Modeling ideal solid electrolytes
The thin film solid electrolyte, lithium phosphorus oxynitride (LiPON),
was invented at Oak Ridge National Laboratory [Ref. Nancy J. Dudney, The
Electrochemical Society Interface 17[3], 44-48 (2008)
http://www.electrochem.org/dl/interface/fal/fal08/fal08_p44-48.pdf] and
is widely used in all-solid-state thin film batteries for which there is
a growing number of applications. During battery operation, electrons
are excluded from the electrolyte and forced through the electrodes and
external circuit by efficiently transporting Li ions within the
electrolyte material. Using first-principles quantum mechanical
techniques, we have studied realistic models of various types of
isolated defects in the related crystalline electrolyte Li_3PO_4 with
the long term goal of finding the optimal stoichiometric and geometric
structures for LiPON electrolytes. For example, we find that mobile Li
ion vacancies are stabilized by removing oxygen from the lattice which,
in turn, causes the rebonding of nearby phosphate groups to form
(O_3)P-O-P(O_3) (POP) structures embedded in the crystal. These POP
structures can be further stabilized by replacing the bridging oxygen
with nitrogen. The PNP structure is a prototype for chain structures
experimentally identified in LiPON thin films. We find that Li ion
vacancies can diffuse with low migration barriers near these stable and
flexible PNP structures.
***
EU10481
How to assemble a virus without assembling it
Viruses exemplify Nature's way of making functional nanometer sized
particles that assemble spontaneously from their constituent parts. The
self-assembly process that viruses use so efficiently in order to
precisely position the proteins that make their protective coat (capsid)
and the
single-stranded RNA molecule (ssRNA) inside it, represents a prototype dream
of and inspiration for today's nanotechnologists. It is thus of interest
to examine the physical origin of the forces that guide the assembly in a
hope that the process may be adopted for creation of artificial functional
nano-particles. In this paper, we have disregarded all interactions
that are chemically specific, i.e. that might lead to precise "locking" of
proteins
and ssRNA in certain configurations, and we focused on non-specific
interactions
mediated by charges residing on proteins (positive), ssRNA (negative), and
salt ions (positive and negative) in the solution.
We have found that the requirement that the viruses assemble spontaneously
restricts the allowed values of these charges. In certain cases, it will be
thermodynamically favorable to form empty capsids and freely floating
ssRNA in the
solution. Our model examines conditions that may lead to abberant structures
and the regimes of parameters that guarantee the assembly of functional
viruses. We
have also found that some viruses may have evolved the structure of their
proteins
so to enhance the robustness of assembly. These viruses have
characteristic protein
"tails" protruding into their interiors. We have theoretically predicted
that such
structures indeed lead to more stable ssRNA/capsid complexes. Our work may
thus
be of help for experimental design of artificial virus-like container
nano-particles that self-assemble in wide range of conditions (saltness,
pH value,
temperature).
***
ES10074
Tracking nature's symmetries for new technology: Newly discovered elusive
liquid crystal phase after decades of search
It took about a century since the discovery of the first liquid crystal
(LC) in 1888 by Reinitzer and Lehmann to the world-wide invasion of these
materials through the LCD industry. Today's LCD screens rely on "nematic"
LC's whose molecules behave as cylindrically symmetric sticks that produce
different optical states when aligned by an applied voltage. In 1970 a new
phase, called "biaxial nematic", was theoretically predicted by Freiser,
based on the breaking of rotational symmetry of hypothetical molecules of
book-like shape. These molecules would ideally produce better LCD's using
a faster molecular rotation similar to turning a book from its cover to
its back. However, the concretization of such phase remained elusive in
spite of the chemists' efforts to synthesize many kinds of flat molecules.
Recently this quest was finally successful with the discovery of the
biaxial nematic phase in special types of molecular systems. As we show in
our paper, using nuclear magnetic resonance experiments, one of the
possibilities to obtain that phase is using molecules with different
connected segments (tetrapodes) that impose a common local orientation by
hindering each-other's movements. The door for a new technological step in
the LCD.s technology is starting to open.
***
BR10600
Modeling novel properties of nanomaterials
Nanomaterials exhibit novel properties, very different from bulk materials, and demonstrate quantum properties on the mesoscopic level. An analytic model is developed to model the growth of the uniform height islands. The formation of bilayer rings on top of stable islands and the absence of nucleation on top of the third layer are particularly intriguing. The analysis is motivated from recent Monet Carlo simulations that have reproduced the bilayer morphology. The analytic model allows better transparency to the role of the different barriers. In particular the diffusion anisotropy (i.e. atoms diffuse faster azimuthally within the ring than towards the island center) explains why nucleation is only observed close to the island edge and not in the middle. Lower barriers to diffuse back to the wetting layer from unstable heights than the barriers from stable heights explain the absence of nucleation on the third layer. The model can have general use in other systems where QSE play a role and the barriers become height dependent, in the study of different materials for various technical applications.
