Showing papers on "Transition temperature published in 1974"

Electrostatic Effects on Lipid Phase Transitions: Membrane Structure and Ionic Environment

Abstract: Ordered → fluid phase transitions in bilayers of charged lipids are accompanied by a decrease in electrostatic free energy mainly as a result of bilayer expansion. For uniform charge distribution the Gouy-Chapman theory of the electrical double layer predicts a decrease of the transition temperature with increasing charge density. We studied the effects of pH and mono- and divalent cations on the phase transition of lecithin, cephalin, phosphatidylserine, and phosphatidic acid bilayers. Phosphatidic acid with two ionizable protons was selected for a systematic investigation. A change in pH from 7 to 9 increases the charge per polar group from one to two elementary charges. This lowers the transition temperature by about 20°C in agreement with the theory. In this pH region rather small changes in pH suffice to induce the phase transition at constant temperature. Divalent cations (Mg++ and Ca++) increase the transition temperature by charge neutralization and thus can be used to induce the fluid → ordered transition at constant temperature. In contrast, monovalent cations (Li+, Na+, K+) lower the transition temperature, or fluidize the bilayer structure at a given temperature. Rather small changes in ionic environment can induce gross alterations in bilayer structure; divalent and monovalent cations have antagonistic effects. This result parallels current theories on nerve excitation and sensory transduction where cation-induced structural changes in biomembranes are invoked.

Influence of stoichiometry on the metal‐semiconductor transition in vanadium dioxide

Abstract: The structure and resistivity of vanadium dioxide films close to the metal‐semiconductor transition have been studied as a function of stoichiometry. Unit‐cell volume is shown to go through a minimum and the transition temperature and resistivity ratio through a maximum at the apparent stoichiometric oxygen level. The observed resistivities can be accounted for by an increased electron concentration due to vanadium interstitials in the low‐oxygen films and by a second phase in the high‐oxygen films. The variation of resistivity and transition temperature with electron concentration provides experimental support for the previous suggestion that the driving force of the transition is mainly an antiferroelectric distortion.

High superconducting transition temperatures in the palladium-noble metal-hydrogen system

Abstract: High superconducting transition temperatures, Tc, of 16.6, 15.6 and 13.6 K have been observed in Pd-(Cu, Ag and Au) alloys, charged with large amounts of H by means of ion implantation at liquid Helium-temperatures. A peculiar phase-transition indicates that weak phonon modes might be responsible for the high Tc-values. The difference between the maximum Tc-values can be described as a type of isotope effect Tc∞M−1/2.

Spreading of solid glycerides and phospholipids at the air—water interface

Abstract: The equilibrium spreading pressures (πe) of a series of glycerides and phospholipids have been determined at various temperatures. Pure mono, di- and triglycerides and phosphatidylethanolamines can spread spontaneously from the crystal to form monomolecular films at the air—water interface, whereas phosphatidylcholines (lecithins) only spread from the liquid-crystal (i.e., at temperatures above the crystal or gel to liquid crystal transition temperature, Tc). Film pressures of up to 50 mN m−1 can be attained. Glycerides and phosphatidylcholines form liquid expanded monolayers in equilibrium with the bulk lipid while phosphatidylethanolamines can form both condensed and liquid expanded films. Liquid expanded monolayers are stable with respect to the bulk, lipid phase when π πe leads to collapse and formation of bulk, lipid phase without significant overcompression. Condensed monolayers of phosphatidylcholine are metastable (πe = 0 when T < Tc) and significant overcompression is observed before collapse to the gel or the monohydrate occurs. The equilibrium spreading pressures increase linearly with temperature until changes in physical state of the bulk lipid occur. On fusion or transition to a liquid crystalline state, the temperature coefficient of the equilibrium spreading pressure changes sign. The enthalpies and entropies of spreading are used to gain information on the structure of the lipid monolayers. The entropy of spreading and thus the order in the resulting monolayer is about midway between that of the crystal and the melt, depending on whether the compound spreads to a condensed or liquid expanded monolayer.

Electron--phonon interaction and superconductivity in transition metals and transition-metal carbides

Abstract: Using the results of self-consistent augmented-plane-wave band-structure calculations, electron mass enhancement factors and superconducting transition temperatures have been calculated for V, Nb, NbC, TaC, and HfC. Good agreement with experiment is obtained.

