Lower critical field and superfluid density of highly underdoped YBa2Cu3O6+x single crystals -: art. no. 117001

The lower critical field H-c1 for highly underdoped YBa2Cu3O6+x with T-c between 8.9 and 22 K has been determined by measurements of magnetization M(H) curves with applied field parallel to the c axis. Hc1 is linear in temperatures below about 0.6T(c), and H-c1(0) is proportional to T-c(1.64) (+/- 0. 04) c, clearly violating the proportionality between p(s) (0) and T-c. Moreover, the slope - dH(c1)/dT decreases steeply toward zero as T-c approaches zero, indicating that the effective charge of the quasiparticles vanishes as the doping is decreased toward the insulating phase.