Temperature-dependent resistivity from phonons in cuprate superconductors

The temperature dependence of the resistivity of the cuprate superconductors arising from phonon scattering is considered within standard Bloch-Gruneisen theory allowing for several different shapes of transport spectral functions _tr ² F(). In contrast to often made comments that linear resistivity is not characteristic of electron-phonon scattering, it is shown rather that it is more difficult to get muchnonlinearity from this theory except below 50–100 K, and in this low-T regime better solutions to the Boltzmann equation might be required. The reasons for the linear behavior are clarified, and the variations to be expected from possible coupling functions are explored. Possible origins of the nonlinear resistivity seen in YBa₂Cu₄O₈ and recently in YBa₂Cu₃O₇ are suggested, and effects not included specifically in these calculations are discussed. It is also pointed out that phonon contributions to the energy () dependence of quasiparticles, which is a closely related consequence of electron-phonon coupling, will differ significantly from the simple ² Fermi liquid behavior that is commonly assumed.     

The Effect of the Phase Separation in the Phase Diagram of Cuprate 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. Calculations with the BdG approach and an analysis of the local density of states (LDOS), yield good agreement with the measured values of T ^* and T _c . This work has been partially supported by CAPES, CNPq and CNPq-Faperj Pronex E-26/171.168/2003.     

Raman Spectroscopy, Structural Modifications, and Phase Transitions in the High-Temperature Superconductors

The formation of phases is investigated from a systematic study of the Raman active phonons in the whole range of oxygen doping of YBa₂Cu₃O _x (6 < x < 7). Changes in the spectral characteristics of the in-phase vibrations of the plane oxygen atoms in the overdoped region are related, with slight modifications of the buckling in the CuO₂ planes. A softening of the A _g symmetry apical oxygen phonon is accompanied with an increase in the intensity of the high-frequency mode at 600 cm^–1 and attributed to the breaking of the chains into small segments. The analysis of the apex and the in-phase phonon modes indicates the formation of superstructures, which are embedded as spatially separated phases in the whole oxygen concentration region of 6.35 < x < 6.95. A similar separation into small domains of phases may also occur in the underdoped compounds (x < 6.35) down to a critical oxygen concentration at x 6.15, where the system breaks into randomly oriented small chains.     

Phase Separation of Electrons Strongly Coupled with Phonons in Cuprates and Manganites

Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron–electron correlations. We argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron–phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott–Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons (real-space electron (hole) pairs dressed by phonons). They form clusters localized by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Fröhlich finite-range EPI with optical phonons as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR of doped manganites due to magnetic break-up of bipolarons in the ferromagnetic phase. Here I briefly present an explanation of high- and low-resistance phase coexistence near the ferromagnetic transition as a mixture of polaronic ferromagnetic and bipolaronic paramagnetic domains due to unavoidable disorder in doped manganites.     

Phase Separation in a Two-Component Model for Cuprates

In a brief review the role of interactions in a two-component model for electronic spectrum in cuprates is discussed. Interactions in the model result in a tendency to phase separation. It is speculated that the latter determines the scale of the pseudogap temperature, T*, while hybridization between localized and itinerant components moderates this tendency to the first-order transition and brings about isotope mass dependence through polaronic effects.     

Lattice Distortions and Charge Carriers in Cuprates

A phenomenological model with itinerant bands and local states trapped by thelattice on the Cu-sites, is discussed to describe global features ofcuprates. Relative energy positions of localized and itinerant states beingtuned (thermodynamically or by doping), the system must undergo first-orderMott metal-insulator transition. Decreasing the local level (from themetallic end of a stoichiometric compound), charge separation instabilityoccurs first before the Mott transition. Crossing and hybridization betweenlocal (flat) and itinerant bands introduce a structure in density of stateswhich may account for pseudogap features in cuprates. Modelresults in polaronic lattice effects and is rich enough to serve as aphenomenology of cuprates.     

