TUNNELING SPECTROSCOPY ON THE HEAVY-FERMION SUPERCONDUCTORS UPD2AL3 AND UNI2AL3 IN THE NORMAL-STATE

Tunnelling measurements were performed on the new heavy-fermion superconductors UPd2Al3 (single crystal, T(c) = 1.8 K) and UNi2Al3 (polycrystal, T(c) = 1.2 K) and on the magnetic heavy-fermion superconductor URu2Si2 above and below the antiferromagnetic-ordering temperatures T(N) for T greater-than-or-equal-to T(c). Tunnelling along the (a, b)-planes on UPd2Al3 shows that below T(N) almost-equal-to 14 K an energy gap of about 13 meV is formed in the density of states. In contrast, no gap is found along the c-direction. For polycrystalline UNi2Al3 a gap of 10 meV is found below T(N) almost-equal-to 4.8 K. Both the observed anisostropy and the values of the gaps of the new compounds prove surprisingly similar to URu2Si2. The values of the gaps appear to be set by crystal field excitation energies.