Effect of deuteration on the ferroelectric transition temperature and the distribution coefficient of deuterium in K(H1−xDx)2PO4

Abstract: The variation in the ferroelectric transition temperature with deuteration is described by a linear function only if the deuteration level in the crystal is considered A least‐squares fit of these data resulted in the relation Tc=1217K + 107m, with m in mole% D The effective distribution coefficient (keff) of deuterium in the system KH2PO4–KD2PO4–D2O was found to vary from 075 to 099 over the range of 25–998 mole% D

Superconducting transition temperature of aluminum, indium, and lead fine particles

Abstract: Aluminum, indium, and lead fine particles with free surfaces were made by evaporation in helium gas to study the size dependence of the superconducting transition temperatureT c in very small particles. The transition temperature was determined from the change of diamagnetic susceptibility with temperature. The present experiment shows thatT c rises with decrease of the average diameter $$\bar d$$ for aluminum and indium, while it remains nearly constant for lead. The enhancement factorsT c /T (T isT c for the bulk material) in the case of $$\bar d = 100 {\AA}$$ are determined as 1.45, 1.15, and 1.0 for aluminum, indium, and lead, respectively. The effect of the soft phonon mode arising from the increasing fraction of surface satisfactorily explains the experiment with respect to both the size dependence and the difference among elements by a rather simple model.

Influence of smectic order on the static dielectric permittivity of liquid crystals

Abstract: In some p, p′‐di‐n‐alkyl‐azoxybenzenes, an anomalous decrease of e‖ and increase of e⊥ is observed when the nematic phase is cooled to below the nematic/smectic‐A transition temperature. This is attributed to dipole‐dipole interaction, which becomes especially important for neighboring molecules in the same smectic layer. The Kirkwood‐Frohlich theory for the dielectric properties of anisotropic polar liquids is used in order to account for these interactions.

Cooperative Jahn-Teller effects in the mixed crystals TbpGd1-pVO4 and DypY1-pVO4

Abstract: The structural phase change arising from the cooperative Jahn-Teller effect has been studied by Raman scattering and optical birefringence methods in the mixed crystals TbpGd1-pVO4 and DypY1-pVO4 with p varying from one to zero. The introduction of Gd and Y dilutes the active components causing the phase change, resulting in a reduction of the transition temperature TD(p). Varying p varies the splitting of Eg phonons below Tp(p) and also the energy of low-lying collective electronic excitations. The Tb system shows unique behaviour with p=0.365; a transition from tetragonal to orthorhombic symmetry occurs at 8.5K but the crystal reverts to tetragonal symmetry below 2.3K. The effect in the Tb system can be explained by mean-field theory. The Dy compounds are explained using series expansion and modified mean-field methods.

Chemical vapor deposition of superconducting Nb3Ge having high transition temperatures

Abstract: Superconducting films of Nb3Ge have been deposited by chemical vapor deposition. Chloride vapors were produced in situ and reduced by hydrogen. The reactor used allowed surveying of phases with varying Nb/Ge ratio. The highest transition onset temperature was 21.7 K with a transition width of 0.4 K. It occurred near the Ge‐richest boundary of the Nb3Ge phase.

Electrical resistivity of amorphous alloys

Abstract: Amorphous alloys of Ga, Sn, Pb and Bi with Cu, Ag and Au are produced by evaporation on a cold substrate. The residual resistivity, the temperature dependence of the resistivity, the transition temperature of superconductivity and the temperature of the amorphous-crystalline transformation are measured. We observe e.g. that the residual resistivity increases with the noble metal concentration, and that the temperature coefficient of the resistivity of the Au alloys is always negative. In these two respects amorphous alloys differ in behaviour from the corresponding liquid alloys. These observations can be correlated with the atomic energy levels of the free atoms.

The synthesis of Nb 3 Ge by RF sputtering

Abstract: The conditions necessary for the formation of Nb 3 Ge by low-pressure rf sputtering with a superconducting transition temperatureT c >21 K have been investigated. Samples have been deposited onto cooled substrates so that the film is first amorphous and then is crystallized by a subsequent annealing, and onto hot substrates, in which case the film is crystalline upon deposition. The highestT c samples were obtained for a substrate temperature of 735±25° C. The optimum substrate temperature is the same as the optimum annealing temperature for crystallizing films which were first deposited onto cooled substrates. Special conditions are necessary for the formation of single-phase A15 samples of Nb 3 Ge with an optimumT c . We have utilized the collisions that the sputtered atoms undergo with the sputtering gas molecules to thermalize the sputtered atoms. We report here on sputtering in both krypton and in argon.

The Transition Between Thermally Activated and Athermal Flow in Polycrystalline Alpha-Zirconium

Abstract: The deformation of α-zirconium has been studied between 375 and 625°C and at strain rates from 5 × 10−6 to 10−1 s−1 by compression testing. The yield stress at the lower temperatures was relatively insensitive to strain rate and temperature, whereas at the higher temperatures, it was highly sensitive to strain-rate and temperature.Various mechanisms are considered as being responsible for the athermal range of behaviour and it is concluded that the Orowan stress required to bow out segments of the Frank network is most likely to be controlling in this region. Similar considerations applied to the rate-sensitive range of behaviour suggest that the unpinning of nodes in the dislocation network is ratecontrolling above the transition temperature. In the transition region between the athermal and thermally activated ranges of flow, a maximum in flow stress was observed that is attributed to a dynamic strain-ageing effect caused by the presence of interstitial oxygen.

