arXiv subjects

E. V. L. de Mello

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

At least 37 records · Page 2Linked to original sources

arXiv2012

Description of the Oxygen Order evolution and its relation to the Superconducting Transition in $La_2CuO_{4+y}$

The segregation of oxygen in the high critical temperature cuprate superconductor $La_2CuO_{4+y}$ has been systematically studied along the years. In a recent set of experiments, Poccia et al related, for the first time, time ordering ($t$) of oxygen interstitials with the corresponding superconducting transition temperature $T_c(t)$. We develop a phenomenological description of the time ordering forming pattern domains and show how it may affect the superconducting interaction. The superconducting self-consistent calculations in a system with electronic granular structure of varying hole doping yields also different local d-wave amplitudes. These amplitudes are of the order of magnitude of scanning tunneling microscopy measurements and they vanish at $T^*(t)> T_c(t)$. Then, calculations with Josephson coupling among the isolated charge domains reveal that the superconducting interaction is likely to be scaled by the local free energy and capture the details of $T_c(t)$. The accurately reproduction of these apparently disconnected phenomena establishes routes to the important physical mechanisms involved in the connection between sample production and on the origin of the superconductivity of cuprates.

cond-mat.supr-con↗

arXiv2012

Theory of the Fermi Arcs, the Pseudogap, $T_c$ and the Anisotropy in k-space of Cuprate Superconductors

The appearance of the Fermi arcs or gapless regions at the nodes of the Fermi surface just above the critical temperature is described through self-consistent calculations in an electronic disordered medium. We develop a model for cuprate superconductors based on an array of Josephson junctions formed by grains of inhomogeneous electronic density derived from a phase separation transition. This approach provides physical insights to the most important properties of these materials like the pseudogap phase as forming by the onset of local (intragrain) superconducting amplitudes and the zero resistivity critical temperature $T_c$ due to phase coherence activated by Josephson coupling. The formation of the Fermi arcs and the dichotomy in k-space follows from the direction dependence of the junctions tunneling current on the d-wave symmetry on the $CuO_2$ planes. We show that this semi-phenomenological approach reproduces also the main future of the cuprates phase diagram.

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arXiv2011

The Disordered Induced Interaction and the Phase Diagram of Cuprates

There are processes in nature that resemble a true force but arise due to the minimization of the local energy. The most well-known case is the exchange interaction that leads to magnetic order in some materials. We discovered a new similar process occurring in connection with an electronic phase separation transition that leads to charge inhomogeneity in cuprate superconductors. The minimization of the local free energy, described here by the Cahn-Hilliard diffusion equation, drives the charges into regions of low and high densities. This motion leads to an effective potential with two-fold effect: creation of tiny isolated regions or micrograins, and two-body attraction, which promotes local or intra-grain superconducting pairing. Consequently, as in granular superconductors, the superconducting transition appears in two steps. First, with local intra-grain superconducting amplitudes and, at lower temperature, the superconducting phase or resistivity transition is attained by intergrain Josephson coupling. We show here that this approach reproduces the main features of the cuprates phase diagram, gives a clear interpretation to the pseudogap phase and yields the position dependent local density of states gap $Δ(\vec r)$ measured by tunnelling experiments.

cond-mat.supr-con↗

arXiv2011

Resistivity Calculations for Cuprate Superconductor Systems using an Electronic Phase Separation

The resistivity as function of temperature of high temperature superconductors is very unusual and despite its importance lacks an unified theoretical explanation. It is linear with the temperature for overdoped compounds but it falls more quickly as the doping level decreases, and for weakly doped samples it has a minimum, increases like an insulator before it drops to zero at low temperatures. We show that this overall behavior can be explained by calculations using an electronic phase segregation into two main component phases with low and high densities. The total resistivity is calculated by the various contributions through several random picking processes of the local resistivities and using the Random Resistor Network approach.

