Showing papers on "Transition temperature published in 1968"
TL;DR: In this paper, the superconducting transition temperature is calculated as a function of the electron-phonon and electron-electron coupling constants within the framework of strong coupling theory.
Abstract: The superconducting transition temperature is calculated as a function of the electron-phonon and electron-electron coupling constants within the framework of the strong-coupling theory. Using this theoretical result, we find empirical values of the coupling constants and the "band-structure" density of states for a number of metals and alloys. It is noted that the electron-phonon coupling constant depends primarily on the phonon frequencies rather than on the electronic properties of the metal. Finally, using these results, one can predict a maximum superconducting transition temperature.
3,895 citations
TL;DR: In other alloying variations some of the nickel is replaced with cobalt, in the formula TiNixCo1−x, is an effective substitution element for lowering the transition temperature as discussed by the authors.
368 citations
TL;DR: In this article, a glass-liquid transition in the Au0.77Ge0.136Si0.094 alloy was reported and the transition was manifested thermally by an abrupt rise of 5.5 cal/g•atom·°K in specific heat, Cp, as the alloy was heated from 285° to 297°K. The apparent glass transition temperature was increased by 1°−3° by increasing the heating rate by a factor of 16.
Abstract: Gold‐rich (73–81 at.% Au) alloys of gold–germanium–silicon were “splat” cooled to an amorphous solid form. Thermal and rheological evidence of a glass‐liquid transition in the Au0.77Ge0.136Si0.094 alloy is reported. The transition was manifested thermally by an abrupt rise of 5.5 cal/g‐atom·°K in specific heat, Cp, as the alloy was heated from 285° to 297°K. Just above 297°K, Cp starts to fall with increasing T on a reasonable extrapolation of the high‐temperature Cp–T relation of the liquid alloy. This thermal behavior could be repeated on the same sample after it had been carried through the transition and chilled again. The apparent glass‐transition temperature was increased by 1°–3° by increasing the heating rate by a factor of 16. The viscosity η of the amorphous alloy increases rapidly with falling temperature from 0.9 × 109 P at 305°K to 1.4 × 1013 P at 285°K and is described by the equation: η = 0.52 exp [136 / (T − 241.3)] P. The steady isothermal viscosity is independent of stress over the range...
336 citations
TL;DR: In this article, the fluctuation of the order parameter in a superconductor in the temperature slightly above the transition temperature is studied in the framework of the current microscopic theory, and it is shown that the elec- trical resistivity, the nuclear spin lattice relaxation time and the ultrasonic attenuation co-efficient decrease like 1-ai1li- 1I2 and 1-a'i1li − 1I3 in the classical and in the critical region. respectively, where 1l= (TITe) -1 and a and a' are constants of
Abstract: The fluctuation of the order parameter in a superconductor in the temperature slightly above the transition temperature is studied in the framework of the current microscopic theory. It is convenient to divide the temperature region where the fluctuation of the order parameter becomes important into two regions; the classical and the' critical region. In the classical region the spatial correlation of the fluctuation is of the Ornstein-Zernike type, while in the critical region it depends on the relative distance like r-3/2. We show that the elec trical resistivity, the nuclear spin lattice relaxation time and the ultrasonic attenuation co efficient decrease like 1-ai1li- 1I2 and 1-a'i1li- 1I3 in the classical and in the critical region . respectively, where 1l= (TITe) -1 and a and a' are constants of the order (Telp.) 1/2 (lPo)-3/2 and (Tel p.) 1/3 (lpo) -1 respectively. Here p. is the chemical potential, l is the electronic mean free path and Po is the fermi momentum. It is also shown that the thermal conductivity has no singular term at the transition temperature.
255 citations
TL;DR: In this paper, the time dependent Ginzburg-Landau equation was used to calculate the current density and the electromagnetic response function for small frequencies and wave numbers, and the dynamical conductivity for bulk material, thin films, and thin wires showed an increasing peak at zero frequency the width of which decreases as the transition temperature is approached.
Abstract: The fluctuations of the local order parameter above the superconducting transition temperature give rise to singularities in the electrical conductivity and the diamagnetic susceptibility. Using the time dependent Ginzburg-Landau equation the fluctuation of the current density is calculated. By means of the fluctuation-dissipation theorem and a dispersion relation the electromagnetic response function is then determined for small frequencies and wave-numbers. The dynamical conductivity for bulk material, thin films, and thin wires shows an increasing peak at zero frequency the width of which decreases as the transition temperature is approached. This structure should be observable in microwave experiments.
