Bismuth Strontium Calcium Copper Oxide - Properties

Properties

BSCCO needs to be hole-doped by an excess of oxygen atoms (δ in the formula) in order to superconduct. As in all high-temperature superconductors (HTS) T_c is sensitive to the exact doping level: the maximum Tc for Bi-2212 (as for most HTS) is achieved with an excess of about 0.16 holes per Cu atom. This is referred to as optimum doping. Samples with lower doping (and hence lower T_c) are generally referred to as underdoped while those with excess doping (also lower T_c) are overdoped. By changing the oxygen content T_c can thus be altered at will. By many measures, overdoped HTS are strong superconductors, even if their T_c is less than optimum, but underdoped HTS become extremely weak. The application of external pressure generally raises T_c in underdoped samples to values that well exceed the maximum at ambient pressure. This is not fully understood though a secondary effect is that pressure increases the doping. Bi-2223 is complicated in that it has three distinct copper-oxygen planes. The two outer copper-oxygen layers are typically close to optimal doping while the remaining inner layer is markedly underdoped. Thus the application of pressure in Bi-2223 results in Tc rising to a maximum of about 123 K due to optimisation of the two outer planes. Following an extended decline, Tc then rises again towards 140 K due to optimisation of the inner plane. A key challenge therefore is to determine how to optimise all copper-oxygen layers simultaneously. Considerable improvements in superconducting properties could yet be achieved using such strategies.

BSCCO is a Type II superconductor. The upper critical field, H_c2, in Bi-2212 polycrystalline samples at 4.2 K has been measured as 200±25 T (cf 168±26 T for YBCO polycrystalline samples). In practise HTS are limited by the irreversibility field, H*, above which magnetic vortices melt or decouple. Even though BSCCO has a higher upper critical field than YBCO it has a much lower H* (typically smaller by a factor of 100) thus limiting its use for making high-field magnets. It is for this reason that conductors of YBCO are preferred to BSCCO though they are much more difficult to fabricate.