arXiv:cond-mat/0111162 · Physical review. B, Condensed matter
Incommensurate charge-density waves in the adiabatic Hubbard-Holstein model
Vieri Mastropietro
Abstract
The adiabatic Holstein-Hubbard model describes electrons on a chain with step a interacting with themselves (with coupling U) and with a classical phonon field ${\ensuremath{\varphi}}_{x}$ (with coupling \ensuremath{\lambda}). There is Peierls instability if the electronic ground-state energy $F(\ensuremath{\varphi})$ as a functional of ${\ensuremath{\varphi}}_{x}$ has a minimum which corresponds to a periodic function with period $\ensuremath{\pi}{/p}_{F},$ where ${p}_{F}$ is the Fermi momentum. We consider ${p}_{F}/\ensuremath{\pi}a$ irrational so that the charge-density wave is incommensurate with the chain. We prove in a rigorous way in the spinless case, when \ensuremath{\lambda}, U are small and $U/\ensuremath{\lambda}$ large, that (a) when the electronic interaction is attractive $U<0$ there is no Peierls instability and (b) when the interaction is repulsive $U>0$ there is Peierls instability in the sense that our convergent expansion for $F(\ensuremath{\varphi}),$ truncated at second order has a minimum which corresponds to an analytical and $\ensuremath{\pi}{/p}_{F}$ periodic ${\ensuremath{\varphi}}_{x}.$ Such a minimum is found solving an infinite set of coupled self-consistent equations, one for each of the infinite Fourier modes of ${\ensuremath{\varphi}}_{x}.$