Scanning tunneling spectroscopy on oxidized surfaces of highly resistive quasicrystalline alloys

We report on a detailed investigation of the one-electron density of states on oxidized surfaces of highly resistive quasicrystalline alloys. Scanning tunneling spectroscopy measurements were performed at low temperatures (resolution around 5 meV) on icosahedral (i-)AlPdMn, i-AlCuFe, and i-AlPdRe. The tunneling spectra depend largely on the scanning tunneling microscope tip location on the oxidized surfaces. Beside spectra characteristic for the Coulomb blockade, we have identified a zero bias dip intrinsic to our quasicrystalline samples. This single feature increases as a square root of the bias voltage up to similar or equal to100 mV. A natural explanation comes from the combined effects of electron-electron interactions and disorder on the one-electron density of states. In this respect, highly resistive icosahedral alloys behave similar to disordered systems, for which such singularity has been predicted and widely observed. We have found no evidence for the quasicrystal specific energy dependence which was expected from anomalous diffusion laws predicted by quasiperiodic models. The predicted multiple gap structure has not been found either. We discuss which density of states is actually measured in tunneling spectroscopy to help clarify the controversial results obtained in this field.