NEUTRON INELASTIC SCATTERING
Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex, France
Historically, neutron inelastic scattering studies using three-axis spectrometers (TAS) are synonymous of measuring dispersion relations of elementary excitations in crystalline solids. At present such kind of experiments yields more and more place to studies of other objects like continuum modes in low-dimensional quantum spin systems or fluctuations related to ordering processes, both on short and long ranges. Although other experimental techniques (NMR, muon spin rotation) can provide valuable information on the same systems, neutron inelastic scattering remains the method yielding the most complete information on the role of space and time correlations and their interplay in the behaviour of condensed matter systems. Moreover, neutrons couple with comparable strength to both the structural and magnetic degrees of freedom and the two scattering components can be quite cleanly separated using polarized neutron techniques. Finally, the low absorption of neutrons facilitates the work at extreme sample environment conditions.
Among related spectroscopic techniques, providing a direct information on frequencies of individual excitation modes, the optical methods (Raman and infrared spectroscopy), although easily accessible in a laboratory, are limited to the investigations of the immediate vicinity of the origin of reciprocal space because of the tiny photon momentum and hence do not provide direct insight into the space correlation aspects. Moreover, they are subject to rather restrictive selection rules, a priori excluding the studies of certain excitation modes. The X-ray inelastic scattering, developed in the last decade at the third generation synchrotron radiation sources, provides, similarly to neutron scattering, the information on excitation spectra at any location throughout the whole Brillouin zone. The comparison between the two techniques follows the general pattern of complementary use and competition between neutron and X-ray scattering techniques, whose most important element is the existence of neutron magnetic scattering, which does not have a direct equivalent with X-rays in this context.