Page 42 - ILLS Annual Report 2018

MAGNETISM
Romain Sibille. French
Laboratory for Neutron Scattering and Imaging, PSI, Switzerland
‘After getting my PhD in chemistry, I joined PSI in 2012. Since 2016, I have been co-responsible for the thermal neutron diffractometer Zebra. My main research
interests are the preparation, crystal chemistry and fundamental properties of magnetic materials with strong fluctuations— especially rare-earth pyrochlores.’
Signatures of a quantum spin ice ground state
Time-of-flight spectrometer IN5
HYSPEC spectrometer
at Oak Ridge National Laboratories
Quantum spin ice is an appealing proposition of a quantum spin liquid— systems in which the magnetic moments
of the constituent electron spins evade classical long-range order to form an exotic state that is quantum entangled and coherent over macroscopic length scales. Such phases are at the edge of our current knowledge on condensed matter, as they go beyond the established paradigm of symmetry-breaking order and associated excitations. Experiments carried out on the disk chopper time-of-flight spectrometer IN5 reveal signatures of a quantum spin ice that were predicted by theory.
Figure 1
In a classical spin ice, uniaxial magnetic moments decorate a pyrochlore lattice (in black). Magnetic moments (blue/red ellipses) on each tetrahedra are constrained
by a local ‘2-in-2-out’ organisation principle. Moments can be viewed as magnetic fluxes forming a diamond lattice (in blue), which can be coarse-grained to define a continuous medium with emergent magnetostatics. Quantum dynamics on a six-member ring creates electric flux variables (in green) that form a second (interpenetrated) diamond lattice. This quantum spin ice ground state can be thought of as a lattice analogue of quantum electrodynamics, making the sample a tiny universe with its own emergent light of gapless magnetic excitations.
AUTHORS
R. Sibille, M. Kenzelmann and T. Fennell (Paul Scherrer Institute, PSI, Villigen, Switzerland)
N. Gauthier (SLAC National Accelerator Laboratory and Stanford University, California, USA)
J. Ollivier (ILL) ARTICLE FROM
Nat. Phys. (2018)—doi: 10.1038/s41567-018-0116-x
REFERENCES
[1] L. Balents, Nature 464 (2010) 199
[2] M.J.P. Gingras and P.A. McClarty, Rep. Prog. Phys. 77
(2014) 056501
[3] S. Petit, E. Lhotel et al., Nat. Phys. 12 (2016) 146
[4] E. Lhotel, S. Petit et al., Nat. Commun. 9 (2018) 3786
Spin ices are crystalline materials with rather peculiar magnetic properties. The magnetic atoms in their lattices are arranged in geometries that resemble that of frozen water, and an analogous local rule for the electronic spins also prevents the formation of a single state of minimal energy—hence the name spin ice. When such systems with degenerate ground states remain dynamic even at zero temperature they are collectively known as quantum spin liquids; and they have long attracted considerable interest from theorists and experimentalists alike, as they harbour a wealth of exotic physics [1].
Discovered in the late ‘90s in magnetic pyrochlore compounds, classical ‘spin ices’ are materials that contain magnetic moments distributed on a network of corner-sharing tetrahedra. Because of the structure of the material, each magnetic moment is constrained to align along the direction joining its position and the centres of two tetrahedra. With ferromagnetic interactions, two of the four magnetic moments in each tetrahedron must
ANNUAL REPORT 2018