325 research outputs found
Ion induced quark-gluon implosion
We investigate nuclear fragmentation in the central proton-nucleus and
nucleus - nucleus collisions at the energies of LHC. We argue that within the
semi-classical approximation because of fast increase with energy of cross
sections of soft and hard interactions each nucleon is stripped in the average
process off ``soft'' partons and fragments into a collection of leading quarks
and gluons with large . Valence quarks and gluons are streaming in the
opposite directions when viewed in the c.m. of the produced system. The
resulting pattern of the fragmentation of the colliding nuclei leads to an
implosion of the quark and gluon constituents of the nuclei. The matter density
produced at the initial stage in the nucleus fragmentation region is estimated
to be 50 GeV/fm at the LHC energies and probably 10
GeV/fm at RHIC.Comment: 5 pages, final version, discussion of the signals of the new phase is
expande
Spin-filter tunnel junction with matched Fermi surfaces
Efficient injection of spin-polarized current into a semiconductor is a basic
prerequisite for building semiconductor-based spintronic devices. Here, we use
inelastic electron tunneling spectroscopy to show that the efficiency of
spin-filter-type spin injectors is limited by spin scattering of the tunneling
electrons. By matching the Fermi-surface shapes of the current injection source
and target electrode material, spin injection efficiency can be significantly
increased in epitaxial ferromagnetic insulator tunnel junctions. Our results
demonstrate that not only structural but also Fermi-surface matching is
important to suppress scattering processes in spintronic devices.Comment: 5 pages, 4 figure
X-ray scattering from surfaces: discrete and continuous components of roughness
Incoherent surface scattering yields a statistical description of the
surface, due to the ensemble averaging over many independently sampled volumes.
Depending on the state of the surface and direction of the scattering vector
relative to the surface normal, the height distribution is discrete,
continuous, or a combination of the two. We present a treatment for the
influence of multimodal surface height distributions on Crystal Truncation Rod
scattering. The effects of a multimodal height distribution are especially
evident during in situ monitoring of layer-by-layer thin-film growth via Pulsed
Laser Deposition. We model the total height distribution as a convolution of
discrete and continuous components, resulting in a broadly applicable
parameterization of surface roughness which can be applied to other scattering
probes, such as electrons and neutrons. Convolution of such distributions could
potentially be applied to interface or chemical scattering. Here we find that
this analysis describes accurately our experimental studies of SrTiO3
annealing and homoepitaxial growth.Comment: 15 pages, 7 figure
Far infrared study of the two dimensional dimer spin system SrCu_2(BO_3)_2
Using far-infrared spectroscopy in magnetic fields up to 12T we have studied
a two-dimensional dimer spin gap system SrCu_2(BO_3)_2. We found several
infrared active modes in the dimerized state (below 10K) in the frequency range
from 3 to 100cm^-1. The measured splitting from the ground state to the excited
triplet M_S=0 sublevel is Delta_1=24.2cm^-1 and the other two triplet state
sublevels in zero magnetic field are 1.4cm^-1 below and above the M_S=0
sublevel. Another multiplet is at Delta_2=37.6cm^-1 from the ground state. A
strong electric dipole active transition polarized in the (ab)-plane is
activated in the dimer spin system below 15K at 52cm^-1.Comment: 4 pages including 5 figures, submitted to PRB, instrumental arte
facts remove
Coulomb induced diffraction of energetic hadrons into jets
The electromagnetic (e.m.) current conservation and renormalizability of QCD
are used to calculate the amplitude of energetic hadron(photon) diffraction
into several jets with large relative transverse momenta off the
nucleon(nucleus) Coulomb field. Numerical estimates of the ratio of e.m. and
strong amplitudes show that within the kinematic range where the leading twist
approximation for the strong amplitude is applicable, the e.m. contribution can
be neglected. In pA scattering at LHC and in the fragmentation of a photon into
two jets in ultraperipheral AA collisions in the black limit (which maybe
