  Charge-density-wave (CDW) phase transition in a ring-shaped crystals,
recently synthesized by Tanda et al. [Nature, 417, 397 (2002)], is studied
based on a mean-field-approximation of Ginzburg-Landau free energy. It is shown
that in a ring-shaped crystals CDW undergoes frustration due to the curvature
(bending) of the ring (geometrical frustration) and, thus, forms a mixed state
analogous to what a type-II superconductor forms under a magnetic field. We
discuss the nature of the phase transition in the ring-CDW in relation to
recent experiments.
