Controlling the structure (or polymorph) in which a molecule
crystallizes is a long-standing issue. Since polymorphs have
different physical properties, it is crucial for many applications
(e.g. in the making of pharmaceuticals) to understand and control
this phenomenon. In this paper, we use molecular simulations to shed
light on the molecular mechanisms underlying the choice of a specific
polymorph during the crystallization of spherical particles. We show
how, by modifying the temperature and pressure of crystallization, we
succeed in manipulating the mechanisms of crystal growth. We not only
determine the conditions enabling us to control the overall structure
of the crystallite but we also control its purity. Furthermore, we
provide new insight on how one structure can form on the surface of
another and on how we can prevent this phenomenon known
experimentally as cross-nucleation. In (a), we show a cross-
nucleation event as large domains of a structure (yellow) grow on top
of another (grey) while cross-nucleation is greatly reduced in (b).
***
The nuclear trigger for X-ray bursts
X-ray bursts belong to the most fascinating of astrophysical phenomena.
They are explained as thermonuclear explosions in the outer atmosphere
of accreting neutron stars. The thermonuclear explosion is triggered by
a single reaction, 15O(alpha,gamma)19Ne which has been experimentally
determined for the first time after two decades of failed attempts. The
paper not only reports on the successful experiment but demonstrates the
impact of the results on X-ray burst ignition and X-ray burst
periodicity in the framework of a thermonuclear explosion model. The new
results define stringent boundaries for the actual accretion rate in
observed X-ray bursters.
***
Lightspeed? Not so fast
What would happen if light were slowed to the speed of sound?
Tasgal, Band, and Malomed looked at pulses of light (solitons)
in an optical fiber with a Bragg grating, i.e., a periodic modulation
of the index of refraction of the fiber. Bragg reflection off the
periodic modulation can act as a uniformly distributed mirror that
couples forward- and backward-moving photons. This kind of fiber
allows light pulses as slow as zero velocity, though the current
experimental velocities are a sixth the speed of light (but dropping).
Interactions with sound had not been included in studies of this system.
Near the speed of sound, the electrostrictive coupling of light and sound
waves can result in braking, bringing the light pulse abruptly from
supersonic to subsonic. For reasons not yet completely understood,
this sonic braking can decelerate the light pulses sharply down to almost
zero velocity. (See the figure.) This form of slowed or stopped light
can exist at room temperature, in relatively unexotic materials
(as opposed to in Bose-Einstein condensates, which exist near absolute
zero temperature). The coupling of the light to sound provides extra
complications for theorists to grapple with, but it may simplify the work
of experimentalists in bringing light to a stop.
Figure caption: A slightly supersonic soliton spins off sound waves
and thereby slows to a velocity well below the speed of sound. The
top panel shows the light intensity versus time and position in the
fiber, and the bottom panel shows the sound wave amplitude, with the
newly generated sound pulses propagating away from the surviving
subsonic soliton.
***
Low energy interaction of the ozone cation with electrons
Ozone is one of the most important triatomic molecules, since it absorbs harmful
solar ultraviolet radiation and therefore plays a large role in protecting life
on Earth. We have studied the low energy interaction between an electron and
the ozone cation. Such an interaction leads to the almost exclusive rupture of
all the covalent bonds in the ozone molecule. Due to the relatively high
exothermicity of the reaction, oxygen fragments are predominantly formed in
{^3}P and {^1}D states, though formation in the {^1}S state is highly
unfavourable. Oxygen in the {^1}D state is responsible for the red aurora and
red airglow.
***
Equilibrium Flux Patterns in Type-I Superconductors
Patterns play an important role in strongly correlated systems with long
range interactions where different phases can co-exist. In type-I
superconductors, topologically rich physics is observed in so-called
intermediate state that appears due to finite size of the system. For about
80 years a model first suggested by Lev Landau was used in textbooks on
superconductivity. In this model stripy (lamellar or labyrinth) pattern was
suggested. Any hysteresis in macroscopic response (such as magnetization)
was considered due to various imperfections or particular edge structure.
In this work we show that 1) topology of the intermediate state depends on
macroscopic sample shape. We show experiments in spheres, hemispheres and
cones where so-called geometric barrier is absent; 2) equilibrium pattern is
represented by flux tubes, not stripes; 3) the difference in pattern
geometry gives raise to topological hysteresis even in perfect samples.
We conclude that it is impossible to deduce equilibrium topology from energy
minimization arguments. We hope it will prompt development of new methods
in the physics of complex systems.
