Specific heat study of single crystalline Pr0.63_{0.63} Ca0.37_{0.37} MnO3_{3} in presence of a magnetic field

We present the results of a study of specific heat on a single crystal of Pr$_{0.63}$Ca$_{0.37}$MnO$_3$ performed over a temperature range 3K-300K in presence of 0 and 8T magnetic fields. An estimate of the entropy and latent heat in a magnetic field at the first order charge ordering (CO) transition is presented. The total entropy change at the CO transition which is $\approx$ 1.8 J/mol K at 0T, decreases to $\sim$ 1.5 J/mol K in presence of 8T magnetic field. Our measurements enable us to estimate the latent heat $L_{CO}$ $\approx$ 235 J/mol involved in the CO transition. Since the entropy of the ferromagnetic metallic (FMM) state is comparable to that of the charge-ordered insulating (COI) state, a subtle change in entropy stabilises either of these two states. Our low temperature specific heat measurements reveal that the linear term is absent in 0T and surprisingly not seen even in the metallic FMM state.Comment: 8 pages (in RevTEX format), 12 figures (in postscript format) Submitted to Phys. Rev.

Effects of capillary waves on the thickness of wetting layers

The free energy contribution of capillary waves is calculated to show its significant dependence on the thickness of the liquid layer, when the thickness is very small. It is shown that these oscillations can play an important role in determining the thermodynamic stability of a wetting layer, close to the critical point of a binary liquid mixture in the case of both short range and long range forces. In particular, the thickness of the wetting layer goes to zero as the temperature T approaches Tc.On calcule la contribution des ondes capillaires à l'énergie libre d'un film mouillant une surface, et on montre que cette contribution dépend fortement de l'épaisseur du film liquide lorsque celle-ci est faible. On montre également que ces oscillations peuvent jouer un rôle important pour la stabilité thermodynamique d'un film mouillant si le liquide est un mélange binaire proche de son point critique, que les forces soient à courte ou à longue portée. En particulier, l'épaisseur du film tend vers zéro lorsque la température T s'approche de Tc

Density inversion in the binary liquid system cyclohexane + aceto nitrile. Effects of doping and simplified models of the phenomena

The binary liquid system acetonitrile + cyclohexane has closely matched densities, equal to within 0.2% at 20 °C, a density inversion in the system at ;$\simeq$ 32 °C and a critical solution temperature off $\simeq$ 76 °C. It is thus an excellent system for simulating buoyancy corrected microgravity in the laboratory. The changes of the density inversion temperature ti caused by doping with water soluble in acetonitrile only and acetic anhydride soluble in both liquids has been studied experimentally $dt_i/dX$ is equal to 1.7 $\pm$ 0.1 mK/ppm weight fraction of water with respect to acetonitrile and is 1.0 $\pm$ 0.1 mK/ppm weight fraction of acetic anhydride with respect to acetonitrile. Simplified models of the phenomena are presented for each case of doping.Les systèmes binaires liquides acétonitrile + cyclohexane ont des densités très voisines, égales à 0,2% près à 20 °C ; ils présentent une inversion de la densité à 32 °C et une température critique à 76 °C. C'est ainsi un système parfait pour la simulation de microgravité corrigée par flottabilité au laboratoire. Nous avons étudié le changement de la température d'inversion de la densité, T$_i$, causé par l'addition d'eau qui seule est soluble dans l'acétonitrile et par l'addition d'anhydride acétique qui est soluble dans les deux liquides. Des modèles simplifiés des phénomènes sont présentés pour chaque cas