Oxygen-free iron arsenide superconductors discovered
A family of oxygen-free iron arsenide superconductors with a not yet
optimized critical temperature (Tc) of 38 K was discovered. The material
is closely related to the recently found iron-arsenide-oxides, but
adopts an even simpler structure. It is meanwhile accepted, that the
crucial building block to superconductivity in LaOFeAs is the
iron-arsenide layer. The latter is sandwiched between lanthanum-oxide
layers, but only by barium atoms in the new material BaFe2As2. In our
recent paper (PRB, BSR1113, in press, arxiv:0805.4021), we show that the
physical properties of the new parent compound BaFe2As2 are amazingly
similar to LaOFeAs. Thus we predicted that BaFe2As2 would be a
superconductor by doping as known from the LaOFeAs materials. Very
recently, we succeed in inducing superconductivity at 38 K in
Ba0.6K0.4Fe2As2 by hole doping. (PRL, submitted, arxiv:0805.4630). This
discovery opens new avenues to find further superconductors in the large
family of BaFe2As2-related compounds. But more important, the new
materials have a simpler structure and are easier to synthesize also as
large single crystals. This gives a world of opportunity for rapid
experimental and theoretical progress and thus a fresh impetus for
finally solving the mystery of high-Tc superconductivity.
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LF11238
Violation of macroscopic realism in every-day life?
The laws of physics are of quantum nature and microscopic systems cannot be
described classically. But the macroscopic objects around us seem to have
objective properties prior to and independent of measurement which can be
inferred without altering them. Is it possible to experience a violation of
this macroscopic realism in every-day life?
Under realistic conditions we are only able to perform coarse-grained
measurements that do not resolve individual quantum levels of a macroscopic
object. We show that this usually allows a realistic description of the
object's dynamics: At every instant of time the quantum state - even if it
is a macroscopic Schrödinger cat-like superposition - appears as a classical
mixture and the time evolution of this mixture can be explained classically.
However, we demonstrate that there exist non-classical time evolutions which
allow to see a violation of macroscopic realism even under classical
coarse-grained measurements. The question why we then do not see such
violations arises again. We finally suggest that the reason for this is that
non-classical time evolutions are of high computational complexity.
Figuratively, this means that if nature spontaneously "chooses" a time
evolution, it is much more likely that a low complex, i.e. a classical, time
evolution is realized and thus our every-day world appears classical under
coarse-grained measurements.
Figure Caption:
The two graphs on the top show the quantum states of a Schrödinger cat-like
superposition of a spin pointing to the north and to the south (left) and a
classical fifty-fifty mixture in which half of the spins is along north and
the other half is along south (right). Under every-day coarse-grained
measurements both states have the same classical description in terms of a
fifty-fifty probability distribution (bottom left and right). However, there
exist non-classical time evolutions producing time-dependent superposition
states that allow to violate macroscopic realism even under coarse-grained
measurements.
***
BQ10572
Riding the wave of silica glass
Surface dynamics measurements can provide insight into the surface structure where direct imaging techniques fail. Silica glass, the chief component of the vast variety of silicate glasses, is an excellent insulator and its surface features very low surface tension values. For these as well as other reasons, commonly applied surface science techniques did not yet succeed in obtaining high-resolution information of the melt-formed silica surface.
In this paper, we have used neutral helium atoms as probes for investigating the silica glass surface. In the course of the experiment, a coherent matter wave of helium particles is created, gets scattered at the surface and the energy distribution of the scattered particles is obtained by measuring their time of flight. General surface dynamics parameters have been determined from an analysis of elastically scattered particles. Furthermore, a new surface dynamics effect peculiar to disordered matter – the surface boson peak – has been discovered; properties of which have also been reported in two recent issues of Physical Review Letters [PRL 99, 035503 (2007) and PRL 100, 135504 (2008)].
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LQ11728
Observing the quantization states of 3d electrons in monoatomic Cu chains.
It is well known that the behavior of d electons in solid is very important and associated with magnetism, superconductivity and transport phenomena. For the first time, this paper reported the quantization of one-dimensional 3d electrons states in monatomic metallic Cu wires. Low dimensional electronic systems with novel and tuneable properties attract high fundamental interest with challenging questions. For example, sp electrons in one dimensional Au atom chains on Si(111) step surface exhibit a number of exotic features, such as Peieris transitions, periodic lattice distortions, and charge density waves. By applying angle-resolved photoelectron spectroscopy, the authors in this paper for the first time observed an anisotropic one-dimensional 3d electron band quantization states in a monatomic array of Cu chains on the Pt(997) step surface. The 3d electrons of Cu are delocalized along the chains and localized in perpendicular direction of chains. Moreover, the authors demonstrated that the one dimensional confinement of Cu 3d electrons quantization states is introduced by the direct wave-function overlap between next-neighbor Cu atoms in the chains. These results are susceptible to have more general impact on the microscopic understanding of electronic low-dimensional systems.
