Anomalous thermal decoherence in a quantum magnet measured with neutron spin echo spectroscopy

The effect of temperature dependent asymmetric line broadening is investigated in Cu(NO3)(2)center dot 2.5D(2)O, a model material for a one-dimensional bond alternating Heisenberg chain, using the high resolution neutron-resonance spin echo (NRSE) technique. Inelastic neutron scattering experiments on dispersive excitations including phase sensitive measurements demonstrate the potential of NRSE to resolve line shapes, which are non-Lorentzian, opening up a new and hitherto unexplored class of experiments for the NRSE method beyond standard linewidth measurements. The particular advantage of NRSE is its direct access to the correlations in the time domain without convolution with the resolution function of the background spectrometer. This application of NRSE is very promising and establishes a basis for further experiments on different systems, since the results for Cu(NO3)(2)center dot 2.5D(2)O are applicable to a broad range of quantum systems.