engleski

Small-cluster study of bipolarons

The stability of bipolarons in the Holstein-Hubbard model in the wide range of parameter's space is explored via the exact diagonalization of small clusters. We examined the model for both the repulsive and the attractive on-site electron interaction U. In the intermediate region of the adiabaticity parameter $\frac{\omega}{t}$ solutions are governed by the anomalous (non-Gaussian) fluctuation of the oscillator's coordinates. We showed that the adiabaticity parameter determines the crossover between on-site and inter-site bipolaronic solutions in the g-U "phase diagram". For repulsive U, the stability of solution, on-site or inter-site bipolaron, is a result of the competition of the electron-phonon coupling g and the repulsive U, but depends strongly on the phonon frequency. For attractive U, even the inter-site bipolaron appears to be a stable solution, provided phonons are fast enough. We speculate on the applicability of our results to the so called Uemura-plot of exotic superconductors.

Bipolarons

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