Superconductors, high coherence length

The first step towards the fabrication of thin-film devices is the growth of a single-layer film. The anisotropic layered structure and short superconducting coherence lengths of most high Tc superconductors introduce aspects of film growth that need not to be considered for conventional low-temperature superconductors. 

Electrical properties of junctions formed between superconducting material, S, and a non-superconducting metallic material, N, which may be a metal or a degenerate semiconductor, are determined by special boundary conditions. If we consider a superconductor-semiconductor (S-N) interface with high transparency, a proximity effect is observed due to injection of electron pairs (Cooper pairs) from the superconductor into the semiconductor where they decay over a characteristic length, the induced coherence length. 

Since oxide high-temperature superconductors have a strong crystalline and electronic anisotropy, the materials must be synthesized in the form of oriented, virtually single crystal films. Furthermore, due to the extremely short coherence length of those materials, the quality of the films must be well controlled at the surface or interface as well as inside the film for device applications. The critical current density is one of the indications of the quality and the highestT obtained so far is >5 x 10 A/cm at 77K in OT. 

In contrast to conventional superconductors, thermal fluctuations of the vortex positions become very important in high-Tc materials, in particular close to Hc2(T). This is a consequence of both the small coherence length and the fact that the pinning centers in the cuprates are mainly provided by point defects. 

Upper Critical Field. The upper critical fields Hc2 in high-7[ superconductors give information about microscopic parameters, e.g., the coherence length and their anisotropies in the superconducting state. The temperature dependence of the upper critical field Hci(T) for H c and // c measured on a polydomain sample [2.67] is given in Fig. 4.2-44. A linear dependence is observed for both directions with slopes of the critical field of —1.9 and — 10.5TK for // c and H c, respectively. The same dependence measured by the resistivity method is shown by the dashed lines. Corresponding to these measurements one can calculate 77 (7 = 0K) = 122T, H (T = OK) = 674 T, 4 = 3.0 A, and = 16.4 A. 

Therefore, high currents can flow across the grain boundaries of bulk polycrystalline MgB2. This is due to the relatively large coherence length of this superconductor (Table 4.2-31). On the other hand, this large coherence length is responsible for the relatively low values of the upper critical field Hc2 observed,... 

Finally, spintronics represents a new field of research, development, and appUca-tions that utilizes electron spins for high-control quantum electronics. Recently, the depairing of Cooper pairs in low-T superconductors such as A1 metal was observed experimentally in superconductor-ferromagnet hybrid structures (Beckmann et al, 2004). As the paired state can survive within a characteristic distance when soaked from the superconductor into normal metal wires, electrons with oppositely oriented spins can be sampled in adequately magnetized wires, provided that the wire separation is within, but close to, the coherence length of the Cooper pairs. 

Type 11 superconductors are normally limited by the Ginzburg-Landau ratio k = X/ > l/ /l. One special attribute of a high-temperature superconductor (HTS) materials is its small coherence length in comparison with a low-temperature superconductor (LTS), and its anisotropy due to the two-dimensional (2-D) character of the layered crystal structures. The coherence length is on the order of unit cell dimensions, where For example, yttrium barium copper oxide (YBCO)... 

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