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E. Cappelluti

Publications and source records attributed to E. Cappelluti.

At least 55 records · Page 3Linked to original sources
arXiv2006

Spin-Hall Conductivity in Electron-Phonon Coupled Systems

We derive the ac spin-Hall conductivity $σ_{\rm sH}(ω)$ of two-dimensional spin-orbit coupled systems interacting with dispersionless phonons of frequency $ω_0$. For the linear Rashba model we show that the electron-phonon contribution to the spin-vertex corrections breaks the universality of $σ_{\rm sH}(ω)$ at low-frequencies and provides a non-trivial renormalization of the interband resonance. On the contrary, in a generalized Rashba model for which the spin-vertex contributions are absent, the coupling to the phonons enters only through the self-energy, leaving the low frequency behavior of $σ_{\rm sH}(ω)$ unaffected by the electron-phonon interaction.

cond-mat.mes-hall↗
arXiv2006

Electron-phonon effects on the Raman spectrum in MgB$_2$

The anomalous features of the Raman spectroscopy measurement in MgB$_2$ represent a still unresolved puzzle. In particular highly debated are the origin of the huge $E_{2g}$ phonon linewidth, the nature of the low energy ($ω< ω_{E_{2g}}$) background and the evolution of the Raman spectra with Al doping. In this paper we compute the self-energy of the $E_{2g}$ phonon mode in a fully self-consistent way taking into account electron-phonon effects on the electronic properties. We show that all the anomalous features can be naturally understood in a framework where the whole electron-phonon spectrum $α^2F(ω)$ gives rise to significant damping processes for the electronic excitations and consequently for the $E_{2g}$ phonon itself. The two-peak structure as function of the Al doping is ascribed to finite bandwidth effects arising as the Fermi level approaches the $σ$ band edge.

cond-mat.supr-con↗
arXiv2006

Tuning topological disorder in MgB$_{2}$

We carried out Raman measurements on neutron-irradiated and Al-doped MgB$_2$ samples. The irradiation-induced topological disorder causes an unexpected appearance of high frequency spectral structures, similar to those observed in lightly Al-doped samples. Our results show that disorder-induced violations of the selection rules are responsible for the modification of the Raman spectrum in both irradiated and Al-doped samples. Theoretical calculations of the phonon density of states support this hypothesis, and demonstrate that the high frequency structures arise mostly from contributions at ${\bf q}\not=0$ of the E$_{2g}$ phonon mode.

cond-mat.supr-con↗
arXiv2006

Off-Fermi surface cancellation effects in spin-Hall conductivity of a two-dimensional Rashba electron gas

We calculate the spin-Hall conductivity of a disordered two-dimensional Rashba electron gas within the self-consistent Born approximation and for arbitrary values of the electron density, parametrized by the ratio $E_F /E_0$, where $E_F$ is the Fermi level and $E_0$ is the spin-orbit energy. We confirm earlier results indicating that in the limit $E_F/E_0 \gg 1$ the vertex corrections suppress the spin-Hall conductivity. However, for sufficiently low electron density such that $E_F\lesssim E_0$, we find that the vertex corrections no longer cancel contributions from the Fermi surface, and they cannot therefore suppress the spin current. This is instead achieved by contributions away from the Fermi surface, disregarded in earlier studies, which become large when $E_F\lesssim E_0$.

cond-mat.mes-hall↗
arXiv2006

Infrared properties of Mg$_{1-x}$Al$_x($B$_{1-y}$C$_{y}$)$_2$ single crystals in the normal and superconducting state

The reflectivity $R (ω)$ of $ab$-oriented Mg$_{1-x}$Al$_x$(B$_{1-y }$C$_y$)$_2$ single crystals has been measured by means of infrared microspectroscopy for $1300<ω<17000$ cm$^{-1}$. An increase with doping of the scattering rates in the $π$ and $σ$ bands is observed, being more pronounced in the C doped crystals. The $σ$-band plasma frequency also changes with doping due to the electron doping, while the $π$-band one is almost unchanged. Moreover, a $σ\toσ$ interband excitation, predicted by theory, is observed at $ω_{IB} \simeq 0.47$ eV in the undoped sample, and shifts to lower energies with doping. By performing theoretical calculation of the doping dependence $ω_{IB}$, the experimental observations can be explained with the increase with electron doping of the Fermi energy of the holes in the $σ$-band. On the other hand, the $σ$ band density of states seems not to change substantially. This points towards a $T_c$ reduction driven mainly by disorder, at least for the doping level studied here. The superconducting state has been also probed by infrared synchrotron radiation for $30<ω<150$ cm$^{-1}$ in one pure and one C-doped sample. In the undoped sample ($T_c$ = 38.5 K) a signature of the $π$-gap only is observed. At $y$ = 0.08 ($T_c$ = 31.9 K), the presence of the contribution of the $σ$-gap indicates dirty-limit superconductivity in both bands.

