Showing papers on "Transition temperature published in 1971"
TL;DR: In this article, two distinguishable mechanisms of the monoclinic-to-tetragonal transition in VO 2 are identified: an antiferroelectric-toparaelectric transition at a temperature T t as well as a change from homopolar to metallic VV bonding at T t.
1,154 citations
TL;DR: In this paper, the high-temperature phase equilibria of nonstoichiometric lithium niobate were accurately determined by 93Nb NMR and by shifts in the ferroelectric transition temperature.
Abstract: Deviations from stoichiometry occurring during the crystal growth of lithium niobate have been studied by 93Nb NMR and by shifts in the ferroelectric transition temperature. The high‐temperature phase equilibria of nonstoichiometric lithium niobate were accurately determined by these techniques together with DTA and crystal‐growth experiments. The congruently melting composition is located at 48.6 mole% Li2O and the solid‐solution region at room temperature extends from 48% to 50% Li2O. The implications of growth at noncongruently melting compositions on the compositional uniformity of lithium niobate are discussed in detail.
312 citations
TL;DR: In this article, the volume change at the crystalline-liquid crystalline phase transition was measured for the dipalmitoylphosphatidylcholine-water system containing lamellar bimolecular structures.
183 citations
TL;DR: X-ray diffraction techniques have been used to investigate the physical state of the lipids in the membrane of an unsaturated fatty acid auxotroph of Escherichia coli, and it is concluded that the distribution of lipids within the membrane is heterogeneous.
Abstract: X-ray diffraction techniques have been used to investigate the physical state of the lipids in the membrane of an unsaturated fatty acid auxotroph of Escherichia coli. Thermotropic phase transitions have been detected in membranes prepared from cells grown on various fatty acid supplements. Below the transition temperature, the X-ray diffraction pattern features a sharp ring at 420 pm (4.2 A) due to the close hexagonal packing of the apolar groups of the lipids; above the transition temperature the lipids are in a less organized liquid-crystalline state that gives rise to a diffuse band centered at a Bragg spacing of 460 pm (4.6 A). This transition occurs at or below the temperature at which the cells were grown. Disparities between this transition temperature and the temperature of discontinuities in the Arrhenius plots for proline transport and succinic dehydrogenase activity lead us to conclude that the distribution of lipids within the membrane is heterogeneous.
179 citations
TL;DR: In this paper, the effects of temperature and hydrostatic pressure on the ferroelectric (FE) properties of PbTiO3 and on certain properties of Ba TiO3 which have not been treated or emphasized earlier were investigated.
Abstract: The effects of temperature and hydrostatic pressure on the ferroelectric (FE) properties of PbTiO3 and on certain properties of BaTiO3 which have not been treated or emphasized earlier were investigated. For PbTiO3, the FE-paraelectric (PE) transition is of first-order, and the transition temperature, Tc, decreases with pressure with an initial slope of -8.4 ± 0.3°K/kbar. It is estimated that the latent heat associated with the transition is 350 ± 45 cal/mole, with a corresponding entropy change of 0.46 ± 0.05 cal/mole°K. In the PE phase, the static dielectric constant, e, obeys the Curie-Weiss law e = C/(T- T0) over a wide temperature range. Both C and T0 decrease with pressure. At constant % e varies with pressure according to e = C*/(p-po), where p0 decreases strongly with increasing T while C* is very weakly T-dependent. The results can be explained in terms of an increase of the frequency of the soft FE mode with pressure, and the Gruneisen parameter and its Tdependence are determined for this mode. ...
166 citations
TL;DR: In this paper, the authors observed a second transition back into the normal state at temperatures below the transition temperature Tc for certain (La 1− x Cescx Al2) alloys, in agreement with the recent prediction that such an effect could exist in a Kondo superconductor with TK.
117 citations
TL;DR: In this article, the superconducting transition temperature, the critical magnetic field Hc, and the room temperature conductivity have been measured as a function of thickness d. The results indicate that films as thin as 30 A act essentially as uniform layers in which the crystal size is approximately equal to the film thickness.
Abstract: Thin films of aluminum have been produced in the thickness range of 1000–30 A in small area samples with photoetched edges. The superconducting transition temperature Tc, the critical magnetic field Hc, and the room‐temperature conductivity have been measured as a function of thickness d. The results indicate that films as thin as 30 A act essentially as uniform layers in which the crystal size is approximately equal to the film thickness. The transition temperature was found to vary linearly with d−1. Hc (T) was measured from Tc to 0.4°K. For thickness from 1000 to 200 A, Hc∼d−3/2, as expected from the Ginzburg‐Landau theory. For d <200 A, Hc is paramagnetically limited to about 49 kOe = 19.6Tc, slightly above the Clogston limit.
