# High pressure Raman study of the quantum magnet (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$

## TL;DR

This study investigates magnetic and lattice excitations in the quantum antiferromagnet (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$ under high pressure using Raman spectroscopy, revealing pressure-dependent magnetic changes and strong magnetoelastic coupling.

## Contribution

It provides new insights into pressure-induced phase transitions in PHCC, showing significant magnetic and phononic changes not observed in previous neutron studies.

## Key findings

- Magnetic scattering varies significantly with pressure.
- Phonons exhibit anomalous shifts linked to magnetoelastic effects.
- Transitions are not due to structural changes.

## Abstract

Magnetic and lattice excitations in the quantum antiferromagnet (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$ (PHCC) are studied across two pressure-induced phase transition at $P_c=4.3~\mathrm{kbar}$ and $P_1=13.4~\mathrm{kbar}$ using Raman spectroscopy. It is confirmed that neither transition is a result of a structural transformation. Magnetic scattering is detected. It shows a pronounced pressure dependence and undergoes substantial changes at both transitions. The results are in clear contradiction with previous neutron studies, which detected only minor changes of the magnon spectrum at $P_1$. A number of phonons show anomalous frequency shifts at low temperatures. This effect is pressure dependent and for two of the observed phonons dramatically reverses sign at around $P_1$. The anomalous behavior is attributed to strong magnetoelastic coupling in PHCC.