Naslov
Quantum effects in hydrogen-bonded ferroelectrics

Autori
Merunka, Dalibor ; Rakvin, Boris

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
EMF-2007 Programme and Book of Abstracts / Zalar, Boštjan ; Malič, Barbara ; Bobnar, Vid - Ljubljana : Institut Jožef Stefan, 2007, 153-153

ISBN
978-961-6303-93-4

Skup
EMF-2007, 11th European Meeting on Ferroelectricity

Mjesto i datum
Bled, Slovenija, 03.09.2007. - 07.09.2007

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
quantum effects; hydrogen bond; ferroelectrics

Sažetak
Two quantum effects are generally detected in the hydrogen-bonded ferroelectric crystals such as KH2PO4 (KDP). These are the isotope effect and the quantum saturation effect detected at low temperatures and/or high pressures. The isotope effect is a strong effect of the proton-deuteron replacement on quantities that characterize ferroelectric transition, while the quantum saturation effect is the opposing effect of quantum fluctuations of atoms on the ferroelectric ordering. These two quantum effects, as well as the phase transition mechanism, are not yet satisfactory explained. The traditional model of interacting tunneling protons predicts the phase transition of displacive type. It also predicts that the proton tunneling causes the quantum saturation effect. Since this effect depends on the isotope-dependent tunneling energy D, the model explains the isotope effect as the isotope-dependent quantum saturation effect. On the other hand, several experiments suggest the phase transition of order-disorder type governed by interacting dipoles of the heavy-ions groups (PO4, AsO4, etc.). However, the quantum effects within a model of interacting heavy-ions dipoles still remain to be explained. In order to resolve this problem, we have considered the coupled proton-dipole model for hydrogen-bonded ferroelectrics and tried to separate the roles of tunneling protons from the heavy-ions dipoles in the phase transition mechanism by applying appropriate adiabatic approximation. When the dipoles, represented as harmonic oscillators with frequency w0, are faster than protons (w0>D/&#295; ), the adiabatic approximation is applied to the dipoles, and the model of interacting tunneling protons follows from the coupled proton-dipole model. In the opposite case, &#61472; w0<D/&#295; , the adiabatic approximation is applied to the protons, resulting in the model of interacting dipoles that is able to explain the order-disorder behavior of the dipoles and the quantum effects. According to this model, the quantum saturation effect originates from quantum fluctuations of the dipoles and depends on w0. Therefore, it is not isotope-dependent nor is it responsible for the isotope effect. The origin of the isotope effect is effective interaction between the dipoles, which depends on D. The MSDPC model for KDP [1], as an example of the model of interacting dipoles, was successfully applied to describe the isotope effect and order-disorder behavior in KDP without taking into account quantum fluctuations of the dipoles. In order to estimate the quantum saturation effect in KDP, these quantum fluctuations have been taken into account and the path-integral Monte Carlo method has been applied. The calculated results show good agreement with the experimental results. [1] D. Merunka and B. Rakvin, Struct. Bond. 124, 149 (2007).

Izvorni jezik
Engleski

Znanstvena područja
Fizika

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