arXiv subjects

G. De Filippis

Publications and source records attributed to G. De Filippis.

At least 73 records · Page 4Linked to original sources

arXiv2010

Interplay between charge-lattice interaction and strong electron correlations in cuprates: phonon anomaly and spectral kinks

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 $\vec{q}_s=\{(\pm π/ 2, 0), (0, \pm π/ 2) \}$ 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.

cond-mat.str-el↗

arXiv2009

Evolution of magnetic phases and orbital occupation in (SrMnO3)n/(LaMnO3)2n superlattices

The magnetic and electronic modifications induced at the interfaces in (SrMnO$_{3}$)$_{n}$/(LaMnO$_{3}$)$_{2n}$ superlattices have been investigated by linear and circular magnetic dichroism in the Mn 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 $e_{g}(x^{2}-y^{2})$ orbital occupation, in agreement with the average strain. Far from interfaces, inside LaMnO$_3$, electron localization and local strain favor antiferromagnetism (AFM) and $e_{g}(3z^{2}-r^{2})$ 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 $n \geq 5$ the FM layers are separated by AFM regions having out-of-plane spin orientation.

cond-mat.str-el↗

arXiv2008

Localization-delocalization transition of a polaron near an impurity

We solve the problem of polaron localization on an attractive impurity by means of direct-space Diagrammatic Monte Carlo implemented for the system in the thermodynamic limit. In particular we determine the ground state phase diagram in dependence on the electron-phonon coupling and impurity potential strength for the whole phonon frequency range. Including the quantum phonon dynamics we find and characterize a new phase which is missing in the zero phonon-frequency limit (adiabatic approximation), where self-trapped polarons are not localized at shallow impurities. We predict and show that in the vicinity of the localization transition a region with a mixture of weak- and strong-coupling spectral response is realized.

cond-mat.str-el↗

arXiv2008

Charge dynamics of doped holes in high Tc cuprates - A clue from optical conductivity

The charge dynamics in weakly hole doped high temperature superconductors is studied in terms of the accurate numerical solution to a model of a single hole interacting with a quantum lattice in an antiferromagnetic background, and accurate far-infrared ellipsometry measurements. The experimentally observed two electronic bands in the infrared spectrum can be identified in terms of the interplay between the electron correlation and electron-phonon interaction resolving the long standing mystery of the mid-infrared band.

cond-mat.str-el↗

arXiv2007

Non-local composite spin-lattice polarons in high temperature superconductors

The non-local nature of the polaron formation in t-t'-t"-J model is studied in large lattices up to 64 sites by developing a new numerical method. We show that the effect of longer-range hoppings t' and t" is a large anisotropy of the electron-phonon interaction (EPI) leading to a completely different influence of EPI on the nodal and antinodal points in agreement with the experiments. Furthermore, nonlocal EPI preserves polaron's quantum motion, which destroys the antiferromagnetic order effectively, even at strong coupling regime, although the quasi-particle weight in angle-resolved-photoemission spectroscopy is strongly suppressed.

cond-mat.str-el↗

arXiv2007

Temperature dependence of the angle resolved photoemission spectra in the undoped cuprates: self-consistent approach to the t-J-Holstein model

We develop a novel self-consistent approach for studying the angle resolved photoemission spectra (ARPES) of a hole in the t-J-Holstein model giving perfect agreement with numerically exact Diagrammatic Monte Carlo data at zero temperature for all regimes of electron-phonon coupling. Generalizing the approach to finite temperatures we find that the anomalous temperature dependence of the ARPES in undoped cuprates is explained by cooperative interplay of coupling of the hole to magnetic fluctuations and strong electron-phonon interaction.

cond-mat.str-el↗

arXiv2006

On the validity of the Franck-Condon principle in the optical spectroscopy: optical conductivity of the Fröhlich polaron

The optical absorption of the Fröhlich polaron model is obtained by an approximation-free Diagrammatic Monte Carlo method and compared with two new approximate approaches that treat lattice relaxation effects in different ways. We show that: i) a strong coupling expansion, based on the the Franck-Condon principle, well describes the optical conductivity for large coupling strengths ($α>10$); ii) a Memory Function Formalism with phonon broadened levels reproduces the optical response for weak coupling strengths ($α<6$) taking the dynamic lattice relaxation into account. In the coupling regime $6<α<10$ the optical conductivity is a rapidly changing superposition of both Franck-Condon and dynamic contributions.

cond-mat.str-el↗

arXiv2005

Ballistic transport in one-dimensional loops with Rashba and Dresselhaus spin-orbit coupling

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 are found. Furthermore, we investigate the suppression of the Al'tshuler--Aronov--Spivak oscillations that appear, in presence of a magnetic flux, when the electrons are injected and collected at the same node. Finally, we point out that a recent realization of a ballistic spin interferometer can be used to obtain a reliable estimate of the magnitude ratio of the two spin-orbit interactions.\bigskip

cond-mat.mes-hall↗

arXiv2004

Effects of the electron-phonon coupling near and within the insulating Mott phase

The role of the electron-phonon interaction in the Holstein-Hubbard model is investigated in the metallic phase close to the Mott transition and in the insulating Mott phase. The model is studied by means of a variational slave boson technique. At half-filling, mean-field static quantities are in good agreement with the results obtained by numerical techniques. By taking into account gaussian fluctuations, an analytic expression of the spectral density is derived in the Mott insulating phase showing that an increase of the electron-phonon coupling leads to a sensitive reduction of the Mott gap through a reduced effective repulsion. The relation of the results with recent experimental observations in strongly correlated systems is discussed.

