# Dynamical collective memory in fluidized granular materials

## TL;DR

This paper uses simulations to replicate experimental findings of superdiffusive behavior in vibrated granular media, revealing a collective rotational mode that causes superdiffusion and dynamical heterogeneity.

## Contribution

It uncovers a persistent collective rotational mode in dense granular media, linking it to superdiffusive probe dynamics and providing insights not accessible through experiments alone.

## Key findings

- Identification of a collective rotational mode at high density and low temperature.
- Link between the rotational mode and superdiffusive behavior of the probe.
- Observation of dynamical heterogeneity and fluctuation reduction over time.

## Abstract

Recent experiments with rotational diffusion of a probe in a vibrated granular media revealed a rich scenario, ranging from the dilute gas to the dense liquid with cage effects and an unexpected superdiffusive behavior at large times. Here we setup a simulation that reproduces quantitatively the experimental observations and allows us to investigate the properties of the host granular medium, a task not feasible in the experiment. We discover a persistent collective rotational mode which emerges at high density and low granular temperature: a macroscopic fraction of the medium slowly rotates, randomly switching direction after very long times. Such a rotational mode of the host medium is the origin of probe's superdiffusion. Collective motion is accompanied by a kind of dynamical heterogeneity at intermediate times (in the cage stage) followed by a strong reduction of fluctuations at late times, when superdiffusion sets in.