Cuprate Compounds Research Articles

Strain effects on point defects and chain-oxygen order-disorder transition in 123 cuprate compounds

The energetics of Schottky defects in 123 cuprate superconductor series, $\rm REBa_2Cu_3O_7$ (where RE = lanthandies) and $\rm YAE_2Cu_3O_7$ (AE = alkali-earths), were found to have unusual relations if one considers only the volumetric strain. Our calculations reveal the effect of non-uniform changes of interatomic distances within the RE-123 structures, introduced by doping homovalent elements, on the Schottky defect formation energy. The energy of formation of Frenkel Pair defects, which is an elementary disordering event, in 123 compounds can be substantially altered under both stress and chemical doping. Scaling the oxygen-oxygen short-range repulsive parameter using the calculated formation energy of Frenkel pair defects, the transition temperature between orthorhombic and tetragonal phases is computed by quasi-chemical approximations (QCA). The theoretical results illustrate the same trend as the experimental measurements in that the larger the ionic radius of RE, the lower the orthorhombic/tetragonal phase transition temperature. This study provides strong evidence of the strain effects on order-disorder transition due to oxygens in the CuO chain sites.

Optical response for a discrete stripe

The optical response of a charge stripe in the presence of pinning impurities is investigated. We address the issue of a discrete description of the stripe, and discuss its quantitative relevance with respect to a continuum one in the light of recent optical-conductivity measurements in cuprate compounds.

Synchrotron studies of lattice distortions and lattice modulations in complex oxides

Abstract We review some recent studies of lattice modulations and lattice distortions in single crystals of the layered manganite compound La1.2Sr1.8Mn2O7 and the YBa2Cu3O6+ x cuprate superconductors obtained from X-ray diffuse scattering using synchrotron radiation. These results confirm the existence of a polaronic liquid (with short-range order) in the paramagnetic insulator phase of the manganite compound, and of lattice modulations (with short-range order) in the cuprate compounds, which co exist with superconductivity down to the lowest temperatures.

Crystal Structure and Physical Properties of FeSr2GdCu2O7+δ.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Symmetry considerations for detection of time-reversal breaking phases in cuprates by x-ray diffraction and absorption

Time-reversal and inversion breaking in the pseudogap region of the cuprate compounds have been proposed and detected by means of ARPES in the compound Bi-2212. Given the implication of the effect in the physics of the cuprate superconductors, it is important to check the conclusions through quite independent experimental techniques. We show how time-reversal and inversion breaking may be revealed in the Cuprates through resonant x-ray scattering and through non-reciprocal or magnetochiral dichroism.

SUPERCONDUCTIVITY IN THE VICINITY OF A 2D ELECTRONIC TOPOLOGICAL TRANSITION

We review some results concerning the superconducting properties of a 2D superconductor in the proximity to an electronic topological transition (ETT). In contrast to the 3D case, we find that the superconducting gap at T = 0 is characterized by a nonmonotonic behavior, with maxima occurring close to the ETT, both for s- and for d-wave pairing. Such a result is in good qualitative agreement with the phenomenological trend recently observed for [Formula: see text] as a function of the hopping ratio t′/t for several cuprate compounds. We also derive a nonmonotonic dependence of the quasiparticle inverse lifetime due to impurity scattering. We further analyze the effect of an ETT on the Ginzburg-Landau stiffness η, for which we recover an expression analogous to the case of an isotropic dispersion relation.

Symmetry considerations for the detection of second-harmonic generation in cuprates in the pseudogap phase

A proposal to test the proposed time-reversal and inversion breaking phase in the Pseudogap region of the Cuprate compounds through the variation of Second-harmonic generation intensity with temperature and polarization and angle of incidence is presented.

Pair Fermi contour and repulsion-induced superconductivity in cuprates

The pairing of charge carriers with large pair momentum is considered in connection with high-temperature superconductivity of cuprate compounds. The possibility of pairing arises due to some essential features of quasi-two-dimensional electronic structure of cuprates: (i) The Fermi contour with strong nesting features; (ii) The presence of extended saddle point near the Fermi level; (iii) The existence of some ordered state (for example, antiferromagnetic) close to the superconducting one as a reason for an appearing of "pair" Fermi contour resulting from carrier redistribution in momentum space. In an extended vicinity of the saddle point, momentum space has hyperbolic (pseudoeuclidean) metrics, therefore, the principal values of two-dimensional reciprocal reduced effective mass tensor have unlike signs. Rearrangement of holes in momentum space results in a rise of "pair" Fermi contour which may be defined as zero-energy line for relative motion of the pair. The superconducting gap arises just on this line. Pair Fermi contour formation inside the region of momentum space with hyperbolic metrics results in not only superconducting pairing but in a rise of quasi-stationary state in the relative motion of the pair. Such a state has rather small decay and may be related to the pseudogap regime of underdoped cuprates. It is concluded that the pairing in cuprates may be due to screened Coulomb repulsion. In this case, the superconducting energy gap in hole-doped cuprates exists in the region of hole concentration which is bounded both above and below. The superconducting state with positive condensation energy exists in more narrow range of doping level inside this region. Such hole concentration dependence correlates with typical phase diagram of cuprates.

