What a drag - Why do magnets have friction?
When you move an object in a rough environment, there exists a a drag on
the object. In most cases, this mechanical friction can be well
understood through the roughness of the objects and surfaces involved.
Friction comes in two flavors - static friction and kinetic friction.
The latter arises
when the object is actually moving and this type is experienced by spins
- the tiny quantum compass needles which
give rise to magnetism. When spins rotate, they too experience a drag.
However, unlike in the case of mechanical friction, the universal origin
of this friction is not understood. It has been presumed to take an
empirical form in equations for over the past 50 years in order to
explain data. However, two physicists Mark Hickey and Jagadeesh Moodera
from MIT's Francis Bitter Magnet Laboratory have uncovered the origin of
this friction and it lies in the fundamental description of the
electrons themselves in the foundations of quantum field theory - the
Dirac equation. This fundamental description is coupled with the law of
electromagnetic induction (discovered by Michael Faraday and
mathematically described by Maxwell), to show that the spins see a
time-varying magnetic field and
this gives rise to the friction they experience. It may also give rise
to the interaction which allows magnets to emit light. The results are
to be published in Physical Review Letters this month.
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LX11092E
SPECIAL RELATIVISTIC GENERALIZATION OF STATISTICAL THERMODYNAMICS
Does a moving body appear hotter, cooler or the same as the one at rest?
What is the special relativistic generalization of the celebrated Maxwell-Boltzmann
velocity distribution? These questions have been around for about a 100 years
now, and many famous physicists (including Einstein himself) have tried
to answer them. Much controversy surrounds these issues.
In this work, we propose a simple and realistic model of a
relativistic gas in order to investigate these questions. We find that Juttner
function is the correct generalization of the Maxwell-Boltzmann distribution.
Furthermore, we establish local thermal equilibrium for the moving system. Finally,
we show that standard statistical mechanical methods do not suffice to determine a
moving system's temperature uniquely. One is therefore left with temperature as a
system's parameter, same in all inertial frames, much like proper mass in mechanics.
***
LV11564
Surface potential determination for nano-structured materials
We describe a novel method for the determination of the local electronic surface potential of nano-structured template surfaces. Knowledge of the local surface potential is crucial for understanding phenomena sensitive to laterally varying surface properties such as catalytic processes and electron emission. Methods so far applied to this problem either lack of the necessary resolution or allow only a qualitative description of variations of the local surface potential. The presented method is based on the local detection of field emission resonances by scanning tunnelling spectroscopy (STS) and their numerical modelling using an appropriate model potential describing the local electrostatics. Due to their close residence at the surface field emission resonances allow the determination of the surface potential landscape with a lateral resolution below 1 nm. The method is applied to elucidate the site-specific adsorption properties of the strain-relief pattern formed by two monolayers of Ag on Pt(111). For the example of C60 fullerenes it is shown that the variation of the local surface potential is responsible for the site-selective immobilization.
***
LX11672
Squeezing an Image through a Tiny Hole
Common sense tells us that it would be hard to squeeze light through a
tiny hole in an opaque screen. Even harder, would be to transport a
complex information-carrying “image” through such a small aperture.
However, in this paper, using theory and computer simulations we show
that this bottleneck may be overcome: if the small hole is filled with a
material with near-zero dielectric constant and a bundle of metallic
wires, the image can be “squeezed” through the hole with its minute
details intact. Even more curious, counterintuitive fact in this
scenario is that even though the bundle of wires effectively contributes
to obstruct even more the tiny hole, the more closely packed the wires
are, the better the information-carrying wave can squeeze through! The
required materials may be either chosen from available natural materials
(e.g., plasmonic or polaritonic materials) operating near their plasma
frequencies, or they may be engineered as metamaterials with near-zero
permittivity. This phenomenon can provide a useful means for
transporting image-carrying waves through a very tight region and/or
sharp bends with potential applications in nanophotonics, and optical
communication and video transport at the nanoscale, to name a few.
