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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 temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on Bardeen-Cooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB2. Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by band structure calculations. In addition, the observation of a coherence peak indicates that the pi band is in the dirty limit and dominates the total conductivity of our films.