The Green Function Approach to the Two-Dimensional t-J model

We investigate the two-dimensional t-J model by the Green functionapproach with the Hubbard operator. The phase diagram, including the Neeltemperature and the superconducting transition temperaturesimultaneously, is derived. The results are consistent with experimentsof cuprate compounds.

Crystal structure and physical properties of FeSr 2GdCu 2O 7+ δ

A new Fe–Sr–Gd–oxide–cuprate compound FeSr 2GdCu 2O 7+ δ (Fe1212) was successfully synthesized by solid-state reaction. X-Ray diffraction data indicate that the sample is single phase. A Rietveld refinement of the structure was carried out. This compound has a 1212-type structure of a tetragonal symmetry. The lattice parameters are a=3.8457(1) Å, c=11.3875(2) Å and the space group is P4/ mmm. Magnetization measurements show that this compound exhibits paramagnetic behavior above 5 K. Resistance measurements show that the sample has a semiconductor-like behavior.

Effects of anisotropy on the critical temperature in layered nonadiabatic superconductors

The generalized anisotropic Eliashberg theory is employed to study the critical temperature of layered nonadiabatic superconductors where the relevant phonon energy is comparable to the Fermi energy. We consider a two-dimensional model appropriate for cuprate compounds and recently discovered superconductor magnesium-diboride (MgB 2) which also reveals layered structure. By using the McMillan approximation we present the result of calculations of critical temperature T c. It is shown that the critical temperature is enhanced due to the influence of anisotropy and nonadiabaticity.

Fracton pairing mechanism for unconventional superconductors: Self-assembling organic polymers and copper-oxide compounds

Self-assembling organic polymers and copper-oxide compounds are two classes of unconventional superconductors, whose challenging behavior does not comply with the traditional picture of Bardeen-Cooper-Schrieffer (BCS) superconductivity in regular crystals. In this paper, we propose a theoretical model that accounts for the basic superconducting properties of either class of the unconventional materials. These properties are considered as interconnected manifestations of the same phenomenon: We argue that superconductivity occurs in both cases because the charge carriers (i.e., electrons or holes) exchange fracton excitations, quantum oscillations of fractal lattices that mimic the complex microscopic organization of the unconventional superconductors. For the copper oxides, the superconducting transition temperature ${T}_{c}$ as predicted by the fracton mechanism is of the order of $\ensuremath{\sim}150 \mathrm{K}.$ We suggest that the marginal ingredient of the high-temperature superconducting phase is provided by fracton coupled holes that condensate in the conducting copper-oxygen planes owing to the intrinsic field-effect-transistor configuration of the cuprate compounds. For the gate-induced superconducting phase in the electron-doped polymers, we simultaneously find a rather modest transition temperature of $\ensuremath{\sim}2--3 \mathrm{K}$ owing to the limitations imposed by the electron tunneling processes on a fractal geometry. We speculate that the hole-type superconductivity shows a larger onset temperature when compared to its electron-type counterpart. This promises an intriguing possibility of high-temperature superconducting states in hole-doped complex materials. The theoretical methods applied in our study bring together the so-called strange (or fractional) dynamics and the unconventional, topological description of the complex fractal sets underlying the fracton spectrum. A generalized kinetic equation containing integer time and fractional real-space derivatives is found for the fracton excitations in the harmonic approximation. The fracton superconductivity mechanism is further discussed in connection with experimental observations. An important prediction of the present study is the universality of ac conduction of the unconventional materials above the superconducting transition temperature ${T}_{c}.$

Magnetic structure ofLi2CuO2: Fromab initiocalculations to macroscopic simulations

The magnetic structure of the edge sharing cuprate compound Li2CuO2 has been investigated by means of ab initio electronic structure calculations. The first and second neighbor in-chain magnetic interactions are calculated to be -142 K and 22 K, respectively. The ratio between the two parameters is smaller than suggested previously in the literature. The interchain interactions are antiferromagnetic in nature and of the order of a few Kelvins only. Monte Carlo simulations using the ab initio parameters to define the model Hamiltonian result in a Neel temperature in rather good agreement with experiment. Spin population analysis situate the magnetic moment on the copper and oxygen ions somewhere between the completely localized picture derived from experiment and the more delocalized picture based on local density calculations.

Effects of an electronic topological transition for anisotropic low-dimensional superconductors

We study the superconducting properties of a two-dimensional superconductor in the proximity to an electronic topological transition (ETT). In contrast to the 3D case, we find that the superconducting gap at T=0, the critical temperature Tc, and the impurity scattering rate are characterized by a nonmonotonic behavior, with maxima occurring close to the ETT. We derive analytical expressions for the value of such maxima both in the s-wave and in the d-wave case. Such expressions are in good qualitative agreement with the phenomenological trend recently observed for Tc^max as a function of the hopping ratio t'/t across several cuprate compounds. We further analyze the effect of an ETT on the Ginzburg-Landau stiffness eta. Instead of vanishing at the ETT, as could be expected, thus giving rise to an increase of the fluctuation effects, in the case of momentum-independent electron-electron interaction, we find eta different from 0, as a result of an integration over the whole Fermi surface.

