The Isostructural Transition in Cerium: A New Look at a Classic Condensed Matter System
As the first rare earth element with f-electrons, Cerium displays intriguing physical and chemical properties. These have stimulated scientific inquiry over two centuries since its discovery, leading to application of Ce in such diverse technologies as nanomedicine, pollution control, and alloy design. The most unique property of Ce is its first-order isostructural phase transition discovered by P.W. Bridgman between a non-magnetic, low temperature/low volume "alpha-phase" and a magnetic, high temperature/high volume "gamma-phase". To date, an accurate theory of the Ce transition has remained elusive. Nowhere is this more obvious than the 50% to 200% disparities between Ce critical point values from theory and experiment. In this paper, we have constructed an integrated statistic and classic thermodynamic framework by explicitly incorporating phonon density-of-states and finite temperature mixing of the Ce nonmagnetic and magnetic states to predict the critical behavior of the transition. Thus, the Ce gamma-alpha phase transition is placed on a firm theoretical foundation applicable to a whole host of materials ranging from elemental solids to batteries and superconductors.
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BR10526
Thermoelectric properties of bismuth telluride nanowires in the constant relaxation time approximation
Thermoelectric heat management requires high efficient nanostructured
thermoelectric materials which can commercially compete with the Freon
refrigeration systems. Highly anisotropic materials like bismuth
telluride and its solid
solutions are presently the best thermoelectric materials for commercial
applications at room temperature. The efficiency of a thermoelectric
material is characterized by means of the figure of merit ZT which
depends on the electrical
conductivity, the Seebeck coefficient, the thermal conductivity, and the
temperature. The figure of merit for the above materials is about 1. The
recent experiments show that the thermoelectric characteristics of the
quantum well and
quantum wire systems are improved in comparison with their bulk
counterparts. For the bismuth telluride quantum wires, the existing
theoretical models take into consideration the lowest subband of the
electronic structure (Size Quantum Limit) using the
bulk-effective-mass approximation. In this paper, we improved the above
model taking into account all carrier subbands, the effective mass
anisotropy, and the temperature dependence of the band gap. According to
our model, the
maximum value of the figure of merit for the p-type nanowire is equal to
2.8 at temperature 480 K and nanowire thickness 7 nm. At the room
temperature, the figure of merit equals 1.7, respectively.
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LR11540
Pumping electrons in nanotube quantum dots
Tiny ripples of mechanical deformation running along the surface of a quartz substrate have been used to drive a current in carbon nanotube molecules above it. While studying these currents we have observed an unexpected pattern -- electrons could travel in either direction, depending on the voltage of a nearby gate electrode. Peaks and dips in the current grow, but then move apart as the mechanical waves are made stronger. We find that the observed behaviour nicely fits theoretical models of charge pumping in which the wave forces electrons to move around between quantum dots in the nanotube. The sign of the pumped current is directly related to whether the available states of the quantum dots are occupied by electrons or not. These experiments help us to understand how quantum dot systems respond to rapidly changing external parameters and, in future experiments, will be used to study even more subtle quantum properties that carbon nanotubes are known to possess.
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ET10321
Colloidal strings
Dipole colloidal particles can self-assemble in unusual asymmetric
structures. If a small number of such particles are restricted to a
plane and squeezed they can order themselves into concentric circles,
galaxy like spirals, chains and Y-shaped configurations. By varying the
magnetic moment of the particles and the strength of the magnetic field
one can move the system from one configuration into the other. At the
same time the stability of the ordered patterns can be manipulated.
The predictions from our computer simulations can be tested e.g. on
paramagnetic colloidal spheres confined in a two-dimensional circular
cavity and/or on colloidal ferrofluids.
Colloidal ferrofluids consists of roughly spherical ferromagnetic
particles with diameters in the range 10nm-1µm dispersed in a simple
solvent. These materials are of significant technological importance
because their rheological properties can be ¿controlled¿ by an applied
magnetic field. In biomedicine, the size of these magnetic nanoparticles
place them at dimensions that are smaller than or comparable to those of
a cell (10-100 µm), a virus (20-450 nm), a protein (5-50 nm) or a gene
(2 nm wide and 10-100 nm long). This means that they can ¿get close¿ to
a biological entity of interest. Furthermore, they can be coated with
biological molecules to make them interact with or bind to a biological
entity, thereby providing a controllable means of ¿tagging¿ or
addressing it besides to be manipulated by an external magnetic field
gradient. It is expected that our results will have applications in
self-assembly and in the manipulation of biomolecules.
