Spin Control: Modeling the Transistor of the Future
As transistor dimensions continue to shrink while computing demands
grow, researchers want to encode digital information in the spin of
electrons, rather than only the charge. Although "spintronics"
already underlies today's high-volume disk drive technology, building
spintronic transistors has been difficult. Twenty years ago, two
scientists proposed a highly promising design, the Datta-Das
transistor (DDT) (Fig. 1). To date, however, no one has been able to
build one; the desired spin effects are sensitive to minor
imperfections in the system. In this paper, we have proposed
creating a minutely controllable atomic version of the spintronic
transistor using, instead of electrons in a semiconductor, a beam of
ultra-cold atoms passing through a region where three laser beams
overlap (Fig. 2). The atoms in light fields precisely mimic the
physics of electrons in a DDT, with two quantum states of the atoms
playing the role of the electron's spin. Unlike the electronic DDT
-- with its myriad sources of error -- an atom-beam analogue would
offer the opportunity to carefully control the behavior of the
system, allowing physicists to determine which specific factors are
most critical to the performance of a real DDT.
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LU11288
Shedding new light on an old problem to give cheap solar cells a boost.
In this paper a new understanding is presented of amorphous silicon
solar cells that could yield major improvements in their efficiency.
Since the 1970’s, amorphous silicon – a-Si – has been widely used as
a low-cost alternative to more traditional crystalline silicon solar
cells. However, its use has been limited to low-power applications
such as watches or calculators due to a very simple yet perplexing
effect: within just a few hours of exposure to light, the efficiency
of a-Si solar cells degrades by 25-30%. This effect has historically
been attributed to an increase in dangling bonds—silicon atoms with
less than the optimal number of four neighbors. By performing
accurate calculations of the interplay between the atomic and
electronic response of a-Si to light, this work finds that incident
light can cause regions of strain to form in the material that are
still fully four-fold coordinated and yet just as detrimental to the
solar cell performance as dangling bonds.
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LR11694
Environmental variability enhances fitness of competing species
In our computational study of competing predator and prey
populations with spatially varying availability of resources we
have found that environmental diversity can considerably enhance
the survival rate of both species, already in the absence of any
evolutionary effects.
Even simple ecological systems display remarkably rich features.
In the 1920s, Lotka and Volterra designed a mathematical model
for predators whose reproduction requires feeding on prey that
produces regular population oscillations. Facilitated by much
improved computing power, researchers have recently refined the
theoretical description of biological systems to include spatial
spreading and environmental diversity. Then, random fluctuations
in the numbers of individuals often play a crucial role.
In this work, we have investigated the consequences of spatial
variability in predator-prey interactions: e.g., the efficacy of
predators to hunt their prey may vary due to local environmental
influences. Our numerical simulations show that such spatial
variations increase the number of surviving individuals of both
populations to a level not predictable by standard analytical
approaches. Rather, this fitness enhancement is caused by
cooperative behavior of predators and prey that cluster together
near favorable locations. Our results underscore the important
role of spatial correlations and random fluctuations in ecology.
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LV11519
Charge ahead!
X-rays now make it possible to create detailed images of how electrons
are distributed inside a material. Utilizing recent advances in
spectroscopic techniques and a bit of mathematics it is now possible to
create detailed images of the charge distribution in a material. In a
demonstration experiment, the distribution of electrons in common copper
has been mapped out and the result shows that electrons of different
energy favour different positions within the solid.
Angle resolved photoelectron spectroscopy is a common technique for
studying the electronic structure of solids. The technique is based on
the photoelectric effect that was discovered by Heinrich Hertz in the
end of the 19^th century and later described theoretically by Albert
Einstein, meriting him the Nobel Prize in 1921. In the last decades the
technique has evolved rapidly as new sources of X-rays have become
available and detectors have improved. These advances now make it
possible to collect enough detailed data to create three-dimensional
images of the electron density in a solid. This density is intimately
related to the mechanical, optical and electrical properties of the
material.