174 citations
TL;DR: In this paper, a series of vanadium dioxide single crystals containing small concentrations of different impurity ions were grown by a thermal decomposition technique, and it was expected that these ions would produce a systematic change in the semiconductormetallic transition exhibited by the VO 2 phase.
143 citations
TL;DR: In this paper, a theory was presented to explain the dependence of the superconducting transition temperature on the changes in the phonon frequency spectrum and electronic density of states which result from lattice disorder.
Abstract: A theory is presented to explain the dependence of the superconducting transition temperature ${T}_{c}$ on the changes in the phonon frequency spectrum and electronic density of states which result from lattice disorder. Numerical calculations of ${T}_{c}$ are presented for films composed of crystalline granules, for films composed of amorphous granules, and for homogeneous amorphous metals. The calculations are in good agreement with experimental values of ${T}_{c}$.
114 citations
TL;DR: In this paper, a modified KDP model of a two-dimensional ferroelectric with the inclusion of an arbitrary electric field was considered, and the critical behavior of the polarizability was shown to be similar to the problem of close-packed dimers on a hexagonal lattice.
Abstract: The modified potassium dihydrogen phosphate (KDP) model of a two-dimensional ferroelectric with the inclusion of an arbitrary electric field $\mathcal{E}$ is considered. The nature of the phase transition is unaffected by the inclusion of an external field, whereas the transition temperature depends on $\mathcal{E}$ with ${T}_{c}\ensuremath{\rightarrow}\ensuremath{\infty}$ as $\mathcal{E}\ensuremath{\rightarrow}\ensuremath{\infty}$, and ${T}_{c}\ensuremath{\rightarrow}0$ as $\mathcal{E}\ensuremath{\rightarrow}\mathrm{some}\mathrm{special}\mathrm{values}$. We also find that ${T}_{c}$ is determined by different relations in different regions in the $\mathcal{E}$ plane, a property not shared by the Slater KDP model. Among the other thermodynamic properties discussed, the critical behavior of the polarizability is shown to be $\ensuremath{\chi}\ensuremath{\sim}{(T\ensuremath{-}{T}_{c})}^{\ensuremath{-}\frac{1}{2}}$. It is shown that this model is identical to the problem of close-packed dimers on a hexagonal lattice.
76 citations
TL;DR: The effect of the Eu metal on the formation of these crystals and its role in forcing the trivalent rare earth oxide into the rock-salt lattice is discussed in this article.
Abstract: The ferromagnetic transition temperature (TC) of EuO has been increased from 69° to 135°K by selectively doping to increase the electrical conductivity Three different chemical systems have been studied: the EuO‐Eu, the EuO‐Eu‐RE2O3, and the EuO‐RES systems (RE is a rare earth other than europium) Compositions in the latter system were multiphase because the solubility of the rare‐earth sulfides in EuO is very small and it was necessary to add about 10% to cause significant Curie‐temperature increase Single crystals have been grown from ternary compositions in the EuO‐Eu‐RE2O3 system with magnetizations at 77°K as high as 170 G cm3/g and ferromagnetic Curie temperatures (TC) close to the paramagnetic θ values, a condition not observed in the other doped europium chalcogenides The effect of the Eu metal on the formation of these crystals and its role in “forcing” the trivalent rare‐earth oxide into the rock‐salt lattice is discussed From Kerr measurements and the optical absorption data it is shown that the magnitude of the rotation of the doped material at 80°K is equal to that of pure EuO at 18°K This makes this material attractive for use in magneto‐optical devices operating at nitrogen temperature
75 citations
TL;DR: In this paper, an extensive solid solution was observed in the system Pb(Sc1/2/,Nb 1/2,1-x,Tix,O3) consisting of the rhombohedral phase with a small amount of the tetragonal phase present.
