Phys Rev Hot Papers: 5-4-07

Metal Contacts at the Atomic Scale

The atomic process in which two metallic surface contact each other has
been characterized. The formation of a contact between two bodies is a
process which always involves the formation of at least one atomic
contact. Many tribological problems such as adhesion or friction will
then depend on the formation of such a contact. We show that for many
situations this process occurs smoothly without a mechanical instability
that would lead to a jump to contact. We observe that the configuration
and material composition of the electrodes before contact largely
determine the presence or absence of a jump. Through a combination of
experiments, atomistic simulations, and first-principles transport
calculations the first contact in between two metallic surfaces is shown
to be formed, in most of the times, by either a single atom , two atoms
aligned or two parallel atoms.

***


Sending a camel through the eye of a needle, the mechanics of DNA
translocation through nanopores

The Bibilical phrase, according to some historians, actually refers
to a certain gate in Jerusalem called
the "Needle's Eye" that was so narrow that a camel could barely squeeze
through and only if unencumbered by baggage. About ten years ago
scientists were quite amazed to
discover that single molecules of DNA will squeeze through holes that
are only just bigger than their diameter,
if a small electrical voltage is applied across it. Since the entry
of a DNA blocks the current that would normally flow
through the hole, the characteristics of the DNA can be inferred by
monitoring the amount of this current.
The intriguing possibility that information about the actual base
sequence can be coaxed out of this current signal
has become the holy grail of a sizable group of researchers pursuing
this as a possible ultrafast DNA sequencing
technology. A major obstacle is that unlike the proverbial camel
trying to go through the Needle's Eye, the DNA
zips through at the galloping clip of a thousand to a million bases
per second -- too fast for scientists to "read" the
sequence. In this paper, we take the first step towards slowing down
the DNA, which is to provide an understanding
of what determines its speed in the first place! It is shown that the
same hydrodynamic resistance that determines
the speed with which a wire can be drawn through a die in the
classical engineering science of metallurgy
also operate at these ultra small molecular scales and is responsible
for determining how fast the DNA
crosses the pore.

Phys Rev Hot Papers: 5-3-07

Photos of the quantum-classical transition

The transition between the quantum and the classical ¿world¿ has been
visualized experimentally for the first time, providing vivid evidence
of the correctness of the basic ideas of the theory of decoherence. This
theory resolves the longstanding problem of the incompatibility of the
quantum mechanical superposition principle with our everyday experience
of a ¿classical¿ world, a problem most drastically illustrated by the
famous Schrödinger¿s cat paradox. Understanding of decoherence is
essential from a fundamental point of view, and experimental control of
decoherence is crucial for applications, e.g. quantum computers. In the
present experiment, a novel mechanism of decoherence has been studied,
Coulomb interaction of elementary particles without inner degrees of
freedom, namely free electrons in a biprism interferometer, with a truly
macroscopic and dissipative environment, namely the electron gas inside
a semiconducting plate. The closer the electrons pass to the surface of
the plate, the stronger is the disturbance (e.g. heating) of the
electron gas beneath the flight paths of the beam electrons. In turn,
which-path information, entanglement and decoherence increase. This
manifests itself in decreasing contrast of the interference fringes
(which are perpendicular to the surface of the plate) with decreasing
altitude of the electrons above the plate. The decrease in contrast
demonstrates the continuous transition from quantum to classical.


***


How to Rip a Fluid

In a simple experiment on a mixture of water, soap, and salt, we show that
a rigid object (like a knife) passes through a gel-like material as if it
were a liquid at slow speeds, but rips it up like a soft solid if it is
pulled rapidly. Most materials in real life do not follow the textbook
cases of solid, liquid, or gas; examples like blood, saliva, toothpaste,
and cell cytoplasm are called viscoelastic (viscous like a fluid, elastic
like a solid). This article focuses on the response of such a material to
increasingly extreme conditions of flow.

As a child will swish its finger
through an unknown liquid to discover what it is, in this experiment we
pull a cylinder through a viscoelastic gel of surfactant and organic salt
in water, to learn its responses. What happens is: flow at slow speeds,
cutting at intermediate speeds, and tearing at the highest speeds. Because
the material is not a solid however, it heals in the wake of the tear, and
recovers completely after several hours. We find that the material
strength of the solid is essentially the surface tension of the liquid -
this fact unifies the response across the time scales from flow to
fracture.

Phys Rev Hot Papers: 5-2-07

Self-assembly of a model quasicrystal

Although this year marks the 25th anniversary of the discovery of
quasicrystals, the mechanism of their formation is only poorly understood
yet. One reason is the lack of suitable theoretical models of atomic
interactions that lead to stable quasicrystals. A realistic model would be a
formidable task and simplifications have to be made. Michael Engel and
Hans-Rainer Trebin from Stuttgart University in Germany have now developed
interactions that allow the first direct observation of quasicrystal
growth. Their simple model consists of identical particles moving in two
dimensions favoring two distinct interparticle distances. The competition
between the distances can favor a local particle arrangement with ten-fold
symmetry, which is not compatible with periodicity. A quasicrystal is then
self-assembled in computer simulations at elevated temperatures. Upon
cooling
the quasicrystal undergoes a reversible phase transition into a complex
periodic crystal. A remarkably large variety of other crystals and a
quasicrystal with twelve-fold symmetry have also been found. In the
future the
model system might be experimentally realized with colloidal particles.


***


Essential building-block for large-scale quantum communication

In the present paper we have experimentally demonstrated an essential
building-block for large-scale quantum communication. We demonstrate
efficient creation of photonic entanglement using spatially distributed,
narrow-band, and memory built-in single-photon sources. This progress may
pave the way for future global quantum networks.

Currently, the channel length of quantum communication is limited to about
150 km due to photon losses and decoherence. Intuitively, one would expect a
"repeater", widely used in the traditional communication networks, will help
to extend the distance of quantum communication to an arbitrary desired
length. However, the quantum nature requires very special properties for
such a quantum repeater. Efficient schemas require a quantum memory to be
scalable.

In our experiment, atomic ensembles are used as quantum memory (stationary
qubit) while single photons are used as information carrier (flying qubit).
The inherent memory built-in properties of single photon sources make the
generation of entanglement--fundamental resource for quantum
communication--of independent single photons coming from remote sites very
efficient, based on which the quantum network is scalable.

***

Anomalous thermodynamics of Coulomb interacting massless Dirac fermions in two spatial dimensions

Low temperature thermodynamics of metals is a part of standard
condensed matter curriculum. It is well know, yet still remarkable,
that on the basis of the low temperature dependence of specific
heat, an experimentalist would be hard pressed to distinguish a
non-interacting electron gas from Coulomb interacting Fermi liquid.
In both cases the specific heat vanishes linearly with temperature.
This paper considers the effect of Coulomb interactions on the
specific heat of a two dimensional semimetal, which is in some sense
a critical point between a metal and a semiconductor. If the
electrons were free, the specific heat of a semimetal would vanish
quadratically with temperature, the extra power of temperature comes
from the linear depletion of single particle states at low energies.
Remarkably, the Coulomb interactions suppress this non-interacting
result by logarithmic factors, whose strength is given by the
effective fine structure constant, differing from the 1/137 by the
ratio of the speed of light to the Fermi velocity. Experimental
observation of such effect, would firmly place two dimensional
semimetals, such as graphene, into the category of non-Fermi liquid.