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arXiv2010

Derivation of the Disorder Induced Interaction and the Phase Diagram of Cuprate Superconductors

We show that an electronic phase transition described by the Cahn-Hilliard equation has important applications to cuprate superconductors. The simulations of the local charge density and free energy reveal two main features: i) The segregation process creates tiny isolated regions with potential wells where the holes can be bound in single-particle levels. ii) The clustering process also gives rise to an effective two-body pairing interactions and superconducting amplitudes $Δ_{sc}(\vec r)$ at low temperatures. The resulting system resembles a granular superconductor with the resistivity transition driven by Josephson coupling among these nanoscale grains. This approach reproduces the well known critical temperature transition $T_c(p)$ as function of the doping level $p$. The derived $p \times T$ phase diagram reproduces the main features measured by several experiments. Furthermore, the local density of states with spatial dependent gaps $Δ(\vec r)$ is due to the intragrain single-particle bound states that remain above $T_c$, which characterizes the pseudogap phase and reproduces many measurements.

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arXiv2009

The Derivation of the Fundamental Superconducting Interaction and the Phase Diagram of Cuprates

We show that an electronic phase separation (EPS) transition described by the an appropriate theory yields regions of low free energy forming grains of low and high charge densities. These local differences in the potential energy are studied numerically and used here for the first time as the origin of an attractive interaction that gives rise to local pair formation. The resistivity transition $T_c$ occurs due to Josephson coupling among these superconducting regions or grains. Using this approach within the Bogoliubov-deGennes calculations we derive the Gaussian shape of $T_c$ against the entire doping level $p$. We find that every pairing gap develops locally at a temperature $T_p$, following the relation $2Δ/k_BT_p\approx8.0$ in close agreement recent measurements.}

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arXiv2007

Measurements and analysis of the upper critical field $H_{c2}$ on an underdoped and overdoped $La_{2-x}Sr_xCuO_4$ compounds

The upper critical field $H_{c2}$ is one of the many non conventional properties of high-$T_c$ cuprates. It is possible that the $H_{c2}(T)$ anomalies are due to the presence of inhomogeneities in the local charge carrier density $ρ$ of the $CuO_2$ planes. In order to study this point, we have prepared good quality samples of polycrystalline $La_{2-x}Sr_xCuO_{4}$ using the wet-chemical method, which has demonstrated to produce samples with a better cation distribution. In particular, we have studied the temperature dependence of the second critical field, $H_{c2}(T)$, through the magnetization measurements on two samples with opposite average carrier concentration ($ρ_m=x$) and nearly the same critical temperature, namely $ρ_m = 0.08$ (underdoped) and $ρ_m = 0.25$ (overdoped). The results close to $T_c$ do not follow the usual Ginzburg-Landau theory and are interpreted by a theory which takes into account the influence of the inhomogeneities.

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arXiv2007

Theoretical high-$T_c$ d-wave superconducting gap in an inhomogeneous medium

We perform theoretical calculations to obtain a distribution of local d-wave superconducting gaps $Δ_0({r})$ for a high temperature superconducting (HTSC) series in a disordered superconductor with an average doping level $<ρ>$. To reproduce the inhomogeneous medium a nonmagnetic random potential $V^{imp}$, within a Bogoliubov-de Gennes (BdG) formalism, is considered. First the phase diagram $Δ_0 x <ρ>$ for the LSCO HTSC series, with V^{imp}=0, is obtained. Then, we perform calculations considering a fixed value of the disorder strength $V^{imp}$ and obtain a distribution of local superconducting gaps $Δ_0({r})$, and local density of charge carriers $ρ({r})$. It is shown that the underdoped compounds are more inhomogeneous than the overdoped ones, which is in accordance with experimental findings. Also, the spatial variation of $Δ_0({r})$ indicates that as <ρ> increases, the system becomes more homogeneous.

cond-mat.supr-con↗

arXiv2006

Phase Separation and the Phase Diagram in Cuprates Superconductors

We show that the main features of the cuprates superconductors phase diagram can be derived considering the disorder as a key property of these materials. Our basic point is that the high pseudogap line is an onset of phase separation which generates compounds made up of regions with distinct doping levels. We calculate how this continuous temperature dependent phase separation process occurs in high critical temperature superconductors (HTSC) using the Cahn-Hilliard approach, originally applied to study alloys. Since the level of phase separation varies for different cuprates, it is possible that different systems with average doping level pm exhibit different degrees of charge and spin segregation. Calculations on inhomogeneous charge distributions in form of stripes in finite clusters performed by the Bogoliubov-deGennes superconducting approach yield good agreement to the pseudogap temperature T*(pm), the onset of local pairing amplitudes with phase locked and concomitantly, how they develop at low temperatures into the superconducting phase at Tc(pm) by percolation.