In our work, we investigated the structure and the dynamic properties of
few-particle systems for different sizes of the magnetic moment of the
colloidal particles and magnitude of the external magnetic field.
Previous work using non-magnetic particles showed that the stability of
the system depends crucially on the lowest frequency normal mode. We
found that this frequency is also governed by angular oscillations of
the particles, independent of their vibrational motion. Besides, we
observed that the magnetic field plays a remarkable influence on the
structure of the system, modifying the frequency of oscillation of the
particles and as a consequence the stability of the system.
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LS11405
Strong electric field destroys the vacuum and thus destroys itself
Recently, due to new challenges in astrophysics, discovery of quark-gluon plasma, and creation of such materials as graphene (one layer of carbon atoms) the problem of strong electric field impact on the physical vacuum and the corresponding backreaction has already become of practical interest. A super strong electric field destroys the vacuum and thus destroys itself due to the backreaction from the latter. A principal possibility that a strong electric field can violate the physical vacuum follows from basic principles of modern quantum theory. Both magnetic and electric fields polarize the vacuum, but only a super strong electric field can destroy the latter creating real electron-positron pairs from the vacuum. In such a way, the super strong electric field loses its energy; its magnitude diminishes with time. In this article, for the first time, the rate of the super strong electric field depletion with time was estimated from
nonperturbative calculations of the energy density of created pairs. In particular, we have obtained a characteristic time of the electric field existence as a function of its strength. Estimations obtained are important for constructing consistent models in physics of elementary particles, and in studying the electric field impact on the physical vacuum in various situations.
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LS11076
New Insights -- Dark-field neutron tomography
The letter /Neutron Dark-Field Tomography/ introduces scattering to
extremely small angles as an additional contrast mechanism for
tomography. Ultra-small-angle scattering based reconstructions as
derived from so called dark-field images are particularly useful to
identify density variations due to structures in an object on the length
scale of about hundred nanometers to a few micrometers. This range
naturally complements the size range spatially resolved directly by
neutron imaging methods. The presented combination of the formalism for
tomographic reconstruction of dark-field data with the derivation of
such data from highly efficient grating interferometer imaging
measurements enables applications at low brilliant sources like reactor
neutron sources or even laboratory based x-ray sources. Consequently a
broad impact of the technique in scientific as well as industrial and
medical applications surveying material research, food inspection,
tissue examinations and even security scanning can be predicted.
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LP11625
Sorting Nanotubes by Color
Nanotubes floating freely in an aqueous solution have been observed to
aggregate in the focal volume of a highly focussed laser, which
selectively pulled only specific colors of nanotube into the focus spot.
When single walled carbon nanotubes are fabricated they typically show a
variety of sizes and twisting angles, and these various species, known
as "chiralities" of nanotubes, show different electronic and optical
properties. The separation and purification of chiralities is a major
challenge for the further study and application of nanotubes to
engineering. Optical methods for the characterisation of nanotube
chiralities are well known, as nanotubes show strong color-dependant
interactions with light. Researchers often analyse which nanotubes are
present in a given sample by using optical spectroscopy. Extending this
concept, colored light can be used to selectively manipulate given
chiralities of nanotubes. By using a laser tweezer system combined with
spectroscopic detection, in this paper we demonstrated that some species
of tube showed a preferential tendency to aggregate at the focal area of
a focussed laser beam. This experiment is a significant first step
toward developing optical methods for the sorting and purification of
nanotube chiralities.
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LC11460
New contribution to friction is predicted
Friction is of enormous practical importance, not only for industry, but
has a
large implication on everybody's life as the following examples illustrate:
Braking/Acceleration of a car, walking on (icy) surface, constant loss of
energy when bicycling, and so on. As it is an intriguingly complex
phenomenon
the progress in experimental techniques on the micro- and nano-scale as well
as the improved computational power for atomic simulations has led to a
renaissance of this old research field in recent years. Currently a large
variety of microscopic models compete with one another. Major complications
are wear, plastic deformation at the contact, impurities, and lubricants. It
is unlikely that in the general case only a single dissipation mechanism
will
be active. Defect motion, phononic and electronic excitations may be
involved
in a very complex blend. In order to reduce these complications research
has
recently focused on the elementary dissipation processes.
