A new quantum phase transition (QPT) has been interestingly observed in
ferroelectric oxides. In this case, the coulomb interaction, which
creates the long-range order of dipole moments, competes with the
quantum mechanical fluctuation that destroys the order.
Usually, the phase transition, for example the freezing of water, is
governed by the thermal fluctuation; however, if the temperature
approaches the absolute zero temperature, the quantum mechanical effect
emerges in the macroscopic scale. In this study, we have clearly
observed the crossover of the critical behavior of the 腧soft mode腨 from
the classical to the quantum regime, where the soft mode is the
anharmonic lattice vibration that triggers the phase transition. This is
the first QPT brought about by the phonon (the quanta of the lattice
vibration), in contrast to those induced by electronic interactions. In
particular, the novel ferroelectric-paraelectric phase coexistence state
has been revealed in the vicinity of the so-called quantum critical
point for the first time. These findings have been achieved in the solid
solution of SrTi^16 O_3 and SrTi^18 O_3 by carefully controlling the
amplitude of the quantum fluctuation without modulating the chemical
natures of the material by a proper selection on the ^18 O-concentration.
***
Bose-Einstein condensation in the presence of an attractive interaction and its relation to the cosmological Q-balls
(1) It is well known that stable atomic Bose-Einstein condensates can
be formed when the interaction between the atoms is repulsive. An
attractive interaction, unless it is extremely small, leads to the
collapse of the Bose gas. The Bose-Einstein condensate of spin waves
(magnons) with a repulsive interaction was experimentally stabilized
in 1984 in superfluid 3He-B. In this paper it is shown that a
specific feature of this Bose condensate is that it survives the
attractive interaction between the magnons. The magnon condensate
with attractive interaction is formed in the potential well produced
by the texture of the order parameter of 3He, and is observed in
nuclear magnetic resonance experiments as a long-lived coherent
precession of magnetization, which can persist for an hour without
external pumping of energy.
(2) The structure and stability of the magnon condensate with an
attractive interaction are similar to those of certain objects
discussed in particle physics. These are the so-called Q-balls,
which are stabilized due to the conservation of the global charge Q,
typically the baryon or lepton number. In the magnon condensate the
role of the charge Q is played by the projection of the total spin of
the liquid in the direction of the magnetic field. At the quantum
level, Q-balls are formed due to a suitable attractive interaction
that binds the quanta of the scalar field into a large compact
object. Q-balls may contribute significantly to the dark matter and
baryon contents of the Universe. Observation of Q-ball objects in
3He-B confirms the possibility that stable cosmological Q-balls might
exist and should be searched experimentally.
***
Origin of Antiferroelectricity Explained from First Principles
The phenomenon of antiferroelectriciy wherin a compound develops two (or more) oppositely polarized sublattices was first discovered in 1938 in ammonium dihydrogen phosphate, commonly called ADP. ADP and its ferroelectric analogs have been used extensively in electro-optical applications. However, the atomistic details of why ADP becomes antiferroelectric has not been fully understood. A detailed understanding of such properties is needed in order too be able to hone their desirable characteristics. Now efficient algorithms for /ab-initio/ electronic structure calculations have enabled a group of solid state theorists at the Rosario National University in Argentina, working with chemists at Florida State University, to show that ADP's antiferroelectricity is a result of a delicate balance between the energetics of the two different types of hydrogen bonds in the lattice : O-H···O and N-H···O bonds. In a small range of atomic configurations, the optimal N-H···O bonding is enabled by the antiparallel alignment of molecular dipole moments in the unit cell and the crystal thus exhibits antiferroelectricity. The calculations show also that a small change in the atomic displacements could also render ferroelectricity to ADP, thus providing a theoretical basis of a long-standing observation that ADP appears to possess coexisting ferroeelectric and antiferroelectric regions whose relative population changes with temperature. (J. Lasave et al., Phys. Rev. Lett. (2007, in press)).
***
Potassium Bose-Einstein condensate: last but not least
A recent revolution in modern physics has been the achievement of Bose Einstein condensation, i.e. the formation of macroscopic quantum objects offering unprecedented possibilities for the study of the quantum world. This phenomenon has been demonstrated for a number of atomic species including all the stable elements in the first column of the periodic table, except one isotope of potassium. An experimental team at LENS, University of Florence, has now succeeded in creating a Bose-Einstein condensate of such last isotope, potassium-39, and found that, surprisingly, it is one of the most interesting among those studied so far. The team has overcome what appeared to be a fundamental obstacle to condensation of this species, i.e. the presence of an attractive interaction between pairs of atoms. The researchers have found that with the application of appropriate magnetic fields such interaction can be changed to repulsive and a condensate can be produced. Moreover, they have found that in a potassium-39 condensate the interaction can also be nulled with high accuracy, opening a route to the study of an ideal, i.e. non-interacting, quantum object. This novel Bose-Einstein condensate is expected to allow the study of some aspects of the quantum world that have not yet been explored.