cond-mat.supr-con↗
arXiv2006

Study of temperature dependent atomic correlations in MgB$_{2}$

We have studied the evolution with temperature of the local as well as the average crystal structure of MgB$_2$ using the real-space atomic pair distribution function (PDF) measured by high resolution neutron powder diffraction. We have investigated the correlations of the B-B and B-Mg nearest neighbor pair motion by comparing, in the wide temperature range from T=10 K up to T=600 K, the mean-square displacements (MSD) of single atoms with the mean-square relative displacements (MSRD) obtained from the PDF peak linewidths. The results show that the single atom B and Mg vibrations are mostly decoupled from each other, with a small predominance of positive (in phase) correlation factor for both the B-B and B-Mg pairs. The small positive correlation is almost temperature independent, in contrast with our theoretical calculations; this can be a direct consequence of the strong decay processes of the $E_{2g}$ anharmonic phonons.

cond-mat.supr-con↗
arXiv2005

Polarons and Bipolarons in Holstein and Holstein $t-J$ models by Dynamical Mean Field Theory

The results of a comparative study of electronic spectral properties as well as phonon Probability Distribution Function (PDF) are reported. The study is done using Dynamical Mean Field Theory for the half-filled spinless and spinful Holstein model and for a single hole in Holstein t-J model. Qualitative changes in phonon PDF signals a polaronic crossover while an electronic Metal Insulator Transition (MIT) due to pairing into a bipolaronic state can be seen from a gap in the electronic DOS or by the vanishing of the quasi particle spectral weight. MIT can be observed only at sufficiently high density provided the Coulomb repulsion is neglected. In the limit of low density the carrier show a polaronic behavior trough the development of a definite polarization shifting the average value of the phonon PDF. At large densities occupied and empty sites result an a bimodal the phonon PDF. At the polaron crossover fluctuation of phonon coordinates tends to be larger than those at any other coupling. In this regime Born-Oppenheimer approximation is shown to fail also when phonon frequency is much less then electron bandwidth. For a single hole in the t-J model a further source of localization is due to magnetic superexchange energy which tends to localize the spin-defect spreading due to the presence of the hole.

cond-mat.str-el↗
arXiv2005

Transport properties in correlated systems: An analytical model

Several studies have so far investigated transport properties of strongly correlated systems. Interesting features of these materials are the lack of resistivity saturation well beyond the Mott-Ioffe-Regel limit and the scaling of the resistivity with the hole density in underdoped cuprates. Due to the strongly correlated nature of these materials, mainly numerical techniques have been employed. A key role in this regards is thought to be played by the continuous transfer of spectral weight from coherent to incoherent states. In this paper we employ a simple analytical expression for the electronic Green's function to evaluate both quasi-particle and transport properties in correlated systems. Our analytical approach permits to enlighten the specific role of the spectral transfer due to the correlation on different features. In particular we investigate the dependence of both quasi-particle and transport scattering rate on the correlation degree and the criterion for resistivity saturation. systems.

cond-mat.str-el↗
arXiv2004

Thomas-Fermi generalized approach for studying systems under pressure

In a previous work one of the authors proposed a simple model for studying systems under pressure based on the Thomas-Fermi (TF) model of single atom. In this work we intend to extend the previous work to more general Thomas-Fermi models where electronic exchange and correlation are introduced. To do so, we first study numerically the equation obtained by H.W.Lewis (TFDL) which introduces the effects of exchange and correlation into the original TF equation; next the procedure followed in the previous work is extended to the new approach and a specific example is illustrated. Although one could expect that no big differences were produced by the generalized TF model, we show the qualitative as well as quantitative equivalence with detailed numerical results. These results support the robustness of our conclusions with regards to the model proposed in the previous work and give the character of universality (i.e. to pass from one atom to another, the quantities calculated must be simply scaled by a numerical factor) to the properties of compressed systems shown in this work.

cond-mat.stat-mech↗
arXiv2004

Small Fermi energy, zero point fluctuations and nonadiabaticity in MgB$_2$

Small Fermi energy effects are induced in MgB$_2$ by the low hole doping in the $σ$ bands which are characterized by a Fermi energy $E_{\rm F}^σ\sim 0.5$ eV. We show that, due to the particularly strong deformation potential relative to the $E_{2g}$ phonon mode, lattice fluctuations are reflected in strong fluctuations in the electronic band structure. Quantum fluctuations associated to the zero-point lattice motion are responsible for an uncertainty of the Fermi energy of the order of the Fermi energy itself, leading to the breakdown of the adiabatic principle underlying the Born-Oppenheimer approximation in MgB$_2$ even if $ω_{\rm ph}/E_{\rm F} \sim 0.1-0.2$, where $ω_{\rm ph}$ are the characteristic phonon frequencies. This amounts to a new nonadiabatic regime, which could be relevant to other unconventional superconductors.

cond-mat.supr-con↗
arXiv2004

Relevance of multiband Jahn-Teller effects on the electron-phonon interaction in $A_3$C$_{60}$

