Reduction In Tc Research Articles

Proximity effect depression of the critical temperature in two-phase Nb-Ti superconductors

The superconducting critical temperature of high critical current density Nb-Ti composites has been measured at various stages in the critical current optimization process. This process involves heat treatment steps which cause precipitation of normal α-Ti and make the matrix more Nb rich. Tc rises from 9.1 to 9.5 K during this stage. The final optimization stage involves extensive wire drawing during which the α-Ti precipitates are reduced to less than a coherence length in thickness. This does not result in a reduction in Jc but Tc is found to fall from 9.44 to 8.7 K during this step. The depression of Tc is found to be in fair agreement with the predicted proximity effect suppression of Tc. Wires of optimum transport critical current density are seen to have Tc of around 9 K.

Tcdegradation by the Coulomb interaction in anisotropic disordered superconductors

The authors calculate the depression of Tc caused by the effect of a short mean free path on the Coulomb repulsion, for the special case of a very anisotropic superconductor. Specifically, they consider the linear chain model for A15 superconductors with a large difference between the in-chain and inter-chain scattering rates. Previous work based on the McMillan equation for Tc gave a large reduction in Tc. In this work they find Tc numerically from the Eliashberg equations. They find that the reduction of Tc due to this effect is about 25% to 30%, which is considerably less than observed experimentally. The effective Coulomb term is retarded, and as a result is frequency dependent. It is affected by the disorder mainly at small frequencies ( approximately 10 meV).

A cytokinetic study of small-intestinal and colonic mucosa after resection of 70% of the small intestine.

Pulse-labelling with tritiated thymidine and a fraction of labelled mitoses experiments have been performed in order to investigate the proliferative changes induced at various sites in the hyperplastic small-intestinal mucosa of rats previously subjected to resection of 70% of the small intestine. Proliferative activity in the colon was also studied. In the distal ileum there is a significant reduction in cell cycle time (Tc) of cells at all levels within the crypt and the growth fraction falls. In the jejunum and proximal ileum the crypts contain an increased number of proliferating cells, but as the size of the maturation zone is also increased, there is no significant alteration in the relative number of proliferating cells per crypt. Nor does the distribution of proliferating cells in these crypts seem to alter. There is no general reduction in Tc at these sites, but there does appear to be a significant reduction in Tc on the part of the cells in the stem-cell zone at the crypt base. In neither proximal nor distal colon was there any significant proliferative change apparent after small-intestinal resection.

The influence of 17 beta-estradiol on patterns of cell division in the uterus.

Uteri from immature (21-day-old) and adult mice show different patterns of cell division in response to a physiological dose of 17 beta-estradiol. In the immature mouse uterus, estradiol increased stromal and epithelial cell proliferation by shortening the cell generation time (Tc). The epithelial Tc was reduced from 100 to 14.5 h; the stromal Tc was reduced to 16 h. In both cell types, the reduction in Tc was primarily due to a reduction of the G1 phase of the cell cycle. In the adult mouse uterus, estradiol stimulated epithelial but not stromal cell proliferation. Here, estradiol reduced the epithelial Tc from 86 to 13 h, mostly by reducing the G1 phase of the cell cycle. Therefore, estrogens stimulate uterine hyperplasia by selectively decreasing the G1 phase of the cell cycle in specific cell populations. At some point during reproductive tract maturation, uterine stromal cells lose their ability to divide in response to estrogen stimulation. A developmental study showed that in the intact mouse, the stromal cell population gradually lost its ability to divide in response to estrogen stimulation during days 22-52 after birth. In prepubertally ovariectomized mice, the stromal cell population showed a very low mitotic response to estrogen stimulation at all ages. Thus, an ovarian mechanism may regulate the change in stromal responsiveness to estrogen stimulation.

Low temperature specific heats of H(NbIrO) and R(NbPtO)

The specific heats of the hexagonal structures H(NbIrO) and R(NbPtO) have been measured from1.2 to15 K. The R-phase compound is found to be superconducting withTc=3.8 K. The reduction inTc from10.1 to3.8 K between the two phases is qualitatively accounted for by corresponding reductions in the density of states and Debye temperature using the BCS theory.

