Multiple Supercnducting Phases in Palladium Deuteride Induced by Nuclear-Spin Isotope Effect

We study the superconducting properties of high-quality PdD _x films. Similar to PdH _x films, a large residual resistivity remains after a sharp drop in resistivity at T _{c 1} ∼ 1.7 K. In contrast to PdH _x , the resistivity exhibits a distinct rectangular-shaped anomaly prior to the second transition to zero resistivity at T _{c 2} ∼ 0.6 K. The temperature and magnetic-field dependences of the residual resistivity demonstrate that this anomaly is intrinsic to PdD _x and reveal the existence of multiple phases within the superconducting state. These results indicate that the superconducting phase diagram of PdD _x differs significantly from that of PdH _x , highlighting an important role of nuclear spin (I = 1 for deuterons and I = 1/2 for protons).