6 research outputs found
Ring-Exchange Interaction Effects on Magnons in Dirac Magnet CoTiO
In magnetically ordered materials with localized electrons, the fundamental
magnetic interactions are due to exchange of electrons [1-3]. Typically, only
the interaction between pairs of electrons' spins is considered to explain the
nature of the ground state and its excitations, whereas three-, four-, and
six-spin interactions are ignored. When these higher order processes occur in a
loop they are called cyclic or ring exchange. The ring-exchange interaction is
required to explain low temperature behavior in bulk and thin films of solid
He [4-8]. It also plays a crucial role in the quantum magnet LaCuO
[9,10]. Here, we use a combination of time domain THz (TDTS) and magneto-Raman
spectroscopies to measure the low energy magnetic excitations in CoTiO, a
proposed Dirac topological magnon material [11,12] where the origin of the
energy gap in the magnon spectrum at the Brillouin zone center remains unclear.
We measured the magnetic field dependence of the energies of the two lowest
energy magnons and determine that the gap opens due to the ring-exchange
interaction between the six spins in a hexagon. This interaction also explains
the selection rules of the THz magnon absorption. Finally, we clarify that
topological surface magnons are not expected in CoTiO. Our study
demonstrates the power of combining TDTS and Raman spectroscopies with theory
to identify the microscopic origins of the magnetic excitations in quantum
magnets.Comment: 7 pages, 4 figures in main text, 26 pages and 11 figures in
supplemen
Quantum Spin Ice Response to a Magnetic Field in the Dipole-Octupole Pyrochlore CeZrO
We report new heat capacity measurements on single crystal CeZrO
down to 0.1 K in a magnetic field along the direction.
These new measurements show that the broad hump in the zero-field heat capacity
moves higher in temperature with increasing field strength and is split into
two humps by the field at 2 T. These separate features
are due to the decomposition of the pyrochlore lattice into effectively
decoupled chains for fields in this direction: one set of chains
(-chains) is polarized by the field while the other (-chains)
remains free. Our theoretical modelling suggests that the -chains are
close to a critical state, with nearly-gapless excitations. We also report new
elastic and inelastic neutron scattering measurements on single crystal
CeZrO in and magnetic fields at
temperatures down to 0.03 K. The elastic scattering behaves consistently with
the formation of independent chains for a field, while the
field produces a single field-induced magnetic Bragg peak at and equivalent wavevectors, indicating a polarized spin ice for fields
above 3 T. For both and fields, our
inelastic neutron scattering results show an approximately-dispersionless
continuum of scattering that increases in both energy and intensity with
increasing field strength. By modelling the complete set of experimental data
using numerical linked cluster and semiclassical molecular dynamics
calculations, we demonstrate the dominantly multipolar nature of the exchange
interactions in CeZrO and the smallness of the parameter
which controls the mixing between dipolar and octupolar degrees of freedom.
These results support previous estimates of the microscopic exchange
parameters.Comment: 20 pages, 10 figure
Reply to "Comment on: 'Case for a U(1) Quantum Spin Liquid Ground State in the Dipole-Octupole Pyrochlore ' "
In his comment [arXiv:2209.03235], S. W. Lovesey argues that our analysis of
neutron scattering experiments performed on CeZrO is invalid.
Lovesey argues that we have not properly accounted for the higher-order
multipolar contributions to the magnetic scattering and that our use of
pseudospin- operators to describe the scattering is inappropriate. In this
reply, we show that the multipolar corrections discussed by Lovesey only become
significant at scattering wavevectors exceeding those accessed in our
experiments. This in no way contradicts or undermines our work, which never
claimed a direct observation of scattering from higher-order multipoles. We
further show that Lovesey's objections to our use of pseudospins are unfounded,
and that the pseudospin operators are able to describe all magnetic scattering
processes at the energy scale of our experiments, far below the crystal field
gap. Finally, we comment on certain assumptions in Lovesey's calculations of
the scattering amplitude which are inconsistent with experiment.Comment: 6 pages, 1 figur