title: Evidence for dimer crystal melting in the frustrated spin-ladder system BiCu2PO6.
abstract: In the spin ladder compound BiCu(2)PO(6), there exists a decisive dynamics of spin excitations that we classify and characterize using inelastic light scattering. We observe an interladder singlet bound mode at 24  cm(-1) and two intraladder bound states at 62 and 108  cm(-1) in the leg (bb) and the rung (cc) polarization as well as a broad triplon continuum extending from 36  cm(-1) to 700  cm(-1). Though isolated spin ladder physics can roughly account for the observed excitations at high energies, frustration and interladder interactions need to be considered to fully describe the spectral distribution and scattering selection rules at low and intermediate energies. In addition, we attribute the rich spectrum of singlet bound modes to a melting of a dimer crystal. Our study provides evidence for a Z(2) quantum phase transition from a dimer to a resonating valence bond state driven by singlet fluctuations.
authors.affiliation: Department of Physics, Chung-Ang University, 221 Huksuk-Dong, Seoul 156-756, Republic of Korea.
authors.forename: K-Y
authors.initials: KY
authors.lastname: Choi
authors.affiliation: Department of Physics, Chung-Ang University, 221 Huksuk-Dong, Seoul 156-756, Republic of Korea.
authors.forename: J W
authors.initials: JW
authors.lastname: Hwang
authors.affiliation: Institute for Condensed Matter Physics, TU Braunschweig, D-38106 Braunschweig, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Lemmens
authors.affiliation: Institute for Condensed Matter Physics, TU Braunschweig, D-38106 Braunschweig, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Wulferding
authors.affiliation: Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: G J
authors.initials: GJ
authors.lastname: Shu
authors.affiliation: Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: F C
authors.initials: FC
authors.lastname: Chou