Neutron Resonance Spin Echo Spectroscopy

FLEXX provides the opportunity to combine triple-axis spectroscopy and high resolution spin-echo spectroscopy for the investigation of dispersive collective excitations in solid state matter. The achievable energy resolution enables unique access to qualitatively new information about the linewidth of quasiparticles. Most notably, wavevector-dependent relaxation times of individual phonon modes, e.g. arising from the anharmonicity of the interatomic potential of the crystalline lattice, can be measured. Together with the phonon group velocity the relaxation time determines the phonon mean free path, the key parameter for thermal conductivity.

Technically, a non-vanishing group velocity of the excitation mode requires well-defined precession field regions which are tilted with respect to the neutron beam paths in order to measure intrinsic mode linewidths. The neutron resonance spin-echo (NRSE) option  available at FLEXX employs compact resonant spin flippers and is particularly well suited for spin-echo focusing with tilted precession fields. With NRSE spectroscopy linewidths of dispersive elementary excitations can be measured with an energy resolution in the μeV range.

NRSE Option at FLEXX

Fig. 1: Triple-axis spectrometer FLEXX with the NRSE option. Two pairs of resonant spin flippers in front of and behind the sample position are located in µ-metal shielding boxes (closed during operation). Tilted precession field regions which are required for dispersive excitations are easily realized by rotating the spin flippers around their vertical axes