Soft modes in two- and eight-direction order-disorder ferroelectrics

By using broadband dielectric spectroscopy in the radiofrequency and microwave range we studied the magnetoelectric dynamics in the multiferroic chiral antiferromagnet MnWO$_4$. Above the multiferroic phase transition at $T_{N2} \approx 12.6$ K we observe a critical slowing down of the corresponding magnetoelectric fluctuations resembling the soft-mode behavior in canonical ferroelectrics. This electric field driven excitation carries much less spectral weight than ordinary p...

We review the phenomenology of coupled magnetic and electric order parameters for systems in which ferroelectric and incommensurate magnetic order occur simultaneously. We discuss the role that such materials might play in fabricating novel magnetoelectric devices. Then we briefly review the mean-field description of ferroelectricity and modulated magnetic ordering as a preliminary to analyzing the symmetry of the interaction between the spontaneous polarization and the order...

We present a minimal theory of superconductivity enhancement in ferroelectric-type materials. Simple expressions for the optical mode responsible for the soft mode transition are assumed. A key role is played by the anharmonic phonon damping which is modulated by an external control parameter (electron doping or mechanical strain) causing the appearance of the soft mode. It is shown that the enhancement in the superconducting critical temperature $T_{c}$ upon approaching the ...

A variety of compounds, for example doped paraelectrics and polar metals, exhibit both ferroelectricity and correlated electronic phenomena such as low-density superconductivity and anomalous transport. Characterizing such properties is tied to understanding the quantum dynamics of inversion symmetry breaking in the presence of itinerant electrons. Here, we present a comprehensive analysis of the normal state properties of a metal near a quantum critical transition to a ferro...

Lead titanate (PbTiO3) is a classical example of a ferroelectric perovskite oxide illustrating a displacive phase transition accompanied by a softening of a symmetry-breaking mode. The underlying assumption justifying the soft-mode theory is that the crystal is macroscopically sufficiently uniform so that a meaningful free energy function can be formed. In contrast to PbTiO3, experimental studies show that the phase transition behaviour of lead-zirconate-titanate solid soluti...

Inelastic neutron scattering measurements of the relaxor ferroelectric PbMg$_{1 / 3}$Nb$_{2 / 3}$O$_{3}$ (PMN) in the temperature range 490~K$<$T$<$880~K directly observe the soft mode (SM) associated with the Curie-Weiss behavior of the dielectric constant $\varepsilon $(T). The results are treated within the framework of the coupled SM and transverse optic (TO1) mode and the temperature dependence of the SM frequency at q=0.075 a* is determined. The parameters of the SM are...

We analyse the spin wave approximation for the 2D-XY model, directly in reciprocal space. In this limit the model is diagonal and the normal modes are statistically independent. Despite this simplicity non-trivial critical properties are observed and exploited. We confirm that the observed asymmetry for the probability density function for order parameter fluctuations comes from the divergence of the mode amplitudes across the Brillouin zone. We show that the asymmetry is a m...

We suggest a random field based model for calculation of physical properties of mixed ferroelectric relaxors. Our model naturally incorporates the different orientations of electric dipoles (related to different solid solution components) as well as the contribution of nonlinear and correlation effects of random field. We calculate the transition temperature $T_c$ as well as concentrational and temperature dependence of order parameters.

Far-infrared and terahertz spectroscopy of ferroelectric soft and central modes in thin films on substrates is reviewed. In addition to classical displacive proper ferroelectrics, also incipient and relaxor ferroelectrics and multiferroics are discussed. Special attention is paid to differences between the soft-mode behavior in thin films and bulk materials (ceramics and single crystals) and their influence on the low-frequency permittivity. Particularly the effects of the th...

Neutron scattering measurements of the lowest-energy TO phonons in the relaxor Pb(Mg1/3Nb2/3)O3 (PMN) are reported for 10<=T<=750 K. The soft mode, which is overdamped by the polar nanoregions below the Burns temperature T_d = 620 K, surprisingly recovers below 220 K. The square of the soft mode energy hw0^2 increases linearly with decreasing temperature, and is consistent with the behavior of a ferroelectric soft mode. At 10 K, hw0 reaches 11 meV, the same value observed in ...