LG12004
Pulsed Laser Deposition" proved to be the best deposition technique for the preparation of ultra-thin film-based devices
Forthcoming generations of data storage devices based on magnetic tunneling junctions MTJs (e.g., hard-disk read heads) designed to operate in the not-so-distant Tbits/inch^2 technology (current density limit ~100 Gbits/inch^2) demand tunneling junctions optimized in regard to: (i) sensitivity to the magnetic field as high as TMR=200% (described as the relative change of the tunneling resistance with the magnetic field) and (ii) low resistance-area product for the magnetic state of highest conductance (<1 Ohm-micron^2). These requirements are only available using tunneling junctions based on single-crystal barrier layers with homogeneous thicknesses where the coherent tunneling becomes feasible. Recent advances in the fabrication by sputtering of CoFeB/single-crystal MgO/ CoFeB MTJs exhibiting large-area TMRs so high as 400% have been achieved. Despite these progresses, current thin-film technologies based on physical vapor-phase deposition (PVD) techniques present severe limitations for the preparation of layers with homogeneous thicknesses of only a few atomic monolayers. At this thickness scale (<2 nm), the layer roughness, which is mostly determined by the growth kinetics of the PVD technique used, is of the same order of magnitude as the layer thickness. This produces a large dispersion of behaviors for the thickness-dependent properties (as large as 1000% for the tunneling current through a 2 nm-thick MgO layer with a standard roughness of 0.3 nm) that compromises the tunneling device reliability. In this work, the growth kinetics of the mostly used PVD techniques -molecular beam epitaxy, sputtering, flash evaporation and pulsed laser deposition- is theoretically investigated with the aim of testing the suitability of such techniques for preparing ultra-flat ultra-thin layers required for reliable tunneling devices. We demonstrate that Pulsed Laser Deposition is the best technique for preparing the flattest layers due to a combination of two unique key features [use of (i) a supersaturated pulsed flux of (ii) hyperthermal species] that promote the coarsening of the surface species by a distinctive mechanism, baptized as kinetically limited Ostwald ripening.
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LD11076
We provide important insight on a very well-known problem, namely the relation between gravitational dynamics and thermodynamics of horizons. The thermodynamics of black holes suggests a deep connection between gravitation and thermodynamics. Then the connection between Einstein gravity and thermodynamics of horizons was proved in 1995. We showed that the cosmological equation in other theories of gravity can be thought as a thermodynamic equation at the apparent horizon. Our work has important consequences in cosmology because gravitational dynamics and thermodynamics are totally different problems in physics. The connection between the two sheds new light on gravity.
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LE11491
Electrons suffer from the heat in an ion track
A team of physicists from Germany and Brazil found that
the absorption of fast electrons escaping from hot regions
inside solids is significantly influenced by the electronic heat
- similar to long-distance runners who are slowing down or
may even stop as a result of excessive heat. In their
experiments, they irradiated aluminum and beryllium
samples with fast ions and observed the resulting angular
intensity-distribution of the ejected Auger electrons.
Such angular distributions are known to be largely
independent of the type of excitation, following roughly a
cosine law at low ejected-electron energies. A very specific
distribution of excitations, however, is produced by a highly
charged fast ion at around 10% the speed of light. Such an
ion is fast enough to favor high charge states inside solids
and slow enough to interact strongly with the electrons of
the penetrated material. Thus, it leads to extremely strong
inner-shell ionization and electronic heat of some 10000 K
inside a nanometer-sized cylinder surrounding each
individual ion path. The authors of this work have shown
that Auger intensity ratios for different degrees of inner-shell
ionization vs. angle are sensitive to the high energy-deposition
density, consistent with enhanced inelastic electron-energy
losses or electron absorption, respectively. This proves that
the transport of fast electrons is significantly influenced by
the spatial electronic excitation distribution.
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LF11017
Imagine a device that is half submarine and half weather balloon, but shrunk down to the size of a marble. We report novel measurements of thermally driven fluid flows using just such a device: a miniature, submersible, mobile, wireless temperature sensor. This "smart particle" is carried along with the fluid, reporting temperature with a tiny radio transmitter as it goes. In this way, we obtain the first quantitative and direct measurements of how the flow transports heat from point A to point B, i.e. of the dynamics of "thermal plumes". Our results impact understanding of thermally driven fluid motions as found in the atmosphere, the oceans, the molten metal core of the Earth , or the interior of stars. We anticipate that similar devices, outfitted with capabilities for measuring other quantities (e.g. acceleration, pressure, concentration) will open new doors for investigating the physics of mixing and transport by fluid flows.
