TUNNEL SPECTROSCOPY INTO THE YBA2CU4O8 COMPOUND

We have realized planar tunnel junctions using both Y123 and Y124 single crystals as base electrodes and Pb as counterelectrode. The crystals, with T-c = = 90 K (Y123) and T-c = 80 K (Y124), have been chemically etched and natural barriers were formed by a short air exposure. We have compared the tunneling spectra obtained on both systems. The features at about +/-18 mV of the Y123 compound are still observed in the Y124, while the low-bias structures at +/- 5mV are not present in the tunneling characteristics of the Y124 system. Linearity of the background conductance is also observed, with less steep slopes in Y124 that might indicate a reduced interlayer coupling in this system.