A First for Spintronics: Measuring the Strength of a Spin-Polarized Current
Traditional electronic devices depend on manipulating electrical charge, the
negative charge of electrons or the positive charge of 'holes,' but future
information-technology devices will depend not on charge electronics but on
spin electronics, or 'spintronics,' which manipulates the orientation of the
(quantum-mechanical) spin of the electrons. One way to achieve this is to
inject a spin-polarized current into a magnetic element and use the torque
the current exerts on the magnetic moment to reverse its direction. For the
first time, Kasai and colleagues have determined the strength of such spin
currents directly and unambiguously, using time resolved magnetic soft x-ray
microscopy to image the effects of spin-polarized electron currents on
magnetic vortex cores confined to a disk of ferromagnetic Permalloy. Working
at the U.S. Department of Energy's Advanced Light Source, the researchers
used specialized x-ray optics (Fresnel zone-plate lenses) to achieve a
spatial resolution of less than 25 nanometers, with a temporal resolution of
70 picoseconds, as determined by the duration of the x-ray flashes. This
novel technique offers a window to understanding fundamental magnetic
processes, with tremendous potential applications in such areas of
nanoscience as the development of novel magnetic materials,
ultrahigh-density magnetic sensors, and new storage technologies.
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LT11222
A FEW ELECTRONS CONTROL THE HEAT
Much current research with an eye toward technologies involves
materials with physical properties that can be manipulated with
weak electric or magnetic fields. Recently, researchers led by
Prof. J. Cohn (University of Miami), found that weak electric
fields have a surprisingly large effect on how well heat is
conducted in the magnetic oxide compound CaMnO(3) (calcium
manganite).
In magnetic solids -- those composed of ions which have a magnetic
"moment" -- the separation between atoms and the mobility of
electrons can be sensitive to the arrangement of the moments and
vice versa. This is particularly evident at the magnetic
transition or ordering temperature, below which neighboring
moments become aligned (a ferromagnet) or anti-aligned (an
antiferromagnet). Well above the antiferromagnet transition
temperature of CaMnO(3) (125 degrees Kelvin), fluctuating
nanoscale magnetically ordered regions form as a precursor to the
transition. This short-ranged order causes distortions in the
structure that impede the flow of heat. Turning on a modest
electric field mobilizes a small density of loosely bound
electrons and improves the heat conduction by a factor of two.
Because the electrons themselves are too few to account for the
improved heat conduction, the researchers suggest the novel
possibility that the liberated electrons suppress the magnetic
fluctuations.
***
LQ11199E
Shake it stronger and it will get denser.
Until now, experimental evidences showed that the stronger you tap a
collection of beads poured into a box, the fluffier it gets. This new study
shows that, if you go beyond a certain tapping intensity, the material starts
compacting back again. It is known that tapping a granular sample promotes
compaction. However, if the intensity of the taps is very low, the system
compacts better, although one needs to tap for longer. This remarkable fact
is supported by a number of experiments. However, none of these previous
studies probed really strong taps. Testing a number of computational models,
this paper shows that, eventually, high intensity taps will induce denser
arrangements. Therefore, there is an optimum tapping intensity at which the
looser state can be obtained. The paper propose an explanation based on the
formation and breakdown of arches. The results suggest that in a sample
confined to two dimensions the effect is much more evident. Preliminary
experimental research seems to confirm the prediction.
***
LV11004
A reversible breaking of inter-molecular chemical bonds with decreasing temperature and the coexistence of multiple molecular charge states are observed in a novel class of molecular solids known as fullerides. These materials, discovered only fifteen years ago, are composed of hollow spherically-shaped molecules (C60) with a diameter of one nanometer and made entirely of carbon which exhibit features that are rare in solids, such as the possibility of almost free rotation, distortion into elongated shapes, inter-molecular chemical binding, magnetism without transition elements, and charge conduction and superconductivity whereas an insulating electrical behavior is expected. We report on yet more surprises in our study: the strong (covalent) chemical bond linking two C60 molecules is observed to break as the temperature is lowered; while this cannot occur for an isolated pair of molecules, it is allowed for a system of many molecules such as a solid. The breaking/forming of the bond is perfectly reversible. When the bonds break a metallic phase results in which we could distinguish molecules carrying distinct electrical charges. This is quite unusual for such materials and indicates the occurrence of local charge fluctuations, which may provide a clue to solving the puzzle of electrical conductivity in fullerides.
