Spectroscopic evidence for unconventional superconductivity in UBe13

We report on measurements of the differential conductivity G of UBe13-Au co ntacts, which reveal the existence of low-energy Andreev surface bound stat es. These hound states are identified via huge conducranoe peaks at zero bi as that may form only in superconductors with nontrivial energy-gap functio ns. From the voltage dependence of G at T < T-c we also establish a lower l imit of the normalized energy gap, such 2 Delta(0)/k(B)T(c) > 6.7, much in excess of the weak coupling BCS value of 3.5, and directly indicating stron g coupling effects in superconducting UBe13.