https://doi.org/10.1140/epjb/e2004-00155-4
Spin-Peierls lattice fluctuations and disorders in CuGeO3 and its solid solutions
1
Laboratoire de Physique des Solides, CNRS-UMR 8502, Université Paris-Sud, bâtiment 510, 91405 Orsay Cedex, France
2
Laboratoire de Chimie de l'État Solide, CNRS-UMR 8648, Université Paris-Sud, bâtiment 414, 91405 Orsay Cedex, France
Corresponding author: a pouget@lps.u-psud.fr
Received:
22
December
2003
Published online:
8
June
2004
The inorganic quasi-one dimensional (1D) S=1/2 antiferromagnetic (AF) system
CuGeO3 undergoes a 2nd order spin-Peierls (SP) phase transition at
K. In this study we present an X-ray synchrotron radiation
investigation which confirms that the SP instability is announced by an
important regime of pretransitional structural fluctuations which have been
detected until 36 K. Furthermore we show that these fluctuations are 1D above
24 K, a feature expected for a structural instability triggered by the Cu2+ chains of spin 1/2. By extrapolating the thermal dependence of the
correlation length in the chain direction, we show that formation of singlet
dimers begins at about 50 K, a temperature that we identify as the mean field
temperature of the SP chain. The critical nature of the pretransitional
fluctuations does not change when low amounts (<1%) of non-magnetic
dopants substitute either the Cu site (case of Zn and Mg) or the Ge site
(case of Si and Al) of CuGeO3. However, the spatial extension of the
fluctuations is considerably reduced when the magnetic dopant Ni substitutes
the Cu site. In the SP ground state of doped materials we have been able to
detect, in addition to the superlattice SP reflections previously observed,
a very weak anisotropic diffuse scattering. We give evidences that this
scattering originates from dopant-induced quasi-1D domains in which the
dimerisation is perturbed. If we assume that each domain is limited by a
soliton-antisoliton pair, pinned either on the substituent of the Cu site or
by the deformation field induced by the substituent of the Ge site, we
deduce that the soliton and antisoliton are separated by a distance of about
Å, and that the soliton half width amounts to about
Å. With these numbers we are able to account
for the rate of decrease of TSP as a function of the dopant
concentration, and to deduce the critical concentration above which the
long-range SP order vanishes. The overall size of the perturbed domains thus
obtained,
Å, is comparable with the size
of the magnetic inhomogeneities determined by muon spin spectroscopy in the
AF phase of doped CuGeO3.
PACS: 71.27.+a – Strongly correlated electron systems; heavy fermions / 61.72.Dd – Experimental determination of defects by diffraction and scattering / 75.45.+j – Macroscopic quantum phenomena in magnetic systems
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2004