270 research outputs found
Size induced metal insulator transition in nanostructured Niobium thin films: Intragranular and intergranular contributions
With a reduction in the average grain size in nanostructured films of
elemental Nb, we observe a systematic crossover from metallic to
weakly-insulating behavior. An analysis of the temperature dependence of the
resistivity in the insulating phase clearly indicates the existence of two
distinct activation energies corresponding to inter-granular and intra-granular
mechanisms of transport. While the high temperature behavior is dominated by
grain boundary scattering of the conduction electrons, the effect of
discretization of energy levels due to quantum confinement shows up at low
temperatures. We show that the energy barrier at the grain boundary is
proportional to the width of the largely disordered inter-granular region,
which increases with a decrease in the grain size. For a metal-insulator
transition to occur in nano-Nb due to the opening up of an energy gap at the
grain boundary, the critical grain size is ~ 8nm and the corresponding grain
boundary width is ~ 1.1nm
Metal nanoplasmas as bright sources of hard x-ray pulses
We report significant enhancements in light coupling to intense-laser-created solid plasmas via surface plasmon and "lightning rod" effects. We demonstrate this in metal nanoparticle-coated solid targets irradiated with 100 fs, 806 nm laser pulses, focused to intensities ~1014-1015Wcm-2. Our experiments show a 13-fold enhancement in hard x-ray yield (10-200 keV) emitted by copper nanoparticle plasmas formed at the focal volume. A simple model explains the observed enhancement quantitatively and provides pointers to the design of structured surfaces for maximizing such emissions
H-substituted anionic carbon clusters CnH- (n \u3c= 10): Density functional studies and experimental observations
We have studied the interaction of hydrogen with small neutral and anionic carbon clusters using density functional calculations. The geometry, stability, and electronic structure of these clusters show an odd–even alternation originating in the bonding nature of the carbon atoms. Our mass spectrometric measurements of the abundance of CnH− (n⩽10) cluster anions produced by gas-feed Cs sputtering from different crystallographic forms of carbon display similar odd–even alternation with the even-n clusters being relatively more abundant. The calculated trend in the adiabatic electron affinities shows a behavior similar to the experimental abundance pattern. We discuss a possible partial suppression of the chain-to-ring transformation (which normally occurs at n=10 in C−n) in CnH− and compare it with our density functional calculations as well as observations in CnN−. We also observe that the size dependence of the abundance of CnH− clusters sputter ejected from a fullerene target exhibits a distinctly different power-law decline compared to crystalline and amorphous carbon
Role of prepulses in the interaction of intense, ultrashort lasers with "structured" surfaces
We examine enhanced hard x-ray emission (20 - 200 keV) from plasmas produced on nanoparticles coated optically polished copper surface under different prepulse conditions. We observe that enhancement reduces with increasing prepulse intensity. The dynamics of the process is seen to be in the ps regime. We attribute this to preplasma formation on nanoparticles and subsequent modification/destruction of the nanostructure layer before the arrival of the main pulse. It is suggested that high-contrast ultrashort pulses are essential for nanoparticles to function as yield enhancers
Hot ion generation from nanostructured surfaces under intense, femtosecond irradiation
We present the effect of a nanostructured surface on the emission of ions and
electrons from intense (5-36 Petwatt per sq.cm) femtosecond laser produced
plasmas. Electrons from optically polished copper targets coated with copper
nanoparticles (CuNP) are observed to be hotter than those from uncoated
polished targets. A nearly two-fold enhancement is observed for ions in the
range 14-74 keV, while ion yield decreases by a factor of 2 in the 74-2000 keV
range. The total ion yields measured using a large area Faraday cup are more
from CuNP targets than those from polished Cu targets, indicating increased ion
beam divergence due to surface modulations.Comment: 14 pages, 4 figure
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