Showing papers on "Transition temperature published in 1970"

Destruction of Superconductivity in Disordered Near-Monolayer Films

Abstract: Severe decreases are observed in the superconducting transition temperatures of metallic films as they are made thinner and more disordered. Explanations for these decreases in the transition temperature are discussed. These include changes in the phonon spectrum, the effect of the metal-insulator boundary, size quantization in small particles, the effect of an activated conduction mechanism in the normal state, and, finally, the effect of order-parameter fluctuations.

Superconductivity: The Transition Temperature Peak Below Four Electrons per Atom

Abstract: It is shown that the superconducting transition temperature for compounds in seven different structure types exhibits a peak at about 3.7 to 3.9 valence electrons per atom.

Polymorphic Behavior of Thin Evaporated Films of Zirconium and Hafnium Oxides

Abstract: Polymorphism in thin evaporated films of zirconium and hafnium oxides was investigated from 100° to 1500°C by electron diffraction and transmission electron microscopy. The films have metastable cubic structures at room temperature and at moderate temperatures. Zirconium oxide, depending on temperature, exists in cubic, tetragonal, and monoclinic forms, whereas hafnium oxide transforms directly from the cubic to the monoclinic structure. The transformation temperatures depend on the oxygen partial pressure. Air annealing of thin films of ZrO2 and HfO2 lowered the temperature of transformation of the tetragonal and the cubic structure into the monoclinic structure by about 150° and 100°C, respectively. The cubic/tetragonal transformation of ZrO2 is monotropic, whereas the tetragonal monoclinic transformation occurs by the typical nucleation and growth mechanism. Determination of grain size in ZrO2 at the tetragonal/monoclinic transformation temperature showed that the transformation occurs when a constant grain size of about 800 A is reached. The oxygen partial pressure, grain size, and temperatures at which the metastable phases exist were correlated. The rate of grain growth is enhanced by increase in oxygen partial pressure. The accelerated transformation in air is attributed to rapid attainment of the critical size; grain boundary energy is an important controlling factor in transformation.

McMillan Model of the Superconducting Proximity Effect for Dilute Magnetic Alloys

Abstract: We extend the tunneling model of McMillan for the proximity effect in normal-superconducting sandwiches to the case when the normal metal film contains magnetic impurities. The excitation spectrum in each film and the superconducting transition temperature of the sandwich are calculated as functions of the impurity concentration and the film thickness. For a large range of impurity concentrations, the calculations predict that the density of states in both films is gapless, although the transition temperature of the sandwich is nonzero. The calculations are considered in relation to experiment, with reference to possible experimental probes for the Kondo effect in alloys which are nonsuperconducting in the bulk.

Some properties of the ferroelectric lithium thallium tartrate

Abstract: Lithium thallium tartrate monohydrate is ferroelectric below 12 ± 0.1°K. Contrary to earlier measurements, the true initial permittivity shows a peak at the transition temperature. Dielectric measurements suggest the transition to be second order. Below T0 there is evidence of a domain wall contribution to e which persists even to very weak fields. True ferroelectric polarization reversal appears very difficult and it is probable that a single domain state is not achieved even for poling fields of 10 kV/cm. At 6°K, P4 =0.12 μc/cm2 as obtained by extrapolation from the hysteresis loop. In the para-electric region a Curie-Weiss law is fulfilled between 13° and 300 K°with C = 1620, and there is no obvious need for considering any quantum effects. The temperature of the permittivity peak rises with biasing field, this shift reaching 7° for E = 10 kV/cm. The significance of the adiabatic correction in LTT is discussed; thermal data must complement the presented dielectric measurements for a full thermodynamic ...

Magnetic Studies of the Alloy System SnTe–MnTe

Abstract: Magnetic ordering and superconductivity have been investigated in the system Sn0.97‐xMnxTe for 0.00001≤x≤0.16 over the temperature range 0.02≤T≤300°K. Reciprocal Hall coefficient, 1/Re, for all these samples was ∼1021 cm−3. Samples with x>0.005 showed ferromagnetic ordering with the Curie temperatures varying linearly with x. For 0.0001

Specific Heat and Superconductivity of Binary Alloys Containing V, Nb, and Ta

Abstract: The electronic specific heat coefficient, γ, the Debye temperature, θD, and the superconducting transition temperature, Tc, have been measured for binary b.c.c. alloys of V, Nb, and Ta. These results have been used to investigate the effects of compositional changes on parameters used in the theory of superconductivity to describe the transition temperature. It has been shown that changes in transition temperature with changes in composition cannot be explained simply on the basis of an alteration of the density of states except in the case of binary Nb-Ta alloys. It is concluded that, for V-Nb and V-Ta alloys, changes in the interaction responsible for superconductivity also govern changes in the transition temperature.

