Lower critical field, temperature

The alloy niobium titanium (NbTi) and the intermetaUic compound of niobium and tin (Nb.3 Sn) are the most technologically advanced LTS materials presently available. Even though NbTi has a lower critical field and critical current density, it is often selected because its metallurgical properties favor convenient wire fabrication. In contrast, Nb.3Sn is a veiy brittle material and requires wire fabrication under very well-defined temperature conditions. 

In Fig. 3 we have plotted the temperature dependence of the upper and lower critical fields, determined as described earlier. For the upper critical field we observe that, except in the vicinity of the critical temperature... 

The temperature dependence of the lower critical field follows within a few percent the empirical law [8]... 

From the fitting we extrapolate the lower critical field at zero temperature //fi(0) —132iiS Oe and using the formula [8l... 

FIG. 3. (a) The temperature dependence of the lower critical field //, ifT) determined on a powder sample as shown in the inset and explained in the text. Error bars represent estimated uncertainty in determining H . The solid line represents the empirical law //, i(7 )/W, i(0) 1 —(T/T, ). (b) The temperature dependence of the upper critical field H, i(T). Inset The method of determining these values from M(T) curves at fixed field. The error bars in the main plot reflect the round-... 

We have analyzed the thermodynamic, magnetic and ultrasound attenuation data on oriented saaiples of the hlgh-T superconductors within the context of anisotropic Glnzburg-LSndau theory for coupled, even-parity superconducting states. We are able to present a consistent Interpretation of the data In terms of the coexistence of a quasi-two-dimensional d-wave state, with critical temperature T. - T and a more Isotropic mixed (s+d)-wave state with critical tempertaure T < T We predict the possibility of a "kink" in the temperature dependence of the lower critical field near 0 9T, which should be tested by experiments on single crystals. 

Both PNIPAAm and PVME exhibit unique thermo-shrinking properties. Thus, as an aqueous solution is heated beyond a certain point, the polymer shrinks and a phase separation occurs. This temperature is commonly referred to as the lower critical solution temperature (LCST). For PNIPAAm, it lies between ca. 30 and 35°C, the exact temperature being a function of the detailed microstructure of the macromolecule. Below LCST, the polymer is soluble in the aqueous phase, as the chains are extended and surrounded by water molecules. Above the LCST, the polymer becomes insoluble and phase separation occurs. Because of the abrupt nature of these transitions and their reversibility (which allows repeated thermal switching) these polymers have stirred up particular interest in the field of science and engineering since their first appearance in the open literature in 1956. 

Interestingly enough, this change of slope is also found (TaUlefer 1990) in the lower critical field curve shown in fig. 84, which confirms the picture of different superconducting phases. The low-field, high-temperature phase A is extremely sensitive to pressure the change of slope in disappears under an external... 

A discontinuous increase in the slope of the lower critical field, B j(T), along with the absence of any change in the upper critical field Bc2( ) the lower transition temperature (figs 89a, b) indicates an increase in the superconducting condensation energy (Rauchschwalbe et al. 1987c). These data thus suggest... 

Table 4.2-24 Superconducting properties (transition temperature Tc, coherence length, penetration depth k, GL coefficient tc, lower critical field Hd, upper critical field Hc2) for YBCO [2.54]...

S. Riedling, J. Franz, T. Ishida, A. Erb, G. Muller-Vogt, H. Wiihl Phase separation in YBa2Cu307 5 single crystals near 5 = 0, Physica C 200, 271-276 (1992) D.-H. Wu, S. Sridhar Pinning forces and lower critical fields in YBa2Cu30y crystals Temperature dependence and anisotropy, Phys. Rev. Lett. 65, 2074-2077 (1990)... 

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