Temperature Dependence of Spin Excitations in the Frustrated Spin Chain System CuGeO3

Abstract

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) for T<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 ...