***
LW10985
Dressed matter waves - new frontiers for quantum state engineering
When an atom interacts with an electromagnetic field, it becomes
"dressed" by the photons and changes its properties. Similarly, one
can "dress" a quantum mechanical matter wave, consisting of a large
number of ultracold atoms stored in an optical lattice, simply by
shaking that lattice with kilohertz frequencies. This means that
the matter wave responds to the shaking not by being disturbed in an
uncontrollable manner, as one might have expected on naive grounds,
but rather by acquiring new properties which the "bare" (i.e. unshaken)
system did not have. A striking demonstration of this concept had
recently been given by an experimental team from Pisa, Italy: These
authors succceeded to turn a superfluid matter wave into a Mott insulator
by varying the shaking strength [arXiv:0809.0768]. In their paper entitled
"Avoided level crossing spectroscopy with dressed matter waves", which
has just been accepted for publication in the Physical Review Letters,
two theoreticians from the Instituto de Ciencias Fotonicas (Spain) and
Oldenburg University (Germany) now have shown that the analogy between
"dressed atoms" and "dressed mesoscopic matter waves" has further high
potential for systematic quantum state engineering with ultracold atoms.
***
LW10842
Super-diffusive mass transport of Pb on Si(111) at low temperatures
An exceptionally fast and unusual mass transport behavior has been
discovered in the dense Pb wetting layer on the Si(111) surface at
temperatures as low as 150 K. The experiments were carried out using a
single laser pulse to desorb Pb from the wetting layer by heating the
surface locally. Low energy electron microscopy is used to observe the
resulting non-equilibrium coverage profile relax to equilibrium uniform
distribution in real-time. According to the classical textbook
description of surface mass transport, the initial coverage profile is
expected to broaden as it equilibrates progressively slower with time.
Surprisingly, profile evolution in the Pb wetting layer lacks these
classical features and is dominated instead by a much more efficient
convection-like mass transport. This behavior is unprecedented at
crystal surfaces and more characteristic of fluid motion. The authors
demonstrate that the transition to this novel super-diffusive state
occurs above a critical wetting layer coverage. Super-diffusive mass
transport can account for the highly efficient self-organization of
uniform height Pb nanostructures that was reported on widely in the
past. Its discovery also signals the possible existence of similar
intriguing mass transport mechanisms at low temperatures in other
systems that are yet to be discovered.
***
LT11935
To follow or not to follow: Laser-controlled electron motion in molecule
defies Newton's law
We show that ultrashort and intense laser pulses can be used to direct
and control electrons inside molecules. For the specific example of the
most simple molecule, the hydrogen molecular ion, we find that the
electronic probability current either follows or opposes the external
laser electric force, depending on the laser electric field strength.
According to Newton's second law of classical mechanics, an object
responds to an external force by changing its momentum. This classical
concept often applies for quantum systems too, where the correspondence
between classical variables and expectation values of quantum mechanical
operators is guaranteed by the Ehrenfest'S theorem. This correspondence
is frequently taken advantage of in order to explain complex quantum
mechanical phenomena, such as the interaction of intense laser light
with matter. For example, the generation of high harmonics radiation and
attosecond pulses can be understood in terms of the classical motion
of an electron in the laser field and its forced recollison with the parent
ion. The situation changes, if the electron is located inside a
molecule, as our numerical simulations for a dissociating hydrogen
molecular ion in an ultrashort intense laser pulse show. Here, the
momentum distribution of the electron is not only shifted by the
external laser field, but also modulated by an interference effect due
to the two nuclei. Due to this modulation the net electronic probability
current inside the molecular ion, that corresponds to the classical motion
of the electron, can follow or oppose to the laser electric force,
depending on the laser intensity. As ultrashort laser pulses are
acting on the time scale of the electronic motion in atoms and
molecules, our results are relevant for the control of chemical
reaction dynamics with light.
***
LN11497
Tuning nucleation on top of Pb islands by electron confinement.
