Magnetoresistivity in strongly underdoped YBa2Cu3Ox single crystals

We report magnetoresistivity measurements on Y Ba2Cu3O6.36 Single crystals in applied magnetic fields H||c up to 14 T. We observe several unusual feat ures: the out-of-plane magnetoresistivity np,lp, is negative at high temper atures T and becomes positive for T < 150 K. The in-plane magnetoresistivit y <Delta>rho (ab)/rho (ab) is positive and displays a maximum at T approxim ate to 180 K. There is a direct correlation between the sign and T-dependen ce of magnetoresistivities and the corresponding temperature coefficients o f resistivities; i.e. Delta rho (c,ab) similar to d rho (c,ab)/dT over the measured T range for the in-plane component and for T > 150 K for the out-o f-plane component.