Temperature Dependence of the Spin-Peierls Energy Gap and Anomalous Line   Shapes in CuGeO$_3$
  Neutron scattering measurements on a large single crystal of CuGeO$_3$ have been used to determine the temperature-dependence of the spin-Peierls energy gap. While the power law behavior of the intensity of structural superlattice peaks is well fit by $I({\rm T})\propto ({\rm T}_c - {\rm T})^{2\beta}$ with an exponent of $\beta =0.33$, the exponent for the temperature dependence of the energy gap is significantly smaller than expected for conventional spin-Peierls materials. Usual scaling relations relate the energy gap to the superlattice reflection intensity as $\Delta({\rm T}) \propto I^a$ with $a=1/3$; the present results suggest an exponent of $a\approx 1/6$ for CuGeO$_3$. Additional scattering cross-section is observed in constant-$q$ and constant-$E$ scans creating a long `tail' extending to higher energies relating to a proposed scattering continuum. 
Michael C. Martin(1), G. Shirane(1), Y. Fujii(2), M. Nishi(2), O.   Fujita(3), J. Akimitsu(3), M. Hase(4,5) and K. Uchinokur...