Superconductivity and lattice parameters in the zirconium-molybdenum, zirconium-tungsten, hafnium-molybdenum and hafnium-tungsten alloy systems

Abstract: Superconductivity and lattice parameters have been measured on annealed alloys of ZrMo 2 , ZrW 2 , HfMo 2 and HfW 2 Polymorphism for HfMo 2 was reconfirmed and the compound was found to be superconducting at 007 °K in the C 15 structure and normal down to 005 °K in the C 36 modification The C15 compounds ZrMo 2 , ZrW 2 and HfW 2 were normal down to at least 12 °K The bcc phase was investigated by rapid quenching from the melt It was possible to obtain single phase bcc alloys in the range 3–41 at% Mo in ZrMo alloys and about 9 to at least 67 at% Mo in HfMo alloys The lattice parameters at room temperature for these alloys could be accurately determined and extrapolated to room temperature bcc values for Zr and Hf It was possible to obtain HfMo 2 alloys in three different crystal structures The superconducting transition temperature is about 1 °K for the bcc modification and decreases with decreasing lattice symmetry The rapid quenching technique was less successful in the tungsten alloys, but superconductivity in the bcc phase could be observed in all four alloy systems A maximum in the transition temperature was found in all systems at an electron/ atom ratio between 4 and 5 In the ZrW system, a dip in the transition temperature versus concentration was observed at the composition of the compound ZrW 2 Some aspects of the superconductivity in the bcc phase are discussed It is found that the rigid band model is a good approximation in the dilute Zr region An extrapolation to bcc Zr gives 15 °K as the estimate for the transition temperature

Transition temperature of superconducting indium, thallium, and lead grains.

Abstract: Experimental results on the transition temperature of indium, thallium, and lead grains of various sizes, together with preliminary measurements on other metals, can be interpreted in terms of changes in the phonon spectrum which alter the electron-phonon coupling constant, although some anomalies exist.

Observation of a phase transformation in Eu3S4 at 168°K

Abstract: A phase transformation in EU3S4 at 168°K has been established by several different experimental methods. Electrical conductivity and Seebeck coefficient measurements show a discontinuity through the transition. Above the transition temperature, the activation energy for conduction is 0.163 eV and the resistivity is about 3 × 10 4 Ωcm , while below the transition the activation energy is 0.21 eV and the resistivity is about 106 Ωcm. Differential thermal analysis thermograms reveal an endothermic peak on heating with a hysteresis of 5°C. X-ray diffraction experiments show a change in symmetry accompanied by a lattice parameter change of approximately 0.4% for the transition. Measurements of the magnetic susceptibility show the transition not to be magnetic in origin. It is proposed that the transformation in EU3S4 is due to charge ordering which produces tetragonal symmetry in the unit cell.

Experimental Determination of the Pair Susceptibility of a Superconductor

Abstract: An excess tunneling current due to order-parameter fluctuations has been found in tin\char22{}tin oxide\char22{}lead junctions just above the transition temperature of tin. Details of the variation of the excess current with voltage, magnetic field, and temperature are in agreement with a calculation by Scalapino in which the excess-current voltage characteristic is a direct measure of the frequency\char22{} and wave-number\char22{}dependent pair susceptibility characteristic of the superconducting transition. Estimates of the pair relaxation frequency in tin based on the data are 50% greater than theoretical predictions.

Fluctuations in superconductors below T c

Abstract: Thermal fluctuations of the order parameter in the superconducting state are investigated, particularly near the transition temperature, using the time dependent Ginzburg-Landau theory. These fluctuations give rise to a contribution of the dynamical conductivity, which strongly increases as the temperature is raised towardsTc, in contrast to the temperature dependence of the electromagnetic response due to the static order parameter. At the transition temperature this contribution joins continuously to the extra response (conductivity, susceptibility) which has been calculated and observed in the temperature region aboveTc, where it represents the onset of superconductivity in the normal state. Particularly the dynamical conductivity due to fluctuations is calculated and discussed for bulk material, thin films and thin wires belowTc. The temperature and frequency dependence should be observable in microwave experiments.

