Neutron resonant spin-echo spectroscopy

The neutron resonant spin-echo spectroscopy, in combination with triple-axis spectroscopy, affords the determination of lifetimes of dispersive excitations over the entire Brillouin zone with μeV energy resolution (Fig. 1). The energy resolution of this method thus exceeds that of conventional neutron spectroscopy by about three orders of magnitude. The idea underlying the method dates back to the early 1970's, but technical obstacles had prevented its implementation. We have systematically overcome these technical obstacles using a prototype setup at the Hahn-Meitner-Institut in Berlin [1]. Based on this experience, the triple axis resonant spin echo spectrometer TRISP was built and commissioned at the FRM-II research reactor in Garching. The instrument performance is excellent.

During the first year of operation of TRISP, we have completed detailed studies of the lifetimes of acoustic phonons in lead and niobium [2], and of antiferromagnetic magnons in MnF₂. In the former study, we monitored the renormalization of the electron-phonon linewidth across the superconducting transition temperature, which amounts to a few μeV and is hence inaccessible to standard neutron (or x-ray) spectroscopy [3]. The data allowed a detailed comparison with ab-initio density functional calculations performed in Andersen's group. In the latter study, 30-year-old predictions of magnon lifetimes were subjected to a first experimental test [4]. The results revealed that scattering from longitudinal spin fluctuations (and not the extensively studied magnon-magnon scattering channel) limits the magnon lifetimes over a wide range of temperatures. The data also exhibit several anomalous features that provide challenges for future theoretical work. We are currently using TRISP to study spin excitations in low-dimensional quantum magnets and phonons in various transition metal oxides. Finally, highly accurate determinations of lattice parameters under high pressure ("Larmor diffraction") on TRISP are being used to study the critical behavior at pressure-induced quantum phase transitions, for instance in the helimagnet MnSi.