title: Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl.
abstract: We report on the magnetic, thermodynamic, and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g., the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.
authors.affiliation: II. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Rückamp
authors.forename: J
authors.initials: J
authors.lastname: Baier
authors.forename: M
authors.initials: M
authors.lastname: Kriener
authors.forename: M W
authors.initials: MW
authors.lastname: Haverkort
authors.forename: T
authors.initials: T
authors.lastname: Lorenz
authors.forename: G S
authors.initials: GS
authors.lastname: Uhrig
authors.forename: L
authors.initials: L
authors.lastname: Jongen
authors.forename: A
authors.initials: A
authors.lastname: Möller
authors.forename: G
authors.initials: G
authors.lastname: Meyer
authors.forename: M
authors.initials: M
authors.lastname: Grüninger