Anomalous normal state magnetothermopower of electron-doped Nd1.85Ce0.15CuO4+δ crystals

The normal-state in-plane thermopowerS of superconducting Nd_1.85Ce_0.15CuO₄ (NCCO) crystals has been measured in magnetic fieldsB up to 9 T. When the magnetic field is parallel to the CuO₂ planes, we find thatS isB-independent, indicating the absence of spin entropy effects. AsB is oriented perpendicular to the CuO₂ planes, a large negative magnetothermopower is found, suggesting an orbital effect which cannot be understood within existing models. A largeT-dependent magnetothermopower is also seen in an as-grown NCCO crystal whenB is perpendicular to the CuO₂ planes.     

Crystal Chemical Concept of Arrangement and Function of Layered Superconducting Materials

The crystal chemical concept of arrangement and function of layered superconducting materials is supposed. The concept is based on the results of our investigation of crystal chemistry of high-temperature superconductors (HTSC) cuprates, diborides AB₂, and borocarbides of nickel RNi₂B₂C. According to these results (1) the main role in appearance of superconductivity played by the structural fragments—sandwiches A₂(CuO₂) in HTSC cuprates, A₂(B₂) in diborides, and RB(Ni) in nickel borocarbides but not the separate planes of CuO₂, B₂, or Ni; (2) correlations between T _c and crystal chemical parameters of these sandwiches have similar character in all three classes of compounds, despite distinction of a nature of their superconductivity. The central idea of the concept consists of following: in contrast to metallic conduction, for which it is enough to provide only concentration and mobility of charge carriers, for occurrence of a superconductivity it is necessary to create in addition a space (channels) for stream of charge carriers, compression of stream of carriers, and focusing to direction on path of motion.     

Transport Properties of Over-doped Epitaxial NdCeCuO Films

The transport properties of slightly overdoped Nd_2?x Ce _x CuO_4??? (NCCO) c-axis oriented thin films with x=0.17 have been investigated in the temperature range from 2.5 K to 300 K and in magnetic fields up to 6 T applied perpendicular to the CuO₂ planes. The films have been grown by a dc sputtering technique in on-axis configuration. They have been optimized on (001)-oriented SrTiO₃ substrates and successively annealed in pure Argon at 900?°C. Structural and compositional analyses were carried out by means of X-ray diffraction and scanning electron microscopy equipped with a wavelength dispersive spectroscopy detector. Current-voltage characteristics of the NCCO films have been measured and the temperature and the magnetic field dependence of the critical current density J _c(T,H) has been obtained. Finally, an increase of the low-temperature normal state resistance with the field has also been observed.     

Phononic and spin excitations in Y1?xPrxBa2Cu3O6.9 crystals

The Y_1–x Pr _x Ba₂Cu₃O_6.9 system has been investigated by quantitative-Raman measurements. The magnetic response of the antiferromagnetic ordered Cu spins in the CuO₂ plane has been observed via the two-magnon-Raman scattering. The in-plane exchange energy is determined to 780 cm^–1 in the system withx=1. Phonon behavior as well as magnetic scattering indicate that the band structure of the CuO₂ planes in PrBa₂Cu₃O_6.9 is similar to that in insulating YBa₂Cu₃O₆.     

Phase Separation in La2CuO4+δ by electrical transport measurements

The dc conductivity and the high-frequency capacitance of a La₂CuO₄₊ single crystal are determined by impedance spectroscopy in the frequency range 20 Hz to 1 MHz at liquid helium temperature. Using isochronal annealing, the effect of the phase separation on these quantitites is studied for E parallel and perpendicular to the CuO₂ planes as well as for different oxygen doping levels,, of the sample. The results are consistent with the model of a diffusion-controlled formation of a metallic—and belowT _c superconducting—percolation network in an insulating background.     

