Nonstoichiometry and critical temperature of MgB2

The influence of nonstoichiometry of the new high-temperature superconducto r MgB2 on its critical temperature was studied by the direct magnetooptical observations of the penetration and trapping of magnetic flux. To preclude the possible influence of accidental factors, a special sample with transi tion from pure boron to the MgB2 with an excess of Mg was synthesized. In a narrow region near the unreacted boron, the magnetic-field trapping and sc reening disappear at a temperature 1.5 K higher than in the dominant stoich iometric region of the sample. (C) 2001 MAIK "Nauka/Interperiodica".