258 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
ARPES on HTSC: simplicity vs. complexity
A notable role in understanding of microscopic electronic properties of high
temperature superconductors (HTSC) belongs to angle resolved photoemission
spectroscopy (ARPES). This technique supplies a direct window into reciprocal
space of solids: the momentum-energy space where quasiparticles (the electrons
dressed in clouds of interactions) dwell. Any interaction in the electronic
system, e.g. superconducting pairing, leads to modification of the
quasi-particle spectrum--to redistribution of the spectral weight over the
momentum-energy space probed by ARPES. A continued development of the technique
had an effect that the picture seen through the ARPES window became clearer and
sharper until the complexity of the electronic band structure of the cuprates
had been resolved. Now, in an optimal for superconductivity doping range, the
cuprates much resemble a normal metal with well predicted electronic structure,
though with rather strong electron-electron interaction. This principal
disentanglement of the complex physics from complex structure reduced the
mystery of HTSC to a tangible problem of interaction responsible for
quasi-particle formation. Here we present a short overview of resent ARPES
results, which, we believe, denote a way to resolve the HTSC puzzle.Comment: A review written for a special issue of FN
Photoelectron Escape Depth and Inelastic Secondaries in High Temperature Superconductors
We calculate the photoelectron escape depth in the high temperature
superconductor Bi2212 by use of electron energy-loss spectroscopy data. We find
that the escape depth is only 3 Ang. for photon energies typically used in
angle resolved photoemission measurements. We then use this to estimate the
number of inelastic secondaries, and find this to be quite small near the Fermi
energy. This implies that the large background seen near the Fermi energy in
photoemission measurements is of some other origin.Comment: 2 pages, revtex, 3 encapsulated postscript figure
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
Observation of a warped helical spin-texture in BiSe from circular dichroism angle-resolved photoemission spectroscopy
A differential coupling of topological surface states to left- versus
right-circularly polarized light is the basis of many opto-spintronics
applications of topological insulators. Here we report direct evidence of
circular dichroism from the surface states of BiSe using a laser-based
time-of-flight angle-resolved photoemission spectroscopy. By employing a novel
sample rotational analysis, we resolve unusual modulations in the circular
dichroism photoemission pattern as a function of both energy and momentum,
which perfectly mimic the predicted but hitherto un-observed three-dimensional
warped spin-texture of the surface states. By developing a microscopic theory
of photoemission from topological surface states, we show that this correlation
is a natural consequence of spin-orbit coupling. These results suggest that our
technique may be a powerful probe of the spin-texture of spin-orbit coupled
materials in general.Comment: 16 pages, 4 figure
Geometric Parameterization of Absorption in Heavy Ion Collisions
We calculate the survival probability of particles in various
colliding systems using a Glauber model. An analysis of recent data has
reported a -nucleon breakup cross section of 6.20.7 mb derived
from an exponential fit to the ratio of to Drell-Yan yields as a
function of a simple, linearly-averaged mean path length through the nuclear
medium. Our calculations indicate that, due to the nature of the calculation,
this approach yields an apparent breakup cross section which is systematically
lower than the actual value.Comment: LaTex, 7 pages, 2 figure
Crystalline electric field effects in Ce 3 core-level spectra of heavy-fermion systems: Hard X-ray photoemission spectroscopy on CeNiCoGe
High-resolution hard X-ray photoemission measurements have been performed to
clarify the electronic structure originating from the strong correlation
between electrons in bulk Ce 3 core-level spectra of
CeNiCoGe (0 1). In the Ce 3
core-level spectra, the variation of satellite structures ( peaks) shows
that the hybridization strength between Ce 4- and conduction electrons
gradually increases with Co concentration in good agreement with the results of
Ce 3 and 4 resonant photoemission spectroscopies. Particularly, in
Ce 3 core-level spectra, the multiplet structures of peaks
systematically change with the degeneracy of -states which originates from
crystalline electric field effects.Comment: 10 pages, 4 figures, to appear in Physical Review
Anisotropic suppression in nuclear collisions
The nuclear overlap zone in non-central relativistic heavy ion collisions is
azimuthally very asymmetric. By varying the angle between the axes of
deformation and the transverse direction of the pair momenta, the suppression
of and will oscillate in a characteristic way. Whereas the
average suppression is mostly sensitive to the early and high density stages of
the collision, the amplitude is more sensitive to the late stages. This effect
provides additional information on the suppression mechanisms such as
direct absorption on participating nucleons, comover absorption or formation of
a quark-gluon plasma. The behavior of the average suppression and its
amplitude with centrality of the collisions is discussed for SPS, RHIC and LHC
energies with and without a phase transition.Comment: Revised and extended version, new figure
Systematic Analysis Method for Color Transparency Experiments
We introduce a data analysis procedure for color transparency experiments
which is considerably less model dependent than the transparency ratio method.
The new method is based on fitting the shape of the A dependence of the nuclear
cross section at fixed momentum transfer to determine the effective attenuation
cross section for hadrons propagating through the nucleus. The procedure does
not require assumptions about the hard scattering rate inside the nuclear
medium. Instead, the hard scattering rate is deduced directly from the data.
The only theoretical input necessary is in modelling the attenuation due to the
nuclear medium, for which we use a simple exponential law. We apply this
procedure to the Brookhaven experiment of Carroll et al and find that it
clearly shows color transparency: the effective attenuation cross section in
events with momentum transfer is approximately $40\ mb\ (2.2\
GeV^2/Q^2)$. The fit to the data also supports the idea that the hard
scattering inside the nuclear medium is closer to perturbative QCD predictions
than is the scattering of isolated protons in free space. We also discuss the
application of our approach to electroproduction experiments.Comment: 11 pages, 11 figures (figures not included, available upon request),
report # KU-HEP-92-2
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
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