56 research outputs found
Ordering of the pyrochlore Ising model with the long-range RKKY interaction
The ordering of the Ising model on a pyrochlore lattice interacting via the
long-range RKKY interaction, which models a metallic pyrochlore magnet such as
Pr_2Ir_2O_7, is studied by Monte Carlo simulations. Depending on the parameter
k_F representing the Fermi wavevector, the model exhibits rich ordering
behaviors
Interplay of Spin-Orbit Interaction and Electron Correlation on the Van Vleck Susceptibility in Transition Metal Compounds
We have studied the effects of electron correlation on Van Vleck
susceptibility () in transition metal compounds. A typical
crossover behavior is found for the correlation effect on as
sweeping spin-orbit interaction, . For a small , orbital
fluctuation plays a dominant role in the correlation enhancement of
; however, the enhancement rate is rather small. In contrast,
for an intermediate , shows a substantial increase,
accompanied by the development of spin fluctuation. We will discuss the
behavior of in association with the results of Knight-shift
experiments on SrRuO and an anomalously large magnetic susceptibility
observed for Ir compounds.Comment: 5 pages, 3 figures, to appear in J. Phys. Soc. Jp
Microrheology with optical tweezers: data analysis
We present a data analysis procedure that provides the solution to a long-standing issue in microrheology studies, i.e. the evaluation of the fluids' linear viscoelastic properties from the analysis of a finite set of experimental data, describing (for instance) the time-dependent mean-square displacement of suspended probe particles experiencing Brownian fluctuations. We report, for the first time in the literature, the linear viscoelastic response of an optically trapped bead suspended in a Newtonian fluid, over the entire range of experimentally accessible frequencies. The general validity of the proposed method makes it transferable to the majority of microrheology and rheology techniques
Theory of paramagnetic scattering in highly frustrated magnets with long-range dipole-dipole interactions: The case of the Tb2Ti2O7, pyrochlore antiferromagnet
Highly frustrated antiferromagnets composed of magnetic rare-earth moments
are currently attracting much experimental and theoretical interest. Rare-earth
ions generally have small exchange interactions and large magnetic moments.
This makes it necessary to understand in detail the role of long-range magnetic
dipole-dipole interactions in these systems, in particular in the context of
spin-spin correlations that develop in the paramagnetic phase, but are often
unable to condense into a conventional long-range magnetic ordered phase. This
scenario is most dramatically emphasized in the frustrated pyrochlore
antiferromagnet material Tb2Ti207 which does not order down to 50 mK despite an
antiferromagnetic Curie-Weiss temperature Tcw ~ -20 K. In this paper we report
results from mean-field theory calculations of the paramagnetic elastic
neutron-scattering in highly frustrated magnetic systems with long-range
dipole-dipole interactions, focusing on the Tb2Ti207 system. Modeling Tb2Ti207
as an antiferromagnetic Ising pyrochlore, we find that the mean-field
paramagnetic scattering is inconsistent with the experimentally observed
results. Through simple symmetry arguments we demonstrate that the observed
paramagnetic correlations in Tb2Ti207 are precluded from being generated by any
spin Hamiltonian that considers only Ising spins, but are qualitatively
consistent with Heisenberg-like moments. Explicit calculations of the
paramagnetic scattering pattern for both Ising and Heisenberg models,
which include finite single-ion anisotropy, support these claims. We offer
suggestions for reconciling the need to restore spin isotropy with the Ising
like structure suggested by the single-ion properties of Tb3+.Comment: Revtex4, 18 pages, 3 eps figures (2 color figures). Change in title
and emphasis on Tb2Ti2O7 only. Spin-ice material removed, to appear in a
later publicatio
Geometrical frustration induced (semi-)metal to insulator transition
We study the low-energy properties of the geometrically frustrated Hubbard
model on a three-dimensional pyrochlore lattice and a two-dimensional
checkerboard lattice on the basis of the renormalization group method and mean
field analysis. It is found that in the half-filling case, a (semi-)metal to
insulator transition (MIT) occurs. Also, in the insulating phase, which has a
spin gap, the spin rotational symmetry is not broken, while charge ordering
exists. The results are applied to the description of the MIT observed in the
pyrochlore system .Comment: 4 pages, 5 figure
Emergence of magnetic long-range order in frustrated pyrochlore NdIrO with metal-insulator transition
In this study, we performed powder neutron diffraction and inelastic
scattering measurements of frustrated pyrochlore NdIrO, which
exhibits a metal-insulator transition at a temperature of 33 K.
The diffraction measurements revealed that the pyrochlore has an
antiferromagnetic long-range structure with propagation vector of
(0,0,0) and that it grows with decreasing temperature below 15 K. This
structure was analyzed to be of the all-in all-out type, consisting of highly
anisotropic Nd magnetic moments of magnitude ,
where is the Bohr magneton. The inelastic scattering measurements
revealed that the Kramers ground doublet of Nd splits below . This suggests the appearance of a static internal magnetic field at the
Nd sites, which probably originates from a magnetic order consisting of
Ir magnetic moments. Here, we discuss a magnetic structure model for the
Ir order and the relation of the order to the metal-insulator transition in
terms of frustration.Comment: 6 pages, 1 table, 3 figure
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