Crystallographic imperfections and exotic superconductivity of UBe 13

The symmetry of the superconducting gap is related to the symmetry of the crystal structure. In unconventional superconductors, big changes in the critical temperature, critical field, or even the gap structure can happen even for small perturbations of the lattice. In this paper, we use microstructuring to study aluminum-free micro crystals of UBe13 which are expected to be closer to a "perfect" material than previously studied aluminum-grown single crystals. We compare the effect of minuscule imperfections on the value of the critical temperature and critical field, which, in the case of UBe13, has drastic effects, supporting its unconventional nature. We conjecture that this likely arises from the oxidation state of uranium, which is sensitive to its crystallographic environment. Our findings suggest that uranium-based materials provide not only an excellent reservoir of unconventional phenomena, but also a way to identify the gap symmetry of unconventional superconductors.