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42469-G10
Exotic Magnetic States in Two-Dimensional Organic Superconductors

Rudolf Torsten Clay, Mississippi State University

In the second year of this project we have continued our study of dynamic phonon interactions and the Hubbard-Holstein model (HHM). In crystalline molecular materials, the coupling of molecular vibrations to electrons (electron-phonon (e-p) interactions) can have profound effects on the electronic state of the material. For a one dimensional (1D) chain of molecules, e-p interactions lead to the Peierls transition to an insulating ground state. A second important interaction in low dimensional materials (like the 1D chain) is the Coulomb repulsive electron-electron (e-e) interaction. For the case of the half-filled band (average electron density of 1 per site), this also leads to a insulating state, the Mott insulator.

While both these interactions are often studied separately in model Hamiltonians, accurate calculations including both interactions are much more difficult. We have focused on accurate calculations of the HHM (see Eq. 1). In one dimension at half filling the Holstein e-p coupling promotes a charge-density wave (CDW) Peierls state consisting of onsite pairs of electrons while the Hubbard onsite Coulomb repulsion U promotes antiferromagnetic correlations and the Mott state.

Recent numerical studies have suggested a possible third intermediate phase between Peierls and Mott states. Using quantum Monte Carlo methods we were able to show that

1. As the electron-phonon coupling is increased, first a spin gap opens, followed by the Peierls transition. Between these two transitions the metallic intermediate phase has a spin gap, no charge gap, and properties similar to the negative-U Hubbard model.
2. The transitions between Mott/intermediate and intermediate/Peierls states are of the Kosterlitz-Thouless form.
3. For larger U the two transitions merge at a tritical point into a single first order Mott/Peierls transition.
4. In addition we also studied the quarter-filled model (average electron density of 0.5 per site) and showed that an intermediate phase also occurs there.

Eq. 1, Hamiltonian of the Hubbard-Holstein model. c operators are for electrons, a operators are for phonons. U is the onsite Coulomb interaction, g the electron-phonon coupling constant, and omega the phonon frequency. We are able to solve this system with no approximations using quantum Monte Carlo.

Fig. 1: Finite-size scaling of the staggered charge susceptibility for U=0 and omega=1 at half filling. Data are for system sizes up to 128 sites. In (a), the vanishing of logarithmic size corrections at g~0.7 is consistent with a Kosterlitz-Thouless type quantum phase transition.	Fig. 2: The phase diagram at half filling as determined from quantum Monte Carlo calculations, for small, intermediate, and large omega. Peierls and Mott phases are insulating; the intermediate phase (I) is metallic, with conduction due to mobile electron pairs.