Scanning tunneling spectroscopy characterization of the pseudogap and the x= 1 8 anomaly in La2-x Srx Cu O4 thin films

Using scanning tunneling spectroscopy we examined the local density of states of thin c -axis La2-x Srx Cu O4 films, over wide doping and temperature ranges. We found that the pseudogap exists only at doping levels lower than optimal. For x=0.12, close to the "anomalous" x= 1 8 doping level, a zero-bias conductance peak was the dominant spectral feature, instead of the excepted V-shaped (c -axis tunneling) gap structure. We have established that this surprising effect cannot be explained by tunneling into (110) facets. Possible origins for this unique behavior are discussed.