VARIATIONS OF ELECTRONIC STATES IN LA2-XSRXCUO4 (X=O TO 0.30) INVESTIGATED BY SCANNING TUNNELING SPECTROSCOPY

Scanning-tunneling-spectroscopy measurements have been performed on as -grown and oxygen-annealed La2-xSrxCuO4 thin films with 0 less than or equal to x less than or equal to 0.30 and the variations of the densi ty of states (DOS) near the Fermi level (E(F)) have been systematicall y investigated. Each spectrum has a characteristic shape in each dopin g: semiconductor, superconductor, and metal. The shape of the DOS dire ctly correlates with the carrier concentration rather than Sr concentr ation or oxygen content. The DOS around the Fermi level gradually appe ars with tailing both from the conduction-band and the valence-band ed ges accompanied with the increase of carrier concentration. The change of DOS above E(F) is noticeable compared to that below E(F) within th e underdoped region.