Solidification velocities in deeply undercooled silver
How fast a liquid-solid interface can move in a deeply undercooled liquid in pure metals? Despite decades of research on this problem, we still do not have a definite answer. Limited experimental results show that it moves on the order of 100 m s-1. Indeed, it moves so fast that even quenching a pure metal into a deeply undercooled state becomes difficult. In this work, we use a femtosecond laser to melt a thin surface layer (~20 nm) of Ag. We are able to quench the melt in a controllable manner to temperatures as low as 0.6 Tm (Tm – melting temperature). Using third-harmonic light generation as a probe, we measure the solidification velocity as a function of temperature. We have found very interesting results. The velocity does not increase indefinitely with undercooling; it reaches a maximum (~ 75 m s-1) at about 0.85 Tm and then decreases very slowly with decreasing temperature. By comparing the results to different theoretical models, we show that a small energy barrier is likely to exist for atoms moving across the interface. The experiments also agree with the velocities measured by molecular dynamics simulation.
***
BY10656
How carbon atoms interact in graphene
The way atoms in a material interact with each other, determines the
properties of the material. This paper presents a model potential
that gives the interaction between carbon atoms in graphene, the new
wonder material with potential for powerful new devices. The
potential has been constructed to correctly reproduce some of the
most basic structural characteristics of graphene such as elastic
constants, cohesive energy, lattice constant, and phonon spectra.
This lends credibility to the proposed potential. The potential is
expressed in terms of analytical functions, which makes it convenient
for structural and thermodynamic calculations, and atomistic
simulations. This work should be a valuable resource for a variety of
nanotechnology related atomistic simulations of graphene. Earlier
work in the field consists of numerical ab initio calculations. Ab
initio methods are, in principle, more accurate but computationally
extensive, which may not be convenient for many applications.
***
LZ11673
Type-1.5 superconductivity discovered
Richard Feynman was once asked by his host, Mrs Bethe, "what would you like to drink - tea or coffee", and he answered "both". "Surely you're joking, Mr. Feynman!", replied then Mrs Bethe.
In fact, what are 1.5-superconductors- are they type-1 or type-2 ? The answer is ..."both"!
We are reporting on the discovery of a totally new type of superconductivity, which we coined "Type-1.5 superconductivity". All superconductors known up to now are either type-1 (characterized by attractive vortex-vortex interaction) or type-2 materials (repulsive vortex-vortex interaction). Remarkably, we have discovered a novel superconducting state in the clean two-component superconductors (MgB2 is the most typical two-component superconductor; others are among new superconducting oxypnictides, etc). Remarkably, in this case we can have for the first component of the order parameter type-1 conditions, while for the second component of the order parameter we can apply type-2 conditions. This "type-1.5" superconductivity is a totally new state which combines the better of the two worlds (type-1 and type-2 simultaneously) in the same single material.
The vortex matter in these type-1.5 superconductors behaves in an extremely unusual way: combination of the vortex-vortex repulsion and attraction in the same material leads to the appearance of the exotic novel vortex patterns: gossamer-like vortex arrays with vortex voids and chains of vortices surrounding them, vortex stripes with a denser vortex array combined with simultaneously present Meissner (vortex free) stripes next to them.These novel vortex patterns have been directly visualized by us in magnetic decoration experiments on high quality single crystals. Moreover, analytical modeling and molecular dynamic simulations of the vortex patterns are in a good agreement with our experimental data.
Besides great fundamental importance, combination of the best features of both type-1 and type-2 superconductivity in a single type-1.5 material has also potential for new applications which cannot be realized by using neither type-1 nor type-2 superconductors. Interestingly, exotic vortex patterns, based on the co-existence of short range repulsion with a long range attraction in type-1.5 superconductors, have many features in common with the similar patterns (stripes, bubbles, labyrinths, gossamer patterns) in Langmuir monolayers, gels, water-oil mixtures, magnetic films, etc.
***
LX11045
Chemical Tuning of Graphene: A Road to Advanced Electronics
Lewis acid character of Boron which allows charge transfer can be
exploited to achieve intrinsic room temperature half-metallic behavior
in graphene based nano devices upon chemical modifications of hydrogen
passivated zigzag edge graphene nanoribbons (ZGNRs). In this era of
miniaturized electronic devices, graphene has opened a new dimension of
device fabrication. Experimental sofistications in both top-down
(micromechanical cleavage or lithography) and bottom-up (synthetic
chemistry) approches ensure the realization of such systems and their
device applicabilities. In this letter, we have studied edge-passivated
ZGNRs of various widths with chemical dopants, Boron and Nitrogen,
keeping the whole system isoelectronic. Doping concentrations and dopant
positions regulate the electronic structure of the nanoribbons. Our study
reveals that the zigzag edge Boron Nitride nanoribbons with terminating
polyacene unit exhibit half-metallicity irrespective of the ribbon width.
This property is sustained even in presence of large electric field at
room temperature. A careful look at the spin density profile shows the
Lewis acid character of Boron atoms which drives charge transfer from
adjacent Carbon to Boron atoms creating a potential gradient across the
ribbon width giving rise to half-metallicity. Thus our study shows new
inroads to design room temperature half-metallic materials exploiting
some basic concepts like Lewis acidity, charge transfer and edge states,
opening a huge possibility in spintronics and memory storage device
applications.
***
BZR1082
Earth's core as hot as the Sun
The core of the Earth is a ball of solid iron with a radius of 1220
km, surrounded by
a shell of almost pure liquid iron which extends up to 3480 km from
the centre.
The temperature of the core is unknown, but this is a fundamental
parameter in the
building of any thermal model of our planet. The presence of a solid/
liquid boundary
(called the ICB) makes it possible to estimate it indirectly: since
the core is mainly
formed by iron, then the melting temperature of iron at the pressure
of the ICB gives a good
indication of the temperature of the core.
In this work I have used the formulation of quantum mechanics known as
density functional
theory to perform simulations of solid and liquid
iron in coexistence, including nearly 1000 atoms in the simulation
cell, and obtained a
melting temperature at ICB pressure of 6390 +- 100 K. This result
supports earlier
density functional theory calculations based on free energies, and
confirms that the
centre of the Earth is as hot as the surface of the Sun.
***
LU11652
Society benefits from behavioral diversity
Cooperation is essential in every society, but puzzling from an evolutionary
perspective. In this work, we address the role of behavioral differences –
ubiquitous among Humans - on the evolution of cooperation. We study a model in
which individuals can either cooperate or defect. They engage in a social
dilemma of cooperation, interacting along the edges of a complex network. The
structure of the network changes in time, as individuals regularly engage in new
interactions while abandoning old ones. Social interactions may be long or
brief, depending on the individuals involved. When dissatisfied, some
individuals will try to break contact as soon as possible, whereas others will
remain in touch. We show that cooperation blooms – and society as a whole
benefits – the larger the behavioral diversity in responding to unwanted
interactions. These results support the idea that diversity, on a grand scale,
is instrumental in shaping us as the most sophisticated cooperating entities on
this planet. Mathematically, we show that taking explicitly into account the
feedback between co-evolving mechanisms – here network topology and individual
strategy and behavior - profoundly affects the outcome of those processes, a
result which has widespread consequences in many problems of natural and social
sciences.
