14,040 research outputs found
Liquid-like thermal conduction in a crystalline solid
A solid conducts heat through both transverse and longitudinal acoustic
phonons, but a liquid employs only longitudinal vibrations. Here, we report
that the crystalline solid AgCrSe2 has liquid-like thermal conduction. In this
compound, Ag atoms exhibit a dynamic duality that they are exclusively involved
in intense low-lying transverse acoustic phonons while they also undergo local
fluctuations inherent in an order-to-disorder transition occurring at 450 K. As
a consequence of this extreme disorder-phonon coupling, transverse acoustic
phonons become damped as approaching the transition temperature, above which
they are not defined anymore because their lifetime is shorter than the
relaxation time of local fluctuations. Nevertheless, the damped longitudinal
acoustic phonon survives for thermal transport. This microscopic insight might
reshape the fundamental idea on thermal transport properties of matter and
facilitates the optimization of thermoelectrics.Comment: four figures, supplemental informatio
Observation of ferromagnetism above 900 K in Cr-GaN and Cr-AlN
We report the observation of ferromagnetism at over 900K in Cr-GaN and Cr-AlN
thin films. The saturation magnetization moments in our best films of Cr-GaN
and Cr-AlN at low temperatures are 0.42 and 0.6 u_B/Cr atom, respectively,
indicating that 14% and 20%, of the Cr atoms, respectively, are magnetically
active. While Cr-AlN is highly resistive, Cr-GaN exhibits thermally activated
conduction that follows the exponential law expected for variable range hopping
between localized states. Hall measurements on a Cr-GaN sample indicate a
mobility of 0.06 cm^2/V.s, which falls in the range characteristic of hopping
conduction, and a free carrier density (1.4E20/cm^3), which is similar in
magnitude to the measured magnetically-active Cr concentration (4.9E19/cm^3). A
large negative magnetoresistance is attributed to scattering from loose spins
associated with non-ferromagnetic impurities. The results indicate that
ferromagnetism in Cr-GaN and Cr-AlN can be attributed to the double exchange
mechanism as a result of hopping between near-midgap substitutional Cr impurity
bands.Comment: 14 pages, 4 figures, submitted to AP
Long-range electronic reconstruction to a -dominated Fermi surface below the LaAlO/SrTiO interface
Low dimensionality, broken symmetry and easily-modulated carrier
concentrations provoke novel electronic phase emergence at oxide interfaces.
However, the spatial extent of such reconstructions - i.e. the interfacial
"depth" - remains unclear. Examining LaAlO/SrTiO heterostructures at
previously unexplored carrier densities cm,
we observe a Shubnikov-de Haas effect for small in-plane fields, characteristic
of an anisotropic 3D Fermi surface with preferential orbital
occupancy extending over at least 100~nm perpendicular to the interface.
Quantum oscillations from the 3D Fermi surface of bulk doped SrTiO emerge
simultaneously at higher . We distinguish three areas in doped
perovskite heterostructures: narrow ( nm) 2D interfaces housing
superconductivity and/or other emergent phases, electronically isotropic
regions far ( nm) from the interface and new intermediate zones where
interfacial proximity renormalises the electronic structure relative to the
bulk.Comment: Supplementary material available at Scientific Reports websit
The role of Cr substitution on the ferromagnetic properties of Ga1-xCrxN
Angular-dependent channeling Rutherford Backscattering Spectroscopy (c-RBS)
has been used to quantify the fraction of Cr atoms on substitutional,
interstitial, and random sites in epitaxial Ga1-xCrxN films grown by reactive
molecular-beam epitaxy. The morphology of these films and correlation with
their magnetic properties has been investigated. Films grown at temperatures
below ~ 750oC have up to 90% of Cr occupying substitutional sites. Post-growth
annealing at 825oC results in a systematic drop in the fraction of
substitutional Cr as well as a fall off in the ferromagnetic signal. The roles
of non-substitutional Cr in transferring charge from the Cr t2 band and
segregation of substitutional Cr in the loss of magnetism are discussed.
