Nonstoichiometry and critical temperature of MgB$_2$

The influence of nonstoichiometry of the new high-temperature superconductor MgB$_2$ 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 transition from pure boron to the MgB$_2$ with an excess of Mg was synthesized. In a narrow region near the unreacted boron, the magnetic-field trapping and screening disappear at a temperature $1.5$ K higher than in the dominant stoichiometric region of the sample.