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arXiv2005

Phase Separation and the Dual Nature of the Electronic Structure in Cuprates

The dual nature of the electronic structure of stripes in $La_{2-x}Sr_xCuO_4$ was characterized by experimental observations, mainly by ARPES, of nodal spectral weight together with the straight segments near antinodal regions. We present here an attempt to understand this dual behavior in terms of the competition of order and disorder, by applying the phase separation theory of Cahn-Hilliard (CH) to the high pseudogap temperature, which is very large in the far underdoping region and vanishs near the doping level p=0.2. The spinodal phase separation predictions together with the Bogoliubov-deGennes (BdG) superconducting theory provides several interesting insights. For instance, we find that the disorder enhances the local superconducting gap which scales with the leading edge shift and that, upon doping, the size of the hole-rich stripes increases, yielding to the system their metallic properties.

cond-mat.supr-con↗

arXiv2004

The Effects of Phase Separation in the Cuprate Superconductors

Phase separation has been observed by several different experiments and it is believed to be closely related with the physics of cuprates but its exactly role is not yet well known. We propose that the onset of pseudogap phenomenon or the upper pseudogap temperature $T^*$ has its origin in a spontaneous phase separation transition at the temperature $T_{ps}=T^*$. In order to perform quantitative calculations, we use a Cahn-Hilliard (CH) differential equation originally proposed to the studies of alloys and on a spinodal decomposition mechanism. Solving numerically the CH equation it is possible to follow the time evolution of a coarse-grained order parameter which satisfies a Ginzburg-Landau free-energy functional commonly used to model superconductors. In this approach, we follow the process of charge segregation into two main equilibrium hole density branches and the energy gap normally attributed to the upper pseudogap arises as the free-energy potential barrier between these two equilibrium densities below $T_{ps}$. This simulation provides quantitative results %on the hole doping and temperature %dependence of the degree of the charge inhomogeneity in agreement with %some experiments and the simulations reproduce the observed stripe and granular pattern of segregation. Furthermore, with a Bogoliubov-deGennes (BdG) local superconducting critical temperature calculation for the lower pseudogap or the onset of local superconductivity, it yields novel interpretation of several non-conventional measurements on cuprates.

cond-mat.supr-con↗

arXiv2004

Numerical Study of the Cahn-Hilliard Equation in One, Two and Three Dimensions

The Cahn-Hilliard equation is related with a number of interesting physical phenomena like the spinodal decomposition, phase separation and phase ordering dynamics. On the other hand this equation is very stiff an the difficulty to solve it numerically increases with the dimensionality and therefore, there are several published numerical studies in one dimension (1D), dealing with different approaches, and much fewer in two dimensions (2D). In three dimensions (3D) there are very few publications, usually concentrate in some specific result without the details of the used numerical scheme. We present here a stable and fast conservative finite difference scheme to solve the Cahn-Hilliard with two improvements: a splitting potential into a implicit and explicit in time part and a the use of free boundary conditions. We show that gradient stability is achieved in one, two and three dimensions with large time marching steps than normal methods.

cond-mat.stat-mech↗

arXiv2004

Theory of the Diamagnetism Above the Critical Temperature for Cuprates

Recently experiments on high critical temperature superconductors has shown that the doping levels and the superconducting gap are usually not uniform properties but strongly dependent on their positions inside a given sample. Local superconducting regions develop at the pseudogap temperature ($T^*$) and upon cooling, grow continuously. As one of the consequences a large diamagnetic signal above the critical temperature ($T_c$) has been measured by different groups. Here we apply a critical-state model for the magnetic response to the local superconducting domains between $T^*$ and $T_c$ and show that the resulting diamagnetic signal is in agreement with the experimental results.

cond-mat.supr-con↗

arXiv2004

Upper critical field $H_{c2}$ calculations for the high critical temperature superconductors considering inhomogeneities

We perform calculations to obtain the $H_{c2}$ curve of high temperature superconductors (HTSC). We consider explicitly the fact that the HTSC possess intrinsic inhomogeneities by taking into account a non uniform charge density $ρ(r)$. The transition to a coherent superconducting phase at a critical temperature $T_c$ corresponds to a percolation threshold among different superconducting regions, each one characterized by a given $T_c(ρ(r))$. Within this model we calculate the upper critical field $H_{c2}$ by means of an average linearized Ginzburg-Landau (GL) equation to take into account the distribution of local superconducting temperatures $T_c(ρ(r))$. This approach explains some of the anomalies associated with $H_{c2}$ and why several properties like the Meissner and Nernst effects are detected at temperatures much higher than $T_c$.