Resistance of superconducting-normal interfaces

Abstract: The work of Pippard, Shepherd & Tindall (P. S. T.) on the resistance of copper foil sandwiched between superconducting lead electrodes has been extended (1) to study the influence of alloying the lead with bismuth, (2) to investigate other pairs of metals, Pb-Re, Pb-Mo, Pb-Al, Pb-Sn, Sn-Al. In addition, different techniques for depositing lead electrodes on copper foil have been investigated; evaporation in vacuo after cleaning the copper surface by sputtering has been found to give the most consistent results and minimal oxide contamination at the metallic interface. Thin oxide layers give rise to a characteristic rise in the resistance of the sandwich as the temperature is lowered well below the superconducting transition. The behaviour is fairly consistent with a simple model in which regions of metallic contact carry current in parallel with tunnel junctions; the conductivity of the latter is much higher than for oxide layers used for experiments on superconducting tunnelling, and it is conjectured that they are monomolecular. The rise of resistance towards the transition temperature of lead, as observed by P. S. T., becomes more marked when bismuth is added to the lead, but reaches a limiting behaviour with 5 % Bi which is maintained up to 20 % Bi, at which point the behaviour becomes less reproducible, as might be expected with a two phase superconducting electrode. The resistance of the sandwich at the lowest temperatures is also somewhat increased by the addition of bismuth to the lead. A theory of the observations has been developed by extension of the argument of P. S. T. which attributes the extra resistance to the penetration of excitations from the normal metal into the superconductor, provided they have an energy above the gap. With the addition of bismuth these excitations are more frequently scattered before finally being assimilated into the equilibrium superconducting state, and the extra scattering reveals itself as extra resistance. At the lowest temperatures no excitations can enter the superconductor from the normal metal, and if the superconductor were pure and metallic contact perfect the total reflexion of excitations with reversal of character, as analysed by Andreev, would lead to no extra resistance. The reflected excitations, however, penetrate to something like the coherence length into the superconductor in the form of an evanescent mode. It is the scattering of the evanescent mode by impurities which is responsible for partial reflexion without change of character, and hence for the extra resistance at low temperatures. The scattering process for evanescent modes is analysed in some detail and the problem of developing a full theory in the three dimensional case is discussed but without a final conclusion. The one dimensional model is nevertheless capable of fairly complete solution, though reasons are given for not trusting its predictions in the real situation. Although fully quantitative explanation of the experimental results is not possible either at low temperatures or near the transition temperature (in the latter case because of effects due to short lifetime of excitations in lead), the qualitative agreement between models and experiment is not unsatisfactory. The results on sandwiches in which the centre part becomes superconducting at a lower temperature than the flanking electrodes confirm other observations in showing how the resistance begins to drop as the temperature falls towards the lower transition temperature. The Pb-Mo samples behave as might have been expected if the molybdenum had a transition temperature of about 1K although these samples, possibly due to iron content, had a transition temperature below 0.4 K.

The superconducting transition temperature and its high-pressure behavior of tetragonal Nb3Sn

Abstract: The superconducting transition temperatureTC of the tetragonal phase of a Nb3Sn single crystal was determined calorimetrically to be 17.78±0.02 K. It is suppressed by the application of hydrostatic pressure at a rate of −(1.40±0.05)×10−5 K bar−1. The previous observations of double superconducting transitions in Nb3Sn and other A15 compounds are discussed.

Superconductivity in transition metal alloys: an alternative to the rigid band model. II

Abstract: For part I see abstr. A79497 of 1973. In a recent paper an account has been given of the electronic heat capacities, gamma , of transition metal solid solutions in terms of an empirical model in which the different metals in the alloy are assumed to contribute individually to gamma . The model is applied to the superconductive transition temperature. It is demonstrated that a fair description of the experimental data available on superconductive transition metal solid solutions is obtained if in the alloy the metals are supposed to contribute individually to -(ln(Tc/ theta D)-1. Within a simple BCS formalism the additivity assumed for this quantity is equivalent to the additivity of densities of states and that of electronic heat capacities. In addition, both heat capacity and superconductivity of transition metal random alloys are reconsidered taking into account phonon enhancement and separate attractive and repulsive electron-electron interactions. It is concluded that the density of states at the Fermi level, N(0), shows maxima at Z approximately=3.3, 5.3, 7.3, 8.8 and 10. This result partly contradicts rigid band type conclusions.