College Park, Maryland June 6 - 10 , 2004
TP53: Bose condensation of magnons in anisotropic Haldane spin chains
A. Zheludev (Condensed Matter Sciences Division, Oak Ridge National Laboratory), C. L. Broholm (Department of Physics & Astronomy, Johns Hopkins University; NIST Center for Neutron Research), S. M. Shapiro (Physics Department, Brookhaven National Laboratory), Z. Honda (Faculty of Engineering, Saitama University, Urawa, Saitama 338-8570, Japan), K. Katsumata (The RIKEN Harima Institute, Mikazuki, Sayo, Hyogo 679-5148, Japan), B. Grenier, L.-P. Regnault (DRFMC/SPSMS/MDN, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex, France)
Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet NDMAP are performed at mK temperatures in magnetic fields of almost twice the critical field Hc. Geometries in which he field is applied perpendicular [1,2] or parallel  to the axis of magnetic anisotropy are both explored. The observed closing of the Haldane gap at Hc  leads to a Bose condensation of magnons and the appearance of long-range antiferromagnetic order . This ordered (spin-solid) high-field phase is not to be confused with a conventional antiferromagnet. In the latter model the spectrum consists of two spin wave branches that correspond to spin precessions around the direction of ordered staggered magnetization. In contrast, in NDMAP at H>Hc we clearly obseve three distinct branches of long-lived massive (gapped) excitations [2,3]. These "breather" states can not be understood in the framework of quasiclassical theories. However, more sophisticated quantum models can account for the observed behavior quite well. The main conclusion is that at even H>Hc quantum fluctuations are crucial, and a magnetized anisotropic Haldane spin chain is a quantum spin solid.
 A. Zheludev, Y. Chen, Z. honda, C. Broholm and K. Katsumata, Phys. Rev. lett. 88, 077206 (2002).
 A. Zheludev, Z. Honda, C. L. broholm, K. katsumata, S. M. Shapiro, A. Kolezhuk, S. park and Y. Qiu, Phys. Rev. B 68, 134438 (2003).
 A. Zheludev, S. M. Shapiro, Z. Honda, K. Katsumata, B. Grenier, E. Ressouche, L.-P. Regnault, Y. Chen, P. Vorderwisch, H.-J. Mikeska, and A. K. Kolezhuk, Phys. Rev. B, Phys. Rev. B 69, 054414 (2004).
 Y. Chen, Z. Honda, A. Zheludev, C. Broholm, K. Katsumata and S. M. Shapiro, Phys. Rev. Lett. 86, 1618 (2001).
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