Bulk two-dimensional Pauli-limited superconductor

We present a nearly perfect Pauli-limited critical field phase diagram for the anisotropic organic superconductor \$\alpha$-(ET)$_2$NH$_4$(SCN)$_4$ when the applied magnetic field is oriented parallel to the conducting layers. The critical fields (${H_{c_2}$) were found by use of penetration depth measurements. Because ${H_{c_2}$ is Pauli-limited, the size of the superconducting energy gap can be calculated. The role of spin-orbit scattering and many-body effects play a role in explaining our measurements.