Quantum disorder induced by nuclear tunneling in lattice

TL;DR

This paper introduces a path-integral molecular dynamics method to study quantum disorder caused by nuclear tunneling in lattice systems, revealing a phase transition sequence and entangled lattice motions.

Contribution

Develops a novel computational approach to accurately describe quantum disorder in lattice systems, validated on models and real materials.

Findings

Demonstrates a quantum order-disorder-order phase transition sequence.

Identifies the quantum disorder region using excitation spectra and entanglement entropy.

Suggests the approach's applicability to systems with soft phonon modes beyond quantum paraelectricity.

Abstract

Lattice degrees of freedom (DoFs) may induce quantum disorder (QD) when nuclear tunneling outvies long-range order, but conventional phonon theory is incapable of describing such QD phases. Here we develop a method based on path-integral molecular dynamics to solve this problem. Its accuracy is verified in a double-well chain model and it is applied to a real material from first principles. A quantum order-disorder-order phase transition sequence is demonstrated when varying the strength of quantum fluctuations using the lattice constants as the tuning factor. Combining the excitation spectra and R\'enyi entanglement entropy, we pinpoint the QD region. This picture may be general in lattice systems having soft phonon modes, not limited to quantum paraelectricity, in which novel entangled lattice motion and its coupling with other DoFs can be expected.