110 citations
TL;DR: In this paper, a power series expansion of the susceptibility at high T up to fifth order in (J/kT) to all orders in ( Gamma /kT), where the coefficients are calculated by diagram counting and time ordering perturbation methods.
Abstract: The Ising model (exchange constant J) with a transverse field Gamma can be regarded as a simple model of hydrogen bonded ferroelectrics and of certain magnetic systems. The transition temperature TC decreases as Gamma increases and falls to zero at a critical field Gamma C. This transition is studied in the quadratic and simple cubic lattices with nearest neighbour interactions by a power series expansion of the susceptibility at high T up to fifth order in (J/kT) to all orders in ( Gamma /kT). The coefficients are calculated by diagram counting and time ordering perturbation methods. The critical Gamma against T curve is derived and the critical exponent gamma of the susceptibility. This appears to vary smoothly with Gamma but it is argued that this is an artifact due to the use of a short series and that the critical exponents are probably independent of Gamma as long as TC remains finite but change discontinuity at TC=0.
96 citations
TL;DR: In this article, the authors examined the electrical and superconducting properties of NBN films with respect to their resistivities and transition temperatures depending upon the partial nitrogen pressure in the argon atmosphere and substrate temperature at which the films were deposited.
Abstract: NbN films are examined with respect to their electrical and superconducting properties. The films are deposited by r.f. reactive sputtering technique under argon gas atmosphere added with nitrogen. The films obtained have a wide range of resistivities and transition temperatures depending upon the partial nitrogen pressure in the argon atmosphere and the substrate temperature at which the films were deposited. The maximum transition temperature obtained is 17.3°K, higher by 0.9°K than the highest one reported so far.
83 citations
TL;DR: In this article, the anomalous specific heat of Mn-Zn, Mn-Ga and Mn-Ge alloys was investigated with X-ray and neutron diffraction and specific heat measurements.
Abstract: Manganese-rich γ-phase alloys of Mn-Ni, Mn-Zn, Mn-Ga and Mn-Ge have been investigated with X-ray and neutron diffraction and specific heat measurements. The Mn-Zn, Mn-Ga and Mn-Ge alloys exhibit antiferromagnetism below the transition temperature, at which the cubic lattice is transformed to the tetragonal lattice (the axial ratio c / a 1) at a lower temperature. The magnetic structure for the axial ratio c / a >1 is different from that for the axial ratio c / a <1. The integrated values of the anomalous specific heat are less than the values predicted by the localized moment model of antiferromagnetism.
TL;DR: In this article, it was shown that there always exists at least a whole plane of optical phonons, which are Raman inactive in the high temperature phase, and the frequency of which goes to zero at the transition temperature.
Abstract: Displacive phase transitions which do not change the size of the unit cell may be classified under two types depending on their order parameter : in the elastic transitions, this is the amplitude of an elastic strain ; it is the amplitude of a relative displacement of the atoms in an optic transition In this last case, we prove that there always exists at least a whole plane of optical phonons, which are Raman inactive in the high temperature phase, and the frequency of which goes to zero at the transition temperature These soft modes become Raman active in the low temperature phase Should these phonons be Raman active in the high temperature phase, they would induce an elastic transition as shown by Miller and Axe : an elastic constant will pass through zero for a still finite frequency of the optical phonon In the case of an elastic transition, if the Landau theory allows it to be second order, we show by group theory that there always exists at least one sound velocity which passes through zero at the critical temperature We also prove that the optical or elastic soft mode never carries an electric field with it Nevertheless the dielectric constant becomes infinite at the transition temperature, and at low temperature is polar, either if the optical soft mode is infrared active or if the elastic mode induces a piezoelectric strain The damping effect associated with such transitions is not taken into account in this paper
TL;DR: In this paper, the authors measured the configurational enthalpy of 2,3-dimethylbutane and determined the residual entropy and T2 temperature of the glassy crystal, which was found to be 7.4 JK−1 mol−1 and 65 K.