***
LX11018
Discovery of Pathway from Graphite to Amorphous diamond
Carbon has various forms such as graphite, diamond, fullerene, and nanotubes. Amorphous diamond is
extreme one of diamond with no long-range ordering. A team of Japanese researchers has discovered
the novel transformation process from graphite into amorphous diamond. They have found that
transformation can occur if graphite was irradiated with high energy neutrons before the shock
compression. Wigner defects formed in graphite by the neutron irradiation are considered to make a
high density of diamond nucleation sites under shock compression, thereby leading to the
transformation into amorphous diamond. The team says that the combined method of irradiation and
shock compression is a promising way to synthesize new carbon materials such as the amorphous
diamond, as we can control the introduction of Wigner defects in terms of the kind, the amount, and
the spatial distribution by changing the irradiation condition of dose, temperature and incident
species. Moreover, the team says the present result is very interesting in general physics as it
has an analogy with "Rainmaking". As is well-known, rainmaking is the act of attempting to
artificially induce precipitation. The nucleation process for rain and the atmosphere for
precipitation are the keys for rainmaking, similar to the present study.
***
LK11669A
Quantum resource or classical control? The environment decides...
In this work we show that we can take a photonic device and use it
either as a quantum resources or a classical control field simply by
changing its environment. Quantum communication technologies are a
reality today, and the first steps are now being taken towards other
new technologies that sense, process and store information using
quantum resources. These new technologies get their power by
leveraging properties, such as "spooky action at a distance", only
seen in quantum systems. So we can operate them, these new
technologies must have a conventional - classical - IT interface.
Furthermore, the quantum resources need to be controlled with
classical sources, such as electromagnetic fields. However, we know
that everything is actually made of quantum parts! So this begs a
question: Under what circumstances are fields quantum - and thus part
of the technology resources - and under what conditions are they
classical - and thus part of the control interface? A "standard"
answer to this question is size: A field with one photon (one quantum
of light) is clearly quantum, and a large coherent field containing
many photons is classical. In our work we demonstrate that the actual
answer is rather more subtle than this. Indeed, it is possible to take
a field with fifty or more photons in it, and allow it to be highly
quantum (part of the resources), or force it to be classical (a
control field) by changing its environment. So size is a factor, but
it's not the only thing that matters. In the end how a system behaves
is also determined by what it interacts with.
***
BAR1138
Quantum dot probes the nanoscale magnetic environment
Nuclear spins in atoms of a solid-state material play the key part
in the dynamic magnetic environment influencing electronic properties on the
nano-scale. We show that by use of optical cooling, nuclear spins in a
layered GaAs/AlGaAs semiconductor can be driven in a highly polarized
(aligned) state with a very long lifetime exceeding one minute. The aligned
spins produce an effective magnetic field as high as several Tesla
experienced by the electrons in the layers containing the corresponding
nuclei. Light emitted by a few nanometer GaAs quantum dot formed at the
interface of the layers in our structure, carries information on the
magnitude of the nuclear field. We employ sensitive optical response from
such a quantum dot nano-probe for precise real-time monitoring of the subtle
changes in the nuclear spin system. Understanding the dynamics of the
effective nuclear field is crucially important for manipulation of the
electron spin, a promising qubit candidate for quantum information
processing.
***
LX11642
Extremely short laser pulses as an efficient terahertz source
Recent years have seen a significant advance in generation of extremely short laser pulses which contain but several (1-3) oscillations of the optical field and have very high powers. When such pulses are focused in a gas, they ionize atoms very fast and produce a dense plasma. In the process of plasma generation, the laser pulse accelerates electrons rapidly. As a result, strong electron currents are excited in the produced plasma and produce electromagnetic radiation in the ambient space. Earlier, most papers studied only the high-frequency spectral fraction of this radiation, which corresponds to the extreme-UV and soft X-ray attosecond-duration radiation. However, several recent papers reported on experimental observations and numerical simulations of low-frequency, specifically, terahertz emission from plasmas produced by few-cycle laser pulses. In our paper, we study the phenomenon of excitation of low-frequency residual currents, which generate this emission, in such plasmas. Efficiency of realization of this phenomenon is found as a function of laser pulse parameters. The role of the quantum effects associated with ionization and other stages of the electron behavior is investigated. We also find the optimal conditions, at which the efficiency of excitation of low-frequency currents can be very high, and see that the fewer oscillations fit in a laser pulse, the higher is the maximum achievable efficiency. The phenomenon studied in the paper opens the way for creating new high-efficiency sources of high-power electromagnetic waves in a challenging and underdeveloped terahertz frequency range. Another important task, which can be solved by using this phenomenon, is the development of a relatively simple way to determine parameters of extremely short laser pulses from the detected low-frequency (terahertz) radiation that they produce.
