Normal-metal-superconducting transition

The theoretical picture for the N-SmA transition has been succinctly summarized by Lubensky. [28] He explains that there are several important differences between the normal-metal-superconducting and the N-S mA transitions where AT, the splay elastic constant, emerges as a dangerous irrelevant parameter [28]. 

The idea that new phenomena could be present in 3He at very low temperatures arose from thermal measurements. The first observation was the anomaly in the specific heat at the normal superfluid transition which reminded the behaviour of specific heat at the superconductive transition in metals (Fig. 2.11) [34-36]. 

The normal spinel Li[Ti2]04 is a metallic oxide with a superconducting transition temperature of 13.7 K. The nominal formula is Li+[Ti3+Ti4+]04, in which the Li+ ions occupy the tetrahedral sites while the octahedral sites contain titanium with an average charge Ti3 5+, although as the material is metallic at room temperature the electrons are delocalized in a partly filled 3d band. 

Conductor-Superconductor Transition When some metals or compounds are cooled below a certain temperature, their electrical resistance drops abruptly to zero. This temperature is referred to as the superconducting transition temperature. These materials are classified into two categories, type I or type II superconductors, depending upon how a bulk sample behaves in an external magnetic field. In the absence of an external magnetic field, the (superconductor + normal) transition is continuous in both types of superconductors. When a magnetic field is applied, the transition becomes first order in type I superconductors, but remains continuous in the type II superconductors. 

An interesting series of bismuth cuprates in terms of the variation of the Tc as well as the hole concentration with composition is provided by Bi2Sr2Ca1 a.Lna.Cu208 where Ln = Y or rare earth (Rao et al. 19906). The electrical resistivity data show a metal-insulator transition in the normal state with change in x (figure 12). The Ta as well as the nb show a maximum at a composition of x = 0.25 (figure 13). Note that when Ca is fully substituted by Ln, the material becomes a non-superconducting insulator. Hole concentration in these bismuth cuprates is readily determined by Fen-Fem redox titrations. 

Figure 12. Resistivity data of superconducting Bi2Srs,Ca1 INd l Cu208 showing the occurrence of a metal-insulator transition in the normal state. (From Rao et al. 19906.)...

Figure 12 Superconducting transition on temperatures (normalized relative to maximmn Tg) as a function of stoichiometry for various transition metal nitrides. For 5-H fNi j , different functions have been reported (a,h). (Ref. 64. Reproduced by permission of Wiley)...

The problematic nature of the melting transition can be illustrated by comparison with other well-known first-order phase transitions, for instance the normal metal-(low T ) superconductor transition. The normal metal-superconductor and melting transitions have similar symptomatic definitions, the former being a loss of resistance to current flow, and the latter being a loss of resistance to shear. However, superconductivity can also be neatly described as a phonon-mediated (Cooper) pairing of electrons and condensation of Cooper pairs into a coherent ground state wave function. This mechanistic description of the normal metal-super-conductor transition has required considerable theoretical effort for its development, but nevertheless boils down to a simple statement, indicat-... 

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