Quantum Billiard Pocket Computer
We propose and describe for the first time how quantum computers could
be physically realized by playing billiard with single atoms. The required
miniature billiard tables must have the proportions of an A4 paper sheet.
Experimental means to create such tables with cushions made of laser light
do already exist. So far, they have been used to study the chaotic motion
of atoms confined to the interior of appropriately shaped laser corrals.
Our "A4-billiards", on the contrary, lead to perfectly predictable and
time-periodic motional patterns. Their dynamic behavior becomes particularly
transparent if observed stroboscopically, with a natural time-unit ("clock
cycle") proportional to the billiard area. The character of the resulting
motion is governed by the laws of quantum mechanics. A striking consequence
is that an atomic billiard-ball (i.e., a "blob" of quantum matter) can
"split" and --in a way-- end up at two or more locations at the same time.
Adding a controlled, minimal intervention from outside (e.g., shining short
laser pulses on individual "blobs" at appropriate instants of time) it
becomes possible to manipulate the system's quantum state at will. The
A4-proportion of these billiards allows one also to increase the system's
complexity by introducing onion-skin-like "billiard in a billiard"
architectures.
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BS10711
Relaxor Ferroelctric-like Behaviour in Ca Doped TbMnO3
TbMnO3 is known as the first magnetic induced ferroelectric material. Many ferroelectric materials have large switching (coercive) fields. In this manuscript we show that small amounts of doping with divalent Ca2+ results in relaxor type behaviour. This behaviour is associated with low coercive fields. The manuscript present neutron diffraction, magnetic and electric properties for 2, 5, and 10% doping.
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LG11819B
Mysterious threshold switching finally unveiled
Apply a large voltage to an amorphous semiconductor, and
the electrons hopping through traps in the material will
heat up and gain mobility. Above a certain threshold
voltage, this process culminates in a tremendous
enhancement of conductivity, which goes under the name of
⿿threshold switching⿿. Threshold switching was first
discovered in 1968 by S. Ovshinsky in chalcogenide
glasses, namely selenium- and tellurium-based alloys
lacking an atomic order. Researchers have long debated the
root cause of this mysterious effect, invoking thermal,
electric breakdown or phase transitions. A report appeared
recently in Phys. Rev. B may have unveiled the intimate
physics of threshold switching. Amorphous semiconductors
conduct electricity by electrons/holes hopping through
localized states. The higher the carrier energy, the
higher its ability to escape from traps, hence its
velocity. This is similar to what happens in a river,
where deep waters tend to remain trapped between rocks and
flow slowly, while shallow waters run more freely. High
electric fields can drive electrons/holes to high energy,
thus promoting their mobility and eventually leading to a
flash-like increase of conductivity. This discovery may
help the development of faster and less consuming non
volatile memories and cognitive devices based on
chalcogenide switching.
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BR10880
Photodiode based on carbon monolayer.
Fabricated a few years ago, graphene, a one-atom-thick carbon
layer of surprisingly high electronic and crystal quality, is a
possible candidate for the base material in future
nanoelectronics. However, the absence of the energy gap between
the valence and conduction bands in this two-dimensional
semiconductor hinders the possibility to control currents in
graphene-based devices, e.g. diodes and field-effect transistors.
To resolve this problem
we propose to use external electromagnetic
radiation, which allows to create a dynamical gap in the graphene
energy spectrum. The value of the gap depends on the power and the
frequency of radiation. We show that the electromagnetic field of
a wide frequency range generates photocurrent in this gapless
semiconductor. Such a photocurrent arises as a result of inelastic
electron tunneling accompanied by one- or two-photon absorption.
Applying a sufficiently large radiation power, one can also fully
suppress electron transport in a graphene-based junction.
***
LG11487
Visualizing atomic-scale acoustic waves in nanostructures
The highest frequency acoustic waves in materials, with nearly
atomic-scale wavelengths, are beginning to be utilized as a new kind
of tool to measure the properties of nanostructures but practical
detection of these waves with ultrahigh THz frequencies (10^12 Hz) is
extremely challenging. We have discovered a new physical phenomenon
that enables observation of such high frequency waves.
The highest frequency acoustic waves can form spontaneously at the
front of shock waves or be generated by sub-picosecond pulse length
lasers. Under some circumstances, when such a wave crosses an
interface between two materials, tiny electric currents are generated
at the interface. These currents produce electromagnetic radiation
of THz frequencies that can be detected a few millimeters away from
the interface. Using molecular dynamics simulations, we show that
the time-history of the wave can be determined with potentially
sub-picosecond, nearly atomic time and space resolution by measuring
the electromagnetic field coherently generated. We have studied the
effect for an interface between AlN and GaN which are used in LED
(light-emitting diode) nanostructures and are piezoelectric, i.e.
electric currents are generated when they are squeezed.
Piezoelectric materials have been employed for decades as arrival
time gauges for shock wave experiments but have been limited by
electrical equipment to detection of acoustic frequencies less than
10 GHz, precluding observation of the highest frequency acoustic
waves.
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BSR1110B
Precision STM measurements resolve atomic structure debate
Recent scanning tunneling microscopy measurements by Choi et al. have
resolved a long-running debate concerning the atomic structure of
one-monolayer films of copper nitride (Cu2N). Copper nitride has attracted
considerable interest for nanoscale templating applications, because
one-monolayer films can be grown that self-assemble into a regular array of
square islands. Despite the variety of techniques applied to study this
system over the last 20+ years, the mechanisms for self-assembly are not
well understood. Choi et al., use an STM operating in a low-temperature,
ultrahigh vacuum environment to directly measure the lattice constant of
copper nitride films. The measurements suggest that strain due to the 3%
lattice mismatch with the underlying Cu substrate contributes to
self-assembly in this system.
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BM10809
Electron-hole drops in multivalley semiconductors
The article develops a new formalism that gives the total energy of an
electron gas in common multivalley semiconductors such as Si, Ge, and GaAs.
To demonstrate its usage, the formalism is applied to electron-hole drops.
Every electron in a solid feels forces from all other electrons, creating
correlations, which makes the total energy of the electrons analytically
tractable in just a small handful of systems. In multivalley semiconductors
the electrons can be distinguished by a number categorizing them according
to their band structure valley; the limit of many valleys permits an
approximation that allows the exact total energy to be found. The energy
penalty of a changing electron density is also found, which allows systems
with spatially varying electron density to be analyzed.
The electron density profile for experimentally realisable electron-hole
drops is derived. Drops can be experimentally probed by straining their
semiconductor, which is naturally investigated within the formalism since
strain reduces the main parameter of the theory, the number of valleys.
***
AR10399
Focusing evanescent waves to a tiny spot
Evanescent waves can be shaped and focused to tiny spots using planar
gratings. This effect was first demonstrated by Merlin (Science, Vol.
3127, 927, 2007), who coined it ?radiationless electromagnetic
interference?. Conventionally, focusing is only achieved using
propagating waves, and to date evanescent waves have been completely
forgotten in this respect. In the current work we demonstrate that
evanescent waves exhibiting certain symmetric polarization patterns
can generate focal spots or holes which are much smaller than the
wavelength of light. The work has applications in optical microscopy
and data storage, where intensity distributions smaller than the
wavelength of light are required to resolve nanoscale objects.
