The upper critical field in superconducting MgB2

The upper critical held H-c2(T) of sintered pellets of the recently discovered MgB2 superconductor was investigated by transport, ac susceptibility and de magnetization measurements in magnetic fields up to 16 T covering a temperature range between T(c)similar to 39 K and T=3 K similar to0.1T(c). The H-c2 data from ac susceptibility are consistent with resistance data and represent the upper critical field of the major fraction of the investigated sample which increases up to H-c2(O)=13 T at T=0 corresponding to a coherence length of xi (0)=5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by de magnetization measurements. The temperature dependence of the upper critical field, H-c2(T), shows a positive curvature near T-c and at medium temperatures indicating that MgB2 is in the clean limit. The H-c2(T) dependence can be described within a broad temperature region 0.3T(c) < T less than or equal to T-c by a simple empirical expression H-c2(T)(proportional to)(1 - T/T-c)(1+alpha), where the parameter ct specifies the positive curvature of H-c2(T). This positive curvature of H,,(T) is similar to that found for the borocarbides YNi2B2C and LuNi2B2C. (C) 2001 Elsevier Science B.V. All rights reserved.