Temperature scaling of the integrated dynamical susceptibility in YBa2Cu3O6.5(T c =50 K)

The magnetic neutron scattering cross section in a crystal YBa₂Cu₃O_6.5(T _c =50 K) has been measured for energies between 4.5 meV and 15 meV and temperatures between 10 K and 275 K. From these data we extract the generalized susceptibility integrated around the (, ) position These data are combined with those at 8.3 and 33.1 meV from a previous study. It is found thatI(, T) exhibits the simple temperature scaling form found previously in La_2–x Sr _x CuO₄. Specifically, in YBa₂Cu₃O_6.5,I(, T)=I(, 0)2/tan^–1 (/0.9T). Implications of this behavior for the d.c. and infrared conductivity are discussed.     

Environmental neutron measurements with solid state track recorders

Abstract

The suitability of using ¹⁰B and ⁵LiF in contact with cellulose acetate butyrate (CAB) for the measurement of slow neutron densities has been investigated. The upper energy threshold of clear Diacel cellulose nitrate (CN) and of the CAB have also been measured. The CAB in good contact with thick ¹⁰B or ⁵LiF sources offers good promise for the detection of slow neutrons. For the actual CAB samples used, the CAB-¹⁰B combination had an efficiency of ～1.3 × 10^?2 tracks per thermal neutron incident in a 2π solid angle. The corresponding number for the CAB⁶-Li combination is ～5.6 × 10^?3 tracks per thermal neutron.     

Selection Method and Characterization of Neutron Monochromator Natural Crystals

Thermal neutrons are important analytical tools for microscopic material probe. These neutrons can be selected by diffraction technique using monocrystal, usually artificial. A crystal selection process was implemented and the characteristics of natural specimens were studied by activation analysis-k ₀ method. The representative 120 samples, of which 21 best types, were irradiated in IPR-R1 and measured with a neutron diffractometer at IEA-R1m Brazilian reactors. These results are useful for database build up and ease the choice of appropriate natural crystal, with some advantage options: highest intensity diffracted, enlarging the energy operational interval and optimal performance in special applications.     

Redox‐Active Hybrid Polyoxometalate‐Stabilised Gold Nanoparticles

We report the design and preparation of multifunctional hybrid nanomaterials through the stabilization of gold nanoparticles with thiol‐functionalised hybrid organic–inorganic polyoxometalates (POMs). The covalent attachment of the hybrid POM forms new nanocomposites that are stable at temperatures and pH values which destroy analogous electrostatically functionalised nanocomposites. Photoelectrochemical analysis revealed the unique photochemical and redox properties of these systems.     

On the Mean-Field Spherical Model

Exact solutions are obtained for the mean-field spherical model, with or without an external magnetic field, for any finite or infinite number N of degrees of freedom, both in the microcanonical and in the canonical ensemble. The canonical result allows for an exact discussion of the loci/ of the Fisher zeros of the canonical partition function. The microcanonical entropy is found to be nonanalytic for arbitrary finite N. The mean-field spherical model of finite size N is shown to be equivalent to a mixed isovector/isotensor σ-model on a lattice of two sites. Partial equivalence of statistical ensembles is observed for the mean-field spherical model in the thermodynamic limit. A discussion of the topology of certain state space submanifolds yields insights into the relation of these topological quantities to the thermodynamic behavior of the system in the presence of ensemble nonequivalence.     

Soft phonon behavior and magnetism at the low temperature structural phase transition of La1.65Nd0.35CuO4

We report a neutron scattering investigation of a La_1.65Nd_0.35CuO₄ single crystal which undergoes a second order structural transition with decreasing temperature from a high temperature orthorhombic phase (space group Bmab) to a distinct low temperature orthorhombic phase (space group Pccn) atT _s =76 K. The transition is induced by the softening of the phonon mode that involves rotations of the tilt axis of the CuO₆ octahedra. Antiferromagnetic long range order is established belowT _N =316 K, and a hysteretic reorientation to a noncollinear spin structure occurs belowT _s . The low energy spin dynamics of La_1.65Nd_0.35CuO₄ aboveT _s are closely similar to those of stoichiometric La₂CuO₄; at 100 K the gaps for in-plane and out-of-plane polarized spin waves are 2.3 meV and 5 meV, respectively. Out-of-plane polarized magnons are little affected by the structural transition, but the gap for in-plane polarized magnons increases markedly belowT _s .