In this paper a new contribution to friction is predicted to occur in
systems
with magnetic correlations: Tangential relative motion of two Ising spin
systems pumps energy into the magnetic degrees of freedom (i.e. the spins).
This leads to a friction force proportional to the area of contact and
independent of velocity for small velocities. According to the picture of
Bowden and Tabor also Coulomb friction is independent of the velocity and
proportional to the real contact area, which due to surface roughness is
smaller than the sliding surface macroscopically appears to be, and grows
proportional to the normal load. Therefore, the velocity independent part of
the magnetic friction force behaves like Coulomb friction. Magnetic
friction
is strongest near the critical temperature, below which the spin systems
order
spontaneously. Antiferromagnetic coupling leads to stronger friction than
ferromagnetic coupling with the same exchange constant. The basic
dissipation
mechanism is explained. A surprising effect is observed in the
ferromagnetically ordered phase: The relative motion can act like a heat
pump
cooling the spins in the vicinity of the friction surface.
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EQ10378
The sounds of complexity
There is a diverse set of problems in a range of pure and applied sciences, from graph theory to quantum chaos and finance, which requires computing the eigenvalue spectra of large complex interaction matrices, a task analogous to that of determining the frequency spectrum of a vibrating body such as a violin. In complex systems, interactions are
typically heterogeneous, lacking the regularities of simple force laws governing the dynamics of conventional bodies. In the present paper we describe an efficient algorithm that puts calculating the spectral density of of huge complex interaction matrices, previously a daunting computational task, within reach of a desktop computer. The algorithm utilizes the 'cavity method' originally invented for the study of disordered magnetic systems, which turns the structure of complex networks (the very thing that makes them hard to study in the first place) to its advantage. Together with related work capable of handling the infinite system limit, we resolve a long-standing open problem in random matrix theory, and are putting tools at our fingertips for efficiently exploring the 'sounds of complexity'
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ER10293
Beyond the curse of the single-event, spatial Monte Carlo simulation
A new spatial kinetic Monte Carlo method has been devised that enables executing multiple events at once with considerable time acceleration. The method employs the n-fold method to create groups of reactions in which the tau-leap algorithm of Gillespie, originally proposed for well-mixed systems, is applied. The method is general, easy to implement and can result in substantial computational savings when global updating is employed. It can be applied to all agent-based models, which are among others, prevalent in physics problems (crystal growth, surface reactions, diffusion on surfaces and in microporous materials, defects in materials, microphase separation, etc.), epidemiology, biology and ecology. Thus, the method has potentially far-reaching applicabilty in areas other than just physics.
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BS10715
Finite temperature quantum transport in networks of quantum dots
As the conductance of small, micrometer sized structures is measured
at lower and lower temperatures, the conductance increasingly differs
from the classically expected result. Such "quantum corrections" arise
as more and more of the structure needs to be treated as one quantum
coherent entity. At finite temperatures these quantum corrections are
suppressed due to thermal smearing and processes referred to as
"dephasing". There are few geometries for which the temperature
dependence can be explicitly calculated. We consider networks of
quantum dots – realized, e.g., by micrometer sized structures such as
metallic grains or gated semiconductor heterostructures – and answer
the question how the Coulomb interaction between the electrons leads
to the temperature dependence of these corrections. We have chosen to
consider a network of quantum dots because, for this geometry, the
final result depends only on parameters directly accessible in
experiment: the individual dots' densities of states and the
conductances of the contacts between the dots.
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LS11450
Is there a limit on being loose?
Disordered packings of grains which are able to sustain their own weight
are familiar in our daily life: sugar, coffee, pills and coal are just few
examples.
The density of these systems depends on how the grains are packed, and an
open question is: how loose can such a packing be without loosing
mechanical stability?
This lower bound is often referred to as the Random Loose Packing.
Two recent papers have clarified the nature of this boundary from both an
experimental and theoretical viewpoint:
Jerkins et al. generated mechanically stable packings of spheres by slow
sedimentation. They showed that the volume fraction of the loosest possible
packing depends both on pressure and the friction coefficient of the
spheres. They also established a new lower bound at a volume fraction of
0.550.
Pica Ciamarra and Coniglio confirmed the dependence on friction in their
numerical study of hard disks. Moreover, they measured the number of
stable configurations of their system as a function of density. In this
way they were able to explain Random Loose Packing as the density were
this number becomes a maximum. Packings with a lower volume fraction do
exist, however they cannot be generated via any known experimental
procedure. Which poses an interesting new problem.
