From Magnons to the Resonance Peak: Spin Dynamics in High-T_C Superconducting Cuprates by Inelastic Neutron Scattering

The spin dynamics of high temperature superconductors measured by inelastic neutron scattering is reviewed. The spin susceptibility evolves a lot with increasing doping from the undoped insulating state to the overdoped metallic state. In the superconducting state, a strong magnetic resonance peak occurs in the spin excitation spectrum whose energy is proportional to the superconducting temperature. In underdoped and optimally doped regimes, normal state antiferromagnetic fluctuations are observed, proving the existence of strong electronic correlations in cuprates. They are characterized by a spin pseudo-gap. In contrast, these dynamical spin correlations vanish in overdoped samples.Comment: To appear in "The gap Symmetry and Fluctuations in High Temperature Superconductors" Edited by J. Bok, G, deutscher, D. Pavuna and S.A. Wolf. (Plenum Press, 1998

Spin susceptibility and the pi-excitation in underdoped cuprates

The dynamical spin susceptibility chi'' at wave vector (pi, pi) and the spectrum pi'' of the spin-triplet particle--particle excitation with center of mass momentum (pi, pi) (pi-excitation) are considered in the slave-boson formulation of the t--J-model. Propagators are calculated in a diagrammatic t-matrix approximation in the d-wave superconducting state for a wide doping range. The resulting spectra chi'' and pi'' both show a resonance at a doping dependent energy, in qualitative agreement with recent numerical cluster calculations. In underdoped systems, the peak position is comparable to that found in neutron scattering experiments. The peak in chi'' as well as pi'' is at low doping entirely caused by spin fluctuations, whereas the triplet particle--particle channel does not contribute as a collective mode.Comment: 3 pages, 4 eps-figures included, uses revtex, eps

Lower Pseudogap Phase: A Spin/Vortex Liquid State

The pseudogap phase is considered as a new state of matter in the phase string model of the doped Mott insulator, which is composed of two distinct regimes known as upper and lower pseudogap phases, respectively. The former corresponds to the formation of spin singlet pairing and the latter is characterized by the formation of the Cooper pair amplitude and described by a generalized Gingzburg-Landau theory. Elementary excitation in this phase is a charge-neutral object carrying spin-1/2 and locking with a supercurrent vortex, known as spinon-vortex composite. Here thermally excited spinon-vortices destroy the phase coherence and are responsible for nontrivial Nernst effect and diamagnetism. The transport entropy and core energy associated with a spinon-vortex are determined by the spin degrees of freedom. Such a spontaneous vortex liquid phase can be also considered as a spin liquid with a finite correlation length and gapped S=1/2 excitations, where a resonancelike non-propagating spin mode emerges at the antiferromagnetic wavevector with a doping-dependent characteristic energy. A quantitative phase diagram in the parameter space of doping, temperature, and magnetic field is determined. Comparisons with experiments are also made.Comment: 22 pages, 12 figure

Simposio de la investigacion francesa en Bolivia

Malgré les difficultés d'exploitation, l'intérêt d'un réseau hydrométrique dans la plaine amazonienne bolivienne est mis en évidence par ses nombreuses applications pratiques. Parmi elles, le projet de centrale hydro-électrique de Cachuela Esperanza, sur le rio Beni, et le projet du réseau d'alerte destiné à prévoir les inondations dans la région de Trinidad et sur plus de 200 km à l'amont et à l'aval de cette ville par suite des crues du Mamore. (Résumé d'auteur

Monthly global irradiation on tilted planes : a simple correlation method and its validation

Paper presented at ISES Solar World Congress (International Solar Energy Society), Hamburg, Germany, Sept. 1987International audienceThis paper presents a simple correlation for the computation of monthly global solar irradiation on tilted surfaces, facing due south. The conversion factor, R, from the horizontal plane to the tilted plane, is considered: it has been established from the Solar Radiation European Atlas data (computed by Pr. Page's algorithm). R is linearly correlated to the monthly clearness index, Kt. The correlation slope depends on the month, the latitude and the tilt angle. This set of correlations fits the European Atlas Data within 2% in summer and 5% in winter (tilt ang. <60deg), within 5 to 10% for South-vertical surfaces. Validation results with respect to measured data from 7 locations are also presented. In some cases, there is a slight systematic underestimation by a few percent (Trappes, Carpentras, Albany); but generally the results seem to be consistent with the correlation scattering (Geneva, Ispra, Bracknell, Valentia)

High-Energy Spin Dynamics in La1.69_{1.69}Sr0.31_{0.31}NiO4_4

We have mapped out the spin dynamics in a stripe-ordered nickelate, La$_{2-x}$Sr$_{x}$NiO$_{4}$ with $x \simeq 0.31$, using inelastic neutron scattering. We observe spin-wave excitations up to 80 meV emerging from the incommensurate magnetic peaks with an almost isotropic spin-velocity: $\hbar c_s\sim 0.32$ eV \AA, very similar to the velocity in the undoped, insulating parent compound, La$_{2}$NiO$_{4}$. We also discuss the similarities and differences of the inferred spin-excitation spectrum with those reported in superconducting high-$T_c$ cuprates.Comment: 4 figure

Linear dependence of peak width in \chi(\bq, \omega) vs T_c for YBCO superconductors

It is shown that the momentum space width of the peak in the spin susceptibility, Im$\chi(q,\omega)$, is linearly proportional to the superconducting $T_c$: $T_c = \hbar v^*\Delta q$ with $\hbar v^* \simeq 35 meV$\AA. This relation is similar to the linear relation between incommensurate peak splitting and $T_c$ in LaSrCuO superconductors, as first proposed by Yamada et al. (Phys. Rev. B 57, 6165, (1998)). The velocity $\hbar v^*$ is smaller than Fermi velocity or the spin-wave velocity of the parent compound and remains the same for a wide doping range. This result points towards strong similarities in magnetic state of YBCO and LaSrCuO.Comment: 5 pages, 3 figures, latex fil