BQ10747
Can laser light cool semiconductor devices?
Temperature reduction by suitable laser irradiation is a well
established technique for cooling free atoms and trapped ions. Extending
this technique to condensed matter has been an attractive goal ever
since the first experiments on atomic systems. Particularly
semiconductors would be an attractive target, where laser cooling might
provide vibration-free cooling. Laser cooling works by converting the
thermal energy of the material into optical energy: if the energy of the
laser photons is set below the energy of the photons that the device
emits, the system can combine the energy of the incident photon with
that of the lattice vibrations to generate a photon with higher energy
that is subsequently radiated away. In our paper we present for the
first time a detailed experimental study of this process (known as
photoluminescence up-conversion) in semiconductor nanostructures. In
particular, we determine the optimal choice of the laser wavelength as a
function of temperature. We find that the cooling power of a given laser
beam increases with temperature, in close analogy to conventional
cooling systems.
***
BN10462
Nanoparticles of cobaltites; intriguing and useful
When the size of the magnetic nanoparticles is reduced to few nanometers,
some of their basic magnetic properties are strongly influenced by the
particle size and may differ significantly from the bulk properties. As
the particle size decreases, the surface and interface effects become more
and more important. In our paper, we report magnetic and structural
properties of nanocrystalline LaCoO3 (cobaltites) with particle size
ranging from 25 to 38 nm. We found that with decreasing particle size the
unit cell expands substantially due to surface effects and the
ferromagnetic moment increases simultaneously with lattice expansion,
while the temperature Curie, Tc, remains nearly unchanged. Contrary to the
downsizing effect, we show that an applied pressure suppresses
dramatically the volume of FM phase leading to its full collapse at 10
kbar, whereas Tc does not change under pressure. The unique behavior
strongly suggests that the ferromagnetism in LaCoO3 is controlled by
unit-cell volume through a variation of Co-O bond length. Magnetic
nanoparticles may found technological applications in logic circuits,
magneto-electronic devices, magnetic data storage (a new higher speed
computer) and in biomedicine, e.g., new medical therapies (hyperthermia)
and as effective catalyst in chemical reactions.
***
EN10391
Heat engines convert the heat transferred from a hot body to a cold
body into some mechanical work, for example the lifting of a weight,
while heat pumps perform the opposite operation. According to
Classical Thermodynamics evaluation of the work W produced by a heat
engine per cycle, and its efficiency, requires the introduction of
absolute temperature and entropy. The latter quantity has been the
subject of many discussions and it is often found difficult to
comprehend. With the help of a mechanical equivalent of the quantum
heat engine, this paper provides a method of evaluation of the engine
efficiency that relies on only two elementary considerations. The
first one involves the potential energy of a weighting ball in the
gravitational field. The second is that the probability of picking up
a particular ball among a collection of N similar balls is equal to 1/
N. Thorough mixing of the balls is implied as for any (fair) lottery
machines. From the authors viewpoint, it is this element of chance
that distinguishes heat engines from more conventional mechanical
engines. In other words, by following the authors argument, most of
the classical thermodynamics laws may be understood with almost no
previous knowledge in physics. Negative temperatures (e.g., atomic
reservoirs with population inversion) and quantum heat engines may be
understood in the same manner.
***
BE10551
A new tricitical universality class
We have found a new tri-critical universality in the
anti-ferromagnetic classical XY model on the Kagom'e lattice
with easy-axes single-ion anisotropy. The Kagom'e lattice is a
two-dimensional network of corner-sharing triangles, in the
other way it's composed of three intervening triangular Bravais
lattices. The XY Kagom'e anti-ferromagnet is a prototype of
the systems representing a class fascinating phenomena called
magnetic frustration. The frustration refers to the presence of
degeneracy in the classical ground states arisen from
arrangement of spins on triangular units. It has been previously
found by I. Rittchey, P. Chandra, and P. Coleman (Phys. Rev. B 47, 15342 (1993)),
that this model shows only a Kosterlitz-Thouless transition in the absence of anisotropy,
which is a topological phase transition.
In our work, We added an easy-axes single-ion anisotropy to The
XY Kagom'e anti-ferromagnet, and studied its the critical
properties by employing an optimized Monte Carlo simulation. We
found that for large values of anisotropy, this system exhibits
an continuous transition to a so called all-in all-out state, in
which all the planar spins are located toward the easy-axes
directions from corner to center of the triangles or vise versa.
Since the large value of anisotropy limits the spin degrees of
freedom only along these directions, so this transition is in
Ising universality class. However the transition for small values
of anisotropy is dis-continuous, indicating the existence of a
tri-critical point for this model. The computed critical exponents
near this point, show deviation from those of 2d-$\phi^6$ model
which is, up to now, the only known two-dimensional tri-critical
universality class. We believe that transverse fluctuations
normal to easy-axes directions accompanying with the special
geometry of the Kagom'e lattice are responsible for this new
tri-critical behavior.
***
BR10618
Extremely low electron density with huge mobility revealed in Graphite
Graphite, the material inside your pens, starts to reveal its true
transport properties. Scientists from Germany, Spain and China,
developed an experimentally simple method to obtain basic properties of
the conduction electrons, like their average mean free path, density and
mobility without adjustable parameters. The method is based on the
change of resistance with a small constriction. Applying it to a piece
of oriented graphite, the team demonstrated that lowering temperature
conduction electrons in graphite can move several micrometers without
having scattering whereas their density tends practically to zero, which
can be translated in a huge mobility. According to the authors those
high mobility values are much larger than single graphene layers of
micrometer size can ever have. The results indicate that ballistic
electronics in graphite is possible and phenomena like
conduction-electron diffraction should be observable. Graphite
disconcerts even more when one realizes that its absolute resistivity is
as small as of a good metal. But how is it possible since there are no
electrons? Superconductivity is the authors' suggestion.
