Giant enhancement of superconductivity in ultrathin $a$-Pb films by a parallel magnetic field: effect of magnetic impurity

An ultrathin superconductor containing paramagnetic impurities is predicted$^{1,2}$ to exhibit \textit{enhancement} of superconductivity ($T_{c}$ and $I_{C})$ when subject to a parallel magnetic field. We have recently observed a pronounced enhancement of superconductivity in ultrathin homogeneous amorphous Pb films without any (intentionally added) magnetic impurities in the presence of a parallel magnetic field; the $T_{c}$ enhancement is as large as 13{\%} and persists in field as high as 8 T. Our experiments are carried out in a modified dilution refrigerator capable of \textit{in situ} film growth, sample rotation, and incremental deposition of magnetic (Cr) impurities, which allows for a systematic, unambiguous elucidation of the effect of paramagnetic impurities on the field-enhancement of superconductivity. With increasing Cr density on a Pb film, the magnitude of the $T_{c}$ enhancement is progressively suppressed, contrary to the theoretical predictions. $^{1}$ Kharitonov \textit{et al}., JETP Lett. 82, 473 (2005). $^{2}$ Wei \textit{et al}., Europhys. Lett. 75, 943 (2006).