Ordering Phenomena in Transition Metal Oxides

Multipole orders in strongly correlated electrons   

Orbital orders in rare-earth and actinide systems are influenced by strong spin-orbit interaction. Magnetic moments in Ce or U systems are mainly determined by orbital angular momentum. Therefore dipole, quadrupole, octupole and even hexadecapole moments are coupled to each other, and give rise to rich ordering and fluctuation phenomena. We propose that higher multipoles are candidates of hidden order parameters in rare-earth hexaborides, borocarbides, and possibly in some actinides. As a specific example, an octupole ordering model for phase IV of Ce_1-xLa_xB₆ is introduced. Experimental consequences of multiple order are discussed with reference to NMR, muSR, elastic anomaly, neutron and X-ray scatterings etc.