Tuning the Superconducting Properties of MgB$_{2}$

The relatively high superconducting transition temperature of 39 K in MgB$_{2}$ has garnered much interest over the past several years in both fundamental and applied research. MgB$_{2}$ is a conventional phonon mediated BCS superconductor with the unconventional property of two superconducting gaps. These gaps ($\sigma $ and $\pi )$ arise from the coupling of boron phonons with two different orthogonal sheets of the Fermi surface. In a conventional single gap superconductor the upper critical field can be tuned by the introduction of nonmagnetic impurities. For MgB$_{2}$ the situation becomes more complex because there are 3 important scattering channels (inter and intra-band). Theoretical calculations predict different developments of the upper critical field and anisotropy ratio if the scattering can be selectively tuned to a specific channel. In this talk I will present data on two different types of perturbations to MgB$_{2}$: carbon doping and neutron irradiation. Low level carbon doping enhances the upper critical field with only a minor decrease in T$_{c}$. Whereas T$_{c}$ suppression is the result of carbon electron doping the system, the enhancement of H$_{c2}$(T=0) is the result of an increase in scattering, with evidence to suggest carbon doping enhances intra-$\pi $-band scattering. In contrast, H$_{c2}$(T=0) values tend to scale with T$_{c}$ in heavily neutron irradiated samples and the superconducting properties can be understood in terms of a decrease in the density of states at the Fermi surface and an increase in interband scattering.