Spatially resolved proton momentum distributions in KDP from first-principles

Area of Science:

• Condensed Matter Physics
• Quantum Mechanics
• Materials Science

Background:

• Understanding ferroelectric to paraelectric (PE) phase transitions is crucial for materials science.
• Potassium dihydrogen phosphate (KDP) exhibits complex quantum phenomena during its PE transition.
• Previous methods like path-integral molecular dynamics have limitations in accurately describing quantum nuclear motion.

Purpose of the Study:

• To rigorously test a first-principles, normal modes framework for anharmonic quantum nuclear motion.
• To accurately calculate spatially resolved momentum distribution functions (MDFs) for KDP.
• To elucidate the proton dynamics and Slater configurations in the PE phase of KDP.

Main Methods:

• Direct calculation of nuclear wavefunctions to obtain MDFs.
• Utilizing a first-principles, normal modes framework.
• Comparison of calculated MDFs with experimental neutron Compton scattering data.

Main Results:

• The framework accurately reproduces experimental MDFs for KDP.
• Coherent, correlated proton tunneling across hydrogen bonds in the PE phase is observed.
• The role of Slater's lateral configurations in the PE phase is highlighted.

Conclusions:

• The normal modes framework is highly effective for analyzing anharmonic quantum nuclear motion.
• Collective, correlated proton motion is essential for understanding the PE transition in KDP.
• The framework shows promise for studying deuterated KDP and other quantum systems.