93 research outputs found
Perpendicular transport properties of YBa_2Cu_3O_{7-\delta}/PrBa_2Cu_3O_{7-\delta} superlattices
The coupling between the superconducting planes of YBa2Cu3O{7-\delta}/
PrBa2Cu3O{7-\delta} superlattices has been measured by c-axis transport. We
show that only by changing the thickness of the superconducting
YBa2Cu3O{7-\delta} layers, it is possible to switch between quasi-particle and
Josephson tunneling. From our data we deduce a low temperature c-axis coherence
length of 0.27 nm.Comment: Presented at LT22, contains 2 pages and 2 figures. to appear in
Physica
Nanomechanics of a Hydrogen Molecule Suspended between Two Equally Charged Tips
Geometric configuration and energy of a hydrogen molecule centered between
two point-shaped tips of equal charge are calculated with the variational
quantum Monte-Carlo (QMC) method without the restriction of the
Born-Oppenheimer (BO) approximation. Ground state nuclear distribution,
stability, and low vibrational excitation are investigated. Ground state
results predict significant deviations from the BO treatment that is based on a
potential energy surface (PES) obtained with the same QMC accuracy. The quantum
mechanical distribution of molecular axis direction and bond length at a
sub-nanometer level is fundamental for understanding nanomechanical dynamics
with embedded hydrogen. Because of the tips' arrangement, cylindrical symmetry
yields a uniform azimuthal distribution of the molecular axis vector relative
to the tip-tip axis. With approaching tips towards each other, the QMC sampling
shows an increasing loss of spherical symmetry with the molecular axis still
uniformly distributed over the azimuthal angle but peaked at the tip-tip
direction for negative tip charge while peaked at the equatorial plane for
positive charge. This directional behavior can be switched between both stable
configurations by changing the sign of the tip charge and by controlling the
tip-tip distance. This suggests an application in the field of molecular
machines.Comment: 20 pages, 10 figure
Lifetimes of Shockley electrons and holes at the Cu(111) surface
A theoretical many-body analysis is presented of the electron-electron
inelastic lifetimes of Shockley electrons and holes at the (111) surface of Cu.
For a description of the decay of Shockley states both below and above the
Fermi level, single-particle wave functions have been obtained by solving the
Schr\"odinger equation with the use of an approximate one-dimensional
pseudopotential fitted to reproduce the correct bulk energy bands and
surface-state dispersion. A comparison with previous calculations and
experiment indicates that inelastic lifetimes are very sensitive to the actual
shape of the surface-state single-particle orbitals beyond the
() point, which controls the coupling between the Shockley
electrons and holes.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
Two-particle photoemission from strongly correlated systems: A dynamical-mean field approach
We study theoretically the simultaneous, photo-induced two-particle
excitations of strongly correlated systems on the basis of the Hubbard model.
Under certain conditions specified in this work, the corre- sponding transition
probability is related to the two-particle spectral function which we calculate
using three different methods: the dynamical-mean field theory combined with
quantum Monte Carlo (DMFT- QMC) technique, the first order perturbation theory
and the ladder approximations. The results are analyzed and compared for
systems at the verge of the metal-insulator transitions. The dependencies on
the electronic correlation strength and on doping are explored. In addition,
the account for the orbital degeneracy allows an insight into the influence of
interband correlations on the two particle excitations. A suitable experimental
realization is discussed.Comment: 25 pp, 10 figs. to be published in PR
Valence band photoemission from the GaN(0001) surface
A detailed investigation by one-step photoemission calculations of the
GaN(0001)-(1x1) surface in comparison with recent experiments is presented in
order to clarify its structural properties and electronic structure. The
discussion of normal and off-normal spectra reveals through the identified
surface states clear fingerprints for the applicability of a surface model
proposed by Smith et al. Especially the predicted metallic bonds are confirmed.