***
LX11637
Quantitative
spin-torque measurement
Among the future magnetic memories the domain wall the race track memory is one
of the most promising. The working principle is based on the fact that a
spin-polarized electric current exerts a pressure on a domain wall (spin
torque). Despite the fact that some of the spin-torque features have already
been established, up to now a quantitative direct measurement of the pressure
exerted by the current on the domain wall has not been achieved.
In this paper we report the first quantitative spin-torque measurement in
domain walls. In principle this method is similar to any quasistatic force
measurement. The domain wall is elastically pinned by a geometrical
constriction. Since the spring constant of the effective pinning is unknown, in
order to measure the force exerted by the current we use a reference force
given by an external magnetic field. By comparing the small displacement (down
to 0.1 nm) provoked by current and magnetic field we compare their forces.
Moreover from the point of view of possible applications, the spin-torque
measurement has revealed the unprecedented spin-torque efficiency existing in
one of the studied layers, approaching the maximum theoretical limit.
***
LQ11379
Ultra-High Energy Cosmic Ray May Be a Result of Plasma Wakefield
Surfing
The origin of the observed ultra-high energy cosmic rays (UHECR) is an
exciting scientific mystery. A single such particle would carry an energy
that is equivalent to that of a fast baseball. Where and how were they
produced? Recent UHECR observation data tend to be in favor of the
³bottom-up² scenario, which assumes that these are ordinary particles, such
as protons, that were accelerated by some astrophysical energetic sources.
In a recent study published in Physical Review Letters by Feng-Yin Chang et
al., the authors show, through computer simulations, that intense plasma
wakefields can be excited by the so-called magnetowaves, which are commonly
thought to exist in energetic outflows of astrophysical objects such as the
Active Galactic Nuclei (AGNs). A proton surfing on plasma wakefields can in
principle gain energy much like the speeding up of the surfer riding on an
ocean wave by the beach. Since this mechanism does not require the bending
of the accelerating particle¹s trajectory, the energy loss may be minimized.
The authors believe that this novel mechanism can accelerate cosmic rays
more efficiently than most other existing models. This may help to solve one
of the big mysteries in astrophysics.
***
BA11335
Two dimensional superconductivity in decoupled FeAs planes and large
upper critical field in iron oxypnicides*
Evidence of two-dimensional nature of superconductivity in iron
oxypnictides has been found by analysing resistivity curves in high
magnetic fields up to 28T.
Resistivity curves around the transition temperature are shaped by
fluctuations effects, as a consequence of thermally assisted formation
of superconducting Cooper pairs even above the superconducting
transition temperature. The analysis of such shape allows extract
information on the dimensional character of superconductivity, which
allows to obtain a description of the system and understand dissipative
mechanisms occurring in it.
In this paper, we apply such analysis to the newly discovered F doped
SmFeAsO superconductors.
Beside demonstrating the two-dimensional behaviour, we exploit our
analysis to obtain a thermodynamic and thus intrinsic estimation of the
upper critical field slope dHc2/dT. This parameter, difficult to be
defined according to a universal criterion in oxypnicides, is crucial
for applications, as it marks the onset of dissipative transport in a
magnetic field. A remarkable dHc2/dT value of -12T/K is found in
optimally doped samples.