Real time visualization of magnetic fields is available at:
http://www.cmpgroup.ameslab.gov/supermaglab/video/Pb.html
***
A handle on chaos in electron billiards
Regular or chaotic motion of balls on billiard tables is “win” or “lose”. This paper investigates how electrons, restricted in their motion to nanometer-sized billiards, can win or lose, in technical words can increase or decrease the billiard’s electrical resistance. This “game” is controlled by the application of a variable magnetic field, switching the electron motion between predominantly regular and predominantly chaotic. The detailed interpretation of the experimentally observed quantity – the electrical resistance – involves classical and quantum-mechanical theoretical concepts and relates prominent structure in the resistance to regular or chaotic behaviour. Also, an intriguing question since the advent of quantum theory - the correspondence of these two concepts – is addressed and shown to be applicable to the electron billiards. Regarding technological aspects, the results are of great relevance for the development of nano-electronics within the next decade, when devices will become electron-billiard-like due to further miniaturization.
***
Giving Faraday's law a new spin
Faraday's law of induction is a basic principle of physics dating from 1831 and
which explains the electromotive force that drives electrical currents in
generators and transformers. In Physical Review Letters [date], Barnes and
Maekawa show that, for magnetic materials, there is a correction to this law.
Electrical currents usually correspond to a flow of electrons, elementary
particles that carry a tiny electrical charge. In 1930, Dirac showed that, in
order to be consistent with Einstein's special relativity and quantum mechanics,
an electron must also have "spin", in effect, be a tiny magnet. The above
modifications to Faraday¿s law occur when this spin is accounted for.
"Spintronics" is an emerging electronics technology in which the electron spin
is used in an essential manner. While the Barnes and Maekawa corrections are
small for transformers or generators, they embody the requirements of energy
conservation for the spintronic devices being developed by the likes of IBM,
Freescale and Hitachi. Barnes and Maekawa show that the corrected Faraday's law
can be elegantly restated in terms of the time rate of change of the so called
¿Berry phase¿.
***
Quantum Repeater
For long-distance quantum communication one must realize quantum
network via quantum repeater protocol, a combination of entanglement
swapping, entanglement purification and quantum memory. In a seminar paper,
Duan et al. proposed a promising quantum repeater protocol (DLCZ) with
atomic ensembles and linear optics. However, in DLCZ protocol entanglement
generation and entanglement swapping rely on single-photon Mach-Zehnder-type
interference, which is sensitive to phase instabilities. This drawback is
severe enough to make long-distance quantum communication impossible. In
this paper, we present a robust quantum repeater architecture building on
DLCZ protocol. The architecture is based on two-photon Hong-Ou-Mandel-type
interference which relaxes the long distance stability requirements by about
7 orders of magnitude, from sub wavelength for the single photon
interference required by DLCZ to the coherence length of the photons. The
prize which has to be paid for this benefit are spurious contributions of
states with higher number of excitations. We subtly design the protocol so
that the spurious contributions can be automatically eliminated during the
entanglement connection process. Our protocol provides an exciting
possibility for robust and realistic long-distance quantum communication.
***
Gating electrons at a free silicon surface
Gating electrons at a free silicon surface terminated with a monolayer
of hydrogen enclosed in a vacuum cavity provides a revolutionary
technique in which to create a high quality two-dimensional electron
system (2DES) in silicon. It has been known for some time that a
silicon surface terminated with hydrogen is an ideal electronic surface
with its atomic flatness and low number of defects. In this paper we
present the first systematic low temperature electron transport
measurements on a hydrogen terminated silicon surface whereby the
quality of our 2DES can be attested by the first observation of the
integer quantum Hall effect on a (111) orientated silicon surface.
Unlike 2DES created by conventional methods at an interface between
silicon and an amorphous glass (SiO2), the hydrogen terminated
silicon-vacuum interface results in an order of magnitude less disorder.
As a result we observe the ground state in silicon (111) surfaces to be
sensitive to the wafer orientation and in-plane magnetic fields. We
believe these results are seminal for two reasons: new ground states may
occur in the quantized Hall regime in a multi-component system such as
Si (111). Secondly because the 2DES is directly below the hydrogen
terminated silicon surface, a new breed of atomic-scale quantum devices
may arise whereby electrons can couple to molecules or atoms
specifically grafted on to the surface.
