Inhomogeneous Nanoscale Disorder in Doped Magnesium Diboride*

Using TEM and STEM imaging and microanalysis, we have shown that doped MgB$_{2}$ has a variety of forms of disorder at a length scale of 5-50 nm. In [0001]-normal, carbon-doped HPCVD thin films, there is a thin, amorphous, C-rich layer which separates domains of lightly carbon-doped MgB$_{2}$. Carbon also causes significant spread in the local in-plane orientation and c-axis direction of the MgB$_{2}$. We also observed disorder in the orientation of small MgB$_{2}$ domains in oxygen-doped thin films grown by MBE. Both of types of films show dramatic enhancement in the upper critical magnetic field compared to pure MgB$_{2}$, with extrapolated $H_{c2}$(0 $K)$ reaching 65-70 T for some samples. The origin of this increase may be in the observed disorder, particularly in confinement of the superconducting MgB$_{2}$ domains by non-superconducting second phase layers.