Surface superconducting states in a polycrystalline MgB$_{2}$ sample
  We report results of dc magnetic and ac linear low-frequency study of a polycrystalline MgB$_2$ sample. AC susceptibility measurements at low frequencies, performed under dc fields parallel to the sample surface, provide a clear evidence for surface superconducting states in MgB$_2$. 
Menachem I. Tsindlekht, Grigory I. Leviev, Valery M. Genkin, Israel   Felner, Pavlo Mikheenko, and J. Stuart Abell