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LK10890.
Foaming of magnetic alloy boosts shape-memory effect
Researchers at Boise State University and Northwestern University have
boosted the magnetic shape-memory strain of polycrystalline Ni-Mn-Ga
sixty-fold (from 0.002 to 0.12 %) by casting the material into a foam.
This strain is developed as a magnetic field moves crystal defects
(known as twin boundaries) in a fully reversible manner over millions of
magnetic cycles. This shape-memory strain in the foam is comparable, in
terms of magnitude and response time, to the elastic strain exhibited by
the best commercial magnetostrictive material, Terfenol D, when exposed
to a magnetic field. However, Ni-Mn-Ga foams have lower density and
contain less expensive metals than Terfenol D. Their open porosity may
lead, beyond light-weight actuators, to micro-pumps without moving parts
or magnetic refrigeration near room temperature.
The researchers believe that the strain improvement in the foams is due
to their struts which are spanned by entire grains, thus reducing the
internal constraints present between adjacent grains in non-foamed bulk
polycrystalline Ni-Mn-Ga. Thus, the foam struts are similar to single
crystals, but they are constrained at their nodes, so they exhibit lower
strains than single crystals with achieve up to 10%. Foamed
polycrystalline Ni-Mn-Ga can be easily cast by conventional methods,
unlike Ni-Mn-Ga single crystals.
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LG12016
Liquid-liquid phase transition in supercooled silicon.
Despite the occurrence of advanced materials in recent years, silicon
still keeps the leadership in semiconductor technology. Most
technological applications begin with crystalline silicon wafers
elaborated from the melt, and amorphous silicon made from slightly
supercooled melts. The properties of normal and supercooled liquid
silicon are therefore of importance for the manufacturing process.
Unusual behavior of the density in the supercooled liquid states, as
deep as 200 K below the melting point, was observed leading researchers
to consider plausible the existence of a liquid-liquid transition at
even lower temperature, unfortunately out of reach to state-of-the-art
experimental facilities. Therefore, the absence of direct evidence from
experiments has prompted to look for it using numerical simulations. We
have proposed a new mixed approach by combining efficiently classical
and first-principles molecular dynamics, giving a strong support to the
existence of a transition between a high density liquid to a low density
liquid near 1050 K, regardless of a specific empirical interaction
model. We were able to clarify the nature of atomic and electronic
structures of both the high and low density phase, which were
incorrectly described from early classical simulations. Our finding
indicates the liquid-liquid transition is accompanied by an enhancement
of the local tetrahedral structure and is not characterized by a
semimetal to semiconductor transition.
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LG12007
“Extinction of the Giants”
The birth secret of the carbon buckyball has now been caught on tape. The nanometer-sized soccer ball was discovered and christened Buckminsterfullerene C60 two decades ago, but the intimate details of its formation remained a mystery. One “hot giant” hypothesis was that the atoms assemble first into graphitic planes, crumpled into large distorted polyhedral boxes, but then shed the loosely bound threads and chains. Eventually only the spherical buckyballs survive, thanks to their high symmetry and stability. This hot evolution is so rapid that no one could either prove or disprove it by observation. Now it has been documented for the first time, captured on high-resolution TEM video, matched by the atomic simulations, in a new joint study by scientists at Sandia Laboratory and Rice University. Thanks to a controllable heat bath inside the 10-nm-wide nanotube, they see how initial angular “giant fullerenes”, comprised of thousands of atoms, gradually shrink and perfect their shape. Honed and polished by great heat, like a raw diamond in the hands of a jeweler, they turn into the gem-quality symmetric buckyballs. If heat is sustained, the fullerenes undergo a further shrinking, ingeniously conjectured by the C60 discoverers, and vanish entirely, giving in to the entropy paramount at these conditions. The researchers believe this reveals the fundamental stage in fullerene formation, and also suggests a way to possibly engineer the nanocages for a variety of applications.
