Field-angular anisotropy of multipolar excitations
Final Report Abstract
In our project, we have investigated the role of magnetic measurements in rotating magnetic fields for understanding the complex magnetic phase diagrams and excitations spectra of several Ce-based f-electron systems. We have developed the concept of a piezo-driven sample rotator for neutron scattering to change the sample orientation in-situ within a cryomagnet. Several designs of such rotators have been proposed for time-of-flight and triple-axis spectrometers. The first tests of the rotator for time-of-flight measurements in the conditions of a real experiment were carried out recenrly, and we are currently continuing its optimization based on the results. We have carried out detailed investigations of field-angle magnetic phase diagrams and magnetic-field-induced excitation spectra on a number of Ce-based f-electron systems such as Ce1−x La x B6 , Ce1−x Nd x B6 , Ce3 Pd20 Si6 , Ce3 Pd20 Ge6 , and Ce2 Bi. The common theme of all these measurements was that they were carried out in magnetic fields applied not only along high-symmetry directions of our single-crystal samples but rather along general low-symmetry directions. The results clearly indicate that a substantial amount of additional information about the magnetic properties of the system can be obtained in this way. First of all, the magnetic ground states themselves, i.e. the stability regions of different long-range ordered phases in the magnetic phase diagrams, show a substantial anisotropy in magnetic-field space. This anisotropy can be large even in systems with cubic symmetry due to an interplay of crystal-electric-field, magnetic exchange, and Zeeman interactions. Furthermore, additional spin excitations in the form of dispersive collective spin-wave-like modes appear under these conditions. Magnetic fields applied along a general crystallographic direction are able to lift the degeneracy of certain collective modes that would otherwise merge and become indistinguishable under more symmetric conditions. In systems like Ce3 Pd20 Si6 and Ce3 Pd20 Ge6 , this leads to an amazing cascade of field-driven meta-multipolar phase transitions with a new Goldstone-type mode emanating at every such transition from zero energy and shifting toward higher energies in an increasing field. The results of our project also opened several new research directions by providing unexpected results on magnetic materials that were previously believed to be relatively well understood. For example, our results question the ferroquadrupolar nature of the “hidden-order” phase in Ce3 Pd20 Ge6 . We have shown that this system exhibits critical spin fluctuations in the same magnetic-field range and at the same wave vector as its isostructural counterpart Ce3 Pd20 Si6. These quasielastic gapless fluctuations centered at the corner of the Brillouin zone speak rather in favor of an alternative hypotheses that quadrupoles in Ce3 Pd20 Ge6 order antiferromagnetically, which allows for the existence of a magneticfield-driven quantum critical point. Another unexpected result was the observation of dispersive excitations in Ce2 Bi that were previously believed to be broadened crystal-electric-field lines of purely local origin. The theoretical interpretation of these results is still ongoing, and their publication may require additional effort such as follow-up measurements or model calculations.
Publications
-
Field-angle resolved magnetic excitations as a probe of hidden-order symmetry in CeB6 . Phys. Rev. X 10, 021010.
Portnichenko, P. Y.; Akbari, A.; Nikitin, S. E.; Cameron, A. S.; Dukhnenko, A. V.; Filipov, V. B.; Shitsevalova, N. Yu.; Čermák, P.; Radelytskyi, I.; Schneidewind, A.; Ollivier, J.; Podlesnyak, A.; Huesges, Z.; Xu, J.; Ivanov, A.; Sidis, Y.; Petit, S.; Mignot, J.-M.; Thalmeier, P. & Inosov, D. S.
-
Local origin of the strong field-space anisotropy in the magnetic phase diagrams of Ce1−x La x B6 measured in a rotating magnetic field. Phys. Rev. B 103, 214415.
Inosov, D. S.; Avdoshenko, S.; Portnichenko, P. Y.; Choi, Eun Sang; Schneidewind, A.; Mignot, J.-M. & Nikolo, M.
-
Neutron-Scattering Studies of Spin Dynamics in Pure and Doped CeB6. Rare-Earth Borides, 691-772. Jenny Stanford Publishing.
Portnichenko, Paulo Y.; Cameron, Alistair S. & Inosov, Dmytro S.
-
Rare-Earth Borides. Jenny Stanford Publishing.
Inosov, Dmytro S.
-
Cascade of magnetic-field-driven quantum phase transitions in Ce3 Pd20 Si6. Phys. Rev. B 105, 174429.
Mazza, F.; Portnichenko, P. Y.; Avdoshenko, S.; Steffens, P.; Boehm, M.; Choi, Eun Sang; Nikolo, M.; Yan, X.; Prokofiev, A.; Paschen, S. & Inosov, D. S.
