Seminarium Fizyki Materii Skondensowanej

Mott insulators containing d4 ions with strong spin-orbit coupling may host anunusual "soft" magnetism due to a mixing of a nonmagnetic J=0 ionicgroundstate and low-lying magnetic J=1 levels. A competition of the exchange and spin-orbit couplings results in a quantum critical point (QCP) between nonmagnetic Mott insulator and magnetic order. Since the magnetic order is due to a condensation of the virtual J=1 levels and hence "soft", the amplitude (Higgs) mode is expected. We will discuss the recent neutron scattering and Raman scattering experiments demonstrating that this scenario applies to Ca2RuO4 containing Ru4+ d4 ions and directly probing the amplitude mode.