https://doi.org/10.1007/s100510170253
Microscopic interactions in CuGeO3 and organic Spin-Peierls systems deduced from their pretransitional lattice fluctuations
Laboratoire de Physique des Solides (CNRS-UMR 8502) , Bâtiment 510,
Université de Paris Sud, 91505 Orsay, France
and
Département Sciences Physiques et Mathématiques,
CNRS, 3 rue Michel-Ange 75794 Paris, France
Corresponding author: a pouget@lps.u-psud.fr
Received:
13
September
2000
Revised:
6
February
2001
Published online: 15 April 2001
CuGeO3 exhibits a Spin-Peierls (SP) transition, at K, which is announced
above 19 K by an important regime of one-dimensional (1D) pretransitional lattice fluctuations
which can be detected until about 40 K using X-ray diffuse scattering investigations. A
quantitative analysis of this scattering shows that in this 1D direction the correlation length
follows the "universal"behaviour expected for the thermal fluctuations of a real order
parameter which characterizes the lattice dimerization. This allows to define a 1D mean-field
temperature,
, of about 60 K and invalidates any mean field scenario for the SP transition
of CuGeO3. As
is as high as
we propose that the 3D-SP order is achieved by the
interchain coupling between 1D solitons which form below about 16-20 K. CuGeO3 being in
the non-adiabatic regime, it is also suggested that the observed pretransitional fluctuations of
CuGeO3 originate from the X-ray scattering on a very broad damped critical response of lower
frequency than the "critical"phonon modes. From the quantitative analysis of the 1D
fluctuations we also estimate the microscopic parameters of the SP chain. These parameters
allow to locate CuGeO3 close to the quantum critical boundary separating the gapped SP
ground state to the ungapped anti-ferromagnetic ground state. The vicinity of a quantum
critical point emphasizes the role of the quantum and non-adiabatic fluctuations and the
importance of the interchain coupling in the physics of CuGeO3. Finally we compare these
findings with those obtained for the organic SP systems (BCPTTF)2PF6, (TMTTF)2PF6 and
MEM(TCNQ)2. From a similar analysis of the pretransitional lattice fluctuations it is found
that (BCPTTF)2PF6 and (TMTTF)2PF6 are located on the SP gapped classical-quantum
boundary and are in the adiabatic regime where the fluctuations lead to the formation of a
pseudo-gap in the spin degrees of freedom. Differently, we place MEM(TCNQ)2 inside the SP
quantum phase around the crossover line between the adiabatic and non-adiabatic regimes.
PACS: 71.27.+a – Strongly correlated electron systems; heavy fermions / 71.10.pm – Fermions in reduced dimensions (anyons, composite fermions, Luttinger liquid, etc.)
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2001