18 research outputs found
Intertwined dipolar and multipolar order in the triangular-lattice magnet TmMgGaO
A phase transition is often accompanied by the appearance of an order
parameter and symmetry breaking. Certain magnetic materials exhibit exotic
hidden-order phases, in which the order parameters are not directly accessible
to conventional magnetic measurements. Thus, experimental identification and
theoretical understanding of a hidden order are difficult. Here we combine
neutron scattering and thermodynamic probes to study the newly discovered
rare-earth triangular-lattice magnet TmMgGaO. Clear magnetic Bragg peaks at
K points are observed in the elastic neutron diffraction measurements. More
interesting, however, is the observation of sharp and highly dispersive spin
excitations that cannot be explained by a magnetic dipolar order, but instead
is the direct consequence of the underlying multipolar order that is "hidden"
in the neutron diffraction experiments. We demonstrate that the observed
unusual spin correlations and thermodynamics can be accurately described by a
transverse field Ising model on the triangular lattice with an intertwined
dipolar and ferro-multipolar order.Comment: Published versio
Neutron Spin Resonance in the Heavily Hole-doped KFeAs Superconductor
We report high-resolution neutron scattering measurements of the low energy
spin fluctuations of KFeAs, the end member of the hole-doped
BaKFeAs family with only hole pockets, above and below its
superconducting transition temperature ( 3.5 K). Our data reveals
clear spin fluctuations at the incommensurate wave vector (, 0,
), ( = 0.2)(1-Fe unit cell), which exhibit -modulation peaking at
. Upon cooling to the superconducting state, the incommensurate spin
fluctuations gradually open a spin-gap and form a sharp spin resonance mode.
The incommensurability ( = 0.4) of the resonance mode ( meV)
is considerably larger than the previously reported value (
) at higher energies ( meV). The determination of the
momentum structure of spin fluctuation in the low energy limit allows a direct
comparison with the realistic Fermi surface and superconducting gap structure.
Our results point to an -wave pairing with a reversed sign between the hole
pockets near the zone center in KFeAs.Comment: Accepted for publication in Physical Review Letter
Frustrated magnetic interactions and quenched spin fluctuations in CrAs
The discovery of pressure-induced superconductivity in helimagnets (CrAs,
MnP) has attracted considerable interest in understanding the relationship
between complex magnetism and unconventional superconductivity. However, the
nature of the magnetism and magnetic interactions that drive the unusual
double-helical magnetic order in these materials remains unclear. Here, we
report neutron scattering measurements of magnetic excitations in CrAs single
crystals at ambient pressure. Our experiments reveal well defined spin wave
excitations up to about 150 meV with a pseudogap below 7 meV, which can be
effectively described by the Heisenberg model with nearest neighbor exchange
interactions. Most surprisingly, the spin excitations are largely quenched
above the Neel temperature, in contrast to cuprates and iron pnictides where
the spectral weight is mostly preserved in the paramagnetic state. Our results
suggest that the helimagnetic order is driven by strongly frustrated exchange
interactions, and that CrAs is at the verge of itinerant and
correlation-induced localized states, which is therefore highly
pressure-tunable and favorable for superconductivity.Comment: 6 pages, 4 figure