# Sign-switching of dimer correlations in SrCu$_2$(BO$_3$)$_2$ under hydrostatic pressure

## TL;DR

This study investigates how hydrostatic pressure influences magnetic and vibrational properties in SrCu$_2$(BO$_3$)$_2$, revealing a pressure-induced switch in dimer spin correlations from antiferromagnetic to ferromagnetic.

## Contribution

It provides the first quantitative analysis of pressure-dependent dimer spin correlations and identifies a switch in their magnetic nature in SrCu$_2$(BO$_3$)$_2$.

## Key findings

- Dimer spin correlations are strongly pressure dependent.
- A switch from antiferromagnetic to ferromagnetic correlations occurs around 22 kbar.
- The pantograph phonon mode exhibits anomalous temperature dependence linked to magnetic exchange energy.

## Abstract

Magnetic and vibrational excitations in SrCu$_2$(BO$_3$)$_2$ are studied using Raman spectroscopy at hydrostatic pressures up to 34 kbar and temperatures down to 2.6 K. The frequency of a particular optical phonon, the so-called pantograph mode, shows a very strong anomalous temperature dependence below about 40 K. We link the magnitude of the effect to the magnetic exchange energy on the dimer bonds in the Sutherland-Shastry spin lattice in this material. The corresponding dimer spin correlations are quantitatively estimated and found to be strongly pressure dependent. At around P$_2\sim$22 kbar they switch from antiferromagnetic to being predominantly ferromagnetic.