In the context of direct transitions the calculated spectra allow to determine
the valence band width and to argue in favor of one of two theoretical bulk
band structures. Furthermore a commonly used experimental method to fix the
valence band maximum is critically tested.Comment: 8 pages, 11 eps files, submitted to PR
c-Axis tunneling in YBa2Cu3O7-\delta/PrBa2Cu3O7-\delta superlattices
In this work we report c-axis conductance measurements done on a superlattice
based on a stack of 2 layers YBa2Cu3O{7-\delta} and 7 layers
PrBa2Cu3O{7-\delta} (2:7). We find that these quasi-2D structures show no clear
superconducting coupling along the c-axis. Instead, we observe tunneling with a
gap of \Delta_c=5.0\pm 0.5 meV for the direction perpendicular to the
superconducting planes. The conductance spectrum show well defined
quasi-periodic structures which are attributed to the superlattice structure.
From this data we deduce a low temperature c-axis coherence length of
\xi_c=0.24\pm 0.03 nm.Comment: 15 pages, 5 figures. To appear in Phys.Rev.
Variational quantum Monte Carlo calculations for solid surfaces
Quantum Monte Carlo methods have proven to predict atomic and bulk properties
of light and non-light elements with high accuracy. Here we report on the first
variational quantum Monte Carlo (VMC) calculations for solid surfaces. Taking
the boundary condition for the simulation from a finite layer geometry, the
Hamiltonian, including a nonlocal pseudopotential, is cast in a layer resolved
form and evaluated with a two-dimensional Ewald summation technique. The exact
cancellation of all Jellium contributions to the Hamiltonian is ensured. The
many-body trial wave function consists of a Slater determinant with
parameterized localized orbitals and a Jastrow factor with a common two-body
term plus a new confinement term representing further variational freedom to
take into account the existence of the surface. We present results for the
ideal (110) surface of Galliumarsenide for different system sizes. With the
optimized trial wave function, we determine some properties related to a solid
surface to illustrate that VMC techniques provide standard results under full
inclusion of many-body effects at solid surfaces.Comment: 9 pages with 2 figures (eps) included, Latex 2.09, uses REVTEX style,
submitted to Phys. Rev.
Role of Interfaces in the Proximity Effect in Anisotropic Superconductors
We report measurements of the critical temperature of YBCO-Co doped YBCO
Superconductor-Normal bilayer films. Depending on the morphology of the S-N
interface, the coupling between S and N layers can be turned on to depress the
critical temperature of S by tens of degrees, or turned down so the layers
appear almost totally decoupled. This novel effect can be explained by the
mechanism of quasiparticle transmission into an anisotropic superconductor.Comment: 13 pages, 3 figure
Photoemission Beyond the Sudden Approximation
The many-body theory of photoemission in solids is reviewed with emphasis on
methods based on response theory. The classification of diagrams into loss and
no-loss diagrams is discussed and related to Keldysh path-ordering
book-keeping. Some new results on energy losses in valence-electron
photoemission from free-electron-like metal surfaces are presented. A way to
group diagrams is presented in which spectral intensities acquire a
Golden-Rule-like form which guarantees positiveness. This way of regrouping
should be useful also in other problems involving spectral intensities, such as
the problem of improving the one-electron spectral function away from the
quasiparticle peak.Comment: 18 pages, 11 figure
Energetic and spatial bonding properties from angular distributions of ultraviolet photoelectrons: application to the GaAs(110) surface
Angle-resolved ultraviolet photoemission spectra are interpreted by combining
the energetics and spatial properties of the contributing states. One-step
calculations are in excellent agreement with new azimuthal experimental data
for GaAs(110). Strong variations caused by the dispersion of the surface bands
permit an accurate mapping of the electronic structure. The delocalization of
the valence states is discussed analogous to photoelectron diffraction. The
spatial origin of the electrons is determined, and found to be strongly energy
dependent, with uv excitation probing the bonding region.Comment: 5 pages, 3 figures, submitted for publicatio
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