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BS10835
Sub-picosecond ac spin current pulses produced.
Conventional electronics utilize electron's charge. Recently, there is a
general interest in using spin of electrons for the next generation
electronic technology - spintronics. Spin currents, flow of electron's
angular momentum, or spin, is the counterpart of charge currents in
conventional electronic devices. Although generation of charge currents
only requires application of a voltage, spin currents are not easy to
produce. In the past, several methods have been developed to generate
spin currents that are dc in nature and steady state. Now, ac spin
current pulses shorter than 1 picosecond have been demonstrated by using
coherent control techniques. The current pulses are generated optically
in intrinsic GaAs bulk and quantum wells, without applying any external
voltage or using magnetic materials. The new scheme may find
applications in high-speed spintronic designs where ac currents are
desired.
***
BSR1064
Strong correlations and order in the iron pnictide superconductors.
One of the key questions raised by the discovery of the new iron
based high
temperature superconductors is whether their physics is akin to the
copper
based high temperature superconductors discovered over 2 decades ago.
In our paper we argue that structural and spin ordering transitions
observed in
recent neutron scattering experiments suggest that these two classes
do indeed
share important features of ``strong correlation physics''. We show
that a
model of localized, and strongly correlated, electrons naturally
exhibits a
transition at relatively high temperatures where the square lattice
symmetry of
the Fe ions is reduced to a rectangular symmetry, just as is
observed. We extend
our model to describe a number of properties of the superconducting
and metallic
states, including the pairing symmetry of the Cooper pairs.
***
BR11049
In this paper, we present for the first time a complete description of
non-specular diffracted beams, both theoretically and experimentally.
An interesting and almost unexplored phenomenon occurring in photonic
crystals at wavelengths on the order or smaller than the lattice
parameter is the opening of diffraction channels, that is, a finite
number of diffracted beams emerge from the crystal slab when the
photon energy is greater than a threshold energy or diffraction
cut-off. These diffracted beams are propagating waves that can be
projected on a screen in order to measure their intensities. However,
up to date, most of the experimental and theoretical analyses in this
high energy range have been focused on the intensities of the
specularly reflected and forwardly (or ballistically) transmitted
beams.
We found in the spectral analysis of the diffracted beams strong
intensity modulations that arise from resonances of the three
dimensionally ordered sphere ensemble. Such novel diffraction
phenomena can be put into practice in a number of optoelectronic
devices.
***
CP10108
Pygmy dipole resonance is a very important phenomenon for astrophysical
models of rapid neutron capture occuring in core-collapse supernovae
which is responsible for the production of roughly one half of all heavy
elements beyond iron in the Universe. Since strongly interacting protons
and neutrons, the constituents of atomic nuclei, respond differently to
an external electromagnetic field, their relative motion exhibits different
modes of excitation at various energies. The best-known example is the giant dipole resonance caused by an oscillation of the proton and neutron liquids
against each other. A neutron or proton excess localized on the
nuclear surface forms a kind of a soft nuclear skin and in an external
electromagnetic field this skin can oscillate against the core
formed by an equal numbers of protons and neutrons. This oscillation
is called pygmy resonance. It occurs at low energies and
resembles the motion of sea water during the tides caused by the
gravitational attraction of the moon.
In this paper, we develop an approach which allows us to describe
many-body dynamics of giant and pygmy dipole resonances as well as
other nuclear vibrational excitations. Mode coupling between the
giant and pygmy dipole resonances and other surface vibrations is
taken into account in a fully consistent way. The respective model
predictions are in a very good agreement with the available
experimental data. Due to the consistency and the relatively small
number of the model parameters, which are universal within the
entire nuclear chart, the method allows also a reliable description
of nuclei not yet accessible to the experiment.
***
BP10529
Classification and Analytical Description of States in a Photonic Crystal
Photonic crystal (PC) has revolutionized the entire field of optics. Numerous methods are proposed for the theoretical modeling of PCs. Unfortunately none of these methods is simple, as they all involves numerical method without any analytical details or a formula-like description.
This work solves this problem by reporting a systematic and generalized way of getting analytical descriptions for two-dimensional PCs. The photonic states in the limit of zero modulation were analyzed and a systematic classification system based on integers was introduced for the states with distinct frequencies. The integers can be factored and states with the same factor can be categorized as they have the same form of solution when the spatial modulation is switched on. As it is shown in the paper, such states can be easily solved and typically lead to analytical solutions.
With the presented work, it is now possible to have an analytical description or formulas for many novel optical properties of PC explored previously using numerical methods such as band gaps, refractions, group velocities and density of states. This in turn, may enable researchers to engineer PCs and PC-based devices analytically.
