Soft spin-amplitude fluctuations in a Mott insulating ruthenate
Date : December 30, 2015 16:00 ~ 17:00
Speaker : Dr. Bumjoon Kim(Max Planck Institute for Solid State Research)
Location : Bldg. 19, Rm. 210
Magnetism in transition-metal oxides (TMOs) has
traditionally been associated with spin degrees of freedom, as orbital is often
largely quenched. On the other hand, recent studies of 5d TMOs have revealed
unconventional, orbital-dominated magnetism driven by strong spin-orbit
coupling (SOC). In this talk, I will discuss novel, excitonic magnetism in the
intermediate strength regime of SOC, characterized by soft magnetic moments and
their large amplitude fluctuations. Using inelastic neutron scattering, we
observe in an archetypal 4d Mott insulator Ca
2
RuO
4
a
“
Higgs"
amplitude mode that manifests quantum criticality from a close competition
between SOC and other conventional tendencies to magnetically order. SOC tends
to suppress magnetic ordering via cancellation between spin and orbital
moments, whereas superexchange process generates quantum fluctuations of
virtual magnetic dipoles, which condense upon softening by coupling to a
lattice distortion. This unconventional spin-orbital-lattice dynamics in Ca
2
RuO
4
identifies SOC as a novel source of quantum criticality in TMOs and provides a
new conceptual framework to interpret SOC driven phenomena in 4d TMOs