Sign switching of dimer correlations in SrCu_{2}(BO_{3})_{2} under hydrostatic pressure

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}∼22 kbar they switch from antiferromagnetic to being predominantly ferromagnetic.