MAGNETIC AND CHARGE-TRANSPORT PROPERTIES IN UNDERDOPED YBA2CU3O6+X AND YBA2CU4O8

An explanation of the closely related temperature dependences of both charge and spin excitations in the CuO2 plane for underdoped YBCO syst ems is presented. The spin pseudogap and the anomalous temperature dep endences of the in-plane resistivity and the optical conductivity obse rved in these underdoped compounds are suggested to follow from the en ergy difference between the chemical potential and the energy of hole quasiparticles at the van Hove singularity (vHs). The numerical result s based on the quasiparticle spectrum calculated by Dagatto et al. [Ph ys. Rev. Lett. 74 (1995) 310] are in good agreement with experiments.