***
LT11287
Crystal surface growth
The way a crystal surface develops might be intimately related to
an optimal process of mass rearrangement. The growth of thin films is
a very important technological process. The dynamical control of thin
film deposition would allow obtaining films with the desired
properties for their use as nanotechnological devices. Epitaxial
growth takes place under high vacuum conditions, in which new atoms
are deposited on the growing surface and they contribute to create a
new structure. After being deposited and before becoming attached,
the atoms move randomly on the interface due to temperature effects.
In this work, we have seen that a plausible explanation of mass
redistribution on the surface is that the relation among the initial
and final positions of the deposited material is such that the square
of the distance among them is minimized. Minimization processes have
been found in other areas of physics: the shape of a soap bubble is
the minimal surface enclosing a given volume of air. Understanding
the process of crystal surface formation is key to control the shape
of the growing films, and thus to obtain materials with the desired
properties.
***
LU11698
A watched kettle never boils
In his famous 'arrow' paradox, the greek philosopher Zeno negated
motion, since it has to be a succession of instantaneous rests. A couple
millenia later, we consider this paradox with amusement, as a testimony
of times when derivatives and infinitesimal quantities were still to be
discovered. However, Zeno is back when it comes to quantum mechanics! A
quantum measurement, even an ideal one, strongly affects the measured
system's state, as is described by the projection postulates. In the
quantum world, a watched arrow cannot move, a watched kettle cannot
boil. Experiments have confirmed this strange effect and demonstrated
for instance that a watched spin cannot precess. Now, a team at ENS has
observed a new and spectacular manifestation of the “Quantum Zeno
Effect”. Repeated ideal measurements of the number of photons in a
`photon box', a very high quality superconducting cavity, inhibit the
injection of a coherent field by a classical source. Quantum
measurements stop a runaway process in which the field energy, if
unimpeded, would grow quadratically with time towards mesoscopic values.
This experiment sheds light on the quantum measurement process and on
the back action of an energy measurement onto the field phase.
***
LT11063
Efficient optical Feshbach resonance
Efficient tuning of interatomic interaction for ultracold $^1S_0$ state atoms has been demonstrated by using an optical Feshbach resonance. The optical Feshbach resonance is a method to tune scattering lengths, which give effective interatomic interactions, by optically coupling the colliding atom-pair state to a molecular bound state. While this technique is applicable to any atoms including the $^1S_0$ state atoms such as ytterbium and alkaline-earth atoms for which the conventional magnetically induced Feshbach resonance is not available, it has a disadvantage of incidental two-body atom loss due to spontaneous decay. In this work, we have realized large change of interatomic interaction while keeping the atom loss small for ytterbium atoms. The use of the narrow $^1S_0 - ^3P_1$ optical transition and the use of an isotope ($^{172}$Yb) with a large negative scattering length improve the efficiency by one order of magnitude compared with previous experiments using alkali-me
tal atoms. A similar efficient optical Feshbach resonance is expected for a $^{171}$Yb-$^{173}$Yb fermion-fermion system, and may open the way to explore a heteronuclear BCS state of this system.
***
LU11769
Current research shows long quantum lifetimes
Our paper shows how to use an electric current to see the magnetic
orientation of electrons held in a silicon chip without compromising their
interesting quantum behavior. Electric currents provide the most sensitive
way to do this, but they always reduce the life of the quantum effects they
are probing. We used a current to look at quantum behavior living fifty
times longer than was previously possible. To achieve this record quantum
lifetime required a magnetic field twenty-five times higher than those used
to date. Why is this breakthrough important? The electrons we study are tiny
magnets that could be the building blocks for a revolutionary new
technology: quantum computation. The tiny magnets are not limited to
pointing north or south: we put them into a quantum superposition of both!
This additional possibility is a source of the extra power that a quantum
computer would have. Silicon has dominated the computing industry for
decades; it may now lead us to quantum computing.
***
LV1114 and LQ11659
Elucidating the mechanism of heat transfer in aerogels
In two separate articles, Gaspard and Gilbert [LQ11659] and Gilbert and
Lefevere [LV1114] consider the problem of characterizing the process of
heat transfer from first principles and suggest that there is hope to
achieve such an understanding for a particular class of insulating
materials known as aerogels --?gels whose liquid component has been removed.
Heat transfer in solid materials is characterized by a number ?--the heat
conductivity--? which measures the heat current resulting from the
application of a small temperature difference across the solid. For most
solids the theoretical computation of this number is a very challenging
task.