Magnetic polarization induced by nonmagnetic impurities in high-Tccuprates

The magnetic polarization induced by nonmagnetic impurities such as Zn in high Tc cuprate compounds is studied by the variational Monte Carlo simulation. The variational wave function is constructed from the eigenstates obtained from Bogoliubov de Gennes mean field Hamiltonian for the two-dimensional t-J model. A Jastrow factor is introduced to account for the induced magnetic moment and the repulsion between holes and the impurity. A substantial energy gain is obtained by forming an antiferromagnetic polarization covering 4 or 5 lattice sites around the impurity. We also found the doping dependence for the induced magnetic moment consistent with experiments.

Character of charge transfer excitons inSr2CuO2Cl2

High resolution electron energy loss spectroscopy (EELS) measurements of the insulating, layered cuprate compound ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ have been performed with a transferred momentum near $\mathbf{k}=0$ and in a wide energy range up to 20 eV. To interpret the data a semiquantitative embedded cluster approach is applied to describe the electron-hole excitations in insulating cuprates. The theory includes the complete set of Cu $3d$ and O $2p$ orbitals and predicts a rather wide $(2\textdiv{}13 \mathrm{eV})$ spectral range of the main dipole-allowed one- and two-center charge transfer (CT) transitions whose positions are in good quantitative agreement with the experimental spectra. Both, theory and the experimental data show that the CT gap is determined by a superposition of nearly degenerate one-center, localized CT excitations and two-center CT excitons with sizable dispersion.

Two excitonic problems

Two excitonic problems are discussed in which strong enough interactions admix states of the conduction and valence bands of the non-interacting theory resulting in unusual states as well as unusual theoretical problems. They relate to the observed ferromagnetism of weakly doped hexaborides and the phenomena in the cuprate compounds.

Exchange integrals ofSr2CuO2Cl2andBa2Cu3O4Cl2fromLDA+Ucalculations

The LDA+U method is used to calculate exchange integrals in strongly correlated cuprate compounds. We distinguish two approaches. The first one compares directly the total energies of different collinear spin arrangements with the corresponding ones of Heisenberg-like models. The second approach maps the energy of noncollinear spin-spiral states to the mean-field solutions of the effective spin Hamiltonian. Both approaches are applied to ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ which can be described with good accuracy by a two-dimensional Heisenberg model with only nearest-neighbor exchange. It is shown that the consideration of quantum fluctuations improves the resulting exchange integrals. The variation of the results with U and the difference between the two approaches are small. Both methods have also been applied to ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{4}{\mathrm{Cl}}_{2}$ which has two coupled antiferromagnetic spin systems. The coupling between the two subsystems has been shown to be larger than previously estimated.

Electric field gradients from first-principles and point-ion calculations

Point-ion models have been extensively used to determine ``hole numbers'' at copper and oxygen sites in high-temperature superconducting cuprate compounds from measured nuclear quadrupole frequencies. The present study assesses the reliability of point-ion models to predict electric field gradients accurately and also the implicit assumption that the values can be calculated from the ``holes'' and not the total electronic structure. First-principles cluster calculations using basis sets centered on the nuclei have enabled the determination of the charge- and spin-density distribution in the ${\mathrm{CuO}}_{2}$ plane. The contributions to the electric field gradients and the magnetic hyperfine couplings are analyzed in detail. In particular they are partitioned into regions in an attempt to find a correlation with the most commonly used point-ion model, the Sternheimer equation, which depends on the two parameters R and $\ensuremath{\gamma}.$ Our most optimistic objective was to find expressions for these parameters, which would improve our understanding of them, but although estimates of the R parameter were encouraging, the method used to obtain the $\ensuremath{\gamma}$ parameter indicated that the two parameters may not be independent. The problem seems to stem from the covalently bonded nature of the ${\mathrm{CuO}}_{2}$ planes in these structures which severely questions using the Sternheimer equation for such crystals, since its derivation is heavily reliant on the application of perturbation theory to predominantly ionic structures. Furthermore, it is shown that the complementary contributions of electrons and holes in an isolated ion cannot be applied to estimates of electric field gradients at copper and oxygen nuclei in cuprates.

Pressure Effects on Anisotropic Low-Dimensional Superconductors Close to an Electronic Topological Transition

Hydrostatic pressure or anisotropic stress can modify the electronic properties of a metal, thus inducing a change in the topology of its Fermi surface. In the case of low-dimensional anisotropic superconductors (such as high- T c cuprates, some organic salts, or heavy Fermion compounds), such changes result in a nonmonotonic dependence of several properties on the critical parameter z , measuring the distance of the chemical potential from the electronic topological transition (ETT). This has to be contrasted with the monotonic, nearly step-like z -dependence of the same quantities in the 3D case. Such a non-monotonic behavior is in agreement with the trend observed for T c as a function of pressure and other material specific quantities in several high- T c cuprate compounds. On the other hand, higher pressures than those reported in the literature (∼ 10 kbar) should be investigated, in order to find evidence for ETT effects in the BEDT-TTF-based salts.