Deuteron Momentum Distribution in KD2HPO4

TL;DR

This study measures the proton momentum distribution in KD2PO4 using neutron scattering, revealing a softer potential and a mass-dependent quantum coherence length, with minimal change across the phase transition.

Contribution

It provides the first detailed comparison of proton momentum distributions in KD2PO4 and KDP, highlighting differences in potential softness and quantum coherence.

Findings

No significant difference in distributions across phase transition

Proton tunneling splitting is less than 20 meV

KD2PO4 has a softer effective potential than KDP

Abstract

The momentum distribution in KD2PO4(DKDP) has been measured using neutron Compton scattering above and below the weakly first order paraelectric-ferroelectric phase transition(T=229K). There is very litte difference between the two distributions, and no sign of the coherence over two locations for the proton observed in the paraelectric phase, as in KH2PO4(KDP). We conclude that the tunnel splitting must be much less than 20mev. The width of the distribution indicates that the effective potential for DKDP is significantly softer than that for KDP. As electronic structure calculations indicate that the stiffness of the potential increases with the size of the coherent region locally undergoing soft mode fluctuations, we conclude that there is a mass dependent quantum coherence length in both systems.