3,941 research outputs found
Lifetimes of antiferromagnetic magnons in two and three dimensions: experiment, theory, and numerics
A high-resolution neutron spectroscopic technique is used to measure
momentum-resolved magnon lifetimes in the prototypical two- and
three-dimensional antiferromagnets Rb2MnF4 and MnF2, over the full Brillouin
zone and a wide range of temperatures. We rederived theories of the lifetime
resulting from magnon-magnon scattering, thereby broadening their applicability
beyond asymptotically small regions of wavevector and temperature.
Corresponding computations, combined with a small contribution reflecting
collisions with domain boundaries, yield excellent quantitative agreement with
the data.Comment: 5 pages, 4 figure
On the anomalous thermal conductivity of one-dimensional lattices
The divergence of the thermal conductivity in the thermodynamic limit is
thoroughly investigated. The divergence law is consistently determined with two
different numerical approaches based on equilibrium and non-equilibrium
simulations. A possible explanation in the framework of linear-response theory
is also presented, which traces back the physical origin of this anomaly to the
slow diffusion of the energy of long-wavelength Fourier modes. Finally, the
results of dynamical simulations are compared with the predictions of
mode-coupling theory.Comment: 5 pages, 3 figures, to appear in Europhysics Letter
The influence of constrictivity on the effective transport properties of porous layers in electrolysis and fuel cells
The aim of the present investigation is to define microstructure parameters, which control the effective transport properties in porous materials for energy technology. Recent improvements in 3D-imaging (FIB-nanotomography, synchrotron X-ray tomography) and image analysis (skeletonization and graph analysis, transport simulations) open new possibilities for the study of microstructure effects. In this study, we describe novel procedures for a quantitative analysis of constrictivity, which characterizes the so-called bottleneck effect. In a first experimental part, methodological tests are performed using a porous (La,Sr)CoO3 material (SOFC cathode). The tests indicate that the proposed procedure for quantitative analysis of constrictivity gives reproducible results even for samples with inhomogeneous microstructures (cracks, gradient of porosity). In the second part, 3D analyses are combined with measurements of ionic conductivity by impedance spectroscopy. The investigations are preformed on membranes of electrolysis cells with porosities between 0.27 and 0.8. Surprisingly, the tortuosities remain nearly constant (1.6) for the entire range of porosity. In contrast, the constrictivities vary strongly and correlate well with the measured transport resistances. Hence, constrictivity represents the dominant microstructure parameter, which controls the effective transport properties in the analysed membrane materials. An empirical relationship is then derived for the calculation of effective transport properties based on phase volume fraction, tortuosity, and constrictivit
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A RISC-V Vector Processor With Simultaneous-Switching Switched-Capacitor DC-DC Converters in 28 nm FDSOI
This work demonstrates a RISC-V vector microprocessor implemented in 28 nm FDSOI with fully integrated simultaneous-switching switched-capacitor DC-DC (SC DC-DC) converters and adaptive clocking that generates four on-chip voltages between 0.45 and 1 V using only 1.0 V core and 1.8 V IO voltage inputs. The converters achieve high efficiency at the system level by switching simultaneously to avoid charge-sharing losses and by using an adaptive clock to maximize performance for the resulting voltage ripple. Details about the implementation of the DC-DC switches, DC-DC controller, and adaptive clock are provided, and the sources of conversion loss are analyzed based on measured results. This system pushes the capabilities of dynamic voltage scaling by enabling fast transitions (20 ns), simple packaging (no off-chip passives), low area overhead (16%), high conversion efficiency (80%-86%), and high energy efficiency (26.2 DP GFLOPS/W) for mobile devices
Finite thermal conductivity in 1d lattices
We discuss the thermal conductivity of a chain of coupled rotators, showing
that it is the first example of a 1d nonlinear lattice exhibiting normal
transport properties in the absence of an on-site potential. Numerical
estimates obtained by simulating a chain in contact with two thermal baths at
different temperatures are found to be consistent with those ones based on
linear response theory. The dynamics of the Fourier modes provides direct
evidence of energy diffusion. The finiteness of the conductivity is traced back
to the occurrence of phase-jumps. Our conclusions are confirmed by the analysis
of two variants of this model.Comment: 4 pages, 3 postscript figure
Extraordinary lifespans in ants: a test of evolutionary theories of ageing
Senescence presents not only a medical problem, but also an evolutionary paradox because it should be opposed by natural selection. Evolutionary hypotheses propose that ageing evolves as the necessary cost of processes increasing early reproductive success(1,2), or because of weaker selection against late-acting mutations(3). A prediction of these hypotheses is that the rate of ageing should increase and the average lifespan decrease as the rate of extrinsic mortality increases(1-7). Alternatively, non-adaptive, purely mechanistic hypotheses invoke damage to DNA, cells, tissues and organs as being the unique cause of senescence and ineluctable death of organisms(8). Here we show that the evolution of eusociality is associated with a 100-fold increase in insect lifespan. Such an increase is predicted by evolutionary theories because termite, bee and ant queens live in colonies that are sheltered and heavily defended against predators. Moreover, a comparison of ants with contrasting life histories also reveals an association between lifespan and extrinsic rate of mortality. These results provide strong support for evolutionary theories of ageing, as purely mechanistic hypotheses of senescence do not propose any association between the rate of extrinsic mortality and lifespans
Super Earth Explorer: A Coronagraphic Off-Axis Space Telescope
The Super-Earth Explorer is an Off-Axis Space Telescope (SEE-COAST) designed
for high contrast imaging. Its scientific objective is to make the
physico-chemical characterization of exoplanets possibly down to 2 Earth radii
>. For that purpose it will analyze the spectral and polarimetric properties of
the parent starlight reflected by the planets, in the wavelength range 400-1250
nmComment: Accepted in Experimental Astronom
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