Degeneracy and Inversion of Band Structure for Wigner Crystals on a Toroidal Helix

TL;DR

This paper investigates the behavior of Wigner crystals on a toroidal helix, revealing bifurcations, vibrational spectrum degeneracies, and band structure inversion as the helix radius varies, with implications for localized excitations.

Contribution

It demonstrates the occurrence of degeneracy and band inversion in Wigner crystals on a toroidal helix, highlighting novel vibrational properties and structural bifurcations.

Findings

Ground state undergoes a pitchfork bifurcation with increasing radius.

Vibrational spectrum collapses to a single frequency below a critical radius.

Band structure inverts beyond the degeneracy point, altering mode frequencies.

Abstract

We explore the formation of Wigner crystals for charged particles on a toroidal helix. Focusing on certain commensurate cases we show that the ground state undergoes a pitchfork bifurcation from the totally symmetric polygonic to a zig-zag-like configuration with increasing radius of the helix. Remarkably, we find that for a specific value of the helix radius, below the bifurcation point, the vibrational frequency spectrum collapses to a single frequency. This allows for an essentially independent small-amplitude motion of the individual particles and consequently localized excitations can propagate in time without significant spreading. Increasing the radius beyond the degeneracy point, the band structure is inverted, with the out-of-phase oscillation mode becoming lower in frequency than the mode corresponding to the center of mass motion.