Abstract: Extensive solid solution was observed in the system Pb(Sc1/2/,Nb1/2,)1-x,Tix,O3. In the range 0 ≤ x ≤ 0.425 a rhombohedral ferroelectric phase was stable at 25° C. In the range 0.45 ≤ x ≤ 1.00 a tetragonal ferroelectric phase was stable at this temperature. The phase diagram of the system below 500° C strongly resembles that of PbZrO3−PbTiO3. The compound Pb(Sc1/2Nb1/2)O3 exhibited rhombohedral perovskite cell symmetry below the ferroelectric ↔ paraelectric transition temperature, and the angle a was acute. The radial coupling coefficient was 0.46 for the composition Sc1/2Nb1/2)0.575Ti0.4250O3. At 25°C this composition consisted primarily of the rhombohedral phase with a small amount of the tetragonal phase present. The ferroelectric ↔ paraelectric transition occurred over a temperature range in the rhombohedral phase field. The spontaneous polarization was finite at temperatures considerably above the temperature of the permittivity maximum for a given rhombohedral solid solution.
71 citations
TL;DR: In this article, the dependence of the superconducting transition temperature on the concentration of Gd spins is reported for La 1− x Gd x Al 2, and the results are in good agreement with the theory of Abrikosov and Gor'kov to concentrations very near the critical concentration.
TL;DR: The structural, dielectric, and piezoelectric properties of the solid solution (Na1-x,Lix,NbO2) were studied in this article.
Abstract: The structural, dielectric, and piezoelectric properties of the solid solution (Na1-x,Lix,)NbO2 were studied. Solid solution ceramics with x values up to 0.14 occur as a single phase. The phase diagram from 0° to 600°C is given. The low-temperature phase has a perovskite pseudo-monoclinic structure and inverts to a pseudotetragonal structure at a temperature that decreases with increasing x. The high-temperature phase is pseudocubic, and the minimum transition temperature from the pseudotetragonal to the pseudocubic structure is 350° C near x= 0.04. Anomalies in the dielectric constant vs. temperature curve agree well with the transition temperatures, although no anomaly was observed at the temperature of transition from the pseudotetragonal to the pseudocubic phase when x was less than 0.06. Lithium ions in the system facilitate the transformation from the antiferro-electric to the ferroelectric state produced by applying an electric field. A new ferroelectric phase becomes stable when x is greater than 0.07. Poled ferroelectric specimens had a planar coupling coefficient of 0.232, a dielectric constant of 220, and a frequency constant of 3352 kc-mm. The piezoelectric properties disappear not at the transition temperature but at the temperature at which strain exhibits an anomaly when x is greater than 0.07. A perovskite phase with a pseudorhombohedral subcell exists as an intermediate phase for x greater than 0.10, the exact value depending on the preparation method. The phase can be easily converted to pseudomonoclinic when the ceramic is fired at 1180° to 1220°C.
TL;DR: In this paper, the effects of temperature and water on the crystal transition of nylon 6 from the γ phase to the α phase, which is caused by stretching along the chain direction, were investigated.
Abstract: The effects of temperature and water on the crystal transition of nylon 6 from the γ phase to the α phase, which is caused by stretching along the chain direction, were investigated. The γ-phase fibers with high crystallite orientation were stretched at constant load under various conditions. An inversion of the effect of water on the transition occurs at about −60°C. Stretching in the wet state is more effective for the transition at higher temperature. In contrast, at low temperatures water in the crystalline regions actsasa cohesive agent for the chains and increases the activation energy for the transition. Thus, dry stretching is more effective than wet stretching at very low temperature. The fraction of transformed α-phase crystallites increases abruptly over a narrow range of stress. Thus the critical stress can be determined for the transition. The critical stress changes appreciably with temperature; the higher the temperature, the lower the critical stress. The relation between stretching temperature and critical stress was analyzed by Flory's equation for the shift of transition temperature by stress. About 220°C. was estimated as the zero-stress transition temperature. The heat content of the γ-phase crystal was estimated to be smaller by 500 cal./mole than that of the α-phase crystal. This result suggests that the free energy of the γ-phase crystal is lower than that of the α-phase crystal at temperatures lower than the transition point. The irreversible strain of a sample in which the crystal transition has taken place is very small at low temperature. This small extension of the sample is evidence that the γ → α transition produced by stretching along the chain axis is a crystal-crystal transition.
TL;DR: In this paper, the magnetization of cylinders of synthetic high-field superconductors were measured as a function of magnetic field and temperature, and the current density was calculated from magnetization versus field curves.
