lower Tc) are overdoped. By changing the oxygen content, Tc can thus be altered at will. By many measures, overdoped HTS are strong superconductors, even if their Tc is less than optimal, but underdoped HTS become extremely weak. The application of external pressure generally raises Tc 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 optimization of the two outer planes. Following an extended decline, Tc then rises again towards 140 K due to optimization of the inner plane. A key challenge therefore is to determine how to optimize all copper-oxygen layers simultaneously. BSCCO is a Type-II superconductor. The upper critical field Hc2 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 practice, 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. See also Yttrium barium copper oxide Thallium barium calcium copper oxide References External links Research fabrication of 2212 on MgO in 1993 (pdf) Local Electronic Structure of Bi2Sr2CaCu2O8 near Oxygen Dopants ... Bismuth compounds Strontium compounds Calcium compounds High-temperature superconductors Ceramic materials Copper(II) compounds Oxides