351 research outputs found
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
Dimensional Evolution of Spin Correlations in the Magnetic Pyrochlore Yb2Ti2O7
The pyrochlore material Yb2Ti2O7 displays unexpected quasi-two-dimensional
(2D) magnetic correlations within a cubic lattice environment at low
temperatures, before entering an exotic disordered ground state below T=265mK.
We report neutron scattering measurements of the thermal evolution of the 2D
spin correlations in space and time. Short range three dimensional (3D) spin
correlations develop below 400 mK, accompanied by a suppression in the
quasi-elastic (QE) scattering below ~ 0.2 meV. These show a slowly fluctuating
ground state with spins correlated over short distances within a
kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated
kagome spin-stars at higher temperatures. Furthermore, low-temperature specific
heat results indicate a sample dependence to the putative transition
temperature that is bounded by 265mK, which we discuss in the context of recent
mean field theoretical analysis.Comment: 5 pages, 6 figure
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
Existence of a phase transition under finite magnetic field in the long-range RKKY Ising spin glass DyYRuSi
A phase transition of a model compound of the long-range Ising spin glass
(SG) DyYRuSi, where spins interact via the RKKY
interaction, has been investigated. The static and the dynamic scaling analyses
reveal that the SG phase transition in the model magnet belongs to the
mean-field universality class. Moreover, the characteristic relaxation time in
finite magnetic fields exhibits a critical divergent behavior as well as in
zero field, indicating a stability of the SG phase in finite fields. The
presence of the SG phase transition in field in the model magnet strongly
syggests that the replica symmetry is broken in the long-range Ising SG.Comment: 4 pages, 4 figures, to be published in JPSJ (2010
Field-angle dependence of sound velocity in the Weyl semimetal TaAs
The elastic modulus of a single crystal of the Weyl semimetal TaAs
was investigated by measuring relative changes in the sound velocity under
application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo
technique, we studied the shear response of the crystal when the angle between
the sound wave propagation and the magnetic field is changed. We observe a
broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely
related to a longitudinal negative magnetoresistance and therefore might
provide evidence of the chiral anomaly, one of the main topological signatures
of this class of materials. We also observe quantum oscillations in the sound
velocity whose frequencies vary with magnetic field orientation. A fan diagram
of Landau level indices reveals topological and trivial Berry phases, depending
on the field orientation, indicating a sensitivity to different Fermi surface
pockets that do or do not enclose Weyl nodes respectively
Subclinical infection and asymptomatic carriage of gastrointestinal zoonoses: Occupational exposure, environmental pathways, and the anonymous spread of disease
Asymptomatic carriage of gastrointestinal zoonoses is more common in people whose profession involves them working directly with domesticated animals. Subclinical infections (defined as an infection in which symptoms are either asymptomatic or sufficiently mild to escape diagnosis) are important within a community as unknowing (asymptomatic) carriers of pathogens do not change their behaviour to prevent the spread of disease; therefore the public health significance of asymptomatic human excretion of zoonoses should not be underestimated. However, optimal strategies for managing diseases where asymptomatic carriage instigates further infection remain unresolved, and the impact on disease management is unclear. In this review we consider the environmental pathways associated with prolonged antigenic exposure and critically assess the significance of asymptomatic carriage in disease outbreaks Although screening high-risk groups for occupationally acquired diseases would be logistically problematical, there may be an economic case for identifying and treating asymptomatic carriage if the costs of screening and treatment are less than the costs of identifying and treating those individuals infected by asymptomatic hosts
Spin Dynamics at Very Low Temperature in Spin Ice DyTiO
We have performed AC susceptibility and DC magnetic relaxation measurements
on the spin ice system DyTiO down to 0.08 K. The relaxation time of
the magnetization has been estimated below 2 K down to 0.08 K. The spin
dynamics of DyTiO is well described by using two relaxation times
( (short time) and (long time)). Both and increase on cooling. Assuming the Arrhenius law in the
temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K,
both and show a clear deviation from the thermal
activated dynamics toward temperature independent relaxation, suggesting a
quantum dynamics.Comment: 4 page
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