We have obtained isofield curves for the square root of the average kinetic energy density of the superconducting state for three single crystals of underdoped $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$, an optimally doped single crystal of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$, and Nb. These curves, determined from isofield magnetization versus temperature measurements and the virial theorem of superconductivity, probe the order parameter amplitude near the upper critical field. The striking differences between the Nb and the high-$T_c$ curves clearly indicate for the latter cases the presence of a unique superconducting condensate below and above $T_c$.

• Received 13 August 2007

DOI:https://doi.org/10.1103/PhysRevB.76.132502