Tc Copper Oxides Research Articles

The singlet–triplet magnetism and induced spin fluctuations in the high- Tc copper oxides

High- T c cuprates such as La 2− x Sr x CuO 4 and YBa 2− x Cu 3O 6+ x are considered as a system of the electron and hole polar pseudo-Jahn–Teller CuO 4 centers [CuO 4 7−] JT and [CuO 4 5−] JT, respectively, or a system of the local bosons moving in a lattice of the hole centers. Ground manifold of the polar centers includes three terms 1A 1g (Zhang–Rice singlet), 1E u, 3E u with different spin multiplicity, orbital degeneracy and parity that provides an unconventional multi-mode behavior of the cuprates. The spin subsystem of the copper oxides within the polar Jahn–Teller CuO 4 centers model is a two-component spin liquid and corresponds to a singlet–triplet magnet with, in general, multiparametric non-collinear spin configurations. In the framework of a modified mean field approximation some kinds of spin ordering are discussed including as a trivial singlet or triplet states as a pure quantum singlet–triplet mixed state. A local boson movement is accompanied by a modulation of the spin density on the site resulting in the so-called induced spin fluctuations. Some unconventional features of the induced spin fluctuations are considered including an appearance of the induced longitudinal ferrimagnetism with an appropriate contribution to the spin susceptibility, a possibility to observe and examine the charge fluctuations with the help of the traditional magnetic methods such as the magnetic inelastic neutron scattering and the spin lattice relaxation experiments. The suggested model, in comparison with the nearly antiferromagnetic Fermi-liquid model, represents a new approach to the description of the spin system of the high- T c cuprates.

Large low-symmetry polarons of the high- Tc copper oxides: formation, mobility and ordering

A microscopic model of the evolution from antiferromagnetic insulator to superconductor on oxidation of the parent-phase (CuO 2) 2− sheets of a cuprate superconductor starts with the assumption that strong electron-lattice interactions are dominant and give a heterogeneous electronic distribution. Introduction of pseudo-Jahn-Teller vibronic coupling associated with the δ holes in the (CuO 2) (2−δ) − sheets is shown to stabilize, below a critical temperature T p ≈ 850 K, large non-adiabatic polarons containing 5 to 7 copper centers; cooperative low-symmetry in-plane vibrations also stabilize an elastic attractive force between polarons that can overcome the longer-range Coulomb repulsion between polarons. Utilizing established parameters for isolated CuO 6 complexes gives a calculated polaron size of 5 to 7 copper centers, which compares with a measured mean size of 5.3 copper centers in underdoped samples 0 < δ ≤ 0.10. A large polaron is shown to move by a piece-wise tunneling of a fraction of itself across a peripheral CuO vibronic bond rather than by an activated hopping. This type of motion, which is not described by conventional transport theories, gives a linear increase of the resistivity with temperature above a temperature T ϱ due to scattering of the polaron at its own border, which separates regions inside and outside the polaron of slightly different mean CuO bond length. At lower temperatures, the polaron mobility becomes activated, but at higher concentrations this change is obscured because the elastic interpolaron attractive force causes the polarons to condense into a “polaron liquid,” and below some critical temperature T d ≥ T c the polarons undergo long-range ordering into one-dimensional 〈110〉 polaronic stripes separated by stripes of the parent phase, which support antiferromagnetic spin fluctuations. The zig-zag polaron stripes consist of polaron pairs oriented alternately along [100] and [010] axes of a CuO 2 sheet. Formation of the ordered superstructure permits conduction of hole pairs without scattering from lattice vibrations provided there is also coupling in the third dimension between CuO 2 sheets. The vibronic coupling introduces an anisotropic dispersion curve and superconductive gap for the states within the polarons, which lie within the energy gap between residual Hubbard-band states of the parent phase. The magnitude of the superconductive gap is determined by the elastic forces responsible for ordering the superstructure rather than by the energy of coupling of superconductive pairs.

Effect of nonmagnetic impurities on the gap of a dx2-y2 superconductor as seen by angle-resolved photoemission.

An analysis of angle-resolved photoemission (ARPES) experiments in the superconducting state of the high \tc copper-oxides is presented. It is based on a phenomenological weak-coupling BCS model which incorporates the experimental normal state dispersion extracted from ARPES, and non-magnetic impurity scattering in the presence of a $d_{x^2-y^2}$ order parameter (OP). It is shown, that already in the pure case, the broadening by finite momentum resolution of the analyzer leads to a finite region of apparent `gaplessness' around the true node of the OP. Non-magnetic impurities further amplify this effect by introducing additional spectral weight around zero frequency. At sufficiently large impurity concentrations $n_i\approx 0.02-0.05$, this results in an extended region of `gaplessness' up to $\delta\phi=\pm7$ ($\phi$ the angle on the Fermi surface) around the true node for a large range of moderate to strong impurity potential strengths. Different ways to identify the presence of impurity scattering in the ARPES spectra are proposed.

