Enhancement of superconductivity by a parallel magnetic field in 2-D superconductors

We report on the observation of significant enhancement of superconductivity by an applied \textit{parallel} magnetic field in two different 2-D superconducting systems: ultrathin, homogeneously disordered amorphous Pb films and the 2-D electron gas at the heteroepitaxial interface of 8 u.c. LaAlO$_{3}$ on TiO$_{2}$ terminated SrTiO$_{3}$. For both systems, we observe that the mean field T$_{c}$ is increased by a parallel magnetic field, while any perpendicular magnetic field results in a reduction of T$_{c}$. In the case of ultrathin $a$-Pb films, the magnitude of the T$_{c}$ enhancement is studied as the film thickness (zero-field T$_{c})$ is varied \textit{in situ}; the T$_{c}$ enhancement shows a strong non-monotonic dependence on the film thickness, peaking at 13{\%} in 8 T parallel field for a film with zero-field T$_{c}$ of 2.504 K. A comparison between the enhancement effect observed in ultrathin $a$-Pb films and the LaAlO$_{3}$/SrTiO$_{3}$ interface is presented. We will discuss the possible origin of the parallel magnetic field enhancement of superconductivity in these disparate systems.