30 research outputs found
Theory of the Eigler-swith
We suggest a simple model to describe the reversible field-induced transfer
of a single Xe-atom in a scanning tunneling microscope, --- the Eigler-switch.
The inelasticly tunneling electrons give rise to fluctuating forces on and
damping of the Xe-atom resulting in an effective current dependent temperature.
The rate of transfer is controlled by the well-known Arrhenius law with this
effective temperature. The directionality of atom transfer is discussed, and
the importance of use of non-equlibrium-formalism for the electronic
environment is emphasized. The theory constitutes a formal derivation and
generalization of the so-called Desorption Induced by Multiple Electron
Transitions (DIMET) point of view.Comment: 13 pages (including 2 figures in separate LaTeX-files with
ps-\specials), REVTEX 3.
Excitations in antiferromagnetic cores of superconducting vortices
We study excitations of the predicted antiferromagnetically ordered vortex
cores in the superconducting phase of the newly proposed SO(5) model of
strongly correlated electrons. Using experimental data from the literature we
show that the susceptibilities in the spin sector and the charge sector are
nearly equal, and likewise for the stiffnesses. In the case of strict equality
SO(5) symmetry is possible, and we find that if present the vortices give rise
to an enhanced neutron scattering cross section near the so called pi resonance
at 41 meV. In the case of broken SO(5) symmetry two effects are predicted.
Bound excitations can exist in the vortex cores with ``high'' excitation
energies slightly below 41 meV, and the massless Goldstone modes corresponding
to the antiferromagnetic ordering of the core can acquire a mass and show up as
core excitation with ``low'' excitation energies around 2 meV.Comment: 9 pages, RevTeX, including 3 postscript figures, submitted to Phys.
Rev. B, July 10, 199
Strong polarization-induced reduction of addition energies in single-molecule nanojunctions
We address polarization-induced renormalization of molecular levels in
solid-state based single-molecule transistors and focus on an organic conjugate
molecule where a surprisingly large reduction of the addition energy has been
observed. We have developed a scheme that combines a self-consistent solution
of a quantum chemical calculation with a realistic description of the screening
environment. Our results indeed show a large reduction, and we explain this to
be a consequence of both (a) a reduction of the electrostatic molecular
charging energy and (b) polarization induced level shifts of the HOMO and LUMO
levels. Finally, we calculate the charge stability diagram and explain at a
qualitative level general features observed experimentally.Comment: 9 pages, 5 figure
The magnetic neutron scattering resonance of high-T_c superconductors in external magnetic fields: an SO(5) study
The magnetic resonance at 41 meV observed in neutron scattering studies of Y
Ba_2 Cu_3 O_7 holds a key position in the understanding of high-T_c
superconductivity. Within the SO(5) model for superconductivity and
antiferromagnetism, we have calculated the effect of an applied magnetic field
on the neutron scattering cross-section of the magnetic resonance. In the
presence of vortices, the neutron scattering cross-section shows clear
signatures of not only the fluctuations in the superconducting order parameter
\psi, but also the modulation of the phase of \psi due to vortices. In
reciprocal space we find that i) the scattering amplitude is zero at
(pi/a,pi/a), ii) the resonance peak is split into a ring with radius pi/d
centered at (pi/a,pi/a), d being the vortex lattice constant, and consequently,
iii) the splitting pi/d scales with the magnetic field as sqrt{B}.Comment: 4 pages including 3 eps-figures - minor changes and one reference
added. Accepted for publication in Phys. Rev.
Three-Body and One-Body Channels of the Auger Core-Valence-Valence decay: Simplified Approach
We propose a computationally simple model of Auger and APECS line shapes from
open-band solids. Part of the intensity comes from the decay of unscreened
core-holes and is obtained by the two-body Green's function ,
as in the case of filled bands. The rest of the intensity arises from screened
core-holes and is derived using a variational description of the relaxed ground
state; this involves the two-holes-one-electron propagator , which
also contains one-hole contributions. For many transition metals, the two-hole
Green's function can be well described by the Ladder
Approximation, but the three-body Green's function poses serious further
problems. To calculate , treating electrons and holes on equal
footing, we propose a practical approach to sum the series to all orders. We
achieve that by formally rewriting the problem in terms of a fictitious
three-body interaction. Our method grants non-negative densities of states,
explains the apparent negative-U behavior of the spectra of early transition
metals and interpolates well between weak and strong coupling, as we
demonstrate by test model calculations.Comment: AMS-LaTeX file, 23 pages, 8 eps and 3 ps figures embedded in the text
with epsfig.sty and float.sty, submitted to Phys. Rev.
