International Journal of Modern Physics B, Condensed Matter Physics, Statistical Physics, Applied Physics vol:14 issue:29-31 pages:3747-3752
High temperature superconductors have an unusual temperature (T) - doping (x) phase diagram, in which both antiferromagnetism and superconductivity occur. The spin gap and the superconducting gap are the two essential features of the T(x) plane which evolve with doping. A material allowing investigating the T-x plane in detail, from the underdoped (x<0.15) to the overdoped (x>0.15) regime, is the La2-xSrxCuO4 superconductor. The temperature dependence of the resistivity strongly depends on the substitution ratio x. Overdoped films show metallic behavior, meaning that rho (T) is linear above 100 K, while it saturates for lower temperatures. This saturation persists below T,, when superconductivity is suppressed by sufficiently high pulsed magnetic fields. From the slightly overdoped (0.2 < x) down to the strongly underdoped regime (x <similar to> 0.03) we observe, instead of saturation, a rho (T) dependence with logarithmic divergence in the field-induced normal state at T --> 0. For underdoped films (x < 0.1) a superlinear pm behavior develops at intermediate temperatures (50 K < T < 200 K), indicating the opening of the spin gap.