Electro-optical system

Abstract: The sharp change in optical properties of ferroelectric and other materials at or near their transition temperature (in the case of ferroelectric material, this may be their ferroelectricparaelectric transition temperature, Curie point) is made use of in light deflectors, light valves and the like. The material is heat biased as, for example, by the application of an alternating voltage of an amplitude sufficient to produce the TANDEL or an analogous effect, to a value close to its transition-temperature. An electrical pulse may then be applied to the material for causing sufficient additional heating to raise the temperature of the material to a different value such as one above the transition temperature. The change in optical properties which results may be employed to deflect or otherwise affect a beam of light applied to the material.

Nuclear Magnetic Resonance Study on 23 Na in Sodium Nitrite in the Vicinity of the Phase Transition Temperatures

Abstract: A study on 23 Na nucleus in sodium nitrite, NaNO 2 , in the vicinity of phase transition temperatures was made using nuclear magnetic resonance Simultaneous measurements of the spectrum and the dielectric constant e b made the one to one correspondence between them clear The main results were as follows: A discontinuous jump of an electric field gradient tensor and thermal hysteresis were found at the transition temperature T t The curves of quadrupole splitting versus temperature show a small kink at the transition temperature T n Only one pair of satellite lines was observed in the sinusoidal antiferroelectric phase The asymmetry parameter of the electric field gradient tensor, η, becomes zero at 03^°∼04^° below T t in the cooling runs

Energy gaps and transition temperatures of the strong-coupling superconductors, lead and mercury, as functions of the mean free paths of the electrons

Abstract: The energy gaps and the transition temperatures of the strong-coupling superconductors lead and mercury are measured as functions of the mean free paths of the electrons. The energy gap of lead increases with decreasing mean free path; the transition temperature is found to be constant. The energy gap of mercury runs through a maximum value with decreasing mean free path whereas the transition temperature decreases. The energy gap of pure mercury is extrapolated to the value 1.58 meV and yields the ratio 2Δ0/kTc=4.4. The experimental results confirm the theoretical prediction that the attractive electron-electron interaction in disordered superconductors is increased. But in addition, the strong-coupling effect appears to become more important in disordered superconductors. A comparison with similar experiments for weak-coupling superconductors is made.

Nonlinear Pressure Dependence of the Superconducting Transition Temperature of Copper Sulfide Cus

Abstract: The superconducting transition temperatures of copper sulfide CuS are measured as a function of hydrostatic pressure up to about 9 katm by an ice bomb technique The superconducting transition is detected with the ac Hartshorn mutual inductance bridge In contrast to ordinary superconductors the curve of the shift of the transition temperature with increasing pressure decreases nonlinearly with downward curvature over the whole range of applied perssures and its slope changes discontinuously at about 35 katm The maximum peak of the imaginary part of the ac susceptibility vanishes under pressures above about 16 katm It is discussed that the observed unusual behaviour of the transition temperature is probably attributed to a change of the Fermi surface of the type proposed by Lifshitz

Search for superconductivity in lithium and magnesium

Abstract: Recent theoretical work suggests that magnesium and lithium should be superconducting in the m°K temperature range. We have cooled samples of each of these metals to a temperature of 4 m°K, measured by a gamma-ray anisotropy thermometer. Although the magnetic field was less than 10−2 Oe, no superconducting transitions were observed. The use of a nuclear orientation thermometer employing60Co in single crystal (hcp) cobalt is described.

Magnetism in Austenitic Stainless Steels

Abstract: Mossbauer‐effect measurements of the temperature dependence of the hyperfine field for several austentic stainless steels are reported. Antiferromagnetic transitions were observed for all specimens in the temperature range below 56°K. In addition, a broadening of the single linewidth occurred for each specimen at a temperature near its transition temperature. The transition temperature is interpreted to be a measure of the Neel temperature of antiferromagnetic γ‐phase iron.

Twin Structure of NiCr 2 O 4 Single Crystal Due to Cooperative Jahn-Teller Distortion

Abstract: The tetragonal phase of nickel chromite (NiCr 2 O 4 ) was investigated by X-ray diffraction method for single crystal. The {110} twin structure was found, below the transition temperature (29°C), as a result of the tetragonal domain formation due to the Jahn-Teller distortion. The twin angle was observed to vary in accord with the temperature dependence of c/ a and to be about 1.1° and 1.6° at T =21°C and T =-30°C, respectively.