677 research outputs found
Electron-phonon coupling and its evidence in the photoemission spectra of lead
We present a detailed study on the influence of strong electron-phonon
coupling to the photoemission spectra of lead. Representing the strong-coupling
regime of superconductivity, the spectra of lead show characteristic features
that demonstrate the correspondence of physical properties in the normal and
the superconducting state, as predicted by the Eliashberg theory. These
features appear on an energy scale of a few meV and are accessible for
photoemission only by using modern spectrometers with high resolution in energy
and angle.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
Scanning the Quark-Gluon Plasma with Charmonium
We suggest the variation of charmonium suppression with Feynman x_F in heavy
ion collisions as a novel and sensitive probe for the properties of the matter
created in such reactions. In contrast to the proton-nucleus case where nuclear
suppression is weakest at small x_F, final state interactions with the comoving
matter create a minimum at x_F=0, which is especially deep and narrow if a
quark-gluon plasma is formed. While a particularly strong effect is predicted
at SPS, at the higher RHIC energy it overlaps with the expected sharp variation
with x_F of nuclear effects and needs comparison with proton-nucleus data. If
thermal enhancement of J/\Psi production takes over at the energies of RHIC and
LHC, it will form an easily identified peak, rather than dip in x_F dependence.
We predict a steep dependence on centrality and suggest that this new probe is
complementary to the dependence on transverse energy, and is more sensitive to
a scenario of final state interactions.Comment: 5 pages including 3 figures. Stylistic and clarifying corrections are
mad
Strictly One-Dimensional Electron System in Au Chains on Ge(001) Revealed By Photoelectron K-Space Mapping
Atomic nanowires formed by Au on Ge(001) are scrutinized for the band
topology of the conduction electron system by k-resolved photoemission. Two
metallic electron pockets are observed. Their Fermi surface sheets form
straight lines without undulations perpendicular to the chains within
experimental uncertainty. The electrons hence emerge as strictly confined to
one dimension. Moreover, the system is stable against a Peierls distortion down
to 10 K, lending itself for studies of the spectral function. Indications for
unusually low spectral weight at the chemical potential are discussed.Comment: 4 pages, 4 figures - revised version with added Fig. 2e) and
additional reference
Madelung potentials and covalency effect in strained LaSrMnO thin films studied by core-level photoemission spectroscopy
We have investigated the shifts of the core-level photoemission spectra of
LaSrMnO thin films grown on three kinds of substrates,
SrTiO, (LaAlO)-(SrAlTaO), and
LaAlO. The experimental shifts of the La 4d and Sr 3d core levels are
almost the same as the calculation, which we attribute to the absence of
covalency effects on the Madelung potentials at these atomic sites due to the
nearly ionic character of these atoms. On the other hand, the experimental
shifts of the O and Mn core levels are negligibly small, in
disagreement with the calculation. We consider that this is due to the strong
covalent character of the Mn-O bonds.Comment: 4 pages, 5 figure
Mott Effect and J/Psi Dissociation at the Quark-Hadron Phase Transition
We investigate the in-medium modification of pseudoscalar and vector mesons
in a QCD motivated chiral quark model by solving the Dyson-Schwinger equations
for quarks and mesons at finite temperature for a wide mass range of meson
masses, from light (pi, rho) to open-charm (D, D*) states. At the chiral /
deconfinement phase transition, the quark-antiquark bound states enter the
continuum of unbound states and become broad resonances (the hadronic Mott
effect). We calculate the in-medium cross sections for charmonium dissociation
due to collisions with light hadrons in a chiral Lagrangian approach, and show
that the D and D* meson spectral broadening lowers the threshold for charmonium
dissociation by pi and rho meson impact. This leads to a step-like enhancement
in the reaction rate. We suggest that this mechanism for enhanced charmonium
dissociation may be the physical mechanism underlying the anomalous J/Psi
suppression observed by NA50.Comment: 3 pages, 3 figures, uses EPJ style (included), contribution to the
Proceedings of the International Conference on Quark Nuclear Physics, June 9
- 14, 2002, Juelich, Germany; 1 figure added, text adde
Hadron formation in high energy photonuclear reactions
We present a new method to account for coherence length effects in a
semi-classical transport model. This allows us to describe photo- and
electroproduction at large nuclei (A>12) and high energies using a realistic
coupled channel description of the final state interactions that goes beyond
simple Glauber theory. We show that the purely absorptive treatment of the
final state interactions can lead to wrong estimates of color transparency and
formation time effects in particle production. As an example, we discuss
exclusive rho^0 photoproduction on Pb at a photon energy of 7 GeV as well as
K^+ production in the photon energy range 1-7 GeV.Comment: 14 pages, 6 figures, version published in Phys. Rev.