Abstract: 2,3-Dimethylbutane has two crystalline phases, high temperature form (crystal-I) and low temperature form (crystal-II). On account of the supercooling effect of crystal-I, its glassy crystalline state (non-equilibrium frozen-in state) is established below Tg (76 K) by rapid cooling. The heat capacities for various phases were determined from 13 to 300 K from principal interest in the thermodynamic properties of the glassy crystal. The transition temperature from crystal-II to crystal-I and melting point were determined to be 136.02 K and 145.05 K, respectively. The heats of transition and of fusion were determined to be 6427 J mol−1 and 788 J mol−1, respectively. From these data, the residual entropy and T2 temperature were determined to be 7.4 JK−1 mol−1 and 65 K, respectively. We also calculated the ratio of Tg to T2 for several glassy crystals known hitherto, which amounts to 1.15–1.20. The stabilization effect was studied by the measurement of the change of the configurational enthalpy in the glassy c...
TL;DR: In this paper, the electron-phonon interaction in transition metals in terms of an atomic parameter η and a mean phonon stiffness is reviewed, and it is shown that the most likely materials for higher T c are compounds whose volume/atom is unusually low.
TL;DR: The heat capacity of the clathrate compound of quinol with hydrogen cyanide was measured from 12 to 300 K and the entropy change associated with it was estimated to be 5.71±0.3 J/mol K as discussed by the authors.
Abstract: The heat capacity of the clathrate compound of quinol with hydrogen cyanide was measured from 12 to 300 K. It exhibited a well defined anomaly at 177.8+0.1 K. The entropy change associated with it was estimated to be 5.71±0.3 J/mol K. This value is close to Rln2, implying that the anomalous heat capacity is due to the order-disorder phase transition concerned with the orientation of the trapped molecules. Dielectric study supported this view. Far infrared spectra of the crystal was recorded from 38 to 500 cm -1 over a range of temperature. Two bands at 46 and 95 cm -1 were found to be characteristic of this substance. A possible assignment of the former band was proposed. Finally, the lattice sum calculation showed that the dipole interaction energy between the guest molecules corresponds to 318 K of temperature compared with the observed transition temperature.
TL;DR: In this paper, a method was described whereby intermetallic phases which are not stable at high temperatures may be formed using controlled vacuum evaporation of the elements, and V 3 Al with the A15 structure was formed on substrates held at temperatures in the range 350-450°C.
TL;DR: In this paper, the Ginzburg-Landau theory is used to investigate the resistive transition in a "one-dimensional" superconductor as a function of temperature and current through the wire.
Abstract: Time-dependent Ginzburg-Landau theory is used to investigate the resistive transition in a "one-dimensional" superconductor as a function of temperature and current through the wire. A diagrammatic expansion, in powers of the interaction between fluctuations, is described for the electrical conductivity. The conductivity is then calculated using a Hartree-Fock approximation for the interaction. When large currents flow through the wire, an unstable region is found near the depressed critical temperature, suggesting possible hysteresis effects which may have been observed in recent experiments.
TL;DR: In this paper, the phonon spectrum of an Al film with 3.6 at and 3.5 at was shown to be very similar to that of normal aluminum, and only slightly lower in energy, which corresponds to a 10.5% increase in volume.
Abstract: Mixtures of Al and ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ were sputtered at room temperature, while mixtures of Al and Ge were sputtered at 77 \ifmmode^\circ\else\textdegree\fi{}K yielding, respectively, a maximum transition temperature of 2.5 and 3.6 \ifmmode^\circ\else\textdegree\fi{}K. The phonon spectrum of an Al film with 3.6 at.% $\mathrm{Ge}({T}_{c}=3.48 ^{\ensuremath{\circ}}\mathrm{K})$ is very similar to that of normal $\mathrm{Al} ({T}_{c}=1.2^{\ensuremath{\circ}}\mathrm{K})$ and only slightly lower in energy. The magnitude at 0 \ifmmode^\circ\else\textdegree\fi{}K and the temperature dependence of the energy gap of the enhanced Al are extremely well fitted by the BCS theory. Density measurements were performed in two different ways on the two mixtures (10 wt% of ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ and 10 wt% of Ge) with maximum ${T}_{c}$. Density measurements by a direct weighing technique yielded a decrease in density of 5.5%. Electron diffraction experiments showed that the lattice spacing increased from 4.05 (normal aluminum) to 4.19 \AA{} for the most enhanced aluminum, which corresponds to a 10.5% increase in volume. The observed increase in volume is sufficient to explain the increase in ${T}_{c}$ without invoking any other mechanism.
TL;DR: In this paper, a linear dependence of transition temperature on reciprocal film thickness was observed for amorphous films of Bi, Ga, Al, and Pb containing 18 at. % Bi.
TL;DR: In this article, the authors examined the decrease in superconducting critical temperature Tc with impurity concentration nI for dilute magnetic alloys in the framework of the Kondo-Nagaoka theory.
