Surprising size of the pseudogap
The energy gap determined by specific heat measurements (green), ARPES (blue) and tunnelling experiments (red) on YBCO (yttrium barium copper oxide) and BSCCO (bismuth strontium calcium copper oxide) samples. The gap is substantial: it is about 20 meV at optimal doping and extrapolates to about 100 meV at zero doping. The values measured by ARPES and tunnelling experiments agree with each other but seem higher than the specific heat values, which suggests that the former two techniques must also measure some multiple of the superconducting energy gap. The dashed line is 5kBTc(x). (J W Loram et al. 1997 Physica C 282287 1405).