Superfluid density in the underdoped YBa2Cu3O7-x: evidence for d-density-wave order of the pseudogap.

The investigation of the penetration depth lambda(ab)(T,p) in YBa2Cu3O7-x crystals allowed one to observe the following features of the superfluid density n(s)(T,p) proportional, variant lambda(-2)(ab)(T,p) as a function of temperature T0) depends on p slightly in the optimally and moderately doped regions (0.