173 research outputs found
Momentum dependence of the energy gap in the superconducting state of optimally doped Bi2(Sr,R)2CuOy (R=La and Eu)
The energy gap of optimally doped Bi2(Sr,R)2CuOy (R=La and Eu) was probed by
angle resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet
laser (photon energy 6.994 eV) or He I resonance line (21.218 eV) as photon
source. The results show that the gap around the node at sufficiently low
temperatures can be well described by a monotonic d-wave gap function for both
samples and the gap of the R=La sample is larger reflecting the higher Tc.
However, an abrupt deviation from the d-wave gap function and an opposite R
dependence for the gap size were observed around the antinode, which represent
a clear disentanglement between the antinodal pseudogap and the nodal
superconducting gap.Comment: Submitted as the proceedings of LT2
Effect of Reducing Atmosphere on the Magnetism of Zn1-xCoxO Nanoparticles
We report the crystal structure and magnetic properties of Zn1-xCoxO
nanoparticles synthesized by heating metal acetates in organic solvent. The
nanoparticles were crystallized in wurtzite ZnO structure after annealing in
air and in a forming gas (Ar95%+H5%). The X-ray diffraction and X-ray
photoemission spectroscopy (XPS) data for different Co content show clear
evidence for the Co+2 ions in tetrahedral symmetry, indicating the substitution
of Co+2 in ZnO lattice. However samples with x=0.08 and higher cobalt content
also indicate the presence of Co metal clusters. Only those samples annealed in
the reducing atmosphere of the forming gas, and that showed the presence of
oxygen vacancies, exhibited ferromagnetism at room temperature. The air
annealed samples remained non-magnetic down to 77K. The essential ingredient in
achieving room temperature ferromagnetism in these Zn1-xCoxO nanoparticles was
found to be the presence of additional carriers generated by the presence of
the oxygen vacancies.Comment: 11 pages, 6 figures, submitted to Nanotechnology IO
Equation of state for the 2+1 dimensional Gross-Neveu model at order 1/N
We calculate the equation of state of the Gross-Neveu model in 2+1 dimensions
at order 1/N, where N is the number of fermion species. We make use of a
general formula valid for four-fermion theories, previously applied to the
model in 1+1 dimensions. We consider both the discrete and continuous symmetry
versions of the model. We show that the pion-like excitations give the dominant
contribution at low temperatures. The range of validity for such pion dominance
is analyzed. The complete analysis from low to high temperatures also shows
that in the critical region the role of composite states is relevant, even for
quite large N, and that the free-component behaviour at high T starts at about
twice the mean field critical temperature.Comment: 19 pages, RevTeX, 10 figures.p
Characteristics of alpha projectile fragments emission in interaction of nuclei with emulsion
The properties of the relativistic alpha fragments produced in interactions
of 84^Kr at around 1 A GeV in nuclear emulsion are investigated. The
experimental results are compared with the similar results obtained from
various projectiles with emulsion interactions at different energies. The
total, partial nuclear cross-sections and production rates of alpha
fragmentation channels in relativistic nucleus-nucleus collisions and their
dependence on the mass number and initial energy of the incident projectile
nucleus are investigated. The yields of multiple alpha fragments emitted from
the interactions of projectile nuclei with the nuclei of light, medium and
heavy target groups of emulsion-detector are discussed and they indicate that
the projectile-breakup mechanism seems to be free from the target mass number.
It is found that the multiplicity distributions of alpha fragments are well
described by the Koba-Nielsen-Olesen (KNO) scaling presentation. The mean
multiplicities of the freshly produced newly created charged secondary
particles, normally known as shower and secondary particles associated with
target in the events where the emission of alpha fragments were accompanied by
heavy projectile fragments having Z value larger than 4 seem to be constant as
the alpha fragments multiplicity increases, and exhibit a behavior independent
of the alpha fragments multiplicity.Comment: 33 pages, 8 figures and 3 tables (in press
Leading nucleon and inelasticity in hadron-nucleus interactions
We present in this paper a calculation of the average proton-nucleus ine-
lasticity. Using an Iterative Leading Particle Model and the Glauber model, we
relate the leading particle distribution in nucleon-nucleus interactions with
the respective one in nucleon-proton collisions. To describe the leading
particle distribution in nucleon-proton collisions, we use the Regge-Mueller
formalism. To appear in Journal of Physics G.Comment: 11 pages, 2 figure
Shape resonance for the anisotropic superconducting gaps near a Lifshitz transition: the effect of electron hopping between layers
The multigap superconductivity modulated by quantum confinement effects in a
superlattice of quantum wells is presented. Our theoretical BCS approach
captures the low-energy physics of a shape resonance in the superconducting
gaps when the chemical potential is tuned near a Lifshitz transition. We focus
on the case of weak Cooper-pairing coupling channels and strong pair exchange
interaction driven by repulsive Coulomb interaction that allows to use the BCS
theory in the weak-coupling regime neglecting retardation effects like in
quantum condensates of ultracold gases. The calculated matrix element effects
in the pairing interaction are shown to yield a complex physics near the