Abstract: Measurements of the magnetization of cylinders of synthetic high‐field superconductors were made as a function of magnetic field and temperature. The superconductors were made by impregnating porous glass with indium. The porous glass can be characterized by the pore diameter; the critical field and the transition temperature depend strongly on the pore diameter. Assuming a critical‐state model, the current density Jc was calculated from the magnetization versus field curves and was found to be of the form Jc=A−CB for magnetic fields B less than 0.3 of the critical field. In greater fields, Jc is almost independent of field. A and C depend upon temperature. For values of the ratio T/Tc between 0.9 and 0.5, A increases linearly with decreasing temperature. Several samples with different pore sizes have been measured; no systematic dependence on pore size was found.
TL;DR: In this paper, the particle size dependences of the spin flip transition temperature (Morin temperature) and of the lattice constant of α-Fe 2 O 3 were studied experimentally.
Abstract: The particle size dependences of the spin flip transition temperature (Morin temperature) and of the lattice constant of α-Fe 2 O 3 were studied experimentally. With decreasing particle size, it was found that the Morin temperature shifts downwards and crystal lattice expands both in a - and c -axis directions. As the origin of the Morin temperature shift, the change of the dipolar magnetic field due to the lattice dilatation was considered. Experimental values were compared to the theoretical calculation of Artmann and a qualitative agreement was obtained.
TL;DR: The magnetic properties of hexagonal CeAl3 and orthorhombic CeAl4 between 2.2 and 300°K are reported and discussed in detail in this paper, where it is attributed to a combination of crystal field and exchange interactions.
TL;DR: Polycrystalline thin films of,, and have been prepared in the thickness range from 500 to 4000Aa as mentioned in this paper, which are obtained by a postdeposition heat-treatment of sputtered V or VO films.
Abstract: Polycrystalline thin films of , , and have been prepared in the thickness range from 500 to 4000Aa. The films are obtained by a post‐deposition heat‐treatment of sputtered V or V‐O films. The quality of the films has been studied by x‐ray diffraction techniques and by measuring resistivity as a function of inverse temperature to check the magnitude of the metal to semiconductor transition. Electrically the films compare favorably with bulk crystals, and in addition the films are extremely stable during repeated cycling through the transition temperature.
01 Jan 1968
TL;DR: In this article, the temperature dependence of the ferromagnetic-antiferromagnetic coupling is studied between −190 and 20 °C in polycrystalline thin films of cobalt or nickel oxidized superficially.
Abstract: On a etudie le couplage ferro-antiferromagnetique sur des couches minces polycristallines, a anisotropie uniaxiale, oxydees superficiellement, du type CoCoO ou NiNiO, entre _ 190 et 20 °C. L'anisotropie unidirectionnelle s'annule a une temperature de transition toujours inferieure au point de Neel et liee a l'epaisseur moyenne de la couche antiferromagnetique. L'anisotropie uniaxiale n'est pas modifiee par la presence du couplage. On explique le maximum du champ coercitif et la disparition de l'hysteresis de rotation au voisinage de la temperature de transition en supposant que la couche antiferromagnetique “deformable” du point de vue magnetique aux basses temperatures, devient “rigide” aux plus hautes temperatures. On observe un trainage magnetique de tres grande amplitude qui provient de reorientations des moments dans certains grains antiferromagnetiques et suit une loi logarithmique dans des intervalles de temps de plusieurs heures, par suite de la dispersion de la taille des grains. Les ordres de grandeur de l'anisotropie antiferromagnetique deduits de l'etude detaillee du trainage sont supeieurs a l'anisotropie magnetocristalline des oxydes a l'etat massif.
The temperature dependence of the ferromagnetic-antiferromagnetic coupling is studied between _190 and 20 °C in polycrystalline thin films of cobalt or nickel oxidized superficially. The unidirectional anisotropy vanishes at a transition temperature below the Neel temperature and is related to the average thickness of the antiferromagnetic film. The uniaxial anisotropy is found not to be dependent on the coupling. The observed maximum in the coercive force and the reduction in hysteresis loss at temperatures near the transition temperature are in qualitative agreement with a model in which the moments in the antiferromagnetic atomic planes are “distorted” by the coupling at low temperatures, the film being “rigid” at higher temperatures only. The magnetic aftereffect related to spin re-orientations in some antiferromagnetic regions is found to be considerable and follows a logarithmic law for times of several hours, due to the size distribution of the antiferromagnetic crystallites. Finally, values greater than the magnetocrystalline anisotropy of the bulk materials are deduced for the anisotropy in the antiferromagnetic oxides.
