Sign switching of dimer correlations in SrCu2(BO3)2 under hydrostatic pressure

Magnetic and vibrational excitations in SrCu2(BO3)(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 similar to 22 kbar they switch from antiferromagnetic to being predominantly ferromagnetic.