Inelastic Neutron Scattering from the Spin Ladder Compound (VO)2P2O7

We present results from an inelastic neutron scattering experiment on the candidate Heisenberg spin ladder vanadyl pyrophosphate, (VO)2P2O7. We find evidence for a spin-wave excitation gap of $E_{gap} = 3.7\pm 0.2$ meV, at a band minimum near $Q=0.8 A^{-1}$. This is consistent with expectations for triplet spin waves in (VO)2P2O7 in the spin-ladder model, and is to our knowledge the first confirmation in nature of a Heisenberg antiferromagnetic spin ladder.Comment: 11 pages and 2 figures (available as hard copy or postscript files from the authors, send request to [email protected] or [email protected]), TEX using jnl, reforder and eqnorder, ORNL-CCIP-94-05 / RAL-94-04

Solar collector cover with temperature-controlled solar light transmittance

Our aim is the development of a solar collector cover with temperature-controlled solar light transmittance in order to protect plastic solar collectors against overheating and to prevent collector damage during stagnation. The temperature-dependent reduction of solar transmittance is based on an increase of backscattering of the incident solar radiation (thermotropism). The thermotropic materials consist of two components: 1) a thermotropic additive, namely submicron-sized core-shell particles containing a phase-change material, and 2) an appropriate transparent matrix polymer. Thermotropic samples based on three different matrix polymers (UV-curable cast resin, EVA and PVB) were prepared as sandwich laminates according to industrially relevant processes. Temperature-dependent measurements of the total solar transmittance reveal absolute differences of up to 28 % between OFF and ON state