Phys Rev Hot Papers: 4-20

Light induced terahertz surface plasmons

Summary
Manipulation of electromagnetic waves below wavelength scale has been a challenging task for decades. Recent discovery of higher-than-unity transmission of light in periodic metal nanoholes, primarily due to resonant excitation of surface plasmons, has opened up a new avenue to subwavelength photonics. In this paper, we report light induced terahertz surface plasmons. This unique approach leads to direct observation of instantaneous transition between two interesting optical phenomena, out-of-plane photonic crystal effect and surface-plasmon resonance, in a thin semiconductor film perforated with an array of subwavelength holes. By use of optical pump-terahertz probe technique, the dielectric function of semiconductors is essentially altered by intense ultrafast laser pulses due to photo-generated free carriers. As a result, the semiconductor array becomes metallic and favors the coupling and propagation of surface plasmons in the terahertz frequency region. This finding demonstrates a new path to tunable surface plasmons, particularly, the ultrafast tuning of surface plasmons will be feasible in arrays made from semiconductors that possess fast carrier lifetime. Thus, it would be promising in device applications such as tunable terahertz sources, switches, filters, and modulators.

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Dark Stability -Storage of an Optical Vortex

A joint group of researchers from the Weizmann Institute of Science and the Technion Institute of Technology in Israel stored and later retrieved an optical vortex beam in rubidium vapor, employing Electro-magnetic Induced Transparency (EIT) in the medium.

Imagine yourself circling the earth along the equator while constantly updating your analog clock to the local time. After returning to the origin, the hand of your clock have completed a full circle. The number of complete turns of the clock's hand after this round-trip, is a topological invariant known as the winding number. Small changes in the local time across the globe can not change the winding number of the clock's hand upon completion of the trip.

Facing a light vortex, one observes a ring of light surrounding a dark center. The phase of the light at the rim is winding n times around the dark center, n being an integer. This phase is much like the hands of the clock. When two light waves with opposite phase meet, they destructively interference and cancel each other. The dark center of a light vortex is a result of this destructive interference. Light coming from a point on the rim to the center will be exactly canceled by light coming from the opposite rim since they have opposing phases.

Storing light in an EIT medium is done by continuously transferring a "signal" light pulse onto the atomic level's coherence, by switching off a second "control" beam. By switching back this control beam, the atoms are forced to emit the stored "signal" pulse.

In this work the spatial two dimensional phase and amplitude of an optical vortex was stored. Since this beam's winding number is topologically stable against local deformations, the structure of the restored vortex remained invariant, and the dark center remained dark even though the atoms storing the beam diffused significantly during the storage time. In a control experiment, the researchers used a similar ring shaped beam with a dark center but with a flat phase, and showed that it is not stable against atomic diffusion and was in fact filled with light upon retrieval.

The importance of this experiment lies in the prospect of using optical vortices in quantum communication and computation schemes, as well as in more elaborate two-dimensional information storage schemes.


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Playing with bricks on the atomic scale: making icosahedral and decahedral
particles in a bath of liquid helium.

Small particles of matter, consisting of just a few thousand atoms or
less, are very different from their large cousins. Like bricks in a
child's toy set, they come in a variety of symmetric shapes, including
icosahedra and decahedra of the Platonic shape fame. For a few compounds,
it is possible to make such particles using methods of chemistry. Minute
amounts of others can be produced in the so-called supersonic beams. In
our work, we report a technique of producing atomic-scale icosahedral and
decahedral particles in large amounts. Thus far, noble-gas elements (neon,
argon) were used, and the particles were stored in liquid helium at low
temperatures. However, particles made of other compounds that are stable
at higher temperatures can also be produced with this technique. Small
particles possess unique properties which find applications in such
diverse areas as electronics, energy storage, and catalysis. Our work
opens new opportunities for research of the properties of the atomic-scale
particles, and may lead to future technological applications.

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Unveiling Quasi-Periodicity within Periodic Media

Quasi-periodicity is the intriguing concept of order without periodicity.
Such materials, known as quasicrystals, are obtained by a definite, but not
periodic ordering in space of a certain material parameter - e.g. atom
position in natural quasicrystals (as discovered by Schectman et al in
1984), or the value of the linear or nonlinear electric susceptibility, in
man-made linear and nonlinear photonic crystals. Unlike periodic crystals,
quasicrystals lack translational symmetry, thus they cannot be constructed
using a translated repetition of a single building block. Until recently,
quasi-periodic behavior was studied only in materials which are
quasi-periodically ordered. However, we have found a scattering condition,
which unveils quasi-periodic behavior within a periodic media. This is a
physical manifestation of one of the methods to create models for
quasicrystals by referring to an abstract high-dimensional periodic
structure and then projecting part of it into an irrationally oriented
subspace. To verify our finding experimentally, we used a basic nonlinear
optical process - three-wave-mixing - within an artificially constructed
periodic nonlinear photonic crystal. Under the new scattering condition, the
photons participating in two different processes exhibited an incommensurate
momentum conservation relation, thus revealing quasi-periodicity.