***
Surveying with Electrons: Putting Lloyd's Mirror on the Map
A major puzzle for many researchers working in the field of materials
research is how to obtain 3D movies of surface dynamic events, often
during thin film growth or annealing. In a new approach, Monash
Physicists have shown how in situ electron microscopy can be used to
obtain surface height resolution in real-time. Just as the contour lines
on a topographic map provide mountaineers with the capacity to plan
their assault on a mountain, Lloyd's mirror combined with electron
microscopy can provide a relief map of surfaces in which the spacing
between fringes is directly related to height. In the classic 19th
century Lloyd's mirror experiment, light reflected off a mirror
interferes with light coming directly from the source. PRL XX reports a
novel 21st century version of the famous Lloyd's mirror, in which
ultraviolet light is used to illuminate gallium droplets on a gallium
arsenide substrate. The bright interference fringes result in the
emission of electrons, which are imaged in a surface electron
microscope. The fringes are sensitive to the 3D shape of the gallium
droplets, but are also distorted by the electric field due to the
topographic features. However, these distortions can be corrected using
image processing, to provide a real-time relief map showing surface
dynamics of the evolving metallic droplets. Lloyd's mirror electron
microscopy using a synchrotron light source will allow the technique to
image surface features with nanoscale resolution, opening up new
possibilities to make 3D movies of dynamic events during thin film
growth and processing.
***
Towards ultrafast x-ray physics: a novel method to manipulate x-ray
pulses
Theoretical calculations predict a novel physical effect for x rays that
may facilitate the generation of ultrashort x-ray pulses with tailored
shape. This prediction opens new vistas in science, in connection with
both existing x-ray facilities such as Argonne's Advanced Photon Source
and future x-ray free electron lasers. The effect is called
electromagnetically induced transparency (EIT) for x rays. A gas that
would otherwise strongly absorb, and thus be opaque to, x rays of a
given wavelength can be rendered transparent to the x rays by
simultaneously shining an intense laser into the gas. EIT for visible
light is well known and represents a unique tool for manipulating light.
Inner-shell vacancies produced by x-ray absorption have very short
lifetimes of the order of femtoseconds or shorter. Therefore, the laser
intensity required to induce EIT is so high that the standard
description of EIT becomes invalid. Before this study, it was
questionable whether EIT may exist at all in the x-ray regime. The
theoretical analysis carried out in this paper demonstrates that
existing intense-laser technology may be harnessed to imprint the
temporal shape of optical laser pulses onto x-ray pulses.
***
Unique due to relativity
Heavy element polonium enters harmfully our lives via air, soil and
plants such as tobacco or tea, but very little is known about its
physical and chemical properties. It has not been clear why polonium, as
the only element from the Periodic Table, crystallizes in the simple
cubic structure, one of the simplest structures ever. Our paper shows,
on the basis of the fundamental quantum mechanics, that this
unique structure of polonium is, similarly as the yellow luster of
gold, due to relativistic effects. We also draw the attention to
polonium strong elastic anisotropy, which has no equal in other solids,
and demonstrate that this is an inherent peculiarity of its crystal
structure. Finally, as a challenge for future experimental studies, we
predict a mechanical instability of simple cubic polonium at relatively
low pressures resulting in a mixture of two lower-symmetry structures.
***
The quantum nature of an atom enables its simultaneous propagation in two different regions of space: this property has been used by the research group of J. Vigué (Université P. Sabatier and CNRS, Toulouse) to make an absolute measurement of the refraction index of gases for lithium atomic waves. This experiment is based on an atom interferometer, in which each atom follows two paths. Their maximum distance, equal to 100 micrometers, is sufficient to introduce a material object between them so as to apply a weak gas pressure on one of them.
The only comparable experiments have been done on sodium by the research group of D. Pritchard at MIT but they have not achieved an absolute measurement.
This index describes the attenuation and the delay or advance of the atomic wave induced by collisions with the atoms of the gas. The comparison of the present attenuation measurements with previous values is excellent. The delay or advance of the atomic wave can be accessed only by the present atom interferometry method and the measured values are in agreement with theoretical estimates.
Finally, this type of experiments changes our representation of atoms, from point-like particles to strongly delocalised quantum waves!
***
Plasma Density Fluctuations Are Universal, But This Was Hidden Beyond A
Picturesque Smile
A unique parabolic relation links two statistical quantities related to
plasma density fluctuations. As in many other physical systems, a
significant effort is dedicated to finding universal aspects in a
statistical description of plasma turbulence. The probability of
occurence of any event can be given by its probability density function
(PDF hereafter). For a PDF, the degree of asymmetry and the weight of
its wings are contained in two parameters called skewness and kurtosis,
respectively. In this paper, we compute these two parameters for around
ten thousands signals, measured over the whole cross-section of a
toroidal magnetised plasma for a broad range of experimental conditions.
For the first time in plasma physics, universality has been sought by
plotting the kurtosis against the skewness, revealing two proofs for
universality. Firstly, all the experimental points are not randomly
distributed but form a picturesque smile. Secondly, all the PDF of the
measured signals are universally described by one and only one
analytical distribution. Fluctuations in the frequency range of the
paradigmatic drift-interchange instability are necessary and sufficient
to assure that PDF can be described by this specific distribution.