***
LY11361B
Deformation of vortex lattices: making a soft material hard
We studied the emergence of irreversible deformation phenomena in vortex
lattices. Like ordinary crystalline materials, vortex matter in type II
superconductors can be deformed plastically, or irreversibly, upon
applying large enough currents. Through a combination of theoretical
arguments and numerical simulations, we demonstrated that plastic
activity is mediated by the motion of “scars” in the vortex lattices,
known as grain boundaries, which break the order of the lattice into
pieces. Upon increasing impurity in the system, the number of
topological scars increases and lattice domains become smaller and
smaller. This sort of “polycrystalline” arrangement is able to adjust to
impurities extremely easily, as every single crystallite introduces
additional degrees of freedom in the system. As a result the vortex
lattice, which is usually regarded as a “soft” material, becomes harder,
as larger currents are required to deform it. Our findings emphasize the
crucial role of grain boundary motion in small-scale deformation
phenomena, as observed in experiments on nanocrystalline materials and
disordered colloids.
In the figure: Upon applying a current to the vortex lattice (green
circles) plastic deformation is nucleated around grain boundary scars
(highlighted in blue and red)
***
LD11785B
Modification of surface electronic states by an adsorbed molecular
semiconductor layer
This report provides conclusive evidence that the electronic states in a
metal surface can be modified by the adsorption of an organic
semiconductor layer. While it is well established that molecular
orbitals -- the electronic states within molecules -- can be affected by
their interaction with a metallic substrate, e.g. in organic electronic
devices, the reverse effect of organic layers affecting metallic states
is less studied. In this paper we report about an increase in the
surface state occupation, i.e. an electron transfer to the surface
state, after adsorption of a monolayer of organic molecules, namely
pentacene leading to a shift in the binding energy of the surface state
and a modification of the effective mass describing the electronic
behavior of the substrate. Studying electronic interface phenomena for
technologically relevant materials like pentacene results in a better
understanding of the electronic properties of device interfaces and can
thus lead to improved charge injection behaviour in organic electronic
devices.
***
BY10738
The surface of a photonic crystal can determine its optics
Photonic crystals are leading contenders in the race to replace "slow"
electronic by "fast" photonic elements in optoelectronic circuits. In the
present work, it is experimentally demonstrated that by modifying the
surface of a photonic crystal one can tailor its optical properties. Among
such properties are resonant transmission and, for magnetoactive photonic
crystals, a giant resonance rotation of the polarization plane of incident
light. The change of the optical response of the photonic crystal occurs due
to the appearance of special non-propagating states localized near the
surface of the crystal. These states are analogous to the well-known surface
states in electronic crystals first predicted by Tamm [1] in 1932. In
Ref.[2] such states were predicted to arise at the interface between two
photonic crystals or a photonic crystal and a material with negative
dielectric permeability. When a surface of a photonic crystal is made of an
active material, optical Tamm states open up possibilities for tunability of
photonic elements.
***
LX11347
Vacancies that won't stay on-top
Usually, it is easier to remove an atom from the surface than from
the inside of a material: after all, fewer bonds need to be broken.
This paper shows that the opposite is the case for one particular
surface, TiO2 anatase.
This surface is rather 'stiff', with short bonds and a low surface
energy. Calculations predict that it costs more energy to form an
oxygen vacancy at the surface than deep in the bulk. Moreover, once
formed, the activation energy for a vacancy to migrate from the
surface to the inside of the sample is smaller than the other way
round.
Experiments confirmed this unusual prediction: using Scanning
Tunneling Microscopy, researchers have inspected the surfaces of
anatase (101) single crystals and found no surface oxygen vacancies.
More reduced samples showed clear evidence for ordered, subsurface
defects, however.
The result could be more than a mere curiosity: TiO2 is an important
material for the conversion of solar radiation into chemical and
electrical energy, and defects drive many surface chemical reactions.
If surface oxygen vacancies are formed, they will not survive long in
the ambient - almost immediately they will be covered by water or by
other gas molecules. When such defects hide in a subsurface layer,
they could provide a more subtle, but also more robust influence on
surface reactivity.