The Theory of Local Superconductivity and Phase Separation Applied to Cuprates

In this paper we present a theory of the high critical temperature superconductors (HTSC). As has been shown directly by Scanning Tunneling Microscopy (STM) and indirectly by neutron diffraction, these materials are not homogeneous. We suppose that, at temperatures detected by some experiments, these compounds go through a phase separation which essentially creates regions with low and large doping around the average doping level. Using a Bogoliubov–deGennes local theory, we calculate the superconducting local gap as function of the density, for islands with 10–14 lattice parameters on average. We argue that this procedure is suitable to explain many HTSC properties and their universal phase diagram.     

Phase separation and charge density waves: Possible sources of non-Fermi liquid behavior and pairing in high-temperature superconductors

In the absence of a sufficiently singular interaction between the particles, the Fermi-liquid state is stable above one dimension. To reconcile this finding with the observed anomalies in the cuprates one is therefore led to the search for mechanisms giving singular interactions. A recent proposal indicates the cuprates as being close to a charge instability, which would drive the system toward phase separation (in the absence of long-range Coulomb forces between the carriers) or toward the formation of incommensurate charge-density waves in the more physical case of charged carriers. Close to these instabilities strong singular and attractive scattering arises between the quasiparticles at the Fermi surface. This scattering would easily account for both the anomalous behavior of the normal metallic phase and for the strong pairing mechanism leading to high-temperature superconductivity. The strong momentum dependence of the singular attraction would also give rise to the (likely) observed unusuald-wave symmetry of the superconducting order parameter in these systems.     

非晶态 Li~+ 导体分相和晶化过程的扫描电镜观察

对非晶态 Li~+导体 B_2O_3—0.7Li_2O—0.7LiCl—0.1Al_2O_3在300℃温度下分相和晶化过程进行了扫描电镜(SEM)观察。结果表明;在等温处理的开始阶段,样品内首先发生分相,从母相中分离出均匀分布的新的非晶相粒子,随后长大和聚集;然后,新非晶相发生晶化长成晶粒,其成分为 Li_4B_7O_(12)Cl;最后,背景的非晶相中析出另一种晶粒,成分为 LiBO_2,并有较大的生长速度。     

Effect of interlayer interaction in high-Tc cuprate superconductors

We have studied the role of interlayer attractive interaction in a high-T _c system having two layers per unit cell. The single band two-layer tight binding model Hamiltonian is considered and the double time Green's function technique is applied within the mean field approximation. The expressions for the hole density, transition temperature, and intra- and interlayer order parameters are obtained which are found to be dependent on the interlayer interaction and other parameters appearing in the Hamiltonian. The numerical analysis shows that the coupling of the charge carriers (holes) between the layers provides better conditions for the stabilization of long-range order and high superconducting transition temperature in layered superconductors. It is also observed that superconductivity is confined to a narrow region of hole concentration and the single particle tunneling suppresses the transition temperature.     

Simulation of Copolymer Phase Separation in One-Dimensional Thin Liquid Films

This paper discusses the development of an invariant finite difference scheme to simulate the microphase separation of copolymers in one-dimensional thin liquid films. The film phenomena are modelled using two-phase shallow water equations and the Ohta–Kawasaki potential, which governs the phase separation of the copolymer. Non-positive volume fractions and spurious oscillations are eventually eliminated, in simulating the one-dimensional phase separation lamellar pattern.     

Complex Phase Separation in Oxygen-Doped Cuprates La2CuO4+y Superconductors

The multiple-phase separation with coexistence of two different superconducting phases in the phase diagram of oxygen-doped La₂CuO_{4+ y} cuprates is interpreted with a modified Van der Waals scheme. The model includes the tendency of intercalated oxygen ions to form clusters at preferential densities. We obtain the coexistence and competition of two different superconducting phases with different T _c and interstitial oxygen content y, in agreement with experiments.     

