GAP FORMATION IN KONDO INSULATOR FESI - POINT-CONTACT SPECTROSCOPY

The cubic compound FeSi is referred to as a nonmagnetic narrow-gap sem iconductor or Kondo insulator. Formation of a gap in the electronic ex citation spectrum has been studied by the point-contact tunneling spec troscopy down to 4.2K on the single-crystal grown by the Czochralski m ethod. The results are compared to the optical reflectivity measuremen ts also made between the helium and room temperatures over a large fre quency range from 25 to 5000 cm(-1) on the same crystal. Both techniqu es reveal a gap 2 Delta. In the tunneling conductance of point-contact junctions the gap-like anomalies indicate 2 Delta equal to about 50 m eV. The point-contact gap is filled up at about 60K. The optical spect ra show a significant reduction of the low-frequency reflectivity belo w 200K. Onset of the fall of reflectivity lies at 700 cm(-1). A magnit ude of the optical gap is about 50-90 meV.