On the nanoscale novel phenomena are present as a manifestation of
Quantum Mechanics. For nanotechnology applications it is important
to prepare nanostructures of uniform dimensions which requires
understanding nucleation on the atomistic level. Surprisingly a
dramatic variation of the nucleated island density is found for the
same orientation and the same substrate (i.e. Pb islands of (111)
orientation) as a function of island height. Normally nucleation is
varied only by changing the surface orientation and/or the substrate.
The new discovery is a result of how free electrons within the Pb
islands are confined according to the laws of Quantum Mechanics.
The electrons form standing waves:how well the standing waves fit
the island affects how easy Pb atoms nucleate the second layer
islands. This coupling of how electrons are confined to adatom
diffusion is unexpected and intriguing. The island density is 60
times larger on islands where the standing waves do not fit perfectly
the island. These unusual results suggest that it is possible to
affect atomistic processes not by varying the surface orientation or
substrate, but by simply changing island geometry.
***
BW10457
High-Temperature Superconductivity: Learning from Failure
In attempting to understand a particular phenomenon, the obvious
choice would be to study particularly successful examples; however,
sometimes one can learn as much or more from nominal failures. In
studying high temperature superconductors, we have discovered
potentially important clues to the mechanism of superconductivity by
studying a particular layered copper-oxide compound with anomalously
depressed superconductivity. This particular material is known to
exhibit unusual patterns of charge and magnetism known as "stripes".
It has been a common belief that this stripe order competes with
superconductivity. Thus, it comes as a considerable surprise that our
collective measurements of electronic transport and magnetic
properties, on one hand, and charge and spin order, on the other,
indicate that a fragile sort of short-range superconductivity develops
simultaneously with magnetic stripe order at a relatively high
temperature. Theorists have proposed that this coexistence may involve
an interweaving of the superconductivity and magnetism, much like warp
and weft. In any case, our results provide support for the
theoretical idea that spatial modulations of charge and spin densities
can enhance the strength of electron pairing, a prerequisite for high-
temperature superconductivity.
***
LR11507
A Quantum Look at Surface Plasmons
Recent developments in nanoplasmonics have led to the rapid development of
plasmonic circuitry for guiding and manipulation of light on subwavelength
scales as well as for engineering various devices and applications utilizing
the interaction of plasmonic modes with molecular excitations. The
development of plasmonic-based nanophotonic devices has also attracted a
keen interest from the quantum optics community for their use in quantum
information processing, due to a superior enhancement of nonlinear optical
effects and faster interaction times.
Most research in plasmonics is restricted to a completely classical
description of the underlying physics. This becomes inadequate once quantum
mechanical effects are prevalent in the investigation of plasmon-molecule
interactions or quantum plasmonic devices such as spasers (plasmon-based
lasers). Further development of molecular plasmonics and the next-generation
of photonic and plasmonic-based devices operating at a faithful
single-photon excitation level in nanoscale structures (desirable for the
purposes of integratable quantum information processing) will require a
genuine ab initio quantum approach.
In our article we provide key insights into the physics of photon-surface
plasmon coupling at the quantum level and the statistics of plasmonic
quantum states. We have established that excited surface plasmons can
completely preserve important quantum mechanical features of the original
photons as they travel along metal surfaces under realistic experimental
conditions. Efficient single-photon excitation in addition to few-photon
excitation of surface plasmons is considered. We also derive the
second-order quantum coherence function, which is an accurate measure of the
quantum nature of a surface plasmon.
Our results open up a new route toward the efficient manipulation of surface
plasmons at the quantum level in nanoplasmonic-based quantum information
processing experiments.
***
LS11197
Electric manipulation of molecular spins
Magnetism of single molecules has received much attention in
recent years. However, so far all the studies of molecular
magnetism have focused on the response of spins to magnetic
fields. Our results show that single-molecule magnets also
respond to electric fields by rearranging their spins. We have
identified a novel mechanism, based on broken inversion symmetry and
the concept of chirality, which enables access to the spin texture of
a special class of single-molecule magnets via static electric fields.
This provides a quantum degree of freedom that is amenable to electric
control, and can be exploited in a variety of ways. At the nanoscale,
strong controllable electric fields are readily available near the
tips of scanning tunneling microscopes, while the precise control over
magnetic fields remains a challenge. Spin-electric coupling therefore
enables a new generation of experimental studies as well as new
technologies based on electric manipulation of molecular spins.
