In solid state physics, the degrees of freedom and interactions necessitate the use of a large variety of experimental methods. The neutron has a unique combination of properties that make it indispensable for many problems in solid state physics. The de Broglie wavelength of thermal neutrons is on the same scale as inter-atomic spacings, allowing diffraction experiments to be conducted to locate the positions of atoms.

Neutrons have played a pivotal role in the investigations of phase transitions and co-operative phenomena, magnetism, structure (static and dynamics), as well as in many other fields. Particularly intriguing is the connection between phase transitions and theoretical concepts, such as symmetry breaking, order parameter, universality class, scaling and critical behavior.

A vast panoply of neutron techniques have contributed to this work. Neutron diffraction (from powders and single crystals) is a basic, but essential, technique, providing information on chemical and magnetic structures. Neutron reflectometry using polarised neutrons has given us a clearer picture of the growth and the physics of magnetic thin films and superlattices. Inelastic neutron scattering is the only probe that provides a complete picture of both structural and magnetic dynamics in solids. Emerging techniques include analysis of three dimensional polarisation, and the direct mapping of the full dynamical susceptibility over the entire Brillouin zone.

Reference: ESS project Volume II
New Science and Technology for the 21^st Century

Flagship	Instrument required
Spin glass dynamics	High res NSE High res BS Med res BS Diffuse
Films and superlattices	Cold chopper Thermal chopper High Energy Chopper
Molecular magnets	Cold Chopper
Spin density waves	Thermal Chopper Cold Chopper
Defects at dilute limit	Thermal Chopper Cold Chopper
Coupled excitacions in solids	High energy chopper Cold Chopper Thermal Chopper
Exotic interactions (Propierties of magnetic materials)	High Energy Chopper Thermal Chopper