Collective quantum tunneling of strongly correlated electrons in commensurate mesoscopic rings

We present a new effect that is possible for strongly correlated electrons in commensurate mesoscopic rings: the collective tunneling of electrons bet ween classically equivalent configurations, corresponding. to ordered state s possessing charge and spin density waves (CDW, SDW) and charge separation (CS). Within an extended Hubbar model at half filling studied Ly exact num erical diagonalization, we demonstrate that the ground state phase diagram comprises, besides conventional critical lines separating states characteri zed by different orderings (e.g. CDW, SDW, CS), critical lines separating p hases with the same ordering (e.g. CDW-CDW) but with different symmetries, while the former also exist in infinite systems, the latter are specific fo r mesoscopic systems and directly related to a collective tunnel effect. We emphasize that, in order to construct correctly a phase diagram for mesosc opic rings, tile examination of CDW, SDW anti CS correlation functions alon e is not sufficient, and one should also consider the symmetry of the wave function that cannot Le broken. We present examples demonstrating that tile jumps in relevant physical properties at the conventional and new; critica l lines are of comparable magnitude. These transitions could be studied exp erimentally e.g. by optical absorption in mesoscopic systems. Possible cand idates are cyclic molecules and ring-like nanostructures of quantum dots.