Quantum Dielectric Fluctuation in Electronic Ferroelectricity Studied by Variational Monte-Carlo Method

The electronic structure and dielectric property in electronic ferroelectricity, where electric polarization is driven by an electronic charge order without inversion symmetry, are studied. Motivated by layered iron oxides, the roles of quantum fluctuation in ferroelectricity in a paired-triangular lattice are focused on. Three types of V-t model, where the intersite Coulomb interaction V and the electron transfer t for spinless fermions are taken into account, are examined by the variational Monte-Carlo method with the Gutzwiller-type correlation factor. It is shown that the electron transfer between the triangular layers corresponding to the interlayer polarization fluctuation promotes a three-fold charge order associated with electric polarization. This result is in high contrast to the usual result observed in the hydrogen-bond type ferroelectricities and quantum paraelectric oxides, where the ferroelectric order is suppressed by quantum fluctuation. The spin degree of freedom of electrons and a realistic interlayer geometry for layered iron oxides further stabilize the polar charge ordered state. The implications of the numerical results for layered iron oxides are discussed.