Scanning tunneling spectroscopy of optimally doped and underdoped YBa2Cu3Ox thin films

Recently, experimental and theoretical evidence was given that grain boundaries in cuprate high-temperature superconductors are charge carrier depleted zones. In order to examine, if a charge carrier reduction in YBa2Cu3Ox (YBCO) thin films is detectable by scanning tunneling spectroscopy (STS), we extensively studied optimally doped and underdoped (charge carrier reduced) YBCO films on SrTiO3 substrates. The current-voltage (I-U) characteristics of these pulsed laser deposited YBCO films were recorded by means of STS at growth island plateaus in an ultra-high vacuum (UHV). The outcome of our experiments shows that dI/dU spectra of optimally doped and underdoped films differ significantly. Since even in UHV the YBCO surface degrades, the influence of a degraded surface layer on the tunnel process is studied. It is discussed, to what extent the dI/dU spectra and the changes on doping, respectively, are related to the local density of states of the films