Assessing the effective relevance of multiband effects in the fullerides is of fundamental importance to understand the complex superconducting and transport properties of these compounds. In this paper we investigate in particular the role of the multiband effects on the electron-phonon (el-ph) properties of the $t_{1u}$ bands coupled with the Jahn-Teller intra-molecular $H_g$ vibrational modes in the C$_{60}$ compounds. We show that, assuming perfect degeneracy of the electronic bands, vertex diagrams arising from the breakdown of the adiabatic hypothesis, are one order of magnitude smaller than the non-crossing terms usually retained in the Migdal-Eliashberg (ME) theory. These results permit to understand the robustness on ME theory found by numerical calculations. The effects of the non degeneracy of the $t_{1u}$ in realistic systems are also analyzed. Using a tight-binding model we show that the el-ph interaction is mainly dominated by interband scattering within a single electronic band. Our results question the reliability of a degenerate band modeling and show the importance of these combined effects in the $A_3$C$_{60}$ family.

cond-mat.supr-con↗
arXiv2004

Polaronic and nonadiabatic phase diagram from anomalous isotope effects

Isotope effects (IEs) are powerful tool to probe directly the dependence of many physical properties on the lattice dynamics. In this paper we invenstigate the onset of anomalous IEs in the spinless Holstein model by employing the dynamical mean field theory. We show that the isotope coefficients of the electron effective mass and of the dressed phonon frequency are sizeable also far away from the strong coupling polaronic crossover and mark the importance of nonadiabatic lattice fluctuations in the weak to moderate coupling region. We characterize the polaronic regime by the appearence of huge IEs. We draw a nonadiabatic phase diagram in which we identify a novel crossover, not related to polaronic features, where the IEs attain their largest anomalies.

cond-mat.str-el↗
arXiv2003

Charge fluctuations and electron-phonon interaction in the finite-$U$ Hubbard model

In this paper we employ a gaussian expansion within the finite-$U$ slave-bosons formalism to investigate the momentum structure of the electron-phonon vertex function in the Hubbard model as function of $U$ and $n$. The suppression of large momentum scattering and the onset a small-${\bf q}$ peak structure, parametrized by a cut-off $q_c$, are shown to be essentially ruled by the band narrowing factor $Z_{\rm MF}$ due to the electronic correlation. A phase diagram of $Z_{\rm MF}$ and $q_c$ in the whole $U$-$n$ space is presented. Our results are in more than qualitative agreement with a recent numerical analysis and permit to understand some anomalous features of the Quantum Monte Carlo data.

cond-mat.str-el↗
arXiv2003

Poor screening and nonadiabatic superconductivity in correlated systems

In this paper we investigate the role of the electronic correlation on the hole doping dependence of electron-phonon and superconducting properties of cuprates. We introduce a simple analytical expression for the one-particle Green's function in the presence of electronic correlation and we evaluate the reduction of the screening properties as the electronic correlation increases by approaching half-filling. The poor screening properties play an important role within the context of the nonadiabatic theory of superconductivity. We show that a consistent inclusion of the reduced screening properties in the nonadiabatic theory can account in a natural way for the $T_c$-$δ$ phase diagram of cuprates. Experimental evidences are also discussed.

cond-mat.str-el↗
arXiv2003

Electron-phonon renormalization in small Fermi energy systems

The puzzling features of recent photoemission data in cuprates have been object of several analysis in order to identity the nature of the underlying electron-boson interaction. In this paper we point out that many basilar assumptions of the conventional analysis as expected to fail in small Fermi energy systems when, as the cuprates, the Fermi energy $E_{\rm F}$ is comparable with the boson energy scale. We discuss in details the novel features appearing in the self-energy of small Fermi energy systems and the possible implications on the ARPES data in cuprates.

cond-mat.supr-con↗
arXiv2003

Comment on ``Electron-phonon Vertex in the Two-dimensional One-band Hubbard Model''

In cond-mat/0306131 Huang {\em et al.} reported for the first time a numerical evidence for the predominance of small phonon momenta in the electron-phonon coupling induced by electronic correlation. They report also an upturn of the electron-phonon vertex function for small momenta as function of the Hubbard repulsion $U$ for sufficient large $U$. In this comment we compare their Quantum Monte Carlo data with analytical results based on slave-boson techniques for generic $U$. We associate the upturn of the electron-phonon vertex function with the tendency towards a phase separation instability at large temperatures.

cond-mat.str-el↗
arXiv2003

Band-filling effects on electron-phonon properties of normal and superconducting state

We address the effect of band filling on the effective electron mass $m^*$ and the superconducting critical temperature $T_c$ in a electron-phonon system. We compare the vertex corrected theory with the non-crossing approximation of the Holstein model within a local approximation. We identify two regions of the electron density where $m^*$ and $T_c$ are enhanced or decreased by the inclusion of the vertex diagrams. We show that the crossover between the enhancement at low density and the decrease towards half filling is almost independent of the microscopic electron-phonon parameters. These different behaviors are explained in terms of the net sign of the vertex diagrams which is positive at low densities and negative close to half filling. Predictions of the present theory for doped MgB$_2$, which is argued to be in the low density regime, are discussed.

cond-mat.supr-con↗