講演者: Giniyat Khaliullin氏(Max-Planck-Institut FKF, Stuttgart) 日時: 12月 8日(月) 午後4時30分から [いつもと曜日が異なります] 場所: 青山学院大学 理工学部(相模原キャンパス)L棟6階 L603室 題目: 「FLUCTUATING ORBITALS IN TITANATES AND VANADATES」 要旨: We discuss a collective behavior of orbital angular momentum in Mott insulators with t2g orbital degeneracy. The frustrated nature of the superexchange interactions leads to an infinite degeneracy of classical states. Quantum effects select a particular orbital state depending on spin configuration: (i) In spin one-half antiferromagnetic LaTiO3,   orbitals remain quantum disordered down to low temperature; (ii) In ferromagnetic YTiO3, a weak orbital order is stabilized   by order-from-disorder mechanism; (iii) In large spin (e.g. S=1) systems like cubic vanadates orbitals may form fluctuating quasi one dimensional structures susceptible to an orbital Peierls effect. The role of lattice effects, in particular of tilting of octahedra, will be also considered. The experimental consequences of orbital fluctuations will be discussed which might help to identify orbital states and to detect an elementary orbital excitations. References: LaTiO3: Phys. Rev. Lett. 85, 3950 (2000); 85, 3946 (2000). YTiO3: Phys. Rev. Lett. 89, 167201 (2002); 89, 167202 (2002); Phys. Rev. B 68, in print (2003). (La,Y)VO3: Phys. Rev. Lett. 86, 3879 (2001); Phys. Rev. Lett., in print (2003). --------------------------------- 共催: 青山学院大学 21世紀COEプログラム