College Park, Maryland      June 6 - 10 , 2004

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W2-D5 (2:45 PM): Dynamics of a Z2xZ2 invariant (non-Haldane) S=1 quantum chain

A. Zheludev (Condensed Matter Sciences Division, Oak Ridge National Laboratory), L.-P. Regnault (MDN/DRFMC/SPSMS CEA-Grenoble, France), M. Hagiwara (RIKEN, Waco, Japan), T. Masuda (Condensed Matter Sciences Division, Oak Ridge National Laboratory)

Integral antiferromagnetic spin chains are best known for having an exotic spin liquid ground state and the Haldane gap in the magnetic excitation spectrum. Considerably less attention has been given to a different spin liquid realized in integral spin chains with alternating exchange interactions. In practice, the two ground states and the corresponding excitation spectra are notoriously difficult to distinguish, due to the fact that they only differ in topological "hidden" spin correlators.

We present two series of inelastic neutron scattering experiments on a bond-alternating S=1 quantum spin chain compound NTENP. In the first series of measurements [1] we exploit the Hohenberg-Brinkman 1st moment sum rule to directly measure the distribution of exchange energies in the ground state. The results allow us to unambiguously identify NTENP as a Z2xZ2 invariant (non-Haldane) spin liquid. We then proceed to study the spin dynamics in NTENP in strong external magnetic fields [2]. At H_c=11 T we observe a closing of the spin gap and a field-induced Bose-condensation of magnons. Surprisingly, the high-field state is very similar to that of a classical one-dimensional magnet. The excitation spectrum consists of a doublet of conventional spin-wave-like excitations. Such behavior is in stark contrast with that previously seen in the Haldane-gap antiferromagnet NDMAP in similar conditions [3]. Here the high-field phase is a quantum spin solid characterized by a triplet of coherent breather excitations.

[1] A. Zheludev, T. Masuda, B. Sales, D. Mandrus, T. Papenbrock, T. Barnes, S. Park, cond-mat/0310741.
[2] M. Hagiwara, L.-P. Regnault and A. Zheludev, to be published (2004)
[3] A. Zheludev et al., cond-mat/0308545; Phys. Rev. B 68, 134438 (2003); Phys. Rev. Lett. 88, 077206 (2002), and references therein.

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