Mössbauer Study of the Amorphous Ferromagnet 2826 MB

Mössbauer spectra have been recorded between 10 and 673 K. In the amorphous state, the spectrum displays a distribution in magnetic hyperfine field (B) as is generally found for amorphous ferromagnets. The probability distribution as a function of B is essentially temperature independent between 10 K and room temperature. Noticeable deviations occur for T > 320 K and the half width of the distribution reaches a maximum at 480 K and drops towards zero on approaching the magnetic ordering temperature (Tc) · AT3/2 dependence for the magnetic h.f. field has been found up to T ∼ 0.33Tc. It seems that the crystallization process can be described by an activation energy type of process. A reduction in Tc upon crystallization is found. Annealing at high temperatures results in a reorientation of the magnetization.

The effects of scattering on the CDW transition temperature

A quantitative study is described of the depression of TC(CDW) by scattering off long-wavelength acoustic phonons and random impurities. It is found that, by using parameters appropriate to 2H-NbSe2, large reductions in TC are obtained, for example from 330K to 65K.

The effect of neutron irradiation upon the superconducting critical temperature of some transition-metal carbides, nitrides, and carbonitrides

Samples of TiN, NbC, and Nb(CN) have been irradiated to various neutron doses in the Brookhaven High Flux Beam Reactor. Their superconducting critical temperatures are reduced at a rate which is much less than that for similarly irradiated high-Tc A15 superconductors. A dose of 1.5×1020 n/cm2 (E≳1 MeV) causes a reduction in Tc of 6–27%. This reduction may be explained in terms of the production of metal-lattice vacancies.

Green's Function Approach for High‐Temperature Ferromagnetic Systems in the Presence of Uniaxial Anisotropy

AbstractThe effect of uniaxial anisotropy on the transition temperature for a ferromagnetic spinone system is investigated using the double time Green's function technique. The calculations are carried out up to the second order in the exchange coupling and exactly in the anisotropy term. The localized and the static correlations used in the formulation are calculated selfconsistently. The calculation is compared with corresponding results for an ordered system, which predicts an effective reduction in Tc with anisotropy in high temperature crystalline magnetic systems.

Effect of Surface Charge on the Superconducting Transition Temperature and Normal-State Conductivity of Disordered Metal Films

Measurements were made of the shifts with added electrostatic charge of the normal-state residual conductivity and superconducting transition temperature of films of amorphous Bi and Ga, metastable crystalline Ga, and disordered crystalline Pb. For the amorphous Bi samples adding electrons increased both the superconducting transition temperature Tc and the residual contivity σ0. For the other materials adding electrons lowered both Tc and σ0. In each case removing electrons produced a shift of equal magnitude but of opposite sign to that observed for negative charging. For a given applied charge the shifts in Tc and σ0 were proportional to the reciprocal of the film thickness within the accuracy of the measurements. Results were reproducible from sample to sample. The correlation between the signs of the shifts Tc and σ0 for the metals investigated was the reverse of that observed previously for Sn, In, and Tl. Measurements were also made of the effect of condensed oxygen on the superconducting transition temperature of the films. Physically adsorbed oxygen lowered Tc for all of the materials. Increasing the temperature to the point that a reaction between the metal and the condensed oxygen began produced a further reduction in Tc for Pb and metastable crystalline Ga.

Exchange Scattering in Superconductors

The theory of superconductivity is discussed in view of previous theories and experiments. It was previously suggested that the depression of the superconducting transition temperatare of lanthanum as a function of rare earth impurity content may be traced to the exchange interaction between the conduction electron spins and the f-shell spins of the rare earth ions and conduction electrons interact via the ionic spins to modify the effective V. From present and previous works, it is concluded that the important course of the depression of Tc by exchange scattering is the disparity in the free-energy depressions of the normal and superconducting states. Shift V effects, whether due to changes in wave function or due to electron iateractions via virtual states, are small by comparison. The exchange energy between conduction electrons and ionic spins necessary to account for the observed reductions in Tc is 0.15 volt. (J.H.M.)