In these articles, the authors introduce models of materials which
possess both the spatial structure of solids and collisional dynamics of
gases, and show their heat conductivity is universally given by the
frequency of collisions between gas particles. This universality
manifests itself when the gas particles are individually trapped in a
porous solid material and only rarely collide with each other, mostly
rattling around their traps. As demonstrated in [LV1114], the
universality of this result is independent of the trapping mechanism.
Aerogels share the structure of the models considered by the authors.
Their results point to the possibility of regulating the conductivity of
aerogels by reshaping their porous structure. Conversely, measuring the
heat conductivity of an aerogel provides direct information about its
nanoscale structure.
***
LT12000
Photons for quantum memory
The frequency band-width for the entangled photons through parametric
down-conversion is now reduced to the order several MHz, to meet the
requirement of a quantum memory. The frequency bandwidth required for a
quantum memory is usually on the order of several MHz, while for the
entangled photons through parametric down-conversion the band-width is
on the order of several THz which is about 1 million times larger. A
solution for this problem is to use a cavity to enhance the
down-conversion process. The frequency band-width is now limited by the
cavity line-width which can be easily set to match the atomic lines in a
quantum memory. For the previous experiments, the generated entangled
photons are all multi-mode. In our experiment, by setting a reasonable
cavity length difference between separate polarizations, we reduce the
proportion for the background modes greatly. Further mode cleaning is
realized by using two temperature-controlled etalons. Such a single-mode
narrow-band entangled photon source can merit the field of quantum
information, especially long-distance quantum communication and linear
optical quantum computing where storage of single photons is widely used.
***
LS11770
Turnover in a large scale molecular trajectory
In 1940, Kramers predicted a turnover in chemical
reaction rates resulting from the competition between
two mechanisms, each involving microscopic friction.
While friction between the solvent and reactants
promotes energy diffusion, thereby enhancing the rate
of reaction, it also leads to spatial diffusion which
has the opposite effect.
Also interesting is that these two regimes can be captured
by transition state theory, the other breakthrough taking
place in chemical reaction theory in the late 1930's,
which has been greatly improved by recent works published
in Phys. Rev. Lett.
In spite of all of this, the turnover has not been
conclusively observed experimentally or in realistic
simulations.
In an upcoming Letter, we report an unambiguous observation of
Kramers' turnover in a large scale molecular
trajectory calculation which includes realistic models
of the molecular reactants, the solvent and their
interactions.
These simulations demonstrate that the energy-difusion regime is
realizable
and hence relevant to chemical dynamics, and
many other processes of physical interest,
for example in surface physics.
It should be of significant interest to the general
community because it represents a classic example
of an effect that arises from complexity:
Microscopic friction is an emergent property
arising from the collective behavior of the solvent, and it
drives the energy flows that both accelerate and
decelerate a chemical reaction.
The Letter demonstrates that
this delicate dance can sometimes tip in the direction
of accelerating a reaction as its surroundings become
more crowded.
***
LP11391BJ
Does magnet always levitate above superconductor?
It is well known that all superconductors expel external magnetic
field due to the so-called Meissner effect. This, in particular,
reveals itself as a spectacular phenomenon - levitation of a magnet
above a superconductor. In this paper, in contrast to common belief,
we have theoretically shown that some layered superconductors (such as
organic and high-temperature ones) may attract (concentrate) magnetic
lines at high enough magnetic fields. Under such condition, relatively
weak magnets may levitate above the layered superconductor, while
strong enough magnets may be attracted by the superconductor and,
thus, may fall down (see Fig.1). Another important consequence of such
unusual magnetic properties is that superconducting transition
temperatures of such layered materials may be increase by a magnetic
field.
***
AS10220
Non-classical light meets ultrabroadband generation.
The fast growing field of photonic quantum technology promises drastic
enhancements in performance with respect to classical alternatives.
However, in order to realize this potential we must first develop
non-classical light sources with specific properties and which are
compatible with existing optical fiber networks. Spontaneous four wave
mixing (SFWM) photon-pair sources can be remarkably bright because of
the long interaction lengths possible in fiber, while the fiber
dispersion properties may be readily engineered resulting in tailored
photon-pair entanglement properties. In this work, we focus on
ultra-broadband photon pair sources characterized by an exceptionally
high degree of entanglement, coupled with an exceptionally high degree
of synchronicity (determined by time of emission difference between the
signal and idler photons). These states can lead to exciting
applications such as quantum optical coherence tomography,
large-alphabet quantum key distribution and teleportation of
single-photon wavepackets. We present a comprehensive study of
ultra-broadband photon-pair generation by SFWM in single-mode photonic
crystal fiber. Remarkably, a large emission bandwidth is possible even
for a quasi-monochromatic pump. In fact, the ratio of the emission to
pump bandwidth yields a quantity proportional to the degree of
entanglement. We present specific experimental designs yielding
ultra-broadband photon pair generation, free from spontaneous Raman
contamination.