***
Polytetrahedral nature of the dense disordered packings of hard spheres
by A.V. Anikeenko, N.N. Medvedev
The paper is a new examination of the structure of non-crystalline
and partly-crystalline dense packings of hard spheres. It throws some light
on the unanswered question concerning the disordered dense packing is why
its maximum density is ~0.64. This problem is a "non-crystalline
equivalent" of the long-standing Kepler conjecture about maximal density
(~0.74) for crystalline packing of identical spheres which was solved only
recently. The performed analysis of computer models justifies the
polytetrahedral nature of the dense disordered packings of spheres, i.e.
the packings contain an appreciable fraction of tetrahedral configurations
of spheres that prefer to coalesce via their faces to form a variety of
locally dense aggregates which are inconsistent with the formation of a
lattice. Such tetrahedra are not perfect, the gaps between the neighboring
spheres may be as large as 25% of the diameter. These tetrahedra coincide
with the class of quasi-regular tetrahedra introduced by Hales in his proof
of the Kepler conjecture. The paper is of intense interest to both
mathematicians and condensed matter physicists in general. The model of
hard spheres is very successfully in trying to understand the structures of
liquids, glasses, colloids and granular matter.
***
Novel schemes of measurement-based quantum computation
Imagine one has a quantum system with many constituents prepared in a
certain entangled state in a laboratory. This could be a state of a
many-body system like of cold atoms in optical lattices or of atoms in an
array of cavities, entangled via a light bus. Could this state be used
for quantum computing, based merely on local measurements, but abandoning
the need for any unitary control to realize quantum gates?
In our work [1], we show that with a great deal of flexibility, many-body
states allow for quantum computation in such a fashion that fully
abandons the need for unitary control during the computation. The
celebrated result by Briegel and Raussendorf showed that local
measurements on a single very specific state, the so-called cluster state
[2], gives rise to universal quantum computation. This cluster state has
a number of interesting, but also rare properties. In our work, we hence
address the question: what if the state is simply not a cluster state,
but just some other state?
Our work opens up an avenue to follow such a line of thought: We find
many new models for quantum computing, and also many resource states that
have radically different correlation and entanglement properties. For
example, do we really need to have no long-range correlations in such a
resource state? This would be bad, ruling out many ground states from
scratch. But, fortunately, one can overcome this limitation. This
suggests that to some extent, the theorist does not necessarily have to
approach the experimentalist, asking for the preparation of a particular
state that may possibly be fragile with respect to decoherence effects.
But that the theorist may construct a computational model based on the
very state that can relatively feasibly be prepared.
***
How to make negative charge positive
In this paper we show that some negatively charged particles
may produce a positive charge density. An everyday experience tells us
that it is impossible. Indeed, ordinary particles like negatively charged
electrons produce negative charge density everywhere, any time.
However, the situation is different for W-bosons, fundamental particles
which mediate the weak interaction (that is responsible for
beta-decay of unstable nuclear isotopes).
The reason why W-bosons demonstrate this misterious behavior stems from
the fact that they are described by the relativistic quantum mechanics.
The spin S of W-bosons is large, S=1
(electrons have spin 1/2). In naive terms the W-boson rotates vigorously
around its axes. Precisely this strong rotation, combined with effects
of special relativily, leads to the puzzling phenomenon of the
"wrong sign" of the charge distribution. This effect does not contradict
the charge conservation, i.e. no charge disappered or created.
For example, W-bosons at rest have a given negative charge.
For fast, relativistic W-bosons there are some areas of space with
positive charge and some areas with negative charge, but the total charge
does not change.
***
Are protons getting lighter with time?
It is common knowledge that protons are 1837 times heavier than electrons.
Recent astronomical observations [Reinhold and others, PRL, 96, 151101 (2006)]
suggest that this ratio was even bigger in the past. If proven, that
would require reconsideration of the most fundamental physical theories,
including Einstein's general relativity.
In this paper we have shown that the proton-to-electron mass ratio has not
changed by more than 2.5 parts per million during the last 6 billion years.
We used microwave spectra from the galaxy B0218+357, which is
approximately 6 billion light years away from the Milky Way. These spectra
include famous ammonia transitions, which were used by Townes to build the
first maser [Nobel Prize 1964]. The frequency of these transitions appears to
be extremely sensitive to the mass ratio in question. Comparison of
ammonia lines with microwave lines of other molecules revealed no
relative frequency shift. This allowed us to place the most
stringent limit on the space-time variation of the proton-to-electron
mass ratio.
