189 research outputs found
Persistent current induced by magnetic impurities
We calculate the average persistent current in a normal conducting,
mesoscopic ring in the diffusive regime. In the presence of magnetic
impurities, a contribution to the persistent current is identified, which is
related to fluctuations in the electron spin density. Assuming a spin-flip
scattering rate which is comparable to the Thouless energy E_c and low
temperature, this new contribution to the persistent current is of the order
, which is considerably larger than the persistent
current induced by the electron-electron interaction.Comment: 19 pages, 7 figures, accepted by Z. Phys.
Persistent Currents versus Phase Breaking in Mesoscopic Metallic Samples
Persistent currents in mesoscopic normal metal rings represent, even a decade
after their first experimental observation, a challenge to both, theorists and
experimentalists. After giving a brief review of the existing -- experimental
and theoretical -- results, we concentrate on the (proposed) relationship of
the size of the persistent current to the phase breaking rate. In particular,
we consider effects induced by noise, scattering at two-level systems, and
magnetic impurities.Comment: accepted by JLT
Comment on "Magnetic response of Disordered Metallic Rings: Large Contributions of Far Levels"
Comment on cond-mat/0205390; PRL 90, 026805 (2003
Quantum Coherence in an Exactly Solvable One-dimensional Model with Defects
Using the Quantum Inverse Scattering Method we construct an integrable
Heisenberg-XXZ-model, or equivalently a model for spinless fermions with
nearest-neighbour interaction, with defects. Each defect involves three sites
with a fine tuning between nearest-neighbour and next-nearest-neighbour terms.
We investigate the finite size corrections to the ground state energy and its
dependence on an external flux as a function of a parameter ,
characterizing the strength of the defects. For intermediate values of ,
both quantities become very small, although the ground state wavefunction
remains extended.Comment: accepted by Europhys. Lett., latex, 7 pages. A postscript version
including the figures is available at:
http://www.physik.uni-augsburg.de/theo2/Publications
Density functional theory for a model quantum dot: Beyond the local-density approximation
We study both static and transport properties of model quantum dots,
employing density functional theory as well as (numerically) exact methods. For
the lattice model under consideration the accuracy of the local-density
approximation generally is poor. For weak interaction, however, accurate
results are achieved within the optimized effective potential method, while for
intermediate interaction strengths a method combining the exact diagonalization
of small clusters with density functional theory is very successful. Results
obtained from the latter approach yield very good agreement with density matrix
renormalization group studies, where the full Hamiltonian consisting of the dot
and the attached leads has to be diagonalized. Furthermore we address the
question whether static density functional theory is able to predict the exact
linear conductance through the dot correctly - with, in general, negative
answer.Comment: 8 page
Electronic transport properties of (fluorinated) metal phthalocyanine
The magnetic and transport properties of the metal phthalocyanine (MPc) and
FMPc (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Ag) families of
molecules in contact with S-Au wires are investigated by density functional
theory within the local density approximation, including local electronic
correlations on the central metal atom. The magnetic moments are found to be
considerably modified under fluorination. In addition, they do not depend
exclusively on the configuration of the outer electronic shell of the central
metal atom (as in isolated MPc and FMPc) but also on the interaction
with the leads. Good agreement between the calculated conductance and
experimental results is obtained. For M = Ag, a high spin filter efficiency and
conductance is observed, giving rise to a potentially high sensitivity for
chemical sensor applications.Comment: 8 pages (two-column), 8 figure
From VO2 to V2O3: The Metal-Insulator Transition of the Magneli Phase V6O11
The metal-insulator transition (MIT) of V6O11 is studied by means of
electronic structure calculations using the augmented spherical wave method.
The calculations are based on density functional theory and the local density
approximation. Changes of the electronic structure at the MIT are discussed in
relation to the structural transformations occuring simultaneously. The
analysis is based on a unified point of view of the crystal structures of
V6O11, VO2, and V2O3. This allows to group the electronic bands into states
behaving similar to the dioxide or the sesquioxide. While the sesquioxide-like
V 3d_yz states show rather weak changes on entering the low-temperature
structure, some of the dioxide-like V 3d_x^2-y^2 states display splittings and
shifts similar to those known from VO2. The MIT of V6O11 arises as a
combination of changes appearing in both of these compounds. Our results shed
new light onto the role of particular electronic states for the MIT of V2O3.Comment: 7 pages, 6 figures, accepted by Europhys. Let
Flux Dependence of Persistent Current in a Mesoscopic Disordered Tight Binding Ring
We reconsider the study of persistent currents in a disordered
one-dimensional ring threaded by a magnetic flux, using he one-band
tight-binding model for a ring of N-sites with random site energies. The
secular equation for the eigenenergies expressed in terms of transfer matrices
in the site representation is solved exactly to second order in a perturbation
theory for weak disorder and fluxes differing from half-integer multiples of
the elementary flux quantum. From the equilibrium currents associated with the
one-electron eigenstates we derive closed analytic expressions for the disorder
averaged persistent current for even and odd numbers, Ne, of electrons in the
ground state. Explicit discussion for the half-filled band case confirms that
the persistent current is flux periodic as in the absence of disorder, and that
its amplitude is generally suppressed by the effect of the disorder. In
comparison to previous results, based on an approximate analysis of the secular
equation, the current suppression by disorder is strongly enhanced by a new
flux-dependent factor.Comment: 15 pages, LaTex 2
Diamagnetic orbital response of mesoscopic silver rings
We report measurements of the flux-dependent orbital magnetic susceptibility
of an ensemble of 10^5 disconnected silver rings at 217 MHz. Because of the
strong spin-orbit scattering rate in silver this experiment is a test of
existing theories on orbital magnetism. Below 100 mK the rings exhibit a
magnetic signal with a flux periodicity of h/2 e consistent with averaged
persistent currents, whose amplitude is estimated to be of the order of 0.3 nA.
The sign of the oscillations indicates diamagnetism in the vicinity of zero
magnetic field. This sign is not consistent with theoretical predictions for
average persistent currents unless considering attractive interactions in
silver. We propose an alternative interpretation taking into account spin orbit
scattering and finite frequency.Comment: 4 pages, 4 figures, revtex4, accepted for publication in Physical
Review Letter
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