configuration has allowed to clarify the origin of the temperature dependent mobility observed in these organic semiconductors. With increasing temperature, 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 has been calculated, including vertex corrections that give rise to a transport lifetime one order of magnitude smaller than the spectral lifetime of the states involved in the transport mechanism. The mobility always exhibits a power-law behavior as a function of temperature in agreement with experiments in rubrene. In systems gated with polarizable dielectrics, the electron coupling to interface vibrational modes of the gate has to be included in addition to the intrinsic electron-phonon interaction. While the intrinsic bulk electron-phonon interaction affects the behavior of mobility in the coherent regime below room temperature, the coupling with interface modes is dominant for the activated high temperature contribution of localized polarons. Finally, the effects of a weak disorder largely increase the activation energies of mobility and induce the small polaron formation at lower values of electron-phonon couplings in the experimentally relevant temperature window.
Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in... more Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in the colossal magnetoresistance regime. The calculations are based on a generalization of a variational approach previously proposed for the study of manganite bulk. It is found that a reduction in the thickness of the film causes a decrease of critical temperature and magnetization, and an increase of resistivity at low temperatures. The strain is introduced through the modifications of in-plane and out-of-plane electron hopping amplitudes due to substrate-induced distortions of the film unit cell. The strain effects on the transition temperature and transport properties are in good agreement with experimental data only if the dependence of the hopping matrix elements on the M n − O − M n bond angle is properly taken into account. Finally variations of the electron-phonon coupling linked to the presence of strain turn out important in influencing the balance of coexisting phases in the film.
We study a Thouless pump realized with an elastically deformable quantum dot whose center of mass... more We study a Thouless pump realized with an elastically 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.
Dedicated to Professor Jozef T. Devreese on the occasion of his 65th birthday PACS 31.15.Ew, 47.3... more Dedicated to Professor Jozef T. Devreese on the occasion of his 65th birthday PACS 31.15.Ew, 47.37.+q, 71.15.Mb The occurrence of polaronic behaviour in real solids is often accompanied by strong exchange and correlation effects in the electron gas besides being strongly entangled to the phononic degrees of freedom. Such effects can be relevant for the instability towards Wigner crystallization or charge density waves, or yet for the polaron role in high-T c superconductors and in manganites. The inclusion of exchange and correlation in the coupled electron-phonon system is thus highly desirable. In this work, we exploit the density functional theory and its time-dependent extension to construct an appropriate effective potential useful for studying the properties of interacting polarons. The validity and the possible developments of this approach are discussed, and future applications are proposed.
We introduce a variational wave-function to study the polaron formation when the electronic trans... more We introduce a variational wave-function to study the polaron formation when the electronic transfer integral depends on the relative displacement between nearest-neighbor sites giving rise to a non-local electron-phonon coupling with optical phonon modes. We analyze the ground state properties such as the energy, the electron-lattice correlation function, the phonon number and the spectral weight. Variational results are found in good agreement with analytic weak-coupling perturbative calculations and exact numerical diagonalization of small clusters. We determine the polaronic phase diagram and we find that the tendency towards strong localization is hindered from the pathological sign change of the effective next-nearest-neighbor hopping.
Single-parameter adiabatic charge pumping, induced by a nearby radio-frequency antenna, is achiev... more Single-parameter adiabatic charge pumping, induced by a nearby radio-frequency antenna, is achieved in suspended carbon nanotubes close to the mechanical resonance. The charge pumping is due to an important dynamic adjustment of the oscillating motion to the antenna signal and it is different from the mechanism active in the two-parameter pumping. Finally, the second harmonic oscillator response shows an interesting relationship with the first harmonic that should be experimentally observed.
We study a Thouless pump realized with an elastically deformable quantum dot whose center of mass... more We study a Thouless pump realized with an elastically 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.
Interfaces can drastically modify the electronic properties of correlated electron materials. Rec... more Interfaces can drastically modify the electronic properties of correlated electron materials. Recent examples include the formation of a thin metallic layer at the interface between band and Mott insulators as, for example, between SrTiO3 and LaTiO3 Refs. 1 and 2 or between the band insulators LaAlO3 and SrTiO3. 3 It has been also shown that metallicity and ferromagnetism can be induced at the interface between the antiferromagnetic insulating manganites LaMnO3 LMO Mn3+, t2g 3eg and SrMnO3 SMO Mn4+, t2g 3. 4 In this ...
Journal of Superconductivity and Novel Magnetism, 2012
The anomalous properties of the bond stretching (BS) phonon mode are analyzed in the underdoped r... more 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.
Semiconductors II: Surfaces, interfaces, microstructures, and related topics-Ballistic transport in one-dimensional loops with Rashba and Dresselhaus spin-orbit coupling
An optimized phonon approach for the numerical diagonalization of interacting electron-phonon sys... more An optimized phonon approach for the numerical diagonalization of interacting electron-phonon systems is proposed. The variational method is based on an expansion in coherent states that leads to a dramatic truncation in the phonon space. The reliability of the approach is demonstrated for the extended Holstein model showing that different types of lattice distortions are present at intermediate electron-phonon couplings as observed in strongly correlated systems. The connection with the density matrix renormalization group is discussed.
