Metastable Vortex Lattice Phases in MgB$_{2}$

We present results of recent small-angle neutron scattering (SANS) measurements in MgB$_{2}$ which reveal an unprecedented degree of vortex lattice (VL) metastability. Three different VL phases are observed, all of which have a triangular symmetry. At low fields the VL is aligned with the crystalline $a$-axis. At intermediate fields the VL rotates away from the $a$-axis, leading to two degenerate domain orientations. Once the rotation reaches 30\r{ } a single domain, high field VL is reformed, now aligned along the a*-axis. These three VL orientations are consistent with our earlier reports [R. Cubitt \textit{et al}., PRL \textbf{91}, 047002 (2003)]. Metastable configurations are obtained when crossing the equilibrium VL transition lines by cooling or heating in a constant field. At any given field and temperature the equilibrium VL can be obtained either by inducing vortex motion by a small amplitude field oscillation or by a zero field cooling (ZFC) procedure. The equilibrium VL phase diagram differ significantly from our earlier published report and from theoretical predictions [M.E. Zhitomirsky \textit{et al}., PRB \textbf{69}, 054508 (2004)]. The implications of these results and how they affect our understanding of two-gap superconductivity in MgB$_{2}$ will be discussed.