Two-dimensional electron transport in the presence of magnetic flux vortices
We have considered the conductivity properties of a two dimensional electron
gas (2DEG) in two different kinds of inhomogeneous magnetic fields, i.e. a
disordered distribution of magnetic flux vortices, and a periodic array of
magnetic flux vortices. The work falls in two parts. In the first part we show
how the phase shifts for an electron scattering on an isolated vortex, can be
calculated analytically, and related to the transport properties through a
force balance equation. In the second part we present numerical results for the
Hall conductivity of the 2DEG in a periodic array of flux vortices. We find
characteristic peaks in the Hall conductance, when plotted against the filling
fraction. It is argued that the peaks can be interpreted in terms of
``topological charge'' piling up across local and global gaps in the energy
spectrum.Comment: 47 pages, Revtex 3.0, 18 postscript figures can be obtained from our
WWW-server: http://roemer.fys.ku.dk/vortices.htm , or on request from the
Authors. Note that this paper is a thoroughly revised version of
cond-mat/940405
SO(5) theory of insulating vortex cores in high- materials
We study the fermionic states of the antiferromagnetically ordered vortex
cores predicted to exist in the superconducting phase of the newly proposed
SO(5) model of strongly correlated electrons. Our model calculation gives a
natural explanation of the recent STM measurements on BSCCO, which in
surprising contrast to YBCO revealed completely insulating vortex cores.Comment: 4 pages, 1 figur
Effect of Finite Impurity Mass on the Anderson Orthogonality Catastrophe in One Dimension
A one-dimensional tight-binding Hamiltonian describes the evolution of a
single impurity interacting locally with electrons. The impurity spectral
function has a power-law singularity
with the same exponent
that characterizes the logarithmic decay of the quasiparticle weight
with the number of electrons , . The exponent
is computed by (1) perturbation theory in the interaction strength and
(2) numerical evaluations with exact results for small systems and variational
results for larger systems. A nonanalytical behavior of is observed in
the limit of infinite impurity mass. For large interaction strength, the
exponent depends strongly on the mass of the impurity in contrast to the
perturbative result.Comment: 26 pages, RevTeX, 7 figures included, to be published in Phys. Rev.
Quasiparticle structure and coherent propagation in the model
Numerical studies, from variational calculation to exact diagonalization, all
indicate that the quasiparticle generated by introducing one hole into a
two-dimensional quantum antiferromagnet has the same nature as a string state
in the model. Based on this observation, we attempt to visualize the
quasiparticle formation and subsequent coherent propagation at low energy by
studying the generalized model in which we first
diagonalize the model and then perform a {\em degenerate}
perturbation in . We construct the quasiparticle state and derive an
effective Hamiltonian describing the coherent propagation of the quasiparticle
and its interaction with the spin wave excitations in the presence of the
N\'{e}el order. We expect that qualitative properties of the quasiparticle
remain intact when analytically continuing from the anisotropic
to the isotropic limit, despite the
fact that the spin wave excitations change from gapful to gapless.
Extrapolating to , our quasiparticle dispersion and spectral
weight compare well with the exact numerical results for small clusters.Comment: Revised with minor changes and references updated. To appear in Phys.
Rev. B., Jan. 1996. 10 pages, The complete PostScript file including figures
can be obtained via ftp at ftp://serval.berkeley.edu/tjzjp.ps . It is also
posted in the WEB site of Niels Bohr Institute at
http://roemer.fys.ku.dk/recent.ht
Composite quasiparticle formation and the low-energy effective Hamiltonians of the one- and two-dimensional Hubbard Model
We investigate the effect of hole doping on the strong-coupling Hubbard model
at half-filling in spatial dimensions . We start with an
antiferromagnetic mean-field description of the insulating state, and show that
doping creates solitons in the antiferromagnetic background. In one dimension,
the soliton is topological, spinless, and decoupled from the background
antiferromagnetic fluctuations at low energies. In two dimensions and above,
the soliton is non-topological, has spin quantum number 1/2, and is strongly
coupled to the antiferromagnetic fluctuations. We derive the effective action
governing the quasiparticle motion, study the properties of a single carrier,
and comment on a possible description at finite concentration.Comment: REVTEX 3.0, 22 pages with 14 figures in the PostScript format
compressed using uufile. Submitted to Phys. Rev. B. The complete PostScript
file including figures can be obtained via ftp at
ftp://serval.berkeley.edu/hubbard.ps . It is also available via www at
http://roemer.fys.ku.dk/recent.ht