628 research outputs found
Solvation in atomic liquids: connection between Gaussian field theory and density functional theory
For the problem of molecular solvation, formulated as a liquid submitted to
the external potential field created by a molecular solute of arbitrary shape
dissolved in that solvent, we draw a connection between the Gaussian field
theory derived by David Chandler [Phys. Rev. E, 1993, 48, 2898] and classical
density functional theory. We show that Chandler's results concerning the
solvation of a hard core of arbitrary shape can be recovered by either
minimising a linearised HNC functional using an auxiliary Lagrange multiplier
field to impose a vanishing density inside the core, or by minimising this
functional directly outside the core --- indeed a simpler procedure. Those
equivalent approaches are compared to two other variants of DFT, either in the
HNC, or partially linearised HNC approximation, for the solvation of a
Lennard-Jones solute of increasing size in a Lennard-Jones solvent. Compared to
Monte-Carlo simulations, all those theories give acceptable results for the
inhomogeneous solvent structure, but are completely out-of-range for the
solvation free-energies. This can be fixed in DFT by adding a hard-sphere
bridge correction to the HNC functional.Comment: 14 pages, 4 figure
Massive enhancement of electron-phonon coupling in doped graphene by an electronic singularity
The nature of the coupling leading to superconductivity in layered materials
such as high-Tc superconductors and graphite intercalation compounds (GICs) is
still unresolved. In both systems, interactions of electrons with either
phonons or other electrons or both have been proposed to explain
superconductivity. In the high-Tc cuprates, the presence of a Van Hove
singularity (VHS) in the density of states near the Fermi level was long ago
proposed to enhance the many-body couplings and therefore may play a role in
superconductivity. Such a singularity can cause an anisotropic variation in the
coupling strength, which may partially explain the so-called nodal-antinodal
dichotomy in the cuprates. Here we show that the topology of the graphene band
structure at dopings comparable to the GICs is quite similar to that of the
cuprates and that the quasiparticle dynamics in graphene have a similar
dichotomy. Namely, the electron-phonon coupling is highly anisotropic,
diverging near a saddle point in the graphene electronic band structure. These
results support the important role of the VHS in layered materials and the
possible optimization of Tc by tuning the VHS with respect to the Fermi level.Comment: 8 page
Cluster sum rules for three-body systems with angular-momentum dependent interactions
We derive general expressions for non-energy weighted and energy-weighted
cluster sum rules for systems of three charged particles. The interferences
between pairs of particles are found to play a substantial role. The
energy-weighted sum rule is usually determined by the kinetic energy operator,
but we demonstrate that it has similar additional contributions from the
angular momentum and parity dependence of two- and three-body potentials
frequently used in three-body calculations. The importance of the different
contributions is illustrated with the dipole excitations in He. The results
are compared with the available experimental data.Comment: 11 pages, 3 figures, 2 table
The electronic structure of the high-symmetry perovskite iridate Ba2IrO4
We report angle-resolved photoemission (ARPES) measurements, density
functional and model tight-binding calculations on BaIrO (Ba-214), an
antiferromagnetic ( K) insulator. Ba-214 does not exhibit the
rotational distortion of the IrO octahedra that is present in its sister
compound SrIrO (Sr-214), and is therefore an attractive reference
material to study the electronic structure of layered iridates. We find that
the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at
the predominant role of the spin-orbit interaction in these materials.
Temperature-dependent ARPES data show that the energy gap persists well above
, and favour a Mott over a Slater scenario for this compound.Comment: 13 pages, 9 figure
Electron-Phonon Coupling in Highly-Screened Graphene
Photoemission studies of graphene have resulted in a long-standing
controversy concerning the strength of the experimental electron-phonon
interaction in comparison with theoretical calculations. Using high-resolution
angle-resolved photoemission spectroscopy we study graphene grown on a copper
substrate, where the metallic screening of the substrate substantially reduces
the electron-electron interaction, simplifying the comparison of the
electron-phonon interaction between theory and experiment. By taking the
nonlinear bare bandstructure into account, we are able to show that the
strength of the electron-phonon interaction does indeed agree with theoretical
calculations. In addition, we observe a significant bandgap at the Dirac point
of graphene.Comment: Submitted to Phys. Rev. Lett. on July 20, 201
Quadrupolar Na NMR Relaxation as a Probe of Subpicosecond Collective Dynamics in Aqueous Electrolyte Solutions
Nuclear magnetic resonance relaxometry represents a powerful tool for
extracting dynamic information. Yet, obtaining links to molecular motion is
challenging for many ions that relax through the quadrupolar mechanism, which
is mediated by electric field gradient fluctuations and lacks a detailed
microscopic description. For sodium ions in aqueous electrolytes, we combine ab
initio calculations to account for electron cloud effects with classical
molecular dynamics to sample long-time fluctuations, and obtain relaxation
rates in good agreement with experiments over broad concentration and
temperature ranges. We demonstrate that quadrupolar nuclear relaxation is
sensitive to subpicosecond dynamics not captured by previous models based on
water reorientation or cluster rotation. While ions affect the overall water
retardation, experimental trends are mainly explained by dynamics in the first
two solvation shells of sodium, which contain mostly water. This work thus
paves the way to the quantitative understanding of quadrupolar relaxation in
electrolyte and bioelectrolyte systems.Comment: 36 pages, 25 figures, supplementary information include
Effective range function below threshold
We demonstrate that the kernel of the Lippmann-Schwinger equation, associated
with interactions consisting of a sum of the Coulomb plus a short range nuclear
potential, below threshold becomes degenerate. Taking advantage of this fact,
we present a simple method of calculating the effective range function for
negative energies. This may be useful in practice since the effective range
expansion extrapolated to threshold allows to extract low-energy scattering
parameters: the Coulomb-modified scattering length and the effective range.Comment: 14 pages, 1 figur
Morphology of graphene thin film growth on SiC(0001)
Epitaxial films of graphene on SiC(0001) are interesting from a basic physics
as well as applications-oriented point of view. Here we study the emerging
morphology of in-vacuo prepared graphene films using low energy electron
microscopy (LEEM) and angle-resolved photoemission (ARPES). We obtain an
identification of single and bilayer of graphene film by comparing the
characteristic features in electron reflectivity spectra in LEEM to the PI-band
structure as revealed by ARPES. We demonstrate that LEEM serves as a tool to
accurately determine the local extent of graphene layers as well as the layer
thickness
Interaction of intense vuv radiation with large xenon clusters
The interaction of atomic clusters with short, intense pulses of laser light
to form extremely hot, dense plasmas has attracted extensive experimental and
theoretical interest. The high density of atoms within the cluster greatly
enhances the atom--laser interaction, while the finite size of the cluster
prevents energy from escaping the interaction region. Recent technological
advances have allowed experiments to probe the laser--cluster interaction at
very high photon energies, with interactions much stronger than suggested by
theories for lower photon energies. We present a model of the laser--cluster
interaction which uses non-perturbative R-matrix techniques to calculate
inverse bremsstrahlung and photoionization cross sections for Herman-Skillman
atomic potentials. We describe the evolution of the cluster under the influence
of the processes of inverse bremsstrahlung heating, photoionization,
collisional ionization and recombination, and expansion of the cluster. We
compare charge state distribution, charge state ejection energies, and total
energy absorbed with the Hamburg experiment of Wabnitz {\em et al.} [Nature
{\bf 420}, 482 (2002)] and ejected electron spectra with Laarmann {\em et al.}
[Phys. Rev. Lett. {\bf 95}, 063402 (2005)]
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