Neutron-scattering study of the commensurate-incommensurate transition under field in Si-doped spin-Peierls systems

¹ Institut d'Electronique Fondamentale, CNRS URA 22, Université Paris-Sud, Bât. 220, 91405 Orsay cedex, France
² Département de Recherche Fondamentale sur la Matière Condensée, SPSMS/MDN CEN-G - 38054 Grenoble cedex 9, France
³ Laboratoire de Cristallographie, CNRS - Boîte Postale 166, 38043 Grenoble, France
⁴ Laboratoire Louis Néel, CNRS - Boîte Postale 166, 38043 Grenoble, France
⁵ Institut Laue Langevin - BP156, 38042 Grenoble cedex 9, France
⁶ Laboratoire de Chimie des Solides, CNRS URA 446, Université Paris-Sud, Bât. 414, 91405 Orsay cedex, France

Received: 12 June 1998
Accepted: 1 October 1998

Abstract

We have studied the effects of an applied magnetic field on two silicon-doped spin-Peierls compounds with Si contents x = 0.003 and 0.007. For the first time by means of neutron diffraction, it has been possible to perform measurements above the critical field H_C. In , a sharp commensurate-incommensurate transition is observed upon increasing field at . The transition is characterized by a large hysteresis cycle, and the experimental data are consistent with the soliton-lattice scenario. At higher doping (x=0.007), the superlattice reflections remain peaked at commensurate positions, evolving from a resolution-limited Gaussian lineshape at zero field to a quasi-Lorentzian one at 12 T. By measuring a series of characteristic superlattice peaks on both sides of the transition field in , we have been able to determine the lattice distortion in both the dimerized phase at 11 T and the incommensurate phase at 12 T.