Quantum fluctuations versus thermal fluctuations in KDP studied by neutron incoherent scattering

Abstract

To describe the proton-lattice system in KDP, ‘proton self-trapped’ state model is proposed. Due to the coupling to phonons, protons on the hydrogen bonds tend to be self-trapped at the shifted positions by breaking local symmetry of the surrounding atoms. The same coupling causes the quantum-mechanical transition (phonon assisted tunneling) between the degenerated self-trapped states. At higher temperatures, the transition becomes a thermal-activation type (incoherent tunneling). The results of the recent neutron scattering data have been analyzed using the model. The calculated scattering function shows satisfactory agreement with the observed spectrum at 150K by taking the incoherent tunneling probability to be 2.5 × 10¹² sec⁻¹.