Abstract: The decrease in superconducting critical temperature Tc with impurity concentration nI is examined for dilute magnetic alloys in the framework of the Kondo-Nagaoka theory. When the critical temperature Tco of the host metal is much larger than the Kondo temperature TK the curves of Tc versus nI behave in the manner predicted by the Born approximation theory of Abrikosov and Gorkov, except for an exponential tail for Tc << TK. When Tco << TK these curves show a non-magnetic behaviour which is related to the theory of the Suhl-Nagaoka spin compensated state. The theory is compared to (i) experimental data for LaCe and ThU and (ii) the theory of localized spin fluctuations which provides an alternative explanation for the data.
TL;DR: In this paper, a model for the magnetism in amorphous FexPd80−xSi20 alloys based on a local ferromagnetic order whose direction changes randomly throughout the material is proposed.
TL;DR: The infrared spectrum of crystalline adamantane (tricyclo•3,3,1,1•decane, C10H16) has been studied at temperatures below and above the first-order phase transition at 208°K and as a function of temperature through the transition as discussed by the authors.
Abstract: The infrared spectrum of crystalline adamantane (tricyclo‐3,3,1,1‐decane, C10H16) has been studied at temperatures below and above the first‐order phase transition at 208°K and as a function of temperature through the transition. Splittings of the vibrational transitions and appearance of gas‐phase‐forbidden transitions below 208°K are in agreement with theoretical predictions. A hysteresis is observed in the spectroscopic characteristics over a range of a few degrees around the heat‐capacity peak occurring at the transition temperature. Calculations of lattice energy, frequency splittings, and barriers to molecular rotation have been carried out using central force functions between nonbonded atoms derived from physical properties of a number of crystals. The splittings, heat of sublimation, transition energy, and crystal structural parameters are in semiquantitative agreement with experiment. The vapor pressure of adamantane in the temperature range 312–366°K has been measured, and the heat of sublimati...
TL;DR: The transition temperature of both the cubic and tetragonal phases of several high-temperature beta-W superconductors are observed, and the instability of the cubic lattice appears to be characteristic of high-Temperaturesuperconductors.
Abstract: We have observed the transition temperature of both the cubic and tetragonal phases of several high-temperature β-W superconductors. The instability of the cubic lattice appears to be characteristic of high-temperature superconductors.
TL;DR: In this paper, low temperature heat capacities of nonstoichiometric clathrate compounds with methanol were measured on four specimens of different occupation fractions, corresponding to the occupation fractions 0.974, 0.837, and 0.728.
Abstract: Low temperature heat capacities of the nonstoichiometric clathrate compounds with methanol were measured on four specimens of different occupation fractions. Heat capacity anomalies were found at 65.7. 61.0, 54.4 and 44.4 K, corresponding to the occupation fractions 0.974, 0.897, 0.837 and 0.728, respectively. Magnitude of the entropy change of transition is consistent with the view that the transition is due to randomization of direction of the methanol molecules along the c-axis. Single crystal dielectric data support this interpretation. Variation of the transition temperature with the occupation fraction was compared with data on analogous systems. The lattice energy calculation showed that the dipole interaction between methanol molecules corresponds to 27 or 67 K of temperature depending on the component of the moment taken into account.
TL;DR: In this paper, it was shown that the low temperature electrical resistivity anomaly associated with the Kondo effect in Y-1 at.% Ce decreases with pressure and disappears altogether above ∼ 80 kbar, suggesting that the Ce impurities become nonmagnetic at high pressure.
TL;DR: In this article, the superconducting transition temperatures of both cubic and tetragonal phases of V 3 Si have been observed unambigously for the first time, by heat capacity measurements based on steady state calorimetry technique.
TL;DR: Magnetoresistance and magnetization measurements on the granular Ni-SiO 2 system show the existence of a ferromagnetic to supermagnetic transition as mentioned in this paper, and it is concluded that the intergrain tunneling probability depends on the orientation of the electronic spin.
TL;DR: Optical transmission and reflectance were measured on natural magnetite single crystals in the spectral range $0.15l~h\ensuremath{
u}l~0.75$ eV.
Abstract: Optical transmission and reflectance were measured on natural magnetite single crystals in the spectral range $0.15l~h\ensuremath{
u}l~0.75$ eV. These measurements below and above the semiconductor-to-metal transition temperature ${T}_{\mathrm{v}}$ have shown that, unlike for the vanadium oxides, there is no collapse of the optical gap at the transition. The present results and the existing data on the transport properties of magnetite indicate that different bands contribute to the electric conduction above and below ${T}_{\mathrm{v}}$.