Nanoscale disorder in high critical field, carbon-doped MgB₂ hybrid physical-chemical vapor deposition thin films

Carbon-doped Mg B2 thin films deposited by hybrid physical-chemical vapor deposition (HPCVD) can have Hc2 (0 K) ∼70 T, anomalously high compared to bulk samples which have Hc2 about half this value. Using transmission electron microscopy, the authors show that the carbon content in HPCVD-grown Mg (B1-x Cx) 2 grains is x≤5%, and that most carbon forms an amorphous intergranular phase. The authors also find extensive structural disorder, including rotations about the c axis and small tilts of the c axis, with a domain size of 5-20 nm. The authors propose that strong electron scattering from the nanoscale disorder is responsible for the very high Hc2 in such films.