# Long-wavelength fluctuations and anomalous dynamics in two-dimensional liquids

## TL;DR

This paper investigates how long-wavelength fluctuations, caused by Mermin-Wagner effects, influence the dynamics of two-dimensional liquids, revealing a decoupling between diffusion and relaxation at high temperatures through combined numerical and experimental methods.

## Contribution

It demonstrates that long-wavelength fluctuations affect high-temperature liquid dynamics, causing a decoupling between diffusion and structural relaxation not previously recognized.

## Key findings

- Long-wavelength fluctuations induce a D ∝ τ^(-κ) decoupling in 2D liquids.
- This decoupling occurs even without transient solid-like responses.
- Fluctuations have negligible effects only at extremely high temperatures or low densities.

## Abstract

Long-wavelength Mermin-Wagner fluctuations prevent the existence of translational long-range order, in two-dimensional systems at finite temperature. Their dynamical signature, which is the divergence of the vibrational amplitude with the system size, also affects disordered solids and washes out the transient solid-like response generally exhibited by liquids cooled below their melting temperature. Through a combined numerical and experimental investigation, here we show that long-wavelength fluctuations are also relevant at high temperature, where the liquid dynamics does not reveal a transient solid-like response. In this regime, they induce an unusual but ubiquitous decoupling between long-time diffusion coefficient $D$ and structural relaxation time $\tau$, where $D\propto \tau^{-\kappa}$, with $\kappa > 1$. Long-wavelength fluctuations have a negligible influence on the relaxation dynamics only at extremely high temperatures, in molecular liquids, or extremely low densities, in colloidal systems.