The European Physical Journal B - Condensed Matter, 2003
We study the scattering of an electron of a 2DEG through a large point contact separating a regio... more We study the scattering of an electron of a 2DEG through a large point contact separating a region where the electrons are free and a region where the Rashba spin-orbit coupling is present. The scattering depends dramatically on the electron incidence angle showing double refraction within the Rashba region. For incidence not normal to the interface the electron spin state is not conserved. The calculated conductance exhibits an oscillating behavior as a function of spin state of the incident electrons with different spin down and spin up currents. Our model describes both a ferromagnetic semimetallic source and a simple metallic injection electrode. In the first case the electrons are injected in a pure spin state and in the second one they are unpolarized, that is in a statistical mixture of spin up and down states. In both the cases the passage through the large point contact produces spin polarized currents.
First principles calculations were performed to study the interface electronic structure and the ... more First principles calculations were performed to study the interface electronic structure and the Schottky barrier heights (SBHs) of ZnO–metal interfaces. Different kinds of metals were considered with different chemistries on the polar (0 0 0 1) and (0 0 0 $\bar1$ ) ZnO surfaces. The projection of the density of states on the atomic orbitals of the interface atoms reveals that two kinds of interface electronic states appear: states due to the chemical bonding which appear at well defined energies and conventional metal-induced gap states associated with a smooth density of states in the bulk ZnO band gap region. The relative weight and distribution of the two classes of states depend on both the ZnO substrate termination and on the metal species. SBHs are found to be very sensitive to the specific interface chemical bonding. In particular, it is possible to note the occurrence of either Schottky barriers or Ohmic contacts. Our results have been compared with experiments and with available phenomenological theories, which estimate the SBH from few characteristic material parameters. Finally, the electronic and structural contributions to the SBH have been singled out and related to the different charge transfers occurring at the different interfaces.
Electronic structure: Wide-band, narrow-band, and strongly correlated systems-Effects of electron-phonon coupling near and within the insulating Mott phase
We calculate the spectral function of the one dimensional Hubbard-Holstein model using the time d... more 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. arXiv:1406.6340v1 [cond-mat.str-el]
The so-called proximity effect is the manifestation, across an interface, of the systematic compe... more The so-called proximity effect is the manifestation, across an interface, of the systematic competition between magnetic order and superconductivity. This phenomenon has been well documented and understood for conventional superconductors coupled with metallic ferromagnets; however it is still less known for oxide materials, where much higher critical temperatures are offered by copper oxide-based superconductors. Here we show that, even in the absence of direct Cu-O-Mn covalent bonding, the interfacial CuO2 planes of superconducting La(1.85)Sr(0.15)CuO(4) thin films develop weak ferromagnetism associated to the charge transfer of spin-polarised electrons from the La(0.66)Sr(0.33)MnO(3) ferromagnet. Theoretical modelling confirms that this effect is general to all cuprate/manganite heterostructures and the presence of direct bonding only affects the strength of the coupling. The Dzyaloshinskii-Moriya interaction, also at the origin of the weak ferromagnetism of bulk cuprates, propag...
The linear thermoelectric properties of molecular junctions are theoretically studied close to ro... more 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.
Spectral and transport properties of small molecule single-crystal organic semiconductors have be... more 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. For rubrene, a very accurate analysis in the relevant experimental configuration has allowed to clarify the origin of the temperature dependent mobility observed in these organic semiconductors. With increasing temperature, 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 has been calculated, including vertex corrections that give rise to a transport lifetime one order of magnitude smaller than the spectral lifetime of the states involved in the transport mechanism. The mobility always exhibits a power-law behavior as a function of temperature in agreement with experiments in rubrene. In systems gated with polarizable dielectrics, the electron coupling to interface vibrational modes of the gate has to be included in addition to the intrinsic electron-phonon interaction. While the intrinsic bulk electron-phonon interaction affects the behavior of mobility in the coherent regime below room temperature, the coupling with interface modes is dominant for the activated high temperature contribution of localized polarons. Finally, the effects of a weak disorder largely increase the activation energies of mobility and induce the small polaron formation at lower values of electron-phonon couplings in the experimentally relevant temperature window.
Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in... more Thickness dependence and strain effects in films of La 1−x A x M nO 3 perovskites are analyzed in the colossal magnetoresistance regime. The calculations are based on a generalization of a variational approach previously proposed for the study of manganite bulk. It is found that a reduction in the thickness of the film causes a decrease of critical temperature and magnetization, and an increase of resistivity at low temperatures. The strain is introduced through the modifications of in-plane and out-of-plane electron hopping amplitudes due to substrate-induced distortions of the film unit cell. The strain effects on the transition temperature and transport properties are in good agreement with experimental data only if the dependence of the hopping matrix elements on the M n − O − M n bond angle is properly taken into account. Finally variations of the electron-phonon coupling linked to the presence of strain turn out important in influencing the balance of coexisting phases in the film.
We study a Thouless pump realized with an elastically deformable quantum dot whose center of mass... more We study a Thouless pump realized with an elastically 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.
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Papers by C. Perroni