Oxygen analysis for high- Tc copper oxides: an alternative method for the determination of solid solution limits

The variation of oxygen content with cation substitution for several types of high- T c copper oxide solid solutions has been analyzed in terms of both theoretical derivations and experiments. It has been found that for continuous solid solutions the oxygen content varies with substitution in a linear manner over the entire substitution region, while for discontinuous solid solutions the first derivatives of the oxygen content with respect to substitution is discontinuous at the solid solution limit. For the latter, the oxygen content varies in the same way as continuous solid solutions with substitution within the solution limits. However, beyond the solid solution limits, oxygen contents vary with substitution either in a linear manner but with different slope or in a curvy manner, depending on the properties of the component elements of the solid solutions. Therefore, we can determine the solid solution limit or, rather, detect the existence of minor amounts of impurities from the plot of the variation of oxygen content with substitution. The determination of the oxygen content by iodometric titration have verified the above conclusions. The solid solution limits determined by our method are in good agreement with those by powder X-ray diffraction. In some cases, the present method has advantages over conventional powder X-ray diffraction for determining the solid solution limits.

Magnetic anisotropy and superconductivity in a model for high- Tc copper oxides

The phase diagram for the CuO 2-based superconductors is found to be consistent with an extended Hubbard Hamiltonian with competing positive-and negative- U interactions on a 2D lattice where sites are plaquettes formed by clusters of Cu and O atoms. The negative- U effective interactions are implied by the XY anisotropy in the Cu-Cu spin couplings and local hole pairing corresponds to vortex-antivortex spin configurations. The phase progression observed with the variation of dopant fraction x can be obtained via gradual implementation of canonical transformation that maps the properties of the positive- U Hubbard model at half-filling into those of the negative- U model away from half-filling. In the strong-coupling limit this process is described in terms of percolation-driven dilute magnetism for both spins ( U>0) and pseudospins ( U<O) appearing in the model. Other predictions of the model are: (i) An inverse spin-spin correlation length of the form ξ −1 x = ξ −1 o + η −1 x for x→O as seen in La 2- x Sr xCuO 4. (ii) An x-dependent reduction of spin fluctuations at low temperatures that conforms with NMR studies of La 2-xSr xCuO 4. And, (iii) a reduced superconducting transition locus T c(x)/ T cmax in agreement with the universal shape and location revealed by analysis of experimental data.

Electron correlation and optical conductivity in high- Tc copper oxides — origin of the mid-infrared absorption band

Evolution of the optical spectra with doping has been investigated for high- T c La 2− x Sr x CuO 4 and isostructura l Ni and Co oxides. An absorption band (mid-IR band) commonly appearing in the infrared region upon doping is a general feature of strongly correlated electronic systems. The origin of the mid-IR band is discussed in connection with the itinerancy of doped holes.

Electron correlation and charge dynamics in high- Tc copper oxides — C-axis transport and optical properties

Transport and optical properties perpendicular to the CuO 2 layers are extensively studies for La 2− x Sr x CuO 4. The electronic states are two-dimentional in the orthohombic phase where high- T c appears while in the tetragonal phase, at high T and large x, the anisotropy is reduced and the system is an anisotropic three-dimentional metal. The superconducting transition in the orthorhomsic phase restores the reduced anisotropy in the tetragonal phase.

Simple and reliable method for determination of oxygen content in high- Tc copper oxides

We present the details of a simple and reliable iodine-titration method suited for high- T c copper oxides, in which the use of a solution of ammonium acetate and acetic acid as a buffer agent substantially improves the operation. The results applied to La 2− x M x CuO 4− δ (M = Ba, Sr and Ca) are presented.

Significance of interlayer interactions to the superconducting mechanism of high- Tc copper oxides

A systematic analysis of various experimental data on T c and α (isotope effects) within the Eliashberg formalism including both the intralayer and the interlayer pairing leads us to conclude that i) the trends of T c for known high- T c copper oxides are well described in terms of a non-phonon, BCS type (λ<1 and the Cooper-pair like) pairing mechanism whose identity is yet to be understood, and ii) the effective interlayer pair scattering in multilayer compounds, though weaker than the intralayer counterpart, is estimated to be rather substantial (λ interlayer≈ 0.1). The presence of the appreciable interlayer scattering in the system casts doubt on the possibility of anyon superconductivity which requires two-dimensional topology.