Lower critical field and superfluid density of highly underdoped YBa2Cu3O6+x single crystals.

The lower critical field H(c1) for highly underdoped YBa2Cu3O(6+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. H(c1) is linear in temperatures below about 0.6T(c), and H(c1)(0) is proportional to T(1.64+/-0.04)(c), clearly violating the proportionality between rho(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.