In-plane resistivity and an explanation for the characteristic T* in high-T-c cuprates

We offer an explanation for the observed crossover temperature T* in in-pla ne resistivity rho (ab) of biplanar high-T-c cuprates. The key to our pictu re is the existence of nondegenerate and degenerate carriers possessing dif ferent quasiparticle relaxation rates. In the underdoped regime the change of slope d rho (ab)/dT at T* results from the thermal activation of nondege nerate carriers. In the overdoped regime, the nondegenerate carriers tend t o become degenerate and a second small Fermi energy emerges, resulting in a change to T-2 behavior in rho (ab) at low T. We compare our results with d ata on several compounds. We also find an approximate scaling in conductivi ty.