699 research outputs found
Thermal Conductivity of Thermally-Isolating Polymeric and Composite Structural Support Materials Between 0.3 and 4 K
We present measurements of the low-temperature thermal conductivity of a
number of polymeric and composite materials from 0.3 to 4 K. The materials
measured are Vespel SP-1, Vespel SP-22, unfilled PEEK, 30% carbon fiber-filled
PEEK, 30% glass-filled PEEK, carbon fiber Graphlite composite rod, Torlon 4301,
G-10/FR-4 fiberglass, pultruded fiberglass composite, Macor ceramic, and
graphite rod. These materials have moderate to high elastic moduli making them
useful for thermally-isolating structural supports.Comment: Accepted for publication in the journal Cryogenic
Evidence for gapped spin-wave excitations in the frustrated Gd2Sn2O7 pyrochlore antiferromagnet from low-temperature specific heat measurements
We have measured the low-temperature specific heat of the geometrically
frustrated pyrochlore Heisenberg antiferromagnet Gd2Sn2O7 in zero magnetic
field. The specific heat is found to drop exponentially below approximately 350
mK. This provides evidence for a gapped spin-wave spectrum due to an anisotropy
resulting from single ion effects and long-range dipolar interactions. The data
are well fitted by linear spin-wave theory, ruling out unconventional low
energy magnetic excitations in this system, and allowing a determination of the
pertinent exchange interactions in this material
2014 Dahlberg Award Winner: The effects of dietary toughness on occlusopalatal variation in savanna baboons
This study investigates the relationship between dietary toughness and craniofacial variation in two groups of savanna baboons. Standard craniofacial and malocclusion data were collected from a captive, soft-diet experiment group (n=24) and a sample of wild-captured baboons, raised on tougher, natural foods (n=19). We tested the hypothesis that in the absence of normal masticatory stress experienced during the consumption of wild foods, the captive baboons would exhibit higher levels of facial and dental structural irregularities. Principal component analysis indicates separation of the two samples. The soft-diet sample exhibits significantly shorter palates, greater variability in palate position, and higher frequencies of occlusal irregularities that correlate with the shorter palates. Results offer further support that long-term dietary chewing stresses have a measurable effect on adult craniofacial variation
Dental Occlusion in a Split Amazon Indigenous Population: Genetics Prevails over Environment
Background: Studies examining human and nonhuman primates have supported the hypothesis that the recent increase in
the occurrence of misalignment of teeth and/or incorrect relation of dental arches, named dental malocclusion, is mainly
attributed to the availability of a more processed diet and the reduced need for powerful masticatory action. For the first
time on live human populations, genetic and tooth wear influences on occlusal variation were examined in a split
indigenous population. The Arara-Iriri people are descendants of a single couple expelled from a larger village. In the
resultant village, expansion occurred through the mating of close relatives, resulting in marked genetic cohesion with
substantial genetic differences.
Methodology/Principal Findings: Dental malocclusion, tooth wear and inbreeding coefficient were evaluated. The sample
examined was composed of 176 individuals from both villages. Prevalence Ratio and descriptive differences in the
outcomes frequency for each developmental stage of the dentition were considered. Statistical differences between the
villages were examined using the chi-square test or Fisher’s exact statistic. Tooth wear and the inbreeding coefficient (F)
between the villages was tested with Mann-Whitney statistics. All the statistics were performed using two-tailed distribution
at p#0.05. The coefficient inbreeding (F) confirmed the frequent incestuous unions among the Arara-Iriri indigenous group.
Despite the tooth wear similarities, we found a striking difference in occlusal patterns between the two Arara villages. In the
original village, dental malocclusion was present in about one third of the population; whilst in the resultant village, the
occurrence was almost doubled. Furthermore, the morphological characteristics of malocclusion were strongly different
between the groups.
Conclusions/Significance: Our findings downplay the widespread influence of tooth wear, a direct evidence of what an
individual ate in the past, on occlusal variation of living human populations. They also suggest that genetics plays the most
important role on dental malocclusion etiology
Juxtaposition of Spin Freezing and Long Range Order in a Series of Geometrically Frustrated Antiferromagnetic Gadolinium Garnets
Specific heat measurements in zero magnetic field are presented on a
homologous series of geometrically frustrated, antiferromagnetic, Heisenberg
garnet systems. Measurements of Gd3Ga5O12, grown with isotopically pure Gd,
agree well with previous results on samples with naturally abundant Gd, showing
no ordering features. In contrast, samples of Gd3Te2Li3O12 and Gd3Al5O12 are
found to exhibit clear ordering transitions at 243 mK and 175 mK respectively.