Overall, these results provide strong microscopic evidence that Cr-doped III-N
systems are dilute magnetic semiconductors.Comment: 10 pages, 4 figures, submitted to AP
Covariant polarized radiative transfer on cosmological scales for investigating large-scale magnetic field structures
Polarization of radiation is a powerful tool to study cosmic magnetism and
analysis of polarization can be used as a diagnostic tool for large-scale
structures. In this paper, we present a solid theoretical foundation for using
polarized light to investigate large-scale magnetic field structures: the
cosmological polarized radiative transfer (CPRT) formulation. The CPRT
formulation is fully covariant. It accounts for cosmological and relativistic
effects in a self-consistent manner and explicitly treats Faraday rotation, as
well as Faraday conversion, emission, and absorption processes. The formulation
is derived from the first principles of conservation of phase-space volume and
photon number. Without loss of generality, we consider a flat
Friedmann-Robertson-Walker (FRW) space-time metric and construct the
corresponding polarized radiative transfer equations. We propose an all-sky
CPRT calculation algorithm, based on a ray-tracing method, which allows
cosmological simulation results to be incorporated and, thereby, model
templates of polarization maps to be constructed. Such maps will be crucial in
our interpretation of polarized data, such as those to be collected by the
Square Kilometer Array (SKA). We describe several tests which are used for
verifying the code and demonstrate applications in the study of the
polarization signatures in different distributions of electron number density
and magnetic fields. We present a pencil-beam CPRT calculation and an all-sky
calculation, using a simulated galaxy cluster or a model magnetized universe
obtained from GCMHD+ simulations as the respective input structures. The
implications on large-scale magnetic field studies are discussed; remarks on
the standard methods using rotation measure are highlighted.Comment: 32 pages, 14 figure
An Improved Quantum Molecular Dynamics Model and its Applications to Fusion Reaction near Barrier
An improved Quantum Molecular Dynamics model is proposed. By using this
model, the properties of ground state of nuclei from Li to Pb can
be described very well with one set of parameters. The fusion reactions for
Ca+Zr, Ca+Zr and Ca+Zr at energy near
barrier are studied by this model. The experimental data of the fusion cross
sections for Ca+Zr at the energy near barrier can be
reproduced remarkably well without introducing any new parameters. The
mechanism for the enhancement of fusion probability for fusion reactions with
neutron-rich projectile or target is analyzed.Comment: 20 pages, 12 figures, 3 table
Dynamic study on fusion reactions for Ca+Zr around Coulomb barrier
By using the updated improved Quantum Molecular Dynamics model in which a
surface-symmetry potential term has been introduced for the first time, the
excitation functions for fusion reactions of Ca+Zr at
energies around the Coulomb barrier have been studied. The experimental data of
the fusion cross sections for Ca+Zr have been reproduced
remarkably well without introducing any new parameters. The fusion cross
sections for the neutron-rich fusion reactions of Ca+Zr around
the Coulomb barrier are predicted to be enhanced compared with a
non-neutron-rich fusion reaction. In order to clarify the mechanism of the
enhancement of the fusion cross sections for neutron-rich nuclear fusions, we
pay a great attention to study the dynamic lowering of the Coulomb barrier
during a neck formation. The isospin effect on the barrier lowering is
investigated. It is interesting that the effect of the projectile and target
nuclear structure on fusion dynamics can be revealed to a certain extent in our
approach. The time evolution of the N/Z ratio at the neck region has been
firstly illustrated. A large enhancement of the N/Z ratio at neck region for
neutron-rich nuclear fusion reactions is found.Comment: 21 pages, 7 figures,3 table
Collective oscillations of a classical gas confined in harmonic traps
Starting from the Boltzmann equation we calculate the frequency and the
damping of the monopole and quadrupole oscillations of a classical gas confined
in an harmonic potential. The collisional term is treated in the relaxation
time approximation and a gaussian ansatz is used for its evaluation. Our
approach provides an explicit description of the transition between the
hydrodynamic and collisionless regimes in both spherical and deformed traps.
The predictions are compared with the results of a numerical simulation.Comment: 6 pages, revtex, 2 figures include
Circulating and Dietary Omega‐3 and Omega‐6 Polyunsaturated Fatty Acids and Incidence of CVD in the Multi‐Ethnic Study of Atherosclerosis
Background: Dietary guidelines support intake of polyunsaturated fatty acids (PUFAs) in fish and vegetable oils. However, some controversy remains about benefits of PUFAs, and most prior studies have relied on self‐reported dietary assessment in relatively homogeneous populations. Methods and Results: In a multiethnic cohort of 2837 US adults (whites, Hispanics, African Americans, Chinese Americans), plasma phospholipid PUFAs were measured at baseline (2000–2002) using gas chromatography and dietary PUFAs estimated using a food frequency questionnaire. Incident cardiovascular disease (CVD) events (including coronary heart disease and stroke; n=189) were prospectively identified through 2010 during 19 778 person‐years of follow‐up. In multivariable‐adjusted Cox models, circulating n‐3 eicosapentaenoic acid and docosahexaenoic acid were inversely associated with incident CVD, with extreme‐quartile hazard ratios (95% CIs) of 0.49 for eicosapentaenoic acid (0.30 to 0.79; Ptrend=0.01) and 0.39 for docosahexaenoic acid (0.22 to 0.67; Ptrend<0.001). n‐3 Docosapentaenoic acid (DPA) was inversely associated with CVD in whites and Chinese, but not in other race/ethnicities (P‐interaction=0.01). No significant associations with CVD were observed for circulating n‐3 alpha‐linolenic acid or n‐6 PUFA (linoleic acid, arachidonic acid). Associations with CVD of self‐reported dietary PUFA were consistent with those of the PUFA biomarkers. All associations were similar across racial‐ethnic groups, except those of docosapentaenoic acid. Conclusions: Both dietary and circulating eicosapentaenoic acid and docosahexaenoic acid, but not alpha‐linolenic acid or n‐6 PUFA, were inversely associated with CVD incidence. These findings suggest that increased consumption of n‐3 PUFA from seafood may prevent CVD development in a multiethnic population
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