768 research outputs found
The Quasi-1D S=1/2 Antiferromagnet Cs2CuCl4 in a Magnetic Field
Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF)
Cs2CuCl4 have been measured as a function of magnetic field using neutron
scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D
incommensurate ordering. Fields greater than Bc =1.66 T, but less than the
field (~8 T) required to fully align the spins, are observed to decouple the
chains, and the system enters a disordered intermediate-field phase (IFP). The
IFP excitations are in agreement with the predictions of Muller et al. for the
1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the
Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave
theory.Comment: 10 pages, 4 encapsulated postscript figures, LaTeX, to be published
in PRL, e-mail comments to [email protected]
Temperature Evolution of the Quantum Gap in CsNiCl3
Neutron scattering measurements on the one-dimensional gapped S=1
antiferromagnet, CsNiCl3, have shown that the excitation corresponding to the
Haldane mass gap Delta at low temperatures persists as a resonant feature to
high temperatures. We find that the strong upward renormalisation of the gap
excitation, by a factor of three between 5 and 70K, is more than enough to
overcome its decreasing lifetime. We find that the gap lifetime is
substantially shorter than that predicted by the scaling theory of Damle and
Sachdev in its low temperature range of validity. The upward gap
renormalisation agrees with the non-linear sigma model at low temperatures and
even up to T of order 2Delta provided an upper mass cutoff is included.Comment: Latex, 3 figures, accepted by Pysical Review
Spin wave spectrum of the quantum ferromagnet on the pyrochlore lattice Lu2V2O7
Neutron inelastic scattering has been used to probe the spin dynamics of the
quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined
spin waves are observed at all energies and wavevectors, allowing us to
determine the parameters of the Hamiltonian of the system. The data are found
to be in excellent overall agreement with a minimal model that includes a
nearest- neighbour Heisenberg exchange J = 8:22(2) meV and a
Dzyaloshinskii-Moriya interaction (DMI) D =1:5(1) meV. The large DMI term
revealed by our study is broadly consistent with the model developed by Onose
et al. to explain the magnon Hall effect they observed in Lu2V2O7 [1], although
our ratio of D=J = 0:18(1) is roughly half of their value and three times
larger than calculated by ab initio methods [2].Comment: 5 pages, 4 figure
Determination of the Antiferroquadrupolar Order Parameters in UPd3
By combining accurate heat capacity and X-ray resonant scattering results we
have resolved the long standing question regarding the nature of the
quadrupolar ordered phases in UPd_3. The order parameter of the highest
temperature quadrupolar phase has been uniquely determined to be antiphase
Q_{zx} in contrast to the previous conjecture of Q_{x^2-y^2} . The azimuthal
dependence of the X-ray scattering intensity from the quadrupolar superlattice
reflections indicates that the lower temperature phases are described by a
superposition of order parameters. The heat capacity features associated with
each of the phase transitions characterize their order, which imposes
restrictions on the matrix elements of the quadrupolar operators.Comment: 4 pages, 5 figure
The J_{eff}=1/2 insulator Sr3Ir2O7 studied by means of angle-resolved photoemission spectroscopy
The low-energy electronic structure of the J_{eff}=1/2 spin-orbit insulator
Sr3Ir2O7 has been studied by means of angle-resolved photoemission
spectroscopy. A comparison of the results for bilayer Sr3Ir2O7 with available
literature data for the related single-layer compound Sr2IrO4 reveals
qualitative similarities and similar J_{eff}=1/2 bandwidths for the two
materials, but also pronounced differences in the distribution of the spectral
weight. In particuar, photoemission from the J_{eff}=1/2 states appears to be
suppressed. Yet, it is found that the Sr3Ir2O7 data are in overall better
agreement with band-structure calculations than the data for Sr2IrO4.Comment: 5 pages, 3 figure
Experimental Proof of a Magnetic Coulomb Phase
Spin ice materials are magnetic substances in which the spin directions map
onto hydrogen positions in water ice. Recently this analogy has been elevated
to an electromagnetic equivalence, indicating that the spin ice state is a
Coulomb phase, with magnetic monopole excitations analogous to ice's mobile
ionic defects. No Coulomb phase has yet been proved in a real magnetic
material, as the key experimental signature is difficult to resolve in most
systems. Here we measure the scattering of polarised neutrons from the
prototypical spin ice Ho2Ti2O7. This enables us to separate different
contributions to the magnetic correlations to clearly demonstrate the existence
of an almost perfect Coulomb phase in this material. The temperature dependence
of the scattering is consistent with the existence of deconfined magnetic
monopoles connected by Dirac strings of divergent length.Comment: 18 pages, 4 fig
Crossover from itinerant to localized magnetic excitations through the metal-insulator transition in NaOsO
NaOsO undergoes a metal-insulator transition (MIT) at 410 K,
concomitant with the onset of antiferromagnetic order. The excitation spectra
have been investigated through the MIT by resonant inelastic x-ray scattering
(RIXS) at the Os L edge. Low resolution ( 300 meV)
measurements over a wide range of energies reveal that local electronic
excitations do not change appreciably through the MIT. This is consistent with
a picture in which structural distortions do not drive the MIT. In contrast,
high resolution ( 56 meV) measurements show that the
well-defined, low energy magnons in the insulating state weaken and dampen upon
approaching the metallic state. Concomitantly, a broad continuum of excitations
develops which is well described by the magnetic fluctuations of a nearly
antiferromagnetic Fermi liquid. By revealing the continuous evolution of the
magnetic quasiparticle spectrum as it changes its character from itinerant to
localized, our results provide unprecedented insight into the nature of the MIT
in \naoso. In particular, the presence of weak correlations in the paramagnetic
phase implies a degree of departure from the ideal Slater limit.Comment: Joint submission with Physical Review Letters [Phys. Rev. Lett. 120,
227203 (2018), accepted version at arXiv:1805.03176]. This article includes
further discussion about the calculations performed, models used, and so o
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