954 research outputs found
Metamagnetism and soliton excitations in the modulated ferromagnetic Ising chain CoV2O6
We report a combination of physical property and neutron scattering
measurements for polycrystalline samples of the one-dimensional spin chain
compound CoV2O6. Heat capacity measurements show that an effective S = 1/2
state is found at low temperatures and that magnetic fluctuations persist up to
6.Tn. Above Tn = 6.3 K, measurements of the magnetic susceptibility as a
function of T and H show that the nearest neighbour exchange is ferromagnetic.
In the ordered state, we have discovered a crossover from a metamagnet with
strong fluctuations between 5 K and Tn to a state with a 1/3 magnetisation
plateau at 2 < T < 5 K. We use neutron powder diffraction measurements to show
that the AFM state has incommensurate long range order and inelastic time of
flight neutron scattering to examine the magnetic fluctuations as a function of
temperature. Above Tn, we find two broad bands between 3.5 and 5 meV and
thermally activated low energy features which correspond to transitions within
these bands. These features show that the excitations are deconfined solitons
rather than the static spin reversals predicted for a uniform FM Ising spin
chain. Below Tn, we find a ladder of states due to the confining effect of the
internal field. A region of weak confinement below Tn, but above 5 K, is
identified which may correspond to a crossover between 2D and 3D magnetic
ordering.Comment: Expanded version, includes results from arXiv:0804.2966 and neutron
powder diffraction. To appear in PR
Charge and orbital order in frustrated Pb3Mn7O15
The candidate magnetoelectric Pb3Mn7O15 has a structure consisting of 1/3
filled Kagome layers linked by ribbons of edge-sharing octahedra in the
stacking direction. Previous reports have indicated a complex
hexagonal-orthorhombic structural transition upon cooling to room temperature,
although its origins are uncertain. Here both structures are revisited using a
combination of neutron and synchrotron X-ray diffraction data. Large shifts of
oxygen positions are detected which show that the interlayer sites and those
which occupy voids in the kagome lattice are trivially charge ordered in both
phases. The symmetry breaking is found to occur due to Mn3+ orbital ordering on
the ribbon sites and charge ordering of the sub-set of layer sites which make
up a Kagome network.Comment: Minor changes, to appear in J. Phys. Cond. Mat
Quasiparticle interference in antiferromagnetic parent compounds of Fe-based superconductors
Recently reported quasiparticle interference imaging in underdoped
Ca(Fe{1-x}Co{x})_2As{2} shows pronounced C{2} asymmetry that is interpreted as
an indication of an electronic nematic phase with a unidirectional electron
band, dispersive predominantly along the -axis of this orthorhombic
material. On the other hand, even more recent transport measurements on
untwinned samples show near isotropy of the resistivity in the plane, with
slightly larger conductivity along a (and not b). We show that in fact both
sets of data are consistent with the calculated ab initio Fermi surfaces, which
has a decisively broken C_{4}, and yet similar Fermi velocity in both
directions. This reconciles completely the apparent contradiction between the
conclusions of the STM and the transport experiments.Comment: A version of this work was posted (arXiv:1005.1761) as a comment on a
Science paper entitled "Nematic Electronic Structure in the Parent State of
the Iron-Based Superconductor Ca(Fe1-xCox)2As2". The comment was rejected by
Science on account of it being posted on the ArXiv. This is a version
published in PRB as a research pape
A valence bond liquid on the honeycomb lattice
The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+
cations on cooling below 270C, where the magnetic susceptibility vanishes. We
use density functional theory calculations to show that this reflects the
formation of a 'valence bond crystal', with a strong bond disproportionation.
On warming, x-ray diffraction shows that discrete three-fold symmetry is
regained on average, and the dimerization apparently disappears. In contrast,
local structural measurements using high-energy x-rays, show that disordered
dimers survive at the nanoscale up to at least 650C. The high temperature phase
of Li2RuO3 is thus an example of a valence bond liquid, where thermal
fluctuations drive resonance between different dimer coverages, a classic
analogue of the resonating valence bond state often discussed in connection
with high T cuprates.Comment: 5 pages, 4 figures, References correcte
Measurement of cardiorespiratory fitness in children from two commonly used field tests after accounting for body fatness and maturity
Body fat and maturation both influence cardiorespiratory fitness, however few studies have taken these variables into account when using field tests to predict children's fitness levels. The purpose of this study was to determine the relationship between two field tests of cardiorespiratory fitness (20 m Maximal Multistage Shuttle Run [20-MST], 550 m distance run [550-m]) and direct measurement of VO2max after adjustment for body fatness and maturity levels. Fifty-three participants (25 boys, 28 girls, age 10.6 ± 1.2 y, mean ± SD) had their body fat levels estimated using bioelectrical impedance (16.6% ± 6.0% and 20.0% ± 5.8% for boys and girls, respectively). Participants performed in random order, the 20-MST and 550-m run followed by a progressive treadmill test to exhaustion during which gas exchange measures were taken. Pearson correlation coefficient analysis revealed that the participants' performance in the 20-MST and 550-m run were highly correlated to VO2 max obtained during the treadmill test to exhaustion (r = 0.70 and 0.59 for 20-MST and 550-m run, respectively). Adjusting for body fatness and maturity levels in a multivariate regression analysis increased the associations between the field tests and VO2max (r = 0.73 for 20-MST and 0.65 for 550-m). We may conclude that both the 20-MST and the 550-m distance run are valid field tests of cardiorespiratory fitness in New Zealand 8-13 year old children and incorporating body fatness and maturity levels explains an additional 5-7% of the variance. © Editorial Committee of Journal of Human Kinetics
Local moments and symmetry breaking in metallic PrMnSbO
We report a combined experimental and theoretical investigation of the
layered antimonide PrMnSbO which is isostructural to the parent phase of the
iron pnictide superconductors. We find linear resistivity near room temperature
and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction
shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K,
followed by a spin-flop coupled to induced Pr order. At T \sim 35 K, we find a
tetragonal to orthorhombic (T-O) transition. First principles calculations show
that the large magnetic moments observed in this metallic compound are of local
origin. Our results are thus inconsistent with either the itinerant or
frustrated models proposed for symmetry breaking in the iron pnictides. We show
that PrMnSbO is instead a rare example of a metal where structural distortions
are driven by f-electron degrees of freedom
Charge order at the frontier between the molecular and solid states in Ba3NaRu2O9
We show that the valence electrons of Ba3NaRu2O9, which has a quasi-molecular
structure, completely crystallize below 210 K. Using an extended Hubbard model,
we show that the charge ordering instability results from long-range Coulomb
interactions. However, orbital ordering, metal-metal bonding and formation of a
partial spin gap enforce the magnitude of the charge separation. The striped
charge order and frustrated hcp lattice of Ru2O9 dimers lead to competition
with a quasi-degenerate charge-melted phase under photo-excitation at low
temperature. Our results establish a broad class of simple metal oxides as
models for emergent phenomena at the border between the molecular and solid
states.Comment: Minor changes, with supporting information. To appear in Phys. Rev.
Let
- …