Properties and Defects of Type II Superconductors
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TIN/DECEMBER1991
In particular, the effects of crystal defects on the superconducting parameters Tc, H c2 and / c will be addressed. Critical Transition Température, Tc As reported in various studies,14 the Tc values differ significantly within one compound when the samples are processed by différent techniques. The wide range in Tc has been explained by Hanak et al. by the concept of long-range crystallographic order (LRO), which is related to the defect structure formed during processing. For an A-15 compound with an A 3 B-type atomic arrangement, the LRO parameter S = 1 corresponds to a perfectly ordered structure (i.e., ail A atoms on the chain sites and ail B atoms on cubic sites; see Figure 1). Extensive investigations indicate that any disorder and departure from stoichiometry (A3B) will decrease the Tc. For instance, in A-15 compounds, the
Tc was found to be only 4.4 K in splatquenched NbguSii&i (nonstoichiometric),6 while a much higher Tc of 18.6 K was obtained in Nb 7 5 Si 2 5 (stoichiometric) prepared by an explosive compression method.7 As crystal defects were induced in the A-15 structure by high-energy neutron bombardment, the Tc was also found to be considerably reduced.8 Matthias et al. 9 reported différent Tc values for slowly and rapidly solidified A-15 Nb 3 Ge. Both slow cooling and rapid quenching decrease the LRO parameter S, in turn decreasing the Tc. The Tc was found to be enhanced when low-temperature anneals were used to optimize both the stoichiometric composition and the degree of order. Gavaler reported that the Tc of sputtered Nb 3 Ge thin films reached 23 K after optimized annealing procédures.10 It is believed that the fundamental mechanism of Tc variation is closely connected with the three-dimensional A atom chains in the A-15 structure. Many expérimental results indicate that superconductivity is sensitively determined by thèse chain structures and that any interruption of the chains will either reduce the Tc or destroy superconductivity, depending on the degree of chain disorder. The origin of the effects of crystal disorder on transition température cannot be well understood until detailed electronic structures are correlated with the LRO parameter, S. According to the BCS theory,u the transition température can be expressed as Tc = 1.14(«>)exp[l/N(0)V], where (
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