Magnetic interactions in transition metal doped ZnO : An abinitio study
We calculate the nature of magnetic interactions in transition-metal doped
ZnO using the local spin density approximation and LSDA+\textit{U} method of
density functional theory. We investigate the following four cases: (i) single
transition metal ion types (Cr, Mn, Fe, Co, Ni and Cu) substituted at Zn sites,
(ii) substitutional magnetic transition metal ions combined with additional Cu
and Li dopants, (iii) substitutional magnetic transition metal ions combined
with oxygen vacancies and (iv) pairs of magnetic ion types (Co and Fe, Co and
Mn, etc.). Extensive convergence tests indicate that the calculated magnetic
ground state is unusually sensitive to the k-point mesh and energy cut-off, the
details of the geometry optimizations and the choice of the
exchange-correlation functional. We find that ferromagnetic coupling is
sometimes favorable for single type substitutional transition metal ions within
the local spin density approximation. However, the nature of magnetic
interactions changes when correlations on the transition-metal ion are treated
within the more realistic LSDA + \textit{U} method, often disfavoring the
ferromagnetic state. The magnetic configuration is sensitive to the detailed
arrangement of the ions and the amount of lattice relaxation, except in the
case of oxygen vacancies when an antiferromagnetic state is always favored.Comment: 11 pages, 17 figure
The ground states of the two-component order parameter superconductor
We show that in presence of an applied external field the two-component order
parameter superconductor falls in two categories of ground states, namely, in
the traditional Abrikosov ground state or in a new ground state fitted to
describe a superconducting layer with texture, that is, patched regions
separated by a phase difference of . The existence of these two kinds of
ground states follows from the sole assumption that the total supercurrent is
the sum of the two individual supercurrents and is independent of any
consideration about the free energy expansion. Uniquely defined relations
between the current density and the superfluid density hold for these two
ground states, which also determine the magnetization in terms of average
values of the order parameters. Because these ground state conditions are also
Bogomolny equations we construct the free energy for the two-component
superconductor which admits the Bogomolny solution at a special coupling value.Comment: 5 page
Elastic Scattering Susceptibility of the High Temperature Superconductor Bi2Sr2CaCu2O8+x: A Comparison between Real and Momentum Space Photoemission Spectroscopies
The joint density of states (JDOS) of Bi2Sr2CaCu2O8+x is calculated by
evaluating the autocorrelation of the single particle spectral function
A(k,omega) measured from angle resolved photoemission spectroscopy (ARPES).
These results are compared with Fourier transformed (FT) conductance
modulations measured by scanning tunneling microscopy (STM). Good agreement
between the two experimental probes is found for two different doping values
examined. In addition, by comparing the FT-STM results to the autocorrelated
ARPES spectra with different photon polarization, new insight on the form of
the STM matrix elements is obtained. This shines new light on unsolved
mysteries in the tunneling data.Comment: Revised now available at: Phys. Rev. Lett. 96, 067005 (2006
Structure and transport in multi-orbital Kondo systems
We consider Kondo impurity systems with multiple local orbitals, such as rare
earth ions in a metallic host or multi--level quantum dots coupled to metallic
leads. It is shown that the multiplet structure of the local orbitals leads to
multiple Kondo peaks above the Fermi energy , and to ``shadow'' peaks
below . We use a slave boson mean field theory, which recovers the strong
coupling Fermi liquid fixed point, to calculate the Kondo peak positions,
widths, and heights analytically at T=0, and NCA calculations to fit the
temperature dependence of high--resolution photoemission spectra of Ce
compounds. In addition, an approximate conductance quantization for transport
through multi--level quantum dots or single--atom transistors in the Kondo
regime due to a generalized Friedel sum rule is demonstrated.Comment: 4 pages, 3 figures. Invited article, 23rd International Conference on
Low Temperature Physics LT23, Hiroshima, Japan 200
- …