arXiv subjects

E. V. L. de Mello

Publications and source records attributed to E. V. L. de Mello.

45 records · Page 3Linked to original sources

arXiv2001

A Novel Theory for High Temperature Superconductors considering Inhomogeneous Charge Distributions

We present a percolation theory for the high-$T_c$ oxides pseudogap and $T_c$ dependence on the hole level. The doping dependent inhomogeneous charge structure is modeled by a distribution which may represent the stripe morphology and yield a spatial distribution of local $T_c(r)$. The temperature onset of spatial dependent superconducting gap is identified with the vanishing of the pseudogap temperature $T^*$. The transition to a superconducting state corresponds to the percolation threshold among regions of different $T_c$. As a paradigm we use a Hubbard Hamiltonian with a mean field approximation to yield a doping and temperature dependent superconducting d-wave gap. We show here that this new approach reproduces the phase diagram, explains and gives new insights on several experimental features of high-$T_c$ oxides.

cond-mat.supr-con↗

arXiv2001

BCS Model in Tsallis' Statistical Framework

We show that there is an effect of nonextensivity acting upon the BCS model for superconductors in the ground state that motivates its study in the Tsallis' statistical framework. We show that the weak-coupling limit superconductors are well described by $q \sim 1$, where q is a real parameter which characterizes the degree of nonextensivity of the Tsallis' entropy. Nevertheless, small deviations with respect to q = 1 provide better agreement when compared with experimental results. To illustrate this point, making use of an approximated Fermi function, we show that measurements of the specific heat, ultrasonic attenuation and tunneling experiments for tin (Sn) are better described with q = 0.99.

cond-mat.supr-con↗

arXiv2001

The Hubbard model applied to the phase diagram and pressure effects in YBa_2Cu_3O_{7-δ} superconductors

We apply a method based on a BCS-type approach to the extended Hubbard model to deal with the YBa_2Cu_3O_{7-δ} superconductor family under pressure. The parameters of the tight-binding band are taken from experiments, and the coupling strength U and V are estimated from the zero pressure phase diagram Tc X n_h, where n_h is the hole content and Tc the critical temperature. With the assumption that the pressure P modifies the magnitude V of the attractive potential besides n_h, we obtain a general expansion of Tc in terms of the pressure P, as Tc(P)=Tc(n_h(P),V(P)). With this we can distinguish the charge transfer and the intrinsic contribution to Tc. We show that the changes of Tc with P for YBa_2Cu_3O_{7-δ} are almost due to the intrinsic term.

cond-mat.supr-con↗

arXiv2001

Phase diagram and dependence of the critical temperature T_c on the pressure for Tl_{0.5}Pb_{0.5}Sr_2Ca_{1-x}Y_xCu_2)_7

Using a mean-field BCS-like approach on the bidimensional extended Hubbard Hamiltonian we calculate the superconducting transition temperature Tc as a function of the hole content nh, for the d-wave and extended-s wave gap symmetries. To describe the pressure effect on Tc we assume it induces a change in the magnitude V of the attractive superconductor potential. This assumption yields an explanation for the intrinsic term, and together with the well known change in nh, we set the critical temperature as Tc=Tc(nh(P),V(P)). With this we obtain a general expansion of Tc in terms of the pressure P and the hole content nh. We apply this expansion to the Tl_{0.5}Pb_{0.5}Sr_2Ca_{1-x}Y_xCu_2)_7 system.

cond-mat.supr-con↗

arXiv1999

The extended Hubbard model applied to phase diagram and the pressure effects in \Bi superconductors

We use the two dimensional extended Hubbard Hamiltonian with the position of the attractive potential as a variable parameter with a BCS type approach to study the interplay between the superconductor transition temperature $T_c$ and hole content for high temperature superconductors. This novel method gives some insight on the range and intensity of the Cooper pair interaction and why different compounds have different values for their measured coherence lengths and it describes well the experimental results of the superconducting phase diagram $T_c \times n$. The calculations may also be used to study the effect of the applied pressure with the assumption that it increases the attractive potential which is accompanied by an increase in the superconductor gap. In this way we obtain a microscopic interpretation for the intrinsic term and a general expansion for $T_c$ in terms of the pressure which reproduces well the experimental measurements on the \Bi superconductors.

cond-mat.supr-con↗

arXiv1999

Crossover behavior for complex order parameter in high-Tc superconductors

A number of recent experiments have suggested the presence of either real or complex components in the gap symmetry of high-$T_c$ superconductors (HTSC). In this paper we introduce a novel approach to study the competition of such complex order parameter mixtures by varying the position of the two-body attractive potential in a two dimensional extended Hubbard Hamiltonian. We show that this procedure explain a number of experimental results and on the theoretical side, it may be related with certain HTSC microscopic models like the spin fluctuation theory. Following current trends we concentrate on the study of $d_{x^2-y^2}$ order parameter with a component of the type $d_{xy}$ or a s-wave like $s_{x^2+y^2}$ and $s_{xy}$ symmetry. We show that the position of the optimal s-component peak changes with the position parameter $b$ while the d-component occurs always in the optimally region around hole content $ρ\approx 0.39$.

cond-mat.supr-con↗

arXiv1997

The extended Hubbard model applied to study the pressure effects for High Temperature Superconductors

We make use of a BCS type approach based on the extended Hubbard Hamiltonian to study the superconductor transition and to give a microscopic interpre- tation of the pressure effects on Tc in high temperature superconductors. This novel method suggests that the applied pressure causes an increase of the superconducting gap and this effect is explored in order to explain the variations of Tc. Our approach is therefore beyond the scope of previously phenomenological models which basically postulate a pressure-induced charge transfer and an intrinsic term linear on the pressure. We obtain a microscopic interpretation of this intrinsic term and a general expansion of Tc in terms of the pressure. To demonstrate the efficiency of the method we apply it to the experimental data of the Hg-base superconductors.

cond-mat.str-el↗

arXiv1997

Dynamical Properties of small Polarons

On the basis of the two-site polaron problem, which we solve by exact diagonalization, we analyse the spectral properties of polaronic systems in view of discerning localized from itinerant polarons and bound polaron pairs from an ensemble of single polarons. The corresponding experimental techniques for that concern photoemission and inverse photoemission spectroscopy. The evolution of the density of states as a function of concentration of charge carriers and strength of the electron-phonon interaction clearly shows the opening up of a gap between single polaronic and bi-polaronic states, in analogy to the Hubbard problem for strongly correlated electron systems. The crossover regime between adiabatic and anti-adiabatic small polarons is triggered by two characteristic time scales: the renormalized electron hopping rate and the renormalized vibrational frequency becoming equal. This crossover regime is then characterized by temporarily alternating self- localization and delocalization of the charge carriers which is accompanied by phase slips in the charge and molecular deformation oscillations and ultimately leads to a dephasing between these two dynamical components of the polaron problem. We visualize these features by a study of the temporal evolution of the charge redistribution and the change in molecular deformations. The spectral and dynamical properties of polarons discussed here are beyond the applicability of the standard Lang Firsov approach to the polaron problem.

cond-mat.str-el↗

arXiv1997

Study of the Phase Diagram and the Tc Pressure Dependence for HgBaCuO Superconductors

We apply a method recently derived based on the extended Hubbard Hamiltonian and the BCS approach to obtain the superconducting critical temperature dependence with carrier density for the HgBaCuO compounds. We show that this procedure can be used to study pressure effect with the assumption that pressure changes the supercondutor gap. The results reproduce well the experimental data for the whole doping regime and the high and low pressure dependence.

cond-mat.supr-con↗