The effects of low level disorder are studied through dilution of Gd3+ with
non-magnetic Y3+ in Gd3Te2Li3O12. A thorough structural characterization, using
X-ray diffraction, is performed on all of the samples studied. We discuss
possible explanations for such diverse behavior in very similar systems.Comment: Accepted for publication in Physical Review
Hidden spin-current conservation in 2d Fermi liquids
We report the existence of regimes of the two dimensional Fermi liquid that
show unusual conservation of the spin current and may be tuned by varying some
parameter like the density of fermions. We show that for reasonable models of
the effective interaction the spin current may be conserved in general in 2d,
not only for a particular regime. Low temperature spin waves propagate
distinctively in these regimes and entirely new ``spin-acoustic'' modes are
predicted for scattering-dominated temperature ranges. These new
high-temperature propagating spin waves provide a clear signature for the
experimental search of such regimes.Comment: 4 pages, no figures, revised version, accepted for pub. in the PR
On the problem of catastrophic relaxation in superfluid 3-He-B
In this Letter we discussed the parametric instability of texture of
homogeneous (in time) spin precession, explaining how spatial inhomogeneity of
the texture may change the threshold of the instability in comparison with
idealized spatial homogeneous case, considered in our JETP Letter \textbf{83},
530 (2006), cond-mat/0605386. This discussion is inspired by critical Comment
of I.A. Fomin (cond-mat/0606760) related to the above questions. In addition we
considered here results of direct numerical simulations of the full
Leggett-Takagi equation of motion for magnetization in superfluid 3He-B and
experimental data for magnetic field dependence of the catastrophic relaxation,
that provide solid support of the theory of this phenomenon, presented in our
2006 JETP Letter.Comment: 5 pages, 1 fig. included, JETP Lett. style, submitted to JETP Lett.
as response to Comment cond-mat/060676
Quantum spin fluctuations in the dipolar Heisenberg-like rare earth pyrochlores
The magnetic pyrochlore oxide materials of general chemical formula R2Ti2O7
and R2Sn2O7 (R = rare earth) display a host of interesting physical behaviours
depending on the flavour of rare earth ion. These properties depend on the
value of the total magnetic moment, the crystal field interactions at each rare
earth site and the complex interplay between magnetic exchange and long-range
dipole-dipole interactions. This work focuses on the low temperature physics of
the dipolar isotropic frustrated antiferromagnetic pyrochlore materials.
Candidate magnetic ground states are numerically determined at zero temperature
and the role of quantum spin fluctuations around these states are studied using
a Holstein-Primakoff spin wave expansion to order 1/S. The results indicate the
strong stability of the proposed classical ground states against quantum
fluctuations. The inclusion of long range dipole interactions causes a
restoration of symmetry and a suppression of the observed anisotropy gap
leading to an increase in quantum fluctuations in the ground state when
compared to a model with truncated dipole interactions. The system retains most
of its classical character and there is little deviation from the fully ordered
moment at zero temperature.Comment: Latex2e, 18 pages, 4 figures, IOP forma
Bonding, Moment Formation, and Magnetic Interactions in Ca14MnBi11 and Ba14MnBi11
The ``14-1-11'' phase compounds based on magnetic Mn ions and typified by
Ca14MnBi11 and Ba14MnBi11 show unusual magnetic behavior, but the large number
(104) of atoms in the primitive cell has precluded any previous full electronic
structure study. Using an efficient, local orbital based method within the
local spin density approximation to study the electronic structure, we find a
gap between a bonding valence band complex and an antibonding conduction band
continuum. The bonding bands lack one electron per formula unit of being
filled, making them low carrier density p-type metals. The hole resides in the
MnBi4 tetrahedral unit and partially compensates the high spin d^5 Mn moment,
leaving a net spin near 4 \mu_B that is consistent with experiment. These
manganites are composed of two disjoint but interpenetrating `jungle gym'
networks of spin 4/2 MnBi4^{9-} units with ferromagnetic interactions within
the same network, and weaker couplings between the networks whose sign and
magnitude is sensitive to materials parameters. Ca14MnBi11 is calculated to be
ferromagnetic as observed, while for Ba14MnBi11 (which is antiferromagnetic)
the ferro- and antiferromagnetic states are calculated to be essentially
degenerate. The band structure of the ferromagnetic states is very close to
half metallic.Comment: 17 pages, containing 10 postscript figures and 5 tables. Two
additional figures (Fig.8 and 11 of the paper) are provided in JPG format in
separate files. Submitted to Phys. Rev. B on September 20th 200
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