We have determined the principal components of the Saupe ordering matrix for anthracene-dlo dissolved in the nematogen trans-4-n-heptyl(cyanophenylcyclohexane) as a function of both temperature and pressure, using deuterium NMR spectroscopy. The availability of an equation of state for the solvent has allowed us to extract the temperature variation of the solute ordering matrix at constant volume as well as the usual constant pressure. This provides the first real test of molecular field theories of the ordering of biaxial particles in a liquid crystal solvent, since such theories assume that the system is at constant volume. As we had expected, the predictions of theory are found to be in better accord with the results at constant volume than at constant pressure, although the agreement is not perfect even at constant volume. We have measured the solute equivalents of the thermodynamic parameter F~l){---(d In T/d In V)C2.o } suggested by Alben for pure nematogens to gauge implicitly the relative importance of attractive and repulsive forces in stabilising nematic order. The relationship of the solute to the solvent F parameters is discussed together with the significance of their values.
We have performed magnetisation measurements on Bi2Sr2CaCu2Os+y and obtained the pinning force Fp=Jc.B at different temperatures as a function of field for sintered pellets and for single crystals. Using the flux creep model we have fitted theoretical curves to the data and obtained the pinning activation energy U and the pinned volume 8,. We have performed relaxation measurements and obtained U(B,T), which we have put explicitly into the model.The behaviour of the vortex state in Bi2Sr2CaCU20gfy remains controversial, with many theories being proposed to explain experimentally observed results. These include, amongst others, the ideas of collective pinning [l], a vortex glass [2], and flux lattice melting 131, as well as the more conventional flux creep theories [4]. The latter are well developed and so we proposed to use the predictions of the flux creep model to describe experimentally determined pinning force curves Fp(B), then to examine the parameters involved for any signs of dramatic change that might, for example, accompany a sudden softening or melting of the vortex state.he pinning force curves are shown in figure i. Hysteresis loops up to IZT were performed on an Oxford Instruments VSM system, with a field sweep rate of ImT/s. The sintered material was pressed into a pellet, in order to prevent grains moving due to the vibration. The pellet was not subsequently
The JCt value of 89 A cm-= obtained for the'base composition Lat.a5Sro.15Cu04_, at 7.5 K is greater than previously reported values, indicating a reduced contribution from intergranular weak links. A reduction in Ja observed with increasing vanadium concentration over the range y 5 0.05 is considered to be a consequence of the decrease in superconducting transition temperature, from 34 to 20 K, rather than an increase in the weak link contribution.the absence of a distinct drop in Jn at a temperature intermediate between 7.5 K and T, confirms that these ceramics do not exhibit flux lattice melting. The Jot of the samples declines steeply, to less than half the zero-field value, when measured in an applied field of 20 mT and thereafter decreases at a vety much lower rate to a limiting value as the field is increased to 0.5 T. The initial steep drop in the field dependence of JCt is accentuated with increasing vanadium concentration, indicating that the critical field HZ for the penetration of the weak link regions is sensitive to increasing vanadium concentration, in accordance with the known compositional dependence of HCl in these ceramics. As the JCt of the vanadium solid solutions all declines to a common limiting value with increasing applied magnetic fields, the weak link contribution to J, " is also considered to be independent of the specific vanadium concentration these solid solutions.
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