Abstract
The vortex lattice (VL) symmetry and orientation in clean type-II superconductors depends sensitively
on the host material anisotropy, vortex density and temperature, frequently leading to rich phase diagrams.
Typically, a well-ordered VL is taken to imply a ground-state configuration for the vortex-vortex
interaction. Using neutron scattering we studied the VL in MgB2 for a number of field-temperature
histories, discovering an unprecedented degree of metastability in connection with a known, second-order
rotation transition. This allows, for the first time, structural studies of a well-ordered, nonequilibrium VL.
While the mechanism responsible for the longevity of the metastable states is not resolved, we speculate it
is due to a jamming of VL domains, preventing a rotation to the ground-state orientation.