cond-mat.str-el↗

arXiv2004

Effects of electron-phonon coupling range on the polaron formation

The polaron features due to electron-phonon interactions with different coupling ranges are investigated by adopting a variational approach. The ground-state energy, the spectral weight, the average kinetic energy, the mean number of phonons, and the electron-lattice correlation function are discussed for the system with coupling to local and nearest neighbor lattice displacements comparing the results with the long range case. For large values of the coupling with nearest neighbor sites, most physical quantities show a strong resemblance with those obtained for the long range electron-phonon interaction. Moreover, for intermediate values of interaction strength, the correlation function between electron and nearest neighbor lattice displacements is characterized by an upturn as function of the electron-phonon coupling constant.

cond-mat.str-el↗

arXiv2004

Spin polarization of electrons with Rashba double-refraction

We demonstrate how the Rashba spin-orbit coupling in semiconductor heterostructures can produce and control a spin-polarized current without ferromagnetic leads. Key idea is to use spin-double refraction of an electronic beam with a nonzero incidence angle. A region where the spin-orbit coupling is present separates the source and the drain without spin-orbit coupling. We show how the transmission and the beam spin-polarization critically depend on the incidence angle. The transmission halves when the incidence angle is greater than a limit angle and a significant spin-polarization appears. Increasing the spin-orbit coupling one can obtain the modulation of the intensity and of the spin-polarization of the output electronic current when the input current is unpolarized. Our analysis shows the possibility to realize a spin-field-effect transistor based on the propagation of only one mode with the region with spin-orbit coupling. Where the original Datta and Das device [Appl.Phys.Lett. {\bf 56}, 665 (1990)] use the spin-precession that originates from the interference between two modes with orthogonal spin.

cond-mat.mes-hall↗

arXiv2004

Transport properties in manganite thin films

The resistivity of thin $La_{0.7}A_{0.3}MnO_{3}$ films ($A=Ca, Sr$) is investigated in a wide temperature range. The comparison of the resistivities is made among films grown by different techniques and on several substrates allowing to analyze samples with different amounts of disorder. In the low-temperature nearly half-metallic ferromagnetic state the prominent contribution to the resistivity scales as $T^α$ with $α\simeq 2.5$ for intermediate strengths of disorder supporting the theoretical proposal of single magnon scattering in presence of minority spin states localized by the disorder. For large values of disorder the low-temperature behavior of the resistivity is well described by the law $T^{3}$ characteristic of anomalous single magnon scattering processes, while in the regime of low disorder the $α$ exponent tends to a value near 2. In the high temperature insulating paramagnetic phase the resistivity shows the activated behavior characteristic of polaronic carriers. Finally in the whole range of temperatures the experimental data are found to be consistent with a phase separation scenario also in films doped with strontium ($A=Sr$).

cond-mat.str-el↗

arXiv2004

A variational approach to the optimized phonon technique for electron-phonon problems

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.

cond-mat.str-el↗

arXiv2004

Polaron features for long-range electron-phonon interaction

The polaron features for long-range electron-phonon interaction are investigated by extending a variational approach previously proposed for the study of systems with local coupling. The ground-state spectral weight, the average kinetic energy, the mean number of phonons, and the electron-lattice correlation function are discussed for a wide range of model parameters focusing on the adiabatic regime and comparing the results with the short-range case (Holstein model). A strong mixing of electronic and phononic degrees of freedom for small values of the electron-phonon coupling constant is found in the adiabatic case due to the long-range interaction. Finally a polaron "phase diagram" is proposed.

cond-mat.str-el↗

arXiv2003

Modelling of strain effects in manganite films

Thickness dependence and strain effects in films of $La_{1-x}A_xMnO_3$ 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 $Mn-O-Mn$ 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

cond-mat.str-el↗

arXiv2003

Ground state features of the Frohlich model

Following the ideas behind the Feynman approach, a variational wave function is proposed for the Fröhlich model. It is shown that it provides, for any value of the electron-phonon coupling constant, an estimate of the polaron ground state energy better than the Feynman method based on path integrals. The mean number of phonons, the average electronic kinetic and interaction energies, the ground state spectral weight and the electron-lattice correlation function are calculated and successfully compared with the best available results.

cond-mat.str-el↗

arXiv2003

Infrared conductivity of a one-dimensional charge-ordered state: quantum lattice effects

The optical properties of the charge-ordering ($CO$) phase of the one-dimensional (1D) half-filled spinless Holstein model are derived at zero temperature within a well-known variational approach improved including second-order lattice fluctuations. Within the $CO$ phase, the static lattice distortions give rise to the optical interband gap, that broadens as the strength of the electron-phonon ($el-ph$) interaction increases. The lattice fluctuation effects induce a long subgap tail in the infrared conductivity and a wide band above the gap energy. The first term is due to the multi-phonon emission by the charge carriers, the second to the interband transitions accompanied by the multi-phonon scattering. The results show a good agreement with experimental spectra.

cond-mat.str-el↗

arXiv2003

Infrared absorption of the charge-ordering phase: Lattice effects

The phase diagram of the half-filled spinless Holstein model for electrons interacting with quantum phonons is derived in three dimensions extending at finite temperature $T$ a variational approach introduced for the one-dimensional T=0 case. Employing the variational scheme, the spectral and optical properties of the system are evaluated in the different regimes that characterize the normal and ordered state. The effects of the charge-ordering ($CO$) induce a transfer of spectral weight from low to high energies in the conductivity spectra, as the temperature decreases or the strength of the electron-phonon ($el-ph$) interaction increases. The inclusion of effects of lattice fluctuations is able to smooth the inverse square-root singularity expected for the case of the mean-field approach and determines a subgap tail absorption. Moreover, in the weak to intermediate coupling regime, a two-component structure is obtained within the $CO$ phase at low frequency: the remnant Drude-like term and the incipient absorption band centered around the gap energy.

cond-mat.str-el↗