High-Resolution Neutron Spectroscopy in a Ferromagnetic Superconductor

by Dr Marc Janoscheck (Los Alamos National Laboratory)

Monday 23 Oct 2017, 14:30 → 15:30 Europe/Berlin

The low-temperature phase diagrams of metallic ferromagnets, including the quantum phase transition (QPT) from the ferromagnetic to the paramagnetic state are remarkably well understood. For example, Belitz-Kirkpatrick-Vojta (BKV) theory shows in excellent agreement with experiment that the ferromagnetic quantum phase transitions (QPTs) in clean metals are generally first-order due to the coupling of the magnetization to electronic soft modes, in contrast to the classical analogue that is an archetypical second-order phase transition [1]. Our recent extensive magnetization study on the disordered ferromagnetic metal UCo1-xFexGe demonstrate that BKV theory even correctly predicts that the second order nature of the QPT is restored because the electronic soft modes change their nature from ballistic to diffusive [2]. Despite this significant progress in understanding ferromagnetic metals, to date, little information is available on the underlying low-energy spin fluctuations that drive exotic behavior such as partial magnetic order, and topological non-Fermi liquid behavior, and unconventional spin-triplet superconductivity that is frequently observed in the vicinity of ferromagnetic QPTs. Here we report our recent extensive investigation of the spin fluctuations in the ferromagnetic superconductor UGe2 carried out at the resonance spin echo spectrometer RESEDA at MLZ. Our results demonstrate that the spin fluctuations in UGe2 are purely longitudinal with critical exponents that are in excellent agreement with a three-dimensional Ising ferromagnet as required for spin-triplet superconductivity [3]. Further, we show that the full spin fluctuation spectrum can only be revealed via the ultra-high momentum and energy resolution available at RESEDA.

[1] M. Brando, D. Belitz, F. M. Grosche and T. R. Kirkpatrick, Rev. Mod. Phys. 88, 025006 (2016). [2] K. Huang, S. Eley, P. F. S. Rosa, L. Civale, E. D. Bauer, R. E. Baumbach, M. B. Maple, and M. Janoschek, Phys. Rev. Lett. 117, 237202 (2016).
[3] F. Haslbeck, S. Säubert, M. Seifert, C. Franz, M. Schulz, A. Heinemann, Pinaki Das, J. D. Thompson, E. D. Bauer, C. Pfleiderer,1 and M. Janoschek, in preparation.