现代连续相变理论在玻璃分相中的应用

玻璃分相区中存在一临界点,在此临界点的转变属于连续相变范畴。通过本文的讨论表明。对二组元玻璃系统的分相,可以用现代连续相变理论描述。在临界点附近区域内,其分相曲线为以临界点为界,以两曲线方程:(x~l-x_c)/x_c=A[(T_c-T)/T_c]~(1/3),(x~(l′)-x_c)/x_c=A′[(T_c-T)/T_c]~(1/3)来表达。这里 A、A′为二组元系统分相曲线的普适常数。本文中对 Li_2O-SiO_2,Na_2O-SiO_2,BaO-SiO_2系统分相实验值的拟合结果是 A=-1.8305;A′=2.3551。相应的计算曲线与实验值符合较好。     

The Material-Dependent Parameter Controlling the Universal Phase Diagram of Cuprates

The critical temperature T _c in the universal phase diagram of cuprate superconductors is a function of two variables: the hole-doping δ and a material dependent parameter. Here we focus on the behavior of T _c,max as a function of the material dependent parameter (MDP) at the optimum hole doping. We have discussed the correlation between (1) the average Cu—O (planar) distance, or the strain of the Cu—O bond, (2) the nearest-neighbor hopping t′ and (3) the Lifshitz parameter z. These Lifshitz parameter z = μ_{δ = 0.16}−E _vHs which are all material dependent parameters, where μ_{δ = 0.16} is the chemical potential at optimum doping and E_vHs is the energy of the Van Hove singularity, defines the proximity to the Fermi surface topological transition from electron-like to hole-like. The results show that the striped phases occur for z < 0, the highest T _c,max for and the drop of T _c,max for z > 75 meV.     

Al(PO_3)_3-NaBF_4-AlF3 系统玻璃的分相及晶化的研究

本工作利用 DTA、RTEM、X 射线及 IR 技术研究了 Al(PO_3)_s-NaBF_4-AlF_3系统玻璃的分相机理及分相与晶化之间的关系。从实验结果得出,分相玻璃的 DTA 曲线上存在有两个明显的 T_g 温度。其中,高 T_g 相为富磷氧相,低T_g 相为贫磷氧相。且分相玻璃的第二相呈粒子形态,属成核生长机理。粒子长大服从扩散机制。该系统玻璃分相导致整体析晶。高磷区析晶相为 Al(PO_3)_3,低磷区析晶相为 NaAlP_2O_7。     

K_2O-MgO-Al_2O_3-P_2O_5-B_2O_3系统玻璃分相和光色性之间的关系

为了获得一种具有良好光色性能的磷酸盐光色玻璃,选择了 K_2O-MgO-Al_2O_3-P_2O_5-B_2O_3系统作为基础玻璃,以 AgX 为光敏成分,研究了玻璃分相和光色性之间的相互关系。用分子光谱、SEM,EDAX 和 XRD 等研究了2~#分相玻璃的相组成、相形貌。实验结果表明,光色玻璃中光敏相不仅是 AgCl 和 NaCl 固溶相组成,而且富集于液滴相中。玻璃的热处理以及相应热处理条件下分相形貌的观察结果表明:在一定的温度范围内,随着热处理温度升高,玻璃的分相程度增大,增强了玻璃的变暗程度,但相应地降低了光色玻璃的退色速率。     

Lattice Effects on Charge Localization in Cuprates

Recent experimental results on cuprates and manganites, including those of elastic and inelastic neutron scattering measurements, suggest that charges are not homogeneously distributed even in the metallic state in these compounds. Charge inhomogeneity results from spin/lattice charge constriction. In cuprates the LO phonons strongly reflect the temperature- and composition-dependent charge inhomogeneity and may possibly be involved in causing it. Unlike the static stripes that compete against superconductivity, the charge inhomogeneity seen by the LO phonons is markedly increased in the superconducting phase. A new mechanism of high-temperature superconductivity involving lattice/spin charge constriction is proposed.