Parity represents the mirror inversion symmetry in physics. Commonly, interaction parity between atoms is broken in a crystalline solid due to the asymmetric interatomic repulsion. Symmetry breaking often involves a phase transition, however, it is still unclear if there are any comparable behaviors in a crystalline solid due to the broken interaction parity.

Motivated by such a challenge, a research team led by Prof. Jige Chen at Shanghai Advanced Research Institute (SARI) of the Chinese Academy of Sciences reported a crossover of thermal conduction with a third-order-like singularity in a Fermi-Pasta-Ulam-Tsingou (FPUT) superlattice.

The research results were published in Physical Review E  in May 2025.

The superlattice is consisted of repeating periodic cells of arithmetically increased cubic nonlinearity. Researchersfind that, before a critical point of interaction asymmetry, thermal conduction is cell-length-independent. After crossing the critical point, thermal conduction relies on the unit cell length.

The calculation of reflection symmetry and configuration entropy of the atomic displacement reveals a third-order-like singularity at the critical point, which further determines the peculiar dynamics of energy carriers.

The result indicatesa high-order singularity arising from the broken interaction parity in a crystalline solid and a promising strategy in adjusting phononic superlattice materials. It enlightens our understanding of the critical role of interatomic interaction parity in lattice thermal behaviors since thermal expansion is the most common feature in a crystalline solid.

Schematic of the FPUT Superlattice and its complete Hamiltonian (Image by SARI)