Tuning the Vortex Lattice Phase Diagram of MgB₂ through Doping

Small angle neutron scattering (SANS) on superconducting MgB₂ has revealed a complex vortex lattice (VL) phase diagram with three hexagonal phases: F, L, and I. A large degree of metastability has been observed in conjunction with supercooling or superheating across these phase boundaries. Remarkably, this metastability is neither a result of vortex pinning nor single domain free energy [C. Rastovski et al., Phys. Rev. Lett. 111, 107002 (2013)]. To better understand this behavior, we have modified the underlying vortex-vortex interaction by doping the crystal with either magnetic (Mn) or non-magnetic (C) atoms on the Mg and B sites, respectively. SANS on the Mn doped crystal revealed a phase diagram that remains largely the same as that for the pure crystal, but a decrease in the longitudinal VL correlation length was observed. Conversely, the C doped sample demonstrated a significant suppression of the L phase, while the correlation length remained comparable.