Inelastic-neutron-scattering investigation of the spin-Peierls system CuGeO3

We have investigated the spin dynamics of the spin-Peierls system CuGe${\mathrm{O}}_3$ by inelastic neutron scattering. The measurements have been performed as a function of wave vector, temperature, and magnetic field, for fields perpendicular to the chain axis (up to 10 T). Our neutron results confirm the occurrence of a crystalline distortion below $T_{\mathrm{SP}}\approx 14.2$ K, corroborated by the appearance of superlattice peaks indexed with a commensurate wave vector ${\mathbf{k}}_{\mathrm{SP}}$=(½,0,½). At low temperature, the spin-excitation spectrum exhibits a well defined energy gap $\Delta \approx 2 \mathrm{meV}\approx 1.66 k{T}_{\mathrm{SP}}$ at the antiferromagnetic point ${\mathbf{k}}_{\mathrm{AF}}$=(0,1,½), distinct from ${\mathbf{k}}_{\mathrm{SP}}$. The experimental results for $T\ll T_{\mathrm{SP}}$ are quantitatively understood from an alternating-exchange Hamiltonian with an exchange parameter $\left(2\mathrm{}{J}_1\right)\approx 10.6$ meV and an alternation parameter $\alpha =\frac{J_2}{J_1}\approx 0.92$, despite the existence of sizable interchain couplings both along the $a$ and $b$ axes ($\frac{J_a^{\prime }}{J_1}\approx 0.011$ and $\frac{J_b^{\prime }}{J_1}\approx 0.11$, respectively). We present the temperature dependence of the spin dynamics on both sides of $T_{\mathrm{SP}}$. Our results confirm the persistence of a pseudogap in the excitation spectrum for q=${\mathbf{k}}_{\mathrm{AF}}$, but only in the immediate vicinity of the spin-Peierls transition temperature. The pseudogap vanishes rapidly with $T$, following decreasing size of dimerized segments. The stronger inelasticity observed above $T_{\mathrm{SP}}$ at q=${\mathbf{k}}_{\mathrm{SP}}$ has been attributed to an effect of the dispersion of magnetic excitations due to interchain couplings $J_a^{\prime }$ and $J_b^{\prime }$. Under a magnetic field applied perpendicular to the chain axis, the excited triplet is split into three components, with gap values depending linearly on the field. The spin-Peierls transition temperature $T_{\mathrm{SP}}$ decreases with increasing field, following a quadratic dependence on $H$, in agreement with theoretical as well as other experimental results. We have determined the field dependence of the dimerization peak intensities up to 10 T, which show very small changes. In particular, no trace of pretransitional incommensurability could be...