The ferroelectric polarization of triangular-lattice antiferromagnets induced by helical spin-spiral order is not explained by any existing model of magnetic-order-driven ferroelectricity. We resolve this problem by developing a general theory for the ferroelectric polarization induced by spin-spiral order and then by evaluating the coefficients needed to specify the general theory on the basis of density functional calculations. Our theory correctly describes the ferroelectricity of triangular-lattice antiferromagnets driven by helical spin-spiral order and incorporates known models of magnetic-order-driven ferroelectricity as special cases.

It is known that the unusually large dielectric constant in single crystals of CaCu3Ti4O12 (CCTO) requires the existence of barrier layers in the form of twin boundaries or planar defects within the crystals, and even the high dielectric constant of ceramic CCTO samples requires one to consider domain boundaries within the crystallites. As a probable origin of such domain boundaries, we proposed a structural model of plane defect that can result from a twinning parallel to the (100), (010), and (001) planes and examined its local electronic structures. This plane defect model provides a plausible microscopic explanation for the origin of gigantic dielectric constants in CCTO.

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Several puzzling observations in the scanning tunneling miucroscopy (STM) and atomic force microscopy (AFM) studies of highly oriented pyrolytic graphite (HOPG) and its intercalation compounds MCg (M = K, Rb, Cs) were investigated on the basis of atom-atom potential and Coulombic interaction energy calculations. The charge or spin density wave state of a graphite monolayer is found inconsistent with an identical peak registry of the HOPG STM images obtained at plus and minus bias voltages. Simultaneous STM/AFM measurements of HOPG show the STM and AFM images to have an identical peak registry, which implies that the local hardness of the surface monolayer is larger at the B-site than at the A-site. We confirm this implication by estimating the local hardness in the surface monolayer of a graphite bilayer in terms of atomatom potential calculations. The essential characteristics of the Moire STM images of HOPG are correctly predicted by the local hardness map obtained for the surface monolayer of a graphite bilayer in terms of atom-atom potential calculations. This supports the notion that the tip-force-induced topography change in the surface monolayer is generally responsible for Moire STM patterns in layered materials. It is most likely that the surface charge density wave (CDW) of MCg (M = K, Rb, Cs) observed by STM is associated with the P band electrons of the surface graphite monolayer and is not caused either by a Fermi surface nesting driven electronic instability or by a possible topography change induced by the tip force. We calculate the Coulombic interaction energy of the surface KCg blayer for several different negative charge (transferred from K) distributions in the graphite monolayer. This energy is increasingly lowered when the charge distribution becomes more nonuniform, thereby suggesting that the surface CDW of MCg (M = K, Rb, Cs) occurs most likely to lower the Coulombic interaction energy in the surface MCg bilayer.

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