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arXiv2003

Heavy fermion superconductivity: X-boson treatment

We use the X-boson method to study the heavy fermion superconductivity employing an extension of the periodic Anderson model in the $ U = \infty $ limit with a nearest neighbor attractive interaction between the localized $ f $-electrons and we show that higher values of the hybridization parameter implies in lower values associated to the maximum of the superconducting critical temperature $ T_{c} $, indicating that the real charge transfer between bands tends to destabilizes the Cooper pairing. Moreover, we show that superconductivity are constrained to the vicinity of a range of occupations where the $ f $-band density of states at the Fermi level $ ρ_{f} (μ) $ have sufficiently high values, though superconductivity was found both for configurations where the system present intermediate valence and heavy fermion behavior, as experimentally observed. Finally, as the total occupation is raised and for increased hybridization parameter $ V $ there is a transition insulator-superconductor, which is related to the existence of a hybridization gap that cannot be obtained by the slave-boson method.

cond-mat.supr-con↗

arXiv2003

Is the Superconductiong State for the Cuprates Reached Through a Percolation Transition?

Several recent experiments have revealed that the charge density $ρ$ in a given compound (mostly underdoped) is intrinsic inhomogeneous with large nanoscale spatial variations. Therefore it is appropriate to define a local charge density $ρ(r)$. These differences in the local charge concentration yield insulator and metallic regions, either in an intrinsic granular or in a stripe morphology. In the metallic region, the inhomogeneous charge density produces spatial or local distributions of superconducting critical temperatures $T_c(r)$ and zero temperature gap $Δ_0(r)$. We propose that the superconducting phase in high-$T_c$ oxides is reached when the temperature reachs a value which superconduction regions with different critical temperatures percolates. We show also that this novel approach is able to reproduce the phase diagram for a family of cuprates and provides new insights on several experimental features of high-$T_c$ oxides

cond-mat.supr-con↗

arXiv2003

A Theory for High-$T_c$ Superconductors Considering Inhomogeneous Charge Distribution

We propose a general theory for the critical $T_c$ and pseudogap $T^*$ temperature dependence on the doping concentration for high-$T_c$ oxides, taking into account the charge inhomogeneities in the $CuO_2$ planes. The well measured experimental inhomogeneous charge density in a given compound is assumed to produce a spatial distribution of local $ρ(r)$. These differences in the local charge concentration is assumed to yield insulator and metallic regions, possibly in a stripe morphology. In the metallic region, the inhomogeneous charge density yields also spatial distributions of superconducting critical temperatures $T_c(r)$ and zero temperature gap $Δ_0(r)$. For a given sample, the measured onset of vanishing gap temperature is identified as the pseudogap temperature, that is, $T^*$, which is the maximum of all $T_c(r)$. Below $T^*$, due to the distribution of $T_c(r)$'s, there are some superconducting regions surrounded by insulator or metallic medium. The transition to a superconducting state corresponds to the percolation threshold among the superconducting regions with different $T_c(r)$'s. To model the charge inhomogeneities we use a double branched Poisson-Gaussian distribution. To make definite calculations and compare with the experimental results, we derive phase diagrams for the BSCO, LSCO and YBCO families, with a mean field theory for superconductivity using an extended Hubbard Hamiltonian. We show also that this novel approach provides new insights on several experimental features of high-$T_c$ oxides.

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arXiv2002

Pressure Studies on a High-$T_c$ Superconductor Pseudogap and Critical Temperatures

We report simultaneous hydrostatic pressure studies on the critical temperature $T_c$ and on the pseudogap temperature $T^*$ performed through resistivity measurements on an optimally doped high-$T_c$ oxide $Hg_{0.82}Re_{0.18}Ba_2Ca_2Cu_3O_{8+δ}$. The resistivity is measured as function of the temperature for several different applied pressure below 1GPa. We find that both $T_c$ and $T^*$ increases linearly with the pressure. This result demonstrate that the well known intrinsic pressure effect on $T_c$ is also present at $T^*$ and both temperatures are originated by the same superconducting mechanism.

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