Abstract
The temperature and field dependence of reversible magnetization has been measured on a YBa2Cu3O7-delta single crystal at six different doping concentrations. It is found that the data above 2 T can be described by a scaling law based on critical fluctuation theory in a lowest-Landau-level approach in Ginzburg-Landau theory yielding the values of the slope of upper critical field -dH(c2)(T)/dT near T-c. A universal scaling has been found in the five underdoped samples. Based on a simple Ginzburg-Landau approach, we determined the doping dependence of the upper critical field H-c2(0) and the coherence length xi. Our results may indicate a growing coherence length with increasing underdoping.