Eur. Phys. J. B 38, 581–598 (2004) DOI: 10.1140/epjb/e2004-00155-4 T HE EUROPEAN P HYSICAL JOURNAL B Spin-Peierls lattice fluctuations and disorders in CuGeO 3 and its solid solutions J.-P. Pouget 1, a , S. Ravy 1 , J.P. Schoeffel 1 , G. Dhalenne 2 , and A. Revcolevschi 2 1 Laboratoire de Physique des Solides, CNRS-UMR 8502, Universit´ e Paris-Sud, bˆatiment 510, 91405 Orsay Cedex, France 2 Laboratoire de Chimie de l’ ´ Etat Solide, CNRS-UMR 8648, Universit´ e Paris-Sud, bˆatiment 414, 91405 Orsay Cedex, France Received 22 December 2003 Published online 8 June 2004 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2004 Abstract. The inorganic quasi-one dimensional (1D) S =1/2 antiferromagnetic (AF) system CuGeO3 undergoes a 2nd order spin-Peierls (SP) phase transition at TSP = 14.2 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 Cu 2+ 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 L0 ∼ 28-45 ˚ A, and that the soliton half width amounts to about ξSP ∼ 16-20 ˚ A. 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, L0 +2ξSP ∼ 70 ˚ A, 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 1 Introduction One of the major achievements of solid state physics studies of the end of the 20th century was the discov- ery of quantum cooperative phenomena [1] which tend to promote collective behaviour such as superconductivity, charge, spin or orbital density waves, together with charge or spin gapped states [2]. These features are particularly well documented in systems of reduced electronic or mag- netic dimensionality [3]. The description of these quantum ground states and of their phase diagram when external parameters such as pressure, magnetic field or chemical composition is varied, is one of major issues of to day studies. Among these systems quantum one-dimensional (1D) antiferromagnetic (AF) spin chains and ladders have been particularly studied [4]. The main reason is that their a e-mail: pouget@lps.u-psud.fr ground state properties do not vary continuously when either the spin S of the chain or the number of legs in the ladder changes. For example, the S =1/2 chain develops at T = 0 K local AF correlations due to quantum fluctu- ations [4], while the S = 1 chain exhibits a non-magnetic singlet (S = 0) ground state with a finite gap in its spin excitation spectrum [3]. An S =1/2 spin ladder with an odd number of legs behaves as the S =1/2 AF chain, while an S =1/2 spin ladder with an even number of legs behaves as the S = 1 AF chain [4]. Even for an S =1/2 AF chain, an important fraction of the ground state fluc- tuations contains non-magnetic singlet components. These components can be picked out of the quantum fluctuations in presence of a sizeable spin-phonon coupling allowing the chain to dimerize [5]. This gives rise to a spin-Peierls (SP) phase transition where, below T SP , the dimerisation achieves a lattice of non-magnetic S = 0 singlet pairs, whose magnetic excitations are separated by a gap ∆ from