The upper critical field in superconducting MgB 2

. The upper critical field H c2 ( T ) of sintered pellets of the recently discovered MgB 2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between T c ~ 39 K and T = 3 K ~ 0.1 T 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 (0) = 13 T at T = 0 corresponding to a coherence length of x o = 5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, H c2 ( T ), shows a positive curvature near T c and at intermediate temperatures indicating that MgB 2 is in the clean limit. The H c2 ( T ) dependence can be described within a broad temperature region 0.3 T c < T £ T c by a simple empirical expression H c2 ( T ) (cid:181) (1- T / T c ) 1 + a , where the parameter a specifies the positive curvature of H c2 ( T ). This positive curvature of H c2 ( T ) is similar to that found for the borocarbides YNi 2 B 2 C and LuNi 2 B 2 C.