|
|
Close Help | ||||||||||||||
Elastic
scattering of protons at very high energies
In mutual proton collisions not only an amount of secondary particles is
formed, but also elastic scattering occurs when both the particles are only
deviated from original directions. And just this elastic scattering represents
one of the central theoretical problems of particle physics, as at difference
to inelastic collisions any theory does not exist for them until now. At
present a phenomenological model is being used, according to which the elastic
proton (or antiproton) scattering should occur with the maximum probability in
the case of central collisions; i.e., when the mutual transverse distance of
colliding particles, the so-called impact parameter, is much less than average
proton dimension (dashed line in figure left). The more general model proposed
by us [1] enables, however, to interpret elastic scattering as „peripheral“
process that occurs with maximum probability at impact parameters corresponding
to average proton dimension (full line in figure left). In newly built LHC
collider in CERN the TOTEM experiment is being prepared in which also our group
of physicists takes part. The experiment will be performed with extraordinary
high statistics; it should decide also between these two alternative models.
The measurement of the elastic scattering into very small angles is very
important for determining the accelerator luminosity, i.e., the efficient
frequency of mutual collisions of two proton beams going one against another.
The luminosity is the basic quantity, from which one goes out in establishing
all characteristics of other collision processes. The scattered protons at
energy of 14 TeV will be registered by very sophisticated detectors, the
so-called „Roman pots“, the main parts of which are produced by Vacuum Praha;
the prototype is shown in figure right.
[1] V. Kundrát, M. Lokajíček: "Interference
between Coulomb and hadronic scattering in elastic high-energy nucleon
collisions"; Phys. Lett. B 611:
102-110 (2005).
In the solid-state
physics, an increasing attention is devoted to crystals which are chemically
homogeneous, but their microscopic order is varying on the length scale of
several nanometers. Materials with such nanometric fluctuations or clusters
display unique properties as high-temperature superconductivity, colossal
magnetoresistence or magnetic
switching by electronic spin injections. Also so called relaxor ferroelectrics
belong to such materials. Their record breaking dielectric and piezoelectric
parametries are caused by small regions with ordered dipole moment, so called
polar nanoregions. Using far-infrared reflectivity technique together with
novel assignment of the spectra, a group of workers from the Department of
Dielectrics was able to demonstrate that some lattice-vibrational frequencies
in individual clusters are increased by up to 20-30 %, owing to their
interaction with local dipole moment and the dielectric constant inside the
polar nanoregions is strongly anisotropic in the infrared frequency range. The
dielectric response of relaxor ferroelectrics and piezoelectrics should be therefore
evaluated using the effective medium
approximation, considering also the cluster geometry and topology. The
results indicate that the polar cluster density in such materials is much
higher than assumed before so that they remind rather a ferroelectric crystal
with nano-domain structure or nanoscopically polycrystalline ferroelectric.
[1] J. Hlinka, T. Ostapchuk, D. Noujni, S. Kamba, and J. Petzelt: Anisotropic dielctric function in polar nano-regions of relaxor ferroelectrics, Phys.Rev. Lett. (2005), in print.
[2] J.
Hlinka, J. Petzelt, S. Kamba, D. Noujni and T. Ostapchuk: Infrared dielectric
response of relaxor ferroelectrics, invited review article into a special issue
of Phase Transitions on "Phase Transitions in Giant
Piezoelectrics".
Recently, significant
increase or decrease of temperature has been revealed in several materials
(GdDy, MnFeAs) as they were put in or taken out of magnetic field,
respectively. A study of intermetallic compounds with such very pronounced
(giant) magneto-caloric effect near room temperature has accelerated a rapid development
of the new, ecological and economical technology of cooling. Tests of the first units of magnetic
refrigerators are just running.
In the
Institute of Physics AS CR, a coincidence of structural and magnetic phase
transitions has been induced in some intermetallic compounds (Gd5Si2Ge2;
Tb5Si2Ge2; Ni2MnGa) by
application of a very high hydrostatic pressure and an increase of the accompanied
magneto-caloric effects has been revealed. First measurements of changes in the
magnetic entropy of solids have been performed under high hydrostatic pressure
in the framework of this study. The results of a wide collaboration with
laboratories in Spain, USA and Italy serve consequently in a prospecting of the
new perspective materials for the magnetic cooling.
