Lower critical field

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. 

The parameters for /t-(ET)2Cu(NCS)2 in Table 2.2 are obtained from recent magnetization data in higher fields for Bc2 [189] and lower fields for Bel [203]. In an earlier experiment a lower critical field Bd = 16 mT was extracted in a somewhat different method from the magnetization curves [204]. In a numerical approximation demagnetization effects have been tried taking into account Bd as the fitting parameter [205]. This resulted in a... 

From the specific-heat data the thermodynamical critical field Beth can be estimated [102, 194]. For K-(ET)2l3 this results in Beth 17mT. Be can also be calculated from the upper and lower critical fields via... 

Magnetization measurements on the bulk powders of K3C60 allowed the determination of additional parameters of the superconducting state. Upper and lower critical fields were analyzed to determine a penetration depth of A = 2400 A, and a superconducting coherence length of =... 

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. 

Figure 13.30 The variation of the superconducting properties with the external magnetic field (a) a type-I superconductor HdO) is the critical field at OK (b) a t5fpe-II superconductor HdiO) and i ci(0) are, respectively, the upper and lower critical fields at 0 K...

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