The upper critical field in superconducting MgB 2

Abstract

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 ξ 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 medium 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)∝(1− T/ T c) 1+ α , where the parameter α specifies the positive curvature of H c2( T). This positive curvature of H c2( T) is similar to that found for the borocarbides YNi 2B 2C and LuNi 2B 2C.