particular quantum critical points due to Lifshitz transitions in multigap
superconductivity. Strong deviations of the ratio from the
standard BCS value as a function of the position of the chemical potential
relative to the Lifshitz transition point measured by the Lifshitz parameter
are found. The response of the condensate phase to the tuning of the Lifshitz
parameter is compared with the response of ultracold gases in the BCS-BEC
crossover tuned by an external magnetic field. The results provide the
description of the condensates in this regime where matrix element effects play
a key role.Comment: 12 pages, 6 figure
Consistent LDA'+DMFT approach to electronic structure of transition metal oxides: charge transfer insulators and correlated metals
We discuss the recently proposed LDA'+DMFT approach providing consistent
parameter free treatment of the so called double counting problem arising
within the LDA+DMFT hybrid computational method for realistic strongly
correlated materials. In this approach the local exchange-correlation portion
of electron-electron interaction is excluded from self consistent LDA
calculations for strongly correlated electronic shells, e.g. d-states of
transition metal compounds. Then the corresponding double counting term in
LDA+DMFT Hamiltonian is consistently set in the local Hartree (fully localized
limit - FLL) form of the Hubbard model interaction term. We present the results
of extensive LDA'+DMFT calculations of densities of states, spectral densities
and optical conductivity for most typical representatives of two wide classes
of strongly correlated systems in paramagnetic phase: charge transfer
insulators (MnO, CoO and NiO) and strongly correlated metals (SrVO3 and
Sr2RuO4). It is shown that for NiO and CoO systems LDA'+DMFT qualitatively
improves the conventional LDA+DMFT results with FLL type of double counting,
where CoO and NiO were obtained to be metals. We also include in our
calculations transition metal 4s-states located near the Fermi level missed in
previous LDA+DMFT studies of these monooxides. General agreement with optical
and X-ray experiments is obtained. For strongly correlated metals
LDA+DMFT results agree well with earlier LDA+DMFT calculations and
existing experiments. However, in general LDA'+DMFT results give better
quantitative agreement with experimental data for band gap sizes and oxygen
states positions, as compared to the conventional LDA+DMFT.Comment: 13 pages, 11 figures, 1 table. In v2 there some additional
clarifications are include
J/Psi suppression in colliding nuclei: statistical model analysis
We consider the suppression at a high energy heavy ion collision. An
ideal gas of massive hadrons in thermal and chemical equilibrium is formed in
the central region. The finite-size gas expands longitudinally in accordance
with Bjorken law. The transverse expansion in a form of the rarefaction wave is
taken into account. We show that suppression in such an environment,
when combined with the disintegration in nuclear matter, gives correct
evaluation of NA38 and NA50 data in a broad range of initial energy densities.Comment: 14 pages, 13 figures. Accepted for publication in Phys. Rev.
A topological Dirac insulator in a quantum spin Hall phase : Experimental observation of first strong topological insulator
When electrons are subject to a large external magnetic field, the
conventional charge quantum Hall effect \cite{Klitzing,Tsui} dictates that an
electronic excitation gap is generated in the sample bulk, but metallic
conduction is permitted at the boundary. Recent theoretical models suggest that
certain bulk insulators with large spin-orbit interactions may also naturally
support conducting topological boundary states in the extreme quantum limit,
which opens up the possibility for studying unusual quantum Hall-like phenomena
in zero external magnetic field. Bulk BiSb single crystals are
expected to be prime candidates for one such unusual Hall phase of matter known
as the topological insulator. The hallmark of a topological insulator is the
existence of metallic surface states that are higher dimensional analogues of
the edge states that characterize a spin Hall insulator. In addition to its
interesting boundary states, the bulk of BiSb is predicted to
exhibit three-dimensional Dirac particles, another topic of heightened current
interest. Here, using incident-photon-energy-modulated (IPEM-ARPES), we report
the first direct observation of massive Dirac particles in the bulk of
BiSb, locate the Kramers' points at the sample's boundary and
provide a comprehensive mapping of the topological Dirac insulator's gapless
surface modes. These findings taken together suggest that the observed surface
state on the boundary of the bulk insulator is a realization of the much sought
exotic "topological metal". They also suggest that this material has potential
application in developing next-generation quantum computing devices.Comment: 16 pages, 3 Figures. Submitted to NATURE on 25th November(2007
ARPES study of Pb doped Bi_2Sr_2CaCu_2O_8 - a new Fermi surface picture
High resolution angle resolved photoemission data from Pb doped
Bi_2Sr_2CaCu_2O_8 (Bi2212) with suppressed superstructure is presented.
Improved resolution and very high momentum space sampling at various photon
energies reveal the presence of two Fermi surface pieces. One has the hole-like
topology, while the other one has its van Hove singularity very close to
(pi,0), its topology at some photon energies resembles the electron-like piece.
This result provides a unifying picture of the Fermi surface in the Bi2212
compound and reconciles the conflicting reports.Comment: 4 pages, 4 figure
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