March 1, 2001
The soft modes in the two- and eight-direction order-disorder ferroelectrics are calculated under the mean-field approximation. We find that the conventional method of the pseudospin model to calculate the soft-mode frequency is incorrect, and present a valid modified method. It is demonstrated that the conventional method does not show the soft-mode frequency going to zero at a critical temperature in the presence of random internal fields, while the frequency calculated by our modified method goes to zero in random fields at a critical temperature. In the eight-direction ferroelectrics, the soft-mode frequency decreases to zero at the first-order phase transition temperature though the symmetry has been destroyed at high temperatures under an external field. The promotion effect of random fields on the phase transition is testified by the calculation results of the soft modes in the paraelectric phase.
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August 28, 2000
An eight-potential-well order-disorder ferroelectric model was presented and the phase transition was studied under the mean-field approximation. It was shown that the two-body interactions are able to account for the first-order and the second order phase transitions. With increasing the random fields in the system, a first-order phase transition is transformed into a second-order phase transition, and furthermore, a second-order phase transition is inhibited. However, pro...
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Historically, the soft mode theory of ferroelectric phase transitions has been developed for the high-temperature (paraelectric) phase, where the phonon mode softens upon decreasing the temperature. In the low-temperature ferroelectric phase, a similar phonon softening occurs, also leading to a bosonic condensation of the frozen-in mode at the transition, but in this case the phonon softening occurs upon increasing the temperature. Here we present a soft mode theory of ferroe...
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Perovskite oxide ferroelectrics show classical soft mode behaviour typical for the onset of a homogeneous long-range polar state and a displacive phase transition. Besides these long wave length properties, local effects are observed by different probes which reveal that dynamical symmetry breaking already takes place already far above the actual instability. It is shown here that displacive mean-field type dynamics can indeed coexist with local dynamical symmetry breaking.
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Type-II multiferroics, where spin interactions induce a ferroelectric polarization, are interesting for new device functionalities due to large magnetoelectric coupling. We report on a new type of multiferroicity in the quadruple-perovskite $\text{BiMn}_{\text{3}}\text{Cr}_{\text{4}}\text{O}_{\text{12}}$, where an antiferromagnetic phase is induced by the structural change at the ferroelectric phase transition. The displacive nature of the ferroelectric phase transition at 12...
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In this paper, we present a microscopic model for heterogeneous ferroelectric and an order parameter for relaxor phase. We write a Landau theory based on this model and its application to ferroelectric PbFe$_{1/2}$Ta$_{1/2}$O$_3$ (PFT) and antiferroelectric NaNbO$_3$:Gd. We later discuss the coupling between soft mode and domain walls, soft mode and quasi-local vibration and resulting susceptibility function.
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Using the Landau Ginzburg Devonshire theory and scalar approximation, we derived analytical expressions for the singular points (zeros, complex ranges) of the acoustic phonon mode (A mode) frequency in dependence on the wave vector k and examined the conditions of the soft A modes appearance in a ferroelectric depending on the magnitude of the flexoelectric coefficient f and temperature T. We predict that if the magnitude of the flexocoefficient f is equal to the temperature-...
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The dielectric constant of the incipient ferroelectric EuTiO$_3$ exhibits a sharp decrease at about 5.5K, at which temperature antiferromagnetic ordering of the Eu spins simultaneously appears, indicating coupling between the magnetism and dielectric properties. This may be attributed to the modification of the soft-phonon mode, $T_{1\mu}$, which is the main contribution to the large dielectric constant, by the Eu spins(7$\mu_B$ per Eu). By adding the coupling term between th...
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We analyze the ``ferron" excitations in order-disorder ferroelectrics by a microscopic pseudo-spin model. We demonstrate that analogous to magnons, the quanta of spin waves in magnetic materials, ferrons carry both static and oscillating electric dipole moments, exhibit a Stark effect, and may be parametrically excited by THz radiation. The anti-crossing gap of the ferron-photon hybrid depends strongly on propagation direction and an applied static electric field. We predict ...
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Ferroelectric soft mode in Pb(Mg1/3Nb2/3)O3 (PMN) has been clearly resolved by precision Raman scattering measurements for the first time. A polarization direction of the incident laser was chosen along [110] in cubic configuration in order to eliminate strong scattering components around 45 cm-1, which always smeared the low-frequency spectra of PMN. The soft mode frequency omega_s (=\sqrt{omega_0^2-gamma^2}) softens around 200 K, where omega_0 and gamma are a harmonic frequ...
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