High-pressure Raman study of the quantum magnet (C4H12N2)Cu2Cl6

Magnetic and lattice excitations in the quantum antiferromagnet $({\mathrm{C}}₄{\mathrm{H}}₁₂{\mathrm{N}}₂){\mathrm{Cu}}₂{\mathrm{Cl}}₆$ (PHCC) are studied across two pressure-induced phase transition at ${P}_c=4.3$ kbar and ${P}₁=13.4$ 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.