119 research outputs found
Direct resolution of unoccupied states in solids via two photon photoemission
Non-linear effects in photoemission are shown to open a new access to the
band structure of unoccupied states in solids, totally different from hitherto
used photoemission spectroscopy. Despite its second-order nature, strong
resonant transitions occur, obeying exact selection rules of energy, crystal
symmetry, and momentum. Ab-initio calculations are used to demonstrate that
such structures are present in low-energy laser spectroscopy experimental
measurements on Si previously published. Similar resonances are expected in
ultraviolet angle-resolved photoemission spectra, as shown in a model
calculation on Al.Comment: 12 pages, including 4 figure
The role of surface plasmons in the decay of image-potential states on silver surfaces
The combined effect of single-particle and collective surface excitations in
the decay of image-potential states on Ag surfaces is investigated, and the
origin of the long-standing discrepancy between experimental measurements and
previous theoretical predictions for the lifetime of these states is
elucidated. Although surface-plasmon excitation had been expected to reduce the
image-state lifetime, we demonstrate that the subtle combination of the spatial
variation of s-d polarization in Ag and the characteristic non-locality of
many-electron interactions near the surface yields surprisingly long
image-state lifetimes, in agreement with experiment.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Unoccupied Topological States on Bismuth Chalcogenides
The unoccupied part of the band structure of topological insulators
BiTeSe () is studied by angle-resolved two-photon
photoemission and density functional theory. For all surfaces
linearly-dispersing surface states are found at the center of the surface
Brillouin zone at energies around 1.3 eV above the Fermi level. Theoretical
analysis shows that this feature appears in a spin-orbit-interaction induced
and inverted local energy gap. This inversion is insensitive to variation of
electronic and structural parameters in BiSe and BiTeSe. In
BiTe small structural variations can change the character of the local
energy gap depending on which an unoccupied Dirac state does or does not exist.
Circular dichroism measurements confirm the expected spin texture. From these
findings we assign the observed state to an unoccupied topological surface
state
Adiabatic-Connection-Fluctuation-Dissipation approach to the long-range behavior of the exchange-correlation energy at metal surfaces: A numerical study for jellium slabs
A still open issue in many-body theory is the asymptotic behavior of the
exchange-correlation energy and potential in the vacuum region of a metal
surface. Here we report a numerical study of the position-dependent
exchange-correlation energy for jellium slabs, as obtained by combining the
formally exact adiabatic-connection-fluctuation-dissipation theorem with either
time-dependent density-functional theory or an inhomogeneous
Singwi-Tosi-Land-Sj\"olander approach. We find that the inclusion of
correlation allows to obtain well-converged semi-infinite-jellium results
(independent of the slab thickness) that exhibit an image-like asymptotic
behavior close to the classical image potential .Comment: 6 pages, 4 Figure
Lifetimes of image-potential states on copper surfaces
The lifetime of image states, which represent a key quantity to probe the
coupling of surface electronic states with the solid substrate, have been
recently determined for quantum numbers on Cu(100) by using
time-resolved two-photon photoemission in combination with the coherent
excitation of several states (U. H\"ofer et al, Science 277, 1480 (1997)). We
here report theoretical investigations of the lifetime of image states on
copper surfaces. We evaluate the lifetimes from the knowledge of the
self-energy of the excited quasiparticle, which we compute within the GW
approximation of many-body theory. Single-particle wave functions are obtained
by solving the Schr\"odinger equation with a realistic one-dimensional model
potential, and the screened interaction is evaluated in the random-phase
approximation (RPA). Our results are in good agreement with the experimentally
determined decay times.Comment: 4 pages, 1 figure, to appear in Phys. Rev. Let
Ab initio simulation of photoemission spectroscopy in solids: Plane-wave pseudopotential approach, with applications to normal-emission spectra of Cu(001) and Cu(111)
We introduce a new method for simulating photoemission spectra from bulk
crystals in the ultra-violet energy range, within a three-step model. Our
method explicitly accounts for transmission and matrix-element effects, as
calculated from state-of-the-art plane-wave pseudopotential techniques within
density-functional theory. Transmission effects, in particular, are included by
extending to the present problem a technique previously employed with success
to deal with ballistic conductance in metal nanowires. The spectra calculated
for normal emission in Cu(001) and Cu(111) are in fair agreement with previous
theoretical results and with experiments, including a newly determined
spectrum. The residual discrepancies between our results and the latter are
mainly due to the well-known deficiencies of density-functional theory in
accounting for correlation effects in quasi-particle spectra. A significant
improvement is obtained by the LDA+U method. Further improvements are obtained
by including surface-optics corrections, as described by Snell's law and
Fresnel's equations.Comment: 25 pages, 7 figures, accepted in PR
Theory of inelastic lifetimes of surface-state electrons and holes at metal surfaces
After the early suggestion by John Pendry to probe unoccupied bands at
surfaces through the time reversal of the photoemission process, the
inverse-photoemission technique yielded the first conclusive experimental
evidence for the existence of image-potential bound states at metal surfaces
and has led over the last two decades to an active area of research in
condensed-matter and surface physics. Here we describe the current status of
the many-body theory of inelastic lifetimes of these image-potential states and
also the Shockley surface states that exist near the Fermi level in the
projected bulk band gap of simple and noble metals. New calculations of the
self-energy and lifetime of surface states on Au surfaces are presented as
well, by using the approximation of many-body theory.Comment: 17 pages, 7 figures, to appear in J Phys-Condens Ma
Quantum point contact on graphite surface
The conductance through a quantum point contact created by a sharp and hard
metal tip on the graphite surface has features which to our knowledge have not
been encountered so far in metal contacts or in nanowires. In this paper we
first investigate these features which emerge from the strongly directional
bonding and electronic structure of graphite, and provide a theoretical
understanding for the electronic conduction through quantum point contacts. Our
study involves the molecular-dynamics simulations to reveal the variation of
interlayer distances and atomic structure at the proximity of the contact that
evolves by the tip pressing toward the surface. The effects of the elastic
deformation on the electronic structure, state density at the Fermi level, and
crystal potential are analyzed by performing self-consistent-field
pseudopotential calculations within the local-density approximation. It is
found that the metallicity of graphite increases under the uniaxial compressive
strain perpendicular to the basal plane. The quantum point contact is modeled
by a constriction with a realistic potential. The conductance is calculated by
representing the current transporting states in Laue representation, and the
variation of conductance with the evolution of contact is explained by taking
the characteristic features of graphite into account. It is shown that the
sequential puncturing of the layers characterizes the conductance.Comment: LaTeX, 11 pages, 9 figures (included), to be published in Phys. Rev.
B, tentatively scheduled for 15 September 1998 (Volume 58, Number 12
- …