[1] C. Magen et al., Hydrostatic pressure control of the magneto-structural phase transition in Gd5Si2Ge2 single crystals, Phys. Rev. B 72, 024416 (2005).
[2] J. Kamarád et al., Effect of hydrostatic pressure on magnetization of Ni2+xMn1-xGa alloys, J. Mag. Magn. Mats. 290-291:660 (2005).
Deposition of
conductive transparent oxide thin films (TCO) on polymer foils at atmospheric
pressure is recently very
desired topic in many laboratories for their technological applications. Many
PECVD (plasma enhanced chemical vapour deposition) methods working mainly at
low pressure were developed up to now but new low cost technologies working at elevated pressure
are still searched. It is also important to implement reliable ´in situ´
plasma diagnostics.
Conductive oxides ZnO and doped ZnO:Al on polymer substrates were deposited by means of plasma jet system working at atmospheric pressure in open environment. ZnO thin films are recently desirable due to their long-term stability and other physical properties mainly in application in solar cells, displays, etc. ´In situ´ plasma diagnostics implemented in our atmospheric plasma system made possible to optimize quality of deposited films and improve reproducibility of deposition process.
Deposition of BaxSr1-xTiO3 (BSTO) thin films by means of low-pressure hollow cathode plasma jet system was investigated. BSTO thin films are recently perspective for microwave tunable applications in 10-GHz range. Perovskite phase was found in these deposited BSTO films with crystallite size of »30 nm. Ferroelectric phase confirmed by detected ferroelectric loop was found at room temperature in certain films with a specific composition of Ba0.6Sr0.4TiO3.
[1]
Z. Hubička, M. Čada, P. Adámek, P. Virostko, J. Olejníček,
A. Deyneka, L. Jastrabík, K. Jurek, G. Suchaneck, G. Guenther, G. Gerlach, P.
Boháč: Investigation of the RF pulse
modulated plasma jet system during the deposition of Pb(ZrxTi1-x)O-3 thin films
on polymer substrates, Surf. Coat. Technol. 200: 940-946
(2005).
[2]
M. Chichina, Z.
Hubička, O. Churpita, M. Tichý: Measurement of the parameters of
atmospheric-pressure barrier-torch discharge, Plasma Processes and
Polymers 2: 501-506 (2005).
X-Ray Holography with Atomic
Resolution
At present, the structure of crystalline samples is usually solved by x-ray, electron or neutron diffraction. However, the problem how to determine the atomic structure of samples without translation periodicity (like small clusters of atoms, organic molecules, viruses, etc.) still remains to be solved.
The method of x-ray diffuse scattering holography proposed in the Institute of Physics seems to be very promising from this point of view, because, in principle, it makes possible to obtain the hologram of a cluster of atoms (and a three-dimensional real-space image of atoms can be reconstructed numerically from the hologram) by measuring anomalous x-ray diffuse scattering. This holographic method was not only proposed theoretically [1], but its feasibility was also proved experimentally by using a rubidium chloride sample (Fig. 1) [2]. Now, we are working on the improvement of data collection accuracy [3] as well as on the development of better reconstruction algorithm in order to be able to determine more complicated, and thus more interesting, unknown structures.
[1]
M.
Kopecký: X-ray diffuse scattering
holography, J. Appl.
Crystallogr. 37:711-715 (2004).
[2]
M.
Kopecký, J. Fábry, J. Kub, E. Busetto, and A. Lausi: X-ray diffuse scattering holography of a centrosymmetric sample. Appl. Phys. Lett. (v tisku).
[3]
J. Fábry, M. Kopecký, J. Kub: A simple method of shielding area detectors
from unwanted Bragg diffractions. J. Appl. Crystallogr. (v tisku).
Figure 1: The hologram obtained as a difference of diffuse scattering patterns recorded at energies of 15.060 keV and 15.120 keV (left). Holographic reconstruction of Rb and O atoms in the (001) crystallographic plane (right).
