4,290 research outputs found
Paramagnetic reentrant effect in high purity mesoscopic AgNb proximity structures
We discuss the magnetic response of clean Ag coated Nb proximity cylinders in
the temperature range 150 \mu K < T < 9 K. In the mesoscopic temperature
regime, the normal metal-superconductor system shows the yet unexplained
paramagnetic reentrant effect, discovered some years ago [P. Visani, A. C.
Mota, and A. Pollini, Phys. Rev. Lett. 65, 1514 (1990)], superimposing on full
Meissner screening. The logarithmic slope of the reentrant paramagnetic
susceptibility chi_para(T) \propto \exp(-L/\xi_N) is limited by the condition
\xi_N=n L, with \xi_N=\hbar v_F/2 \pi k_B T, the thermal coherence length and
n=1,2,4. In wires with perimeters L=72 \mu m and L=130 \mu m, we observe
integer multiples n=1,2,4. At the lowest temperatures, \chi_para compensates
the diamagnetic susceptibility of the \textit{whole} AgNb structure.Comment: 4 pages, 4 figures (color
A variational constitutive model for porous metal plasticity
This paper presents a variational formulation of viscoplastic constitutive updates for porous elastoplastic materials. The material model combines von Mises plasticity with volumetric plastic expansion as induced, e.g., by the growth of voids and defects in metals. The finite deformation theory is based on the multiplicative decomposition of the deformation gradient and an internal variable formulation of continuum thermodynamics. By the use of logarithmic and exponential mappings the stress update algorithms are extended from small strains to finite deformations. Thus the time-discretized version of the porous-viscoplastic constitutive updates is described in a fully variational manner. The range of behavior predicted by the model and the performance of the variational update are demonstrated by its application to the forced expansion and fragmentation of U-6%Nb rings
Pine wilt disease and the pinewood nematode
Pine wilt disease (PWD) is one of the most damaging events affecting conifer forests (in
particular Pinus spp.), in the Far East (Japan, China and Korea), North America (USA and Canada) and,
more recently, in the European Union (Portugal). In Japan it became catastrophic, damaging native pine
species (Pinus thunbergii and P. densiflora), and becoming the main forest problem, forcing some areas
to be totally replaced by other tree species. The pine wilt nematode (PWN) Bursaphelenchus xylophilus,
endemic, with minor damage, to North America, was introduced in Japan in the early XX century and
then spread to Asia (China and Korea) in the 1980s. In 1999 it was detected for the first time in Portugal,
where, due to timely detection and immediate government action, it was initially (1999-2008) contained
to a small area 30 km SE of Lisbon. In 2008, the PWN spread again to central Portugal, the entire country
now being classified as “affected area”. Being an A1 quarantine pest, the EU acted to avoid further PWN
spreading and to eradicate it, by actions including financial support for surveyes and eradication, annual
inspections and research programs. Experience from control actions in Japan included aerial spraying of
insecticides to control the insect vector (the Cerambycid beetle Monochamus alternatus), injection of
nematicides to the trunk of infected trees, slashing and burning of large areas out of control, beetle traps,
biological control and tree breeding programs. These actions allowed some positive results, but also
unsuccessful cases due to the PWN spread and virulence. Other Asian countries also followed similar
strategies, but the nematode is still spreading in many regions. In Portugal, despite lower damage than
Asia, PWD is still significant with high losses to the forestry industry. New ways of containing PWD
include preventing movement of contaminated wood, cutting symptomatic trees and monitoring. Despite
a national and EU legislative body, no successful strategy to control and eventually eradicate the
nematode and the disease will prevail without sound scientific studies regarding the nematode and
vector(s) bioecology and genetics, the ecology and ecophysiology of the pine tree species, P. pinaster and
P. pinea , as well as the genomics and proteomics of pathogenicity (resistance/ susceptibility)
Diamagnetic response of cylindrical normal metal - superconductor proximity structures with low concentration of scattering centers
We have investigated the diamagnetic response of composite NS proximity
wires, consisting of a clean silver or copper coating, in good electrical
contact to a superconducting niobium or tantalum core. The samples show strong
induced diamagnetism in the normal layer, resulting in a nearly complete
Meissner screening at low temperatures. The temperature dependence of the
linear diamagnetic susceptibility data is successfully described by the
quasiclassical Eilenberger theory including elastic scattering characterised by
a mean free path l. Using the mean free path as the only fit parameter we found
values of l in the range 0.1-1 of the normal metal layer thickness d_N, which
are in rough agreement with the ones obtained from residual resistivity
measurements. The fits are satisfactory over the whole temperature range
between 5 mK and 7 K for values of d_N varying between 1.6 my m and 30 my m.
Although a finite mean free path is necessary to correctly describe the
temperature dependence of the linear response diamagnetic susceptibility, the
measured breakdown fields in the nonlinear regime follow the temperature and
thickness dependence given by the clean limit theory. However, there is a
discrepancy in the absolute values. We argue that in order to reach
quantitative agreement one needs to take into account the mean free path from
the fits of the linear response. [PACS numbers: 74.50.+r, 74.80.-g]Comment: 10 pages, 9 figure
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