# Non-classicality of spin structures in condensed matter: An analysis of Sr$_{14}$Cu$_{24}$O$_{41}$

## TL;DR

This paper demonstrates how quantum correlations in mediators can reveal the quantum nature of complex materials like Sr$_{14}$Cu$_{24}$O$_{41}$ through magnetic and thermal measurements.

## Contribution

It introduces a method to study the quantum properties of materials without well-defined structures by analyzing quantum correlations in mediators.

## Key findings

- Quantum mediators exhibit quantum discord and entanglement.
- Long-range coupling in Sr$_{14}$Cu$_{24}$O$_{41}$ involves quantum correlations.
- Experimental data confirms quantum mediators in the material.

## Abstract

When two quantum systems are coupled via a mediator, their dynamics has traces of non-classical properties of the mediator. We show how this observation can be effectively utilised to study the quantum nature of materials without well-established structure. A concrete example considered is Sr$_{14}$Cu$_{24}$O$_{41}$. Measurements of low temperature magnetic and thermal properties of this compound were explained with long-range coupling of unpaired spins through dimerised spin chains. We first show that the required coupling is not provided by the spin chain alone and give alternative compact two-dimensional spin structures compatible with the experimental results. Then we argue that any mediator between the unpaired spins must share with them quantum correlations in the form of quantum discord and in many cases quantum entanglement. In conclusion, present data witnesses quantum mediators between unpaired spins in Sr$_{14}$Cu$_{24}$O$_{41}$.