realistic at LHC) e.m. contribution may win.Comment: 10 page
Europium-based high-temperature superconductors studied by x-ray diffraction and 151Eu Mössbauer spectroscopy
Isotropic powders and magnetically aligned crystallites of EuBa2Cu3O7âÎŽ (1:2:3) and europium-doped Bi2Sr2CaCu2O8 (2:2:1:2) were studied by means of x-ray diffraction and Eu151 Mössbauer spectroscopy. The degree of crystallite orientation of the samples and the values of the lattice constants were determined by x-ray diffraction. The Mössbauer spectra were analyzed considering the full hyperfine Hamiltonian of the nuclear states of the 21.5-keV Îł transition. The Mössbauer hyperfine parameters obtained from the superconducting and semiconducting phases are presented. A small change is seen in the Eu151 isomer shift when the oxygen deficiency ÎŽ of the 1:2:3 compound is varied. The shift can be explained by a decrease in the s-electron density due to lattice expansion. The changes in the oxidation state of the copper atoms with varying ÎŽ were determined from the Mössbauer data: The Cu(2) atoms retain their oxidation state, whereas the Cu(1) atoms adjust their valence according to the value of ÎŽ. In the 2:2:1:2 samples, the Eu concentration clearly affected the value of the electric-field gradient at the Eu nucleus. Using a standard procedure, magnetically aligned 2:2:1:2 samples were prepared. The preferred direction of the crystal c axis changed from parallel to perpendicular alignment with the external magnetic field, when the Eu concentration exceeded 20% of the Ca atoms.Peer reviewe
Ab-initio electronic and magnetic structure in La_0.66Sr_0.33MnO_3: strain and correlation effects
The effects of tetragonal strain on electronic and magnetic properties of
strontium-doped lanthanum manganite, La_{2/3}Sr_{1/3}MnO_3 (LSMO), are
investigated by means of density-functional methods. As far as the structural
properties are concerned, the comparison between theory and experiments for
LSMO strained on the most commonly used substrates, shows an overall good
agreement: the slight overestimate (at most of 1-1.5 %) for the equilibrium
out-of-plane lattice constants points to possible defects in real samples. The
inclusion of a Hubbard-like contribution on the Mn d states, according to the
so-called "LSDA+U" approach, is rather ineffective from the structural point of
view, but much more important from the electronic and magnetic point of view.
In particular, full half-metallicity, which is missed within a bare
density-functional approach, is recovered within LSDA+U, in agreement with
experiments. Moreover, the half-metallic behavior, particularly relevant for
spin-injection purposes, is independent on the chosen substrate and is achieved
for all the considered in-plane lattice constants. More generally, strain
effects are not seen to crucially affect the electronic structure: within the
considered tetragonalization range, the minority gap is only slightly (i.e. by
about 0.1-0.2 eV) affected by a tensile or compressive strain. Nevertheless, we
show that the growth on a smaller in-plane lattice constant can stabilize the
out-of-plane vs in-plane e_g orbital and significatively change their relative
occupancy. Since e_g orbitals are key quantities for the double-exchange
mechanism, strain effects are confirmed to be crucial for the resulting
magnetic coupling.Comment: 16 pages, 7 figures, to be published on J. Phys.: Condensed Matte
LHC Optics Measurement with Proton Tracks Detected by the Roman Pots of the TOTEM Experiment
Precise knowledge of the beam optics at the LHC is crucial to fulfil the
physics goals of the TOTEM experiment, where the kinematics of the scattered
protons is reconstructed with the near-beam telescopes -- so-called Roman Pots
(RP). Before being detected, the protons' trajectories are influenced by the
magnetic fields of the accelerator lattice. Thus precise understanding of the
proton transport is of key importance for the experiment. A novel method of
optics evaluation is proposed which exploits kinematical distributions of
elastically scattered protons observed in the RPs. Theoretical predictions, as
well as Monte Carlo studies, show that the residual uncertainty of this optics
estimation method is smaller than 0.25 percent.Comment: 20 pages, 11 figures, 5 figures, to be submitted to New J. Phy
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