Rigorous treatment of strong electronic correlations in polydiacetylene chains

Abstract

We study theoretically polydiacetylene chains diluted in their monomer matrix. We employ the density-matrix renormalisation group method (DMRG) on finite chains to calculate the ground state and low-lying excitations of the corresponding Peierls–Hubbard-Ohno Hamiltonian that is characterised by the electron transfer amplitude t ₀ between nearest-neighbours, by electron–phonon coupling constant α, by Hubbard interaction U, and by long-range interaction V. We treat lattice relaxation in the adiabatic limit, i.e., we calculate the polaronic lattice distortions for each excited state. We present accurate DMRG results for the single-particle gap, the singlet exciton, the triplet ground state and its optical excitation, and find a good agreement with experimental data for PDA–3BCMU chains. Using a fairly stiff spring constant, the length of our unit cell is about 1% larger than observed in experiment.

Keywords:

• polydiacetylene
• spectrum
• exciton
• density-matrix renormalisation group
• Peierls–Hubbard-Ohno model

Acknowledgements

We thank William Barford, Gerhard Weiser, Michel Schott, and Benjamin Janesko for useful discussions. This work was supported in part by the Deutsche Forschungsgemeinschaft through GRK 790, and by the Hungarian Research Fund (OTKA) Grants Nos. K 100908 and K 73455. Ö.L. acknowledges support from the Alexander-von-Humboldt foundation and from ETH Zurich during his time as a visiting professor.