Spectroscopic evidence for unconventional superconductivity in UBe13
We report on measurements of the differential conductivity G of UBe13-Au contacts, which reveal the existence of low-energy Andreev surface bound states. These bound states are identified via huge conductance peaks at zero bias that may form only in superconductors with nontrivial energy-gap functions. From the voltage dependence of G at T<T(c) we also establish a lower limit of the normalized energy gap, such that 2Delta(0) / k(B)T(c)>6.7, much in excess of the weak coupling BCS value of 3.5, and directly indicating strong coupling effects in superconducting UBe13.
Walti
C
Laboratorium fur Festkorperphysik, ETH-Honggerberg, 8093 Zurich, Switzerland.
Ott
HR
Fisk
Z
Smith
JL