Phys Rev Hot Papers: 4-19

How electron stripes yield ferroelectricity

A novel type of ferroelectricity, called “electronic ferroelectricity” – with an essential connection to charge ordering – is strongly suggested by new micrographic evidence. Conventional theory of solids holds that ferroelectricity in general originates from atomic structural polarizations – a familiar example is the notable off-center shift in the perovskite BaTiO3. Yet our new work appearing in Physical Review Letters (LY10555) reveals that ferroelectric LuFe2O4 has a curious ground state distinguished by electron stripes. We discovered that these electron stripes manifest a frustrated charge density wave with a remarkable ferroelectric polarization. This 3-dimensional charge ordering state, occurring at a low temperature of about 20K, was directly revealed for the first time by our in-situ transmission electron microscopy (TEM). A remarkable series of richly varied structural phenomena were also recorded as we lowered the temperature from 300K to 20K. The clear micrographic results have enabled us to detect new details about spontaneous polarization.

P.S. fig.1 (a) Electron diffraction image showing the weak satellite spots from charge stripe order. (b) Model for charge stripes and ferroelectric polarization.

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Orange reflection from a three-dimensional photonic crystal in the
scales of the weevil Pachyrrhynchus congestus pavonius (Curculionidae)
Welch,Victoria/Lousse,Virginie/Deparis,Olivier/Parker,Andrew/Vigneron, Phys Rev E.


The three-dimensional structure which causes the colouration of the
tropical weevil \textit{Pachyrrhynchus congestus pavonius} was studied,
using a combination of electron microscopy, optical spectroscopy and
numerical modelling. The orange scales which cover the coloured rings
on the animal's body were opened, to display the structure responsible
for the colouration. This structure is a three-dimensional photonic
polycrystal, each grain of which showing a face-centred cubic symmetry.
The measured lattice parameter and the observed filling fraction of
this structure explains the dominant reflected wavelength in the reddish
orange. The long-range disorder introduced by the grain boundaries
explain
the paradoxical observation that the reflectance, although generated by
a photonic-crystal, is insensitive to changes in the viewing angle.


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Turning electrons around : Echo as a measure for reversibility

An echo, a phenomenon known for sound (acoustic) waves, can be
observed with electronic waves in atoms. While a sound echo
results from the reflection of a sound wave at a hard object,
the echo of an electronic wave is a reflection induced by a rapid change
of an external field. With such tricks we can reverse the motion of the
electron leading to the recurrence of the initial wave. Using a technique
similar to the spin echo (the echo of the nuclear spins of molecules) we
demonstrate in this paper the echoes of electronic waves in atoms by
reversing the arrow of time. Like sound waves, electron waves may be
damped (decohere) during the propagation due to interactions with the
environment and the intensity of echoes is reduced. Measurements of echoes
can be thus used to quantify how much information initially stored in
atoms survives during the time propagation and can be retrieved.

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X-ray standing wave detects atoms taking tiny steps.



When you pull a string of a guiter to generate a musical note, a standing wave pattern is generated on the string. This is a mechanical wave. We generate standing waves of X-rays, which are electromagnetic waves, in an artificially made layered structure (multilayer) - like those used in the read head in your computer. Standing waves of X-rays can detect an impurity atom taking tiny steps (~ 0.2 nanometer) in such a multilayer. We cause such minute atomic movements (of Fe) in a Pt/C multilayer containing Fe impurity by shooting an energetic ion beam. We track the movements of Fe using an X-ray standing wave as we shoot more and more ions to the multilayer, where Fe atoms are driven out of C layers and captured in the Pt layers. This converts a nonmagnetic material into a ferromagnetic material by forming FePt magnetic nanoparticles. The method holds promise for future Terabit magnetic storage devices as nanometer sized ferromagnetic dots can be created in a nonmagnetic medium using a focused ion beam.
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Phys Rev Hot Papers: 4-14 to 4-18

Choreographic solution to the general-relativistic three-body
problem
By: Tatsunori Imai, Takamasa Chiba, and Hideki Asada

