On the Role of Fermi Energy in Determining Properties of Superconductors: a Detailed Comparative Study of Two Elemental Superconductors (Sn and Pb), a Non-cuprate (MgB 2 ) and Three Cuprates (YBCO, Bi-2212 and Tl-2212)

摘要

Fermi energies (E (F)s) of high- T (c) superconductors (SCs) have of late been evincing considerable interest because they are believed to be the cause of their high T (c)s and gap structures. Since Bardeen-Cooper-Schrieffer (BCS) equations for elemental and generalized-BCS equations for non-elemental SCs are derived under the blanket of the assumption E (F)/ k theta > > 1 (k = Boltzmann constant, theta = Debye temperature), they cannot shed light on the E (F)s of these SCs. This fact leads us to address the gaps (Delta(0)s) and T (c)s of both types of SCs via recently derived equations which incorporate E (F) as a variable. For the specification of the E (F) of any SC, we now need another of its properties. Choosing j (0), the critical current density of the SC at T = 0, and following an idea due to Pines, we present for both types of SCs new equations for j (0) that depend solely on the following properties of the SC: E (F), theta, gram-atomic volume, electronic specific heat constant and a dimensionless construct where m* is the effective mass of superconducting electrons and P (0) their critical momentum. Appeal to the experimental values of Delta(0), T (c) and j (0) of any SC then not only leads to values of E (F), m* and P (0) but also provides plausible clues about how its j (0)-and therefore T (c)-may be increased.