Temperature dependence of spin excitations in the frustrated spin chain system CuGeO₃

We studied the temperature dependence of the spin excitation spectrum in the frustrated spin chain system CuGeO₃ by a time-of-flight neutron scattering measurement. The intensity hump along the upper boundary (UB) of the continuum excitation, which is a robust feature for significant competition between next-nearest-neighbor (NNN) and nearest-neighbor (NN) exchange interactions, was shown to degrade upon warming. The hump disappears above ∼90 K, whereas the outline of the two-spinon continuum excitation remains even at 180 K. The decrease in intensity at the UB, the energy of which is >kBT (where kB and T are the Boltzmann constant and the temperature, respectively) forT < 90 K, suggests that the contribution of NNN interaction to form the thermodynamic equilibrium state becomes negligible as a result of the thermal perturbation. The intensity of the lowest excited mode at the magnetic zone center weakens concomitantly with that at the UB. This intensity reduction, with remaining sizable energy-integrated intensity at the zone boundary, leads a change of the static structure factor from a single Lorentzian shape to a twopeaked structure in the temperature range between 50 and 75 K. These results can be explained through the degradation of the spin dimer by thermal disturbance with remaining large NNN couplings at high temperature.