Magnetic excitations of the spin dimer system PHCC under pressure as seen by Raman scattering*

The model spin-1/2 dimer system $(C_4H_{12}N_2)Cu_2Cl_6$ (aka PHCC) has recently been shown to undergo a phase transition to a magnetically ordered state upon applying hydrostatic pressure in both muon spin rotation($\mu^+$SR) and inelastic neutron scattering(INS) experiments. In the $\mu^+$SR experiments a second phase transition from incommensurate to commensurate order was detected at 14 kbar. By contrast, the INS experiments found hardly any difference in the spin dynamics at 9 kbar and 18 kbar. To resolve this discrepancy, we performed Raman scattering experiments on single crystals to study the pressure-dependence of both magnetic and lattice excitations from ambient pressure up to 18~kbar. The three regimes found in $\mu^+$SR could be reproduced. Each regime could be associated with a characteristic peak shape of the magnetic scattering. Increasing pressure leads to a shifting of the magnetic excitations to lower energies up to a pressure of ~15 kbar, where the magnetic peak mode reaches a minimum. Increasing pressure further leads to the magnetic excitations' energy increasing again. Moreover, no indication of a structural phase transition could be found. We conclude that the evolution of spin dynamics in the ordered phase is far more intriguing than expected from INS.