title: Observation of two-magnon bound states in the two-leg ladders of (Ca,La)(14)Cu(24)O(41).
abstract: Phonon-assisted two-magnon absorption is studied in the spin- 1/2 two-leg ladders of (Ca,La)(14)Cu(24)O(41) for E parallel c (legs) and E parallel a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at approximately equal to 2140 and 2800 cm(-1). The two-magnon continuum is observed at approximately equal to 4000 cm(-1). Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J parallel approximately equal to 1020-1100 cm(-1), J parallel/J perpendicular approximately equal to 1-1.2. At high energies, the magnetic contribution to sigma(omega) is strikingly similar in the ladders and in the undoped high-T(c) cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.
authors.affiliation: II. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Windt
authors.forename: M
authors.initials: M
authors.lastname: Grüninger
authors.forename: T
authors.initials: T
authors.lastname: Nunner
authors.forename: C
authors.initials: C
authors.lastname: Knetter
authors.forename: K P
authors.initials: KP
authors.lastname: Schmidt
authors.forename: G S
authors.initials: GS
authors.lastname: Uhrig
authors.forename: T
authors.initials: T
authors.lastname: Kopp
authors.forename: A
authors.initials: A
authors.lastname: Freimuth
authors.forename: U
authors.initials: U
authors.lastname: Ammerahl
authors.forename: B
authors.initials: B
authors.lastname: Büchner
authors.forename: A
authors.initials: A
authors.lastname: Revcolevschi