TL;DR: In this paper, the dc conductivity of the excitonic insulator is calculated in the semimetallic region and it is shown that the conductivity decreases below the transition temperature to the excitic state for low impurity concentrations.
Abstract: The dc conductivity of the "excitonic insulator" recently discussed in the literature is calculated in the semimetallic region The calculations are based on recent work on the description of the excitonic phase in the presence of impurities It is shown that the conductivity decreases below the transition temperature to the excitonic state For low impurity concentrations the system acquires insulating properties For higher impurity concentrations the conductivity is still nonzero at $T=0$ Thus, metallic properties prevail in the excitonic phase It is pointed out that this behavior depends essentially on the form of the excitation spectrum of the system, ie, the presence or absence of a gap At the transition temperature the conductivity-versus-temperature curve has a finite slope
TL;DR: In this paper, the effect of cerium and gadolinium impurities on the pressure dependence of the superconducting transition temperature of lanthanum is due to different electronic structures of the rare-earth impurity.
Abstract: The different effect of cerium and gadolinium impurities on the pressure dependence of the superconducting transition temperature of lanthanum is due to different electronic structures of the rare-earth impurity. The ionic model explains the properties of gadolinium alloys, while the resonant scattering theory explains those of cerium alloys.
TL;DR: In this article, a description of preparation for samples of pure niobium nitride without and with admixtures of titanium, zirconium and tantalum by diffusion of nitrogen under pressure into Niobium wires is given.
Abstract: A description of preparation for samples of niobium nitride without and with admixtures of titanium, zirconium and tantalum by diffusion of nitrogen under pressure into niobium wires without and with admixtures of titanium, zirconium and tantalum respectively, is given. For samples of pure niobium nitride with different compositions the influence of the nitrogen content on the transition temperature to superconductivity has been investigated with the result of a transition temperature not exceeding 16.5 °K even for the stoichiometric composition NbN. This is in contrary to other authors. For diffusion wires of the systems (Nb-Ti)N
x
, (Nb-Zr)N
x
and (Nb-Ta)N
x
the critical temperature, lattice parameter and nitrogen content as a function of the admixture of titanium, zirconium and tantalum respectively, are given. For the first time the magnetisation curve of niobium nitride has been measured showing the behaviour of type II superconductors. Concerning the kinetics of formation of niobium nitride by diffusion the nitrogen content is increasing for constant nitrogen pressure with raising temperature of diffusion, but is decreasing in the high temperature region.
TL;DR: In this article, the electrical resistivity of vanadium dioxide single crystals undoped and iron-doped was determined as a function of temperature around the transition temperature in the a monoclinic and a perpendicular directions.
Abstract: Electrical resistivity of vanadium dioxide single crystals undoped and iron‐doped was determined as a function of temperature around the transition temperature in the a monoclinic and a perpendicular directions. Results of this experiment indicated a greater resistivity in the a perpendicular direction than in the a parallel direction for both the doped and the undoped crystals. The appearance of anisotropy is in agreement with Goodenough's explanation for the mechanism of electrical conductivity in this phase.
TL;DR: The data suggest that the thermal transition is predominantly a two-state process involving two widely different macroscopic forms, associated with large changes in spectral properties, in optical rotatory dispersion and in fluorescence.
TL;DR: In this paper, the authors used theoretical expressions to calculate the pressure dependence of the transition temperature in non-transition metal superconductors and the experimentally observed effects can be approximately accounted for.
TL;DR: In this paper, the authors measured superconducting transition curves for a series of amorphous bismuth films having thicknesses ranging from 100 to 2240 A. The constant multiplying this factor was found to be proportional to the residual resistance of the film.
TL;DR: In this paper, Niobium films were deposited by dc and asymmetric ac sputtering techniques in argon with addition of a small amount of nitrogen gas, and a wide range of resistivities and transition temperatures depending on the partial nitrogen pressure in the argon atomosphere were obtained.
Abstract: Niobium films have been examined with respect to their electrical and superconductive properties. The films were deposited by dc and asymmetric ac sputtering techniques in argon with addition of a small amount of nitrogen gas. The films obtained had a wide range of resistivities and transition temperatures depending on the partial nitrogen pressure in the argon atomosphere. The maximum transition temperature of films obtained was 15.3°K, which very close to those reported for bulk NbN. It was found that the asymmetric ac sputtering technique is useful to increase the transition temperature of niobium films sputtered reactively in the presence of a nitrogen gas.