LN11370ER
Toroidal crystals: physics on the surface of a donut
Are there defects in the most effective packing of sugar grains on
the surface of a donut? Remarkably the ordered structure of many
natural systems can be related to this simple and appetizing
question and the answer is frequently yes! In the language of
geometry the surface of a donut is called a torus. Crystalline
assemblages of identical sub-units packed together and bent in the
form of a torus have been discovered in the past ten years in the
protein coats of viruses, self-assembled fatty acids and carbon
nanorings. In our article we provide for the first time a unified
description of the structural properties of toroidal crystals based
on the elasticity of disclination defects. In two-dimensional
crystals disclinations are lattice sites with more or less than the
average number (6) of neighbors. Disclinations typically have 5 or 7
neighbors in a triangular lattice. On a flat surface disclinations
are energetically prohibitive and never appear in the ground state.
As soon as the crystal is curved, however, disclinations may appear
and fundamentally alter the basic order. Disclinations may serve as
active biological sites or places for chemical linkages so that
mesoscopic toroidal surfaces can spontaneously link to form novel
molecules and bulk materials. Donuts continue to surprise.
***
BQR1070
Temperature-induced spin coherence dissipation in quantum dots
Phys. Rev. B Rapid Communication and Editor's Suggestion
Spins in ensembles of quantum dots offer one possible pathway to
implementing quantum information technologies in a solid-state
environment. Unfortunately, the spin interaction with the host lattice
leads to coherence lost of the quantum bit. In this work the authors
reported the first measurement of the temperature dependence of the
electron spin decoherence time T2 in semiconductor quantum dots and
they compared their experimental results to recent theoretical
calculations. It has been possible thanks to: (i) a refocusing
technique using laser pulses called "mode-locking technique" developed
by this research group which allows to avoid the inhomogeneities
within an ensemble, (ii) the investigation of InAs quantum dots where
the electronic confinement is much larger than GaAs dots (gate-defined
GaAs dots which allow only very low temperature measurements). It was
found that T2 remains constant up to 20 K and then it presents a sharp
drop due fluctuations of the electron spin interaction with the
lattice nuclei spins. The topic of the paper is of timely nature. The
results are quite important for the corresponding science community
considering the actual debate in the community about the mechanisms
responsible for spin decoherence.
***
LN11086B
Noise self-pumping in long Josephson junctions
The noise self-pumping effect in a spatially extended system is investigated.
This effect leads to significant degradation of noise properties of spatially
extended systems. In particular, in long Josephson junctions it is realized
in a similar fashion as usual ac self-pumping effect: fluctuating solitons,
radiating from the junction, induce fluctuating magnetic field, which in turn
modulates the dynamics of the soliton chain and increases the spectral
linewidth. Contrary to the theory for short Josephson junctions,
predicting linear decrease of the spectral linewidth with increase
of junction length, the minimum of the linewidth versus the length
is observed both for uniform and non-uniform bias feed
distributions.
***
LH11065E
Re-entrance in vitro
In a recent communication, researchers studied the effect of temperature
on single stranded DNA which can form a hairpin structure as seen in the case of
Molecular Beacon. They showed that in the constant force ensemble (appropriate for the set up like magnetic tweezers) the reaction co-ordinate i.e. extension may increase or decrease with temperature as seen in the recent experiment. Though an increase in extension with temperature is well understood theoretically, but there is no clear understanding about the decrease in the extension with temperature. Their exact solution based on the model Molecular Beacon for short chains showed that the decrease in the extension is an entropic effect. This advancement in knowledge now may resolve a long standing issue related to the prediction of re-entrance in force induced transitions where a double stranded DNA (dsDNA) goes to the zipped state from the unzipped state and again to the unzipped state with temperature. Notably now the prediction of re-entrance is not
only confined to dsDNA but also for other bio-polymers e.g. proteins and homopolymers but it experimentally remained elusive so far. Using the large conformational change
in the reaction co-ordinate and the formation of hairpin because of solvent, they showed
that Molecular Beacon is an ideal candidate to observe re-entrance in vitro.
***
LS11341
A UNIVERSAL ELASTIC ANISOTROPY INDEX FOUND
Practically all elastic materials are anisotropic, which means that their properties are directionally dependent. This calls for an appropriate universal measure to uniquely quantify the degree of anisotropy for any particular material, e.g. crystal. Three well known anisotropy measures used for years lack universality as they are non-unique and ignore contributions from the bulk part of the elastic stiffness (or compliance) tensor. In this letter we introduce a universal anisotropy index that overcomes these limitations and allows one to fully quantify the single crystal anisotropy. Furthermore, we establish special relationships between the proposed anisotropy index and the existing anisotropy measures for special cases. An elastic anisotropy diagram is constructed for over 100 different crystals (from cubic through triclinic), demonstrating that the proposed anisotropy measure is applicable to all types of elastic single crystals, and thus fills an important void in the existing literature.
***
LL11424E
Moving at a constant pace may be key to the formation of coherent
patterns in swarming organisms
Self-propelled particle (SPP) models describe leaderless pattern
formation in biology, such as fish schools and insect swarms. SPP
models are classified into two distinct categories: kinematic and
dynamic. Kinematic SPP models assume that each particle travels at a
near constant speed and senses the position and orientation of its
local neighbors to adjust their orientation. Dynamic SPP models
describe the motion of particles based on Newtonian mechanics that
involve attractive and repulsive forces between particles rather than
alignment principles. In their paper, Newman and Sayama developed a
new dynamic SPP model in which a sensory blind zone is introduced into
each particle's zone of interaction, and studied the effects of the
blind zone on the formation of coherent vortex patterns. The result
indicates that even a slight sensory deficiency makes swarms unable to
form a pattern, which is quite different from kinematic models that
can have large sensory deficits and still form vortex patterns. This
comparison presents a conjecture that certain biological swarming
behaviors may sensitively depend on the ability of individuals to
maintain a constant velocity. Specifically, for organisms that keep
moving autonomously at a near constant pace, coherent pattern
formation emerges relatively easily even with significant sensory
blind zones, but for organisms whose motion strongly depends on
environmental stimuli, coherent pattern formation requires a nearly
complete panoramic range of interaction in order for particles to gain
enough propulsion from behind.
