Theoretical Study Of Multiferroicity In The TTF-CA Organic Molecular Crystal

In this thesis, the multiferroicity of several organic molecular crystals are studied on the basis of Hubbard models by using numerical exact diagonalization and constrained-path Monte Carlo methods. In particular, a detailed study has been made on the TTF-CA organic molecular crystal. Our studies show that in a certain parameter regime, there exist magnetism and ferroelectricity at the same in some organic molecular crystals.Based on the one-dimensional Peierls-Hubbard model, the multiferroicity of the TTF-CA organic molecular crystal is studied by using numerical exact diagonalization and constrained-path Monte Carlo methods. At half filling, the calculated results from both numerical methods show that there exist antiferromagnetism and ferroelectricity in a certain parameter regime. At weak electron correlation, the numerical results are in agreement with the results obtained from Hartree-Fock mean field theory. However, at intermediate and strong electron correlations, the electron-phonon coupling for producing multiferroicity predicted from numerical study is much smaller than the one from mean field prediction. An inclusion of nearest-neighbor Coulomb repulsion has a small effect on the phase diagram, especially in the intermediate and strong correlation regimes. Away from half filling, there exist ferromagnetism and ferroelectricity. The numerical results are useful for understanding and designing novel organic multiferroic materials.We also study the multiferroicity of Benzocyclobutadiene, benzo[8]annulene, calicene and7-(2,4)-Cyclopentadien-1-ylidene)-1,3,5-cycloheptatriene, as well as the corresponding fused molecules based on the Hubbard model by using constrained-path Monte Carlo method. At half filling, we find that the ground state lies in the spin-polarized state in some dihydrogenated and tetrahydrogenated organic molecules, and electron correlations promote the ferromagnetism. The first-principles calculations confirm our model calculations, demonstrating that our results reflect the intrinsic properties of studied molecules. The studies of electric polarization in the corresponding parameter regime demonstrate that there exists ferroelectricity in these organic molecules. These numerical results indicate that the organic molecular crystals formed by these organic molecules are potential multiferroic materials.