Superconducting transition width under magnetic field in MgB2 polycrystalline samples

A systematic study on the superconducting transition width as a function of the applied magnetic field was performed in polycrystalline MgB2. A quantitative, yet universal relation between the two parameters was observed in all of the ceramics. It was found that the width decreases linearly with decreasing field in pure MgB2 samples. Whereas, samples with boron and/or Mg atoms partially replaced by other elements show this linear relation in the temperature range below 0.7–0.8 of the superconducting transition temperature (corresponding to a field of about 2 T), at temperatures higher than this range, an abnormal upturn in the width was found. This upturn is ascribed to multiple superconducting transitions. A core-shell model is proposed to describe the multiple transitions. © 2010 American Institute of Physics. Link to publishers version here