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Summary:
Black Holes Endlessly Playing Tag.
Three black holes can chase each other in a figure-eight-shaped orbit.
This has now been shown by physicists at Hirosaki University in Japan.
The motion of three gravitating celestial bodies is very complicated.
In fact, all the orbits for a three-body system are not known in
spite of many efforts since Isaac Newton. In 1993, C. Moore found
a remarkable solution that all the three bodies chase each other
in a figure eight like in a car chase (Physical Review Letters,
vol 70, p 3675, 1993). Such a solution is called 'choreographic'
if all the massive objects move periodically in a single-closed orbit.
A lot of choreographic solutions in Newtonian mechanics have been
found since the discovery of the figure eight. However, no one
has succeeded yet in Einstein's theory of general relativity. It's
currently the most successful gravitational theory describing the
nature of space and time, and has been well confirmed by observations
in such cases as the time delay in GPS (Global Positioning System).
For the first time, nevertheless, a choreographic solution in general
relativity has been found by a team led by Hideki Asada
(+81-172-39-3554, asada@phys.hirosaki-u.ac.jp). Actually, the
probability of three black holes endlessly playing tag in our
universe seems extremely low according to their estimate
(Imai, Chiba and Asada, Physical Review Letters, upcoming article;
preprint available from gr-qc/0702076). Further investigations
motivated by this work will shed new light into the nature of spacetime.
%%%%% end %%%%%

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Resonance by an Electric Current in a Si
Quantum Well

Controlling the magnetic moment of single electrons (their "spin") is
one of the prerequisites of "spintronics" - a new concept to overcome
present limitations of microelectronics (speed and power dissipation)
where the spin is used instead of the charge of electrons for data
storage and computation. We show that the spin of electrons in silicon
nanostructures can be adjusted most effectively by passing a small
current through it. The resulting "spin-orbit field" is by up to 10000
times bigger than the well known Oersted field, caused by the same
current. Experimentally, the spin-orbit field manifests itself
directly by a shift of the so-called electron-spin-resonance relative
to the externally applied magnetic field. Moreover, using ac currents,
spin precession around the spin-orbit field can be induced which can
be utilized for a most efficient spin adjustment. So far, microwave
pulses with kilowatt power were needed to invert the spin direction
within a time of 10 ns. Now, the current pulses, having the same
effect, produce dissipation in the nanowatt regime only.

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Domain Formation in Coherent Matter

An intriguing feature of the ultracold Rb-87 gas is that it exhibits a coexisting state of Bose-Einstein condensation (BEC) and ferromagnetism at low temperatures [PRA 68, 031604 (2003)]. Its domain structure, which forms spontaneously, minimize the free energy and is present in all kinds of ferromagnet, has even been observed very recently [Nature 443, 312 (2006)]. While the domain structure is subject to thermal agitation, BEC is a very special phase of matter which is governed by quantum mechanical principles. In this paper we present a theory to describe the dynamics of domain formation in this interesting system. We show that, driven by thermal agitation, the magnetization of each domain increases in an oscillating way, a behavior having its nature in quantum coherence. Strikingly, we find that the oscillation is amplified by thermal agitation, which is in sharp contrast to the well accepted knowledge that thermal agitation usually suppresses quantum coherence. The condensation of ultracold Rb-87 atoms demonstrates how quantum coherence and thermal agitation can reconcile in the formation of domains.

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Crystal Chirality

Life is left-handed. At least as far as amino-acids are concerned,
symmetry is radically broken in favor of the left-handed ones and the
equally physically possible right-handed population has been cleansed
from the territory of living organisms. Recently, similar categorical
symmetry breaking with the complete extermination of one of the
handed versions has been seen in laboratory crystallization
experiments. In our paper we demonstrate that the crucial phenomenon
necessary to explain this is Ostwald ripening, an idea expressed in
the Bible as "For unto every one that hath shall be given, and he
shall have abundance: but from him that hath not shall be taken away
even that which he hath". This is characteristic of many growth
processes in which only units - crystals in this case - of size above
a critical value are able to grow, while the smaller ones are
dissolved. We show how the competitive growth of the two species of
crystals together with Ostwald ripening leads to the complete
dominance of one and the extinction of the other, so that all
surviving crystals can trace their lineage back to one first crystal.
This has been dubbed the common-ancestor effect and our paper
provides a physical mechanism for it.

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A new mechanism of electron conduction in a transistor

Electrons can be conveyed via 膃impurity atoms芃 in doped
semiconductors. This mechanism, known as the hopping conduction, has
been a central issue of electron transport, but was a bit old
fashioned. Y. Ono and coworkers, for the first time, theoretically
investigated what happens on the hopping if we put the doped
semiconductor into a transistor structure. They found that, with the
gate of the transistor, electrons that randomly flow in
three-dimensional space could align in a restricted two-dimensional
space, which generates a new class of the transistor current. This
hopping transistor might not be able to beat the state-of-the-art
nano-transistors, but could be applied for building quantum
computers, some of which requires gate-controlled electronic charges
on the impurities.