AF Correlations Research Articles

Magnetic and transport properties of UNiGa

The electrical resistivity of UNiGa is strongly anisotropic and displays a dramatic decrease in magnetic fields above the metamagnetic transition. The negative dρ/d T observed above T c can be attributed to AF correlations.

Use of a 19F NMR probe for the study of the local magnetic structure in the AF and SC HTc YBaCuO series : experimental lights on the HTc superconductivity mechanism

Three series of YBaCuOF powdered samples have been derived from the characteristic compositions O6 , O6.7 and O7 by a soft fluorination. The fluorine atoms occupy at first the O vacancies in the chains, then the bridging sites between the chains. 19 F NMR shows 4 types of lines corresponding respectively to a classical AF state, a very peculiar paramagnetic state in a large temperature range around TN with a very high local AF correlation. This structure is seen again in the superconducting samples and reveals an equilibrium between two magnetic structures at TC with slow magnetic fluctuations and lineshift. When put in the bridges, the F atoms seem to harden the magnetic lattice and push the sample out of the SC domain.

NMR studies of the electronic and magnetic properties of the 123 compounds (invited)

A good knowledge of the magnetic properties of cuprates, from their insulating AF state to their metallic state, is required prior to a clarification of the origin of high temperature superconductivity. NMR techniques have proven to be quite successful in studying the magnetic properties on a local scale. The 89Y NMR, which can be detected even in the AF and paramagnetic phases of YBa2Cu3O6+x, allows one to study the influence of oxygen content or impurity substitutions on the 3D AF ordering. The associated changes of spin correlation length and interlayer exchange couplings will be discussed. In the metallic state of YBa2Cu3O6+x, NMR shift data taken on the various nuclear sites, 89Y, 17O, and 63Cu have allowed one to conclude that the covalency between Cu(3dx2−y2) and O(2pσ) holes result in a spin system which behaves as a single spin fluid. The large decrease of the susceptibility of the CuO2 planes at low T for oxygen depleted samples, as well as the different T dependencies of the 89Y, 17O, and 63Cu relaxation rates, is attributed to the existence of AF correlations at the wave vector Q of the AF structure of YBa2Cu3O6. However, the 89Y and 17O relaxation rates, which filter the Q component of the dynamic susceptibility, exhibit a behavior similar to that of a Fermi liquid, for large enough hole doping. The T variations of the spin susceptibility and of the AF correlations are discussed.

Investigation of the spin dynamics in YBa 2Cu 3O 6+ x by inelastic neutron scattering

Inelastic neutron scattering experiments have been carried out on YBa 2Cu 3O 6+ x single crystals to investigate the spin dynamics in the pure AF-state ( x = 0.15, T N = 410 K), in the doped AF-state ( x = 0.37, T N = 180 K), in the metallic weakly doped superconducting state ( x = 0.45, T c = 37 K; x = 0.51, T c = 47 K) and in the T c = 60 K superconducting phase ( x = 0.69, T c = 59 K). The main results are summarized: the two dimensional character of the spin-wave spectrum in the AF-state, strong renormalization of low-energy excitations and overdamping of in-plane spin fluctuations in the doped AF-state, dynamical AF correlations in the superconducting state, evidence for a pseudo-gap induced by superconductivity and existence of superconducting correlations above T c.

NMR studies of the electronic and magnetic properties of high- Tc materials

In the insulating state of cuprates, NMR techniques allow investigations of the influence of hole doping or impurity substitutions on the 3D AF ordering. In the metallic state of YBa 2Cu 3O 6+ x , NMR shift data taken on the various nuclear sites, 89Y, 17O and 63Cu have allowed to conclude that the covalency between Cu(3d x 2− y 2 ) and O(2pσ) holes results in a spin system which behaves as a single spin fluid. The large decrease of the susceptibility of the CuO 2 planes at low T for oxygen depleted samples, as well as the different T dependences of the 89Y, 17O and 63Cu relaxation rates is attributed to the existence of AF correlations at the wave vector Q of the AF structure of YBa 2Cu 3O 6. However, the 89Y and 17O relaxation rates, which filter the Q component of the dynamic susceptibility, exhibit a behaviour similar to that of a Fermi liquid, for high- T c samples. The T variations of the spin susceptibility and of the AF correlations are discussed.

89Y NMR studies of the electronic and magnetic properties of the 123 compounds

An extensive study of the 89Y NMR in YBa 2Cu 3O 6+x compounds has been performed for a broad range of temperatures and concentrations of Oxygen, from the metallic to the insulating state. The 89Y nuclei are found to probe the susceptibility of the CuO 2 planes through a shift of the 89Y NMR associated with a coupling with the O(2p) orbitals of the planar Oxygen sites. From the negative sign of the hyperfine coupling it is deduced that holes reside on Cu(3d)-O(2pσ) orbitals. The NMR shift 89 ΔK is found T independent for x = 1, but has a large T variation as soon as x departs from unity. Comparison with macroscopic susceptibility data allows us to assign this variation to the susceptibility χ s (Cu) at the planar Cu sites. For x∼0.63, this T dependence is found to scale with the 17O NMR shift, which ensures that any independent contribution χ s h of the doped holes to the susceptibility is much weaker than χ s ( Cu), and that the magnetic properties of the system can be described by a single spin fluid. The 89Y nuclear spin lattice relaxation rate follows a Korringa law T 1T = constant for the O 7 composition. For x<1, large T variations of T 1T, similar to those of 89 ΔK, are evidenced. A correct estimate 89 δ =200 ±20 ppm of the 89Y chemical shift is deduced from the linear scaling of the 17O and 89Y shifts. This allows a quantitative study of the correlation between the variations of T 1T and the spin contribution K s to the NMR shift. From the value of the ratio of the 17O and 89Y Korringa products it is concluded that the AF spin correlations at q values near the Brillouin zone corner ( π a , π a ) are largely screened when seen from the O and Y nuclei. Those probe mainly the spin fluctuations in the remaining part of the Brillouin zone, which have a behaviour quite similar to that of Fermi liquids. The effect of the AF correlations on the spin fluctuation spectrum is discussed.

The possible ground states of doped holes in (La 1−xSr x) 2CuO 4

On the basis of the Anderson lattice Hamiltonian, the ground states of the doped holes in (La 1− x Sr x ) 2CuO 4 are investigated by using a variational analysis. It is found that a single hole is localized within a linear cluster with length of about 8 a ( a being the oxygen-oxygen spacing), and the hopping of holes produces conductivity with temperature dependence exp[−(c/k B T) 1 4 ] at low temperature (critical conductivity) and T −1 exp(− v/ k B T) at higher temperature (thermal-excited hopping conductivity). The hole-cluster also breaks the AF correlation between the copper sites on its two sides, the AF coherence length is then proportional to a/ x . By increasing the doping, an infinite cluster may be formed to hold all the doped holes, the AF background is resolved into isolated spin-singlets or spins, and the Fermi-liquid-like features of the holes appear. The critical doping densities for these transitions are determined. Many unusual properties of this material can be quantitatively explained.

Correlation Choropleth Maps

Correlation choropleth map is such choropleth map whose scale is derived from regression line which was constructed in scatter diagram to find out the correlation between two geographical features. The interval between two lines of line pattern corresponds to the regression coefficient of correlation dependence which was analysed. The basic type af correlation choropleth maps (fig. 3 and 7) uses two systems of line patterns: horizontal and vertical. When two corresponding values of geographical features of an area are near the regression line, both systems of line patterns create squares. The more the values differ from the regression line, the longer oblongs are created by the line patterns.