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The terahertz dielectric response of LuFe2O4 is investigated by terahertz time-domain spectroscopy over a temperature range of 6–290 K. It is revealed that besides the central mode associated with the charge-ordered state, a soft TO1 mode at below ∼240 K is identified indicating the existence of displacing ferroelectricity, in addition to the charge-ordering-induced ferroelectricity at below 320 K. The anomaly of the soft mode at ∼180 K reflects the magnetoelectric correlation between the soft TO1 mode and the spin/charge fluctuations revealed recently. Finally, the magnetic property at below ∼240 K is discussed. [ABSTRACT FROM AUTHOR]

A mean-field theory based on a modified transverse-field Ising model including the multifold interactions in Sr1-xCaxTiO3 over a broad composition range from x = 0.0 to 0.4 is developed and the full-scale phase diagram consistent with experiments is evaluated. In this theory, both the ferroelectric distortion and antiferrodistortive effects induced by Ca2+ substitution of Sr2+ are taken into account. It is revealed that the competition between those two types of lattice distortion effects plus the quantum fluctuations represent the core physics underlying the whole set of characteristics in Sr1-xCaxTiO3 up to x = 0.4. The quantum fluctuations dominant state (quantum paraelectrics state), ferroelectrically distorted state, and antiferrodistortive state are successfully predicted, upon increasing substitution level x. It is also predicted that the antiferrodistortive state in the high substitution level is the origin of antiferroelectric order which eventually suppresses the ferroelectric state. [ABSTRACT FROM AUTHOR]

The terahertz dielectric response of LuFe2O4 is investigated by terahertz time-domain spectroscopy over a temperature range of 6–290 K. It is revealed that besides the central mode associated with the charge-ordered state, a soft TO1 mode at below ∼240 K is identified indicating the existence of displacing ferroelectricity, in addition to the charge-ordering-induced ferroelectricity at below 320 K. The anomaly of the soft mode at ∼180 K reflects the magnetoelectric correlation between the soft TO1 mode and the spin/charge fluctuations revealed recently. Finally, the magnetic property at below ∼240 K is discussed. [ABSTRACT FROM AUTHOR]