Topological instability of two-dimensional conductors

In the present paper we propose a mechanism of the structural instability with a periodic charge ordering in two-dimensional isotropic conductors with a closed Fermi surface which completely excludes the conventional nesting mechanism. We show that the structural instability in such conductors may arise as a topological reconstruction under which the initially closed Fermi surface is transformed into an open one. We have found that the order parameter of the charge ordering ground state may exceed one hundredth of the Fermi energy. Furthermore, this charge ordering is a quantum phase transition with respect to the dimensionless coupling constant $\lambda$ related to the mechanism that drives the band reconstruction (e. g. electron-phonon coupling), with the critical value given by $\lambda_c= (1+2/\pi)^{-1}$. Preliminary estimations show that the suggested mechanism can be the origin of density waves observed in such materials as high$-T_c$ cuprates or graphite intercalates.