Superconductors coherence length

Ito H, Watanabe M, Nogami Y, Ishiguro T, Komatsu T, Saito G, Hosoito N (1991) Magnetic determination of Ginzburg-Landau coherence length for organic superconductor /c-(BEDT-TTF)2X (X = Cu(NCS)2, Cu[N(CN)2]Br) effect of isotope substitution. J Phys Soc Jpn 60 ... 

The range of coherence follows naturally from the BCS theory, and we see now why it becomes short in alloys. The electron mean free path is much shorter in an alloy than in a pure metal, and electron scattering tends to break up the correlated pairs, so dial for very short mean free paths one would expect die coherence length to become comparable to the mean free path. Then the ratio k i/f (called the Ginzburg-Landau order parameter) becomes greater than unity, and the observed magnetic properties of alloy superconductors can be derived. The two kinds of superconductors, namely those with k < 1/-/(2T and those with k > l/,/(2j (the inequalities follow from the detailed theory) are called respectively type I and type II superconductors. 

We fabricated simultaneously a series of Nb-Au bilayers, with a fixed Nb thickness and a varying Au thickness, on a single (6 x 40 mm2) Si substrate. The Nb layer thickness was chosen to be significantly larger than the coherence length s in order to avoid any effect due to the finite thickness of the superconductor. For varying the Au thickness, another Si wafer was used as a mask and moved in situ above the Si substrate. After depositing the 120 nm Nb film, the Au film (from 10 to 260 nm) was deposited within 15 minutes at a pressure below 10-8 mbar. These conditions minimize the interface contamination and should preserve the best Nb-Au interface transparency. The full Si wafer with the bilayer films thus obtained (see Fig. 4a) was cleaved in air to separate the different samples. Individual Au (260 nm) and Nb (120 nm)... 

The coherence length in the organic superconductors is of the order of a lattice constant (as it is for the ceramic oxide superconductors). The organic... 

Table 2 Coherence length f, penetration depth X, and Ginzburg-Landau parameter k =Xj for various superconductors...

However, for the field applied parallel to the a direction the nearly linear behavior expected for normal 3D superconductors is not found but, instead, a concave shape of the Bcz curve. This might be due to the low dimensionality of the salts and a corresponding small coherence length in a direction perpendicular to the field. With the anisotropic GL theory [104, 105, 106, 107] the critical field Bc2 in the direction can be written as... 

The coherence length in the organic superconductors is of the order of a lattice constant (as it is for the ceramic oxide superconductors). The organic superconductors are of type II (they have two critical fields). The dimensionality is between 1 and 2 (it is 3-D for the fullerides). For organic superconductors, the isotope effect results are unclear it is not certain which phonon modes are important for the superconductivity. 

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