1,303 research outputs found
Rocking motion induced charging of C60 on h-BN/Ni(111)
One monolayer of C60 on one monolayer of hexagonal boron nitride on nickel is
investigated by photoemission. Between 150 and 250 K the work function
decreases and the binding energy of the highest occupied molecular orbital
(HOMO) increases by approx. 100 meV. In parallel, the occupancy of the, in the
cold state almost empty, lowest unoccupied molecular orbital (LUMO) changes by
0.4 electrons. This charge redistribution is triggered by onset of molecular
rocking motion, i.e. by orientation dependent tunneling between the LUMO of C60
and the substrate. The magnitude of the charge transfer is large and cannot be
explained within a single particle picture. It is proposed to involve
electron-phonon coupling where C60- polaron formation leads to electron
self-trapping.Comment: 15 pages, 4 figure
Remarks on some new models of interacting quantum fields with indefinite metric
We study quantum field models in indefinite metric. We introduce the modified
Wightman axioms of Morchio and Strocchi as a general framework of indefinite
metric quantum field theory (QFT) and present concrete interacting relativistic
models obtained by analytical continuation from some stochastic processes with
Euclidean invariance. As a first step towards scattering theory in indefinite
metric QFT, we give a proof of the spectral condition on the translation group
for the relativistic models.Comment: 13 page
Continuum Limit of Spin Models with Continuous Symmetry and Conformal Quantum Field Theory
According to the standard classification of Conformal Quantum Field Theory
(CQFT) in two dimensions, the massless continuum limit of the model at
the Kosterlitz-Thouless (KT) transition point should be given by the massless
free scalar field; in particular the Noether current of the model should be
proportional to (the dual of) the gradient of the massless free scalar field,
reflecting a symmetry enhanced from to . More
generally, the massless continuum limit of a spin model with a symmetry given
by a Lie group should have an enhanced symmetry . We point out
that the arguments leading to this conclusion contain two serious gaps: i) the
possibility of `nontrivial local cohomology' and ii) the possibility that the
current is an ultralocal field. For the model we give analytic
arguments which rule out the first possibility and use numerical methods to
dispose of the second one. We conclude that the standard CQFT predictions
appear to be borne out in the model, but give an example where they
would fail. We also point out that all our arguments apply equally well to any
symmetric spin model, provided it has a critical point at a finite
temperature.Comment: 19 page
Tuning of the Rashba effect in Pb quantum well states via a variable Schottky barrier
Spin-orbit interaction (SOI) in low-dimensional systems results in the
fascinating property of spin-momentum locking. In a Rashba system the inversion
symmetry normal to the plane of a two-dimensional (2D) electron gas is broken,
generating a Fermi surface spin texture reminiscent of spin vortices of
different radii. This can be exploited in a spin-based field-effect transistor
(spin- FET), where the Rashba system forms a 2D channel between ferromagnetic
(FM) source and drain electrodes. The electron spin precesses when propagating
through the Rashba channel and spin orientations (anti)parallel to the drain
give (low) high conductivity. Crucial is the possibility to tune the momentum
splitting, and consequently the precession angle, through an external
parameter. Here we show that this can be achieved in Pb quantum well states
through the doping dependence of the Schottky barrier, opening up the
possibility of a terahertz spin-FET.Comment: 8 pages, 7 figure
Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering
The in-plane effective mass of quantum well states in thin Pb films on a Bi
reconstructed Si(111) surface is studied by angle-resolved photoemission
spectroscopy. It is found that this effective mass is a factor of three lower
than the unusually high values reported for Pb films grown on a Pb
reconstructed Si(111) surface. Through a quantitative low-energy electron
diffraction analysis the change in effective mass as a function of coverage and
for the different interfaces is linked to a change of around 2% in the in-plane
lattice constant. To corroborate this correlation, density functional theory
calculations were performed on freestanding Pb slabs with different in-plane
lattice constants. These calculations show an anomalous dependence of the
effective mass on the lattice constant including a change of sign for values
close to the lattice constant of Si(111). This unexpected relation is due to a
combination of reduced orbital overlap of the 6p_z states and altered
hybridization between the 6p_z and 6p_xy derived quantum well states.
Furthermore it is shown by core level spectroscopy that the Pb films are
structurally and temporally stable at temperatures below 100 K.Comment: 7 pages, 6 figure
Negative-Energy Spinors and the Fock Space of Lattice Fermions at Finite Chemical Potential
Recently it was suggested that the problem of species doubling with
Kogut-Susskind lattice fermions entails, at finite chemical potential, a
confusion of particles with antiparticles. What happens instead is that the
familiar correspondence of positive-energy spinors to particles, and of
negative-energy spinors to antiparticles, ceases to hold for the Kogut-Susskind
time derivative. To show this we highlight the role of the spinorial ``energy''
in the Osterwalder-Schrader reconstruction of the Fock space of non-interacting
lattice fermions at zero temperature and nonzero chemical potential. We
consider Kogut-Susskind fermions and, for comparison, fermions with an
asymmetric one-step time derivative.Comment: 14p
Complex Numbers, Quantum Mechanics and the Beginning of Time
A basic problem in quantizing a field in curved space is the decomposition of
the classical modes in positive and negative frequency. The decomposition is
equivalent to a choice of a complex structure in the space of classical
solutions. In our construction the real tunneling geometries provide the link
between the this complex structure and analytic properties of the classical
solutions in a Riemannian section of space. This is related to the Osterwalder-
Schrader approach to Euclidean field theory.Comment: 27 pages LATEX, UCSBTH-93-0
AdS/CFT correspondence in the Euclidean context
We study two possible prescriptions for AdS/CFT correspondence by means of
functional integrals. The considerations are non-perturbative and reveal
certain divergencies which turn out to be harmless, in the sense that
reflection-positivity and conformal invariance are not destroyed.Comment: 20 pages, references and two remarks adde
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