Dynamic Inhomogeneities in the La2CuO4-Based Superconductors

Polarization-dependent X-ray-absorption fine-structure (XAFS) measurements on the local structure of the La₂CuO₄-based high-T _c superconductors La_2–x Sr _x CuO₄, La_2–x Ba _x CuO₄, and La_1.6–x Sr _x Nd_0.4CuO₄ find, among others, orientation disorder induced in the Cu–O₂ planes by doping Sr, Ba, and alloying Nd atoms, all such atoms residing in La-sites. The orientation disorder is of two types: mostly static-buckling disorder, and dynamic disordering of the tilt angles of the Cu–O₆ octahedra correlated in nanoscale regions, with respect to neighboring nanoscale regions. Buckling disorder in the Cu–O₂ planes has the greatest detrimental effect on T _c and conductivity for such foreign atoms.     

Self-doping Effects on the Superconducting Properties of Cu0.5Tl0.25M0.25Ba2Ca2Cu3O10−δ (M = Bi, Hg, Nb, Pd, Li, Na, K)

The effect of self-doping and substitution of elements of higher and lower electronegativity, such as Bi, Hg, Nb, Pd, Li, Na, K, on the superconducting properties of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ with x=0.25 is investigated. These experiments demonstrated that the elements of lower electronegativity such as Li, Na, and K can easily liberate their outer most s-electron that could be supplied to the conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ superconductor, and as a result, we get enhanced superconducting properties. However, highly electro-negative elements hinder the transfer of carriers from charge reservoir layer to the conducting CuO₂ planes and promote inferior superconducting properties. In the present studies, we have investigated the effect of post-annealing in nitrogen and oxygen atmospheres for optimizing the carriers in conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ (M=Bi,Hg,Nb,Pd,Li,Na,K) superconductor. These studies are important since the density of carriers in the conducting CuO₂ planes determines the Fermi-vector k _F and Fermi velocity v _F of the carriers, which ultimately brings about the final superconducting state of the system.      

Knight Shift Data and Superconducting Tc Modeling Based on an Effective Hole Model

The superconducting T _c of cuprates have been modeled on a linear scaling with hole concentration per CuO₂ plane and a deleterious influence of bond resonance with the apical system (effective hole formalism). In cases where distribution between various hole reservoirs is not trivial, Knight shift can provide actual hole concentrations. It is shown here that when Knight shift data are used in an effective hole algorithm, satisfactory T _c predictions can be made, corroborating the deleterious influence on T _c of apical O and earlier assumptions concerning hole distributions. For the case of stacking of more than two CuO₂ planes, the inner plane has to be treated as an infinite layer analog in the effective hole model. A separation into inner and outer planes with different dopings is indeed observed by Knight shift, with higher doping in the latter. This is explained here as being due to a tendency to equalize an effective doping degree, as the outer planes lose holes in resonance of bonding with the apical system.     

Influence of Lattice Parameter Scaling on Local Electronic and Magnetic Properties in La2CuO4

A large amount of static and dynamic NMR experiments have been carried out to determine the spin properties in high temperature superconductors cuprates (HTSC). In the majority of cuprates the magnetic shift at the planar copper nucleus is independent of the temperature when the field is applied perpendicular to the CuO₂ planes and does not exhibit the expected reduction of the spin susceptibility in the superconducting state. This has been explained by a coincidental cancellation of on-site and transferred hyperfine fields at the copper nucleus, which would lead to a vanishing spin shift. To examine this explanation we determine the hyperfine fields by ab initio cluster calculations for La₂CuO₄ within the framework of spin-polarized density functional theory. In particular we investigate the sensitivity of the hyperfine fields on the lattice constants.     

Chemical and structural effects on the electronic states of (Hg, Tl, Pb, Bi)-based cuprate superconductors

The electronic energy bands have been calculated for the new single-layer and triple-layer Hg-based superconductors, HgBa₂CuO₄ and HgBa₂Ca₂Cu₃O_y, for the analogous Tl-and Bi-based materials, and for hypothetical Pb-based materials. As one moves across the last row of the periodic table, from Hg to Bi,s- andp-derived bands pass below the Fermi energy, to influence hole doping of the copper oxide planes and transport in the other layers of the material. The dispersion of these bands is significantly affected by the crystal structure. The calculations were performed with a simple chemical model.