326 research outputs found
Elastic constants of As2S3
We have used Brillouin scattering to measure ten of the thirteen independent elastic constants of crystalline As2S3. We find that the phonon propagation within the layer plane is largely controlled by the orthorhombic symmetry of the layer rather than the monoclinic symmetry of the crystal. The stiffness within the plane displays a large anisotropy, and the smaller of the stiffnesses within the plane is less than that along the axis perpendicular to the plane, an unexpected result for a layered crystal. For phonons propagating near the [001] direction the two branches of the velocity show a mode repulsion and exchange their polarizations. In this direction the velocity of the shear wave is larger than that of the compressional wave
Scaling relations for magnetic nanoparticles
A detailed investigation of the scaling relations recently proposed by [J.
d'Albuquerque e Castro, D. Altbir, J. C. Retamal, and P. Vargas, Phys. Rev.
Lett. 88, 237202 (2002)] to study the magnetic properties of nanoparticles is
presented. Analytical expressions for the total energy of three characteristic
internal configurations of the particles are obtained, in terms of which the
behavior of the magnetic phase diagram for those particles upon scaling of the
exchange interaction is discussed. The exponent in scaling relations is
shown to be dependent on the geometry of the vortex core, and results for
specific cases are presented.Comment: 6 pages, 4 figure
Using resilience assessments to inform the management and conservation of coral reef ecosystems
Climate change is causing the decline of coral reef ecosystems globally. Recent research highlights the importance of reducing CO2 emissions in combination with implementing local management actions to support reef health and recovery, particularly actions that protect sites which are more resilient to extreme events. Resilience assessments quantify the ecological, social, and environmental context of reefs through the lens of resilience, i.e., the capacity of a system to absorb or withstand stressors such that the system maintains its structure and functions and has the capacity to adapt to future disturbances and changes. Resilience assessments are an important tool to help marine managers and decision makers anticipate changes, identify areas with high survival prospects, and prioritize management actions to support resilience. While being widely implemented, however, there has not yet been an evaluation of whether resilience assessments have informed coral reef management. Here, we assess the primary and gray literature and input from coral reef managers to map where resilience assessments have been conducted. We explore if and how they have been used to inform management actions and provide recommendations for improving the likelihood that resilience assessments will result in management actions and positive conservation outcomes. These recommendations are applicable to other ecosystems in which resilience assessments are applied and will become increasingly important as climate impacts intensify and reduce the window of opportunity for protecting natural ecosystems
Unexpected behavior of the antiferromagnetic mode of NiO
Although NiO is often considered the classic example of an antiferromagnetic insulator, recent investigations have revealed unexplained features of the magnon spectrum. The present study of the temperature and polarization behavior of first-order magnetic Raman scattering reveals that the polarization selection rules are not described by the generally accepted antisymmetric scattering tensor. The inclusion of quadratic magneto-optic coupling terms can explain the symmetry of the scattering tensor, but does not lead to results consistent with the accepted [112] spin alignment direction
Pressure-amorphized SiO2 ±-quartz: An anisotropic amorphous solid
Recent experiments [Kruger and Jeanloz, Science 249, 647 (1990)] have shown that the pressure-induced amorphous form of AlPO4 reverts to its original single-crystal form on release of pressure. Here we present the results of a Brillouin scattering study of the sister compound ±-quartz, SiO2 (which can also be pressure amorphized but retains its amorphous structure on release of pressure), which shows that the recovered material is not elastically isotropic but retains a memory of its original crystallographic orientation
Isotopic-mass dependence of the A, B, and C excitonic band gaps in ZnO at low temperatures
Low temperature wavelength-modulated reflectivity measurements of isotopically engineered ZnO samples have yielded the dependence of their A, B, and C excitonic band gaps on the isotopic masses of Zn and O. The observed dependence is analyzed in terms of the band gap renormalization by zero-point vibrations via electron-phonon interaction and the volume dependence on isotopic mass. A simplified, two-oscillator model, employed in the analysis, yields zero-point renormalizations of the band gaps, -154 +/- 14 meV (A), -145 +/- 12 meV (B), and -169 +/- 14 meV (C), for ZnO with natural isotopic composition
Low-temperature heat transfer in nanowires
The new regime of low-temperature heat transfer in suspended nanowires is
predicted. It takes place when (i) only ``acoustic'' phonon modes of the wire
are thermally populated and (ii) phonons are subject to the effective elastic
scattering. Qualitatively, the main peculiarities of heat transfer originate
due to appearance of the flexural modes with high density of states in the wire
phonon spectrum. They give rise to the temperature dependence of the
wire thermal conductance. The experimental situations where the new regime is
likely to be detected are discussed.Comment: RevTex file, 1 PS figur
Fragility and compressibility at the glass transition
Isothermal compressibilities and Brillouin sound velocities from the
literature allow to separate the compressibility at the glass transition into a
high-frequency vibrational and a low-frequency relaxational part. Their ratio
shows the linear fragility relation discovered by x-ray Brillouin scattering
[1], though the data bend away from the line at higher fragilities. Using the
concept of constrained degrees of freedom, one can show that the vibrational
part follows the fragility-independent Lindemann criterion; the fragility
dependence seems to stem from the relaxational part. The physical meaning of
this finding is discussed. [1] T. Scopigno, G. Ruocco, F. Sette and G. Monaco,
Science 302, 849 (2003)Comment: 4 pages, 2 figures, 2 tables, 33 references. Slightly changed after
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