Critical x-ray scattering at the Peierls transition of the blue bronze

We report on the temperature evolution of the 2kF charge-density wave (CDW) in the blue bronze K0.3{MoO}₃$. Using detailed profile analysis of the x-ray scattering, we were able to deduce: (i) from the temperature dependence of the peak intensity the square of the order parameter in the ordered phase (T${T}c$) and the CDW static susceptibility in the disordered phase (T>${T}c$); (ii) from the temperature variation of the scattering width, the correlation lengths of the CDW above ${T}c$. Close to the Peierls transition (${T}c180 K), these variations were, respectively, parametrized by the power laws ({T}c$-T${)}^{2{β}}$, (T-${T}c)^-γ, and (T-Tc{)}^{{{-}}{ν}}$ with {β}{}0.31, {γ}{}1.33, and {ν}{}0.68. The experimental values of these exponents indicate the existence of a three-dimensional (3D) regime of critical fluctuations, compatible with an order parameter of dimension n=2. At about 20 K above Tc we observed a crossover into a regime characterized by quasi-two-dimensional fluctuations, which survive at least up to room temperature.