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

Critical wetting phenomena : observation of hydrodynamic instabilities

In critical wetting phenomena, the situation of a heavier liquid residing on top of lighter liquid is observed. The consequences of the Rayleigh-Taylor mode of instability in the semicritical wetting layer in a binary liquid mixture are investigated. We also report experimental observations in support of our theory, in the critical binary liquid mixture cyclohexane + acetonitrile.Dans le cadre de phénomènes critiques de mouillage, on observe souvent qu'un liquide plus dense peut rester sur un liquide plus léger. Nous avons étudié ici les conséquences du mode d'instabilité de Rayleigh-Taylor dans la couche de mouillage semicritique d'un mélange binaire de liquides. Pour soutenir nos hypothèses, nous rapportons des observations expérimentales dans un mélange binaire de cyclohexane et acétonitrile

Temperature and pressure dependence of direct current electrical resistivity in the sodium oxide-​zinc oxide-​boron oxide glass system

Two series of glasses were prepd. with compns. xNa2O, yZnO, (100 - x - y)​B2O3 and (30 - x)​Na2O, xZnO, 70B2O3 (mol​%)​. The temp. dependence of the d.c. resistivity was detd. from room temp. to ∼700 K. In both series of glasses, a simple Arrhenius type of temp. dependence was obsd. However, the resistivity of the binary alkali glass increased steeply by 2 orders of magnitude with the addn. of even a small quantity of ZnO and remained virtually unaffected by further addn. of ZnO. No resistivity max. was obsd. (due to the mixed cation effect) at any Na​/(Na + Zn) ratio. The resistivity (≤80 kbar) decreased gradually with increasing pressure in Na2O-​B2O3 but increased with increasing pressure with the addn. of ZnO, attaining a fairly well-​defined max. and then slowly decreasing until leveling off at still higher pressures. The resistivity max. had a marked dependence on the ZnO content. These results are probably due to the B anomaly

Temperature and pressure dependence of direct current electrical resistivity in the Na2O-ZnO-B2O3 glass system

Two series of glasses were prepared, xNa2O, yZnO, 100 - x - yB2O3 and 30 - xNa2O, xZnO, 70B2O3 (mol). The temperature dependence of the direct current resistivity was measured from room temperature to about 700 K and in both series of glasses we observed a simple Arrhenius type of temperature dependence. However, the resistivity of the binary alkali glass increased steeply by as much as two orders of magnitude with the addition of even a small quantity of ZnO and remained virtually unaffected by further addition of ZnO. The resistivity decreases gradually with increasing pressure in Na2O-B2O3 but increases with increasing pressure with the addition of ZnO

Nonlinear I-​V characteristics and threshold switching in As-​Te-​In glasses

Nonlinear I-​V behavior and elec. switching exhibited by chalcogenide glassy semiconductors, find applications in variety of areas including information storage and power control. Semiconducting chalcogenide As40Te60-​xInx glasses were prepd. by melt quenching technique. The current-​voltage and elec. switching behavior of these glasses were studied using a custom-​built PC based system. The results obtained clearly indicate that all the glasses studied exhibit current controlled neg. resistance behavior, which leads to the low resistance state. The switching to the low resistance estate is reversible and the samples revert back to the high resistance state on reducing the current. Threshold switching over such a wide range of compns. was obsd. only in very few systems so far. The most interesting outcome of the present studies is the variation of the switching voltage with compn. There is an increase in the switching voltage Vt with the increase in In concn. in the compn. range 7.5 ≤ x ≤ 12.5. Further, the compn. dependence of switching field is found to exhibit a distinct change in slope at x is 12.5 and Vt continues to increase with x until x = 13.5. Around x = 13.5, the trend is reversed and Vt starts decreasing with x. A min. in Vt is seen around the compn. x = 14.3, which corresponds to the chem. threshold of the As-​Te-​In system. Beyond x = 14.3, switching voltage increases with compn. again. The present result are consistent with earlier observations, which indicate the compn. dependence of switching voltages of chalcogenide glasses are influenced by chem. ordering and rigidity percolation