228 research outputs found
On the Magnetic Nature of Quantum Point Contacts
We present results for a model that describes a quantum point contact. We
show how electron-electron correlations, within the unrestricted Hartree-Fock
approximation, generate a magnetic moment in the point contact. Having
characterized the magnetic structure of the contact, we map the problem onto a
simple one-channel model and calculate the temperature dependence of the
conductance for different gate voltages. Our results are in good agreement with
experimental results obtained in GaAs devices and support the idea of Kondo
effect in these systems.Comment: 7 pages, 4 figure
Theory of Core-Level Photoemission and the X-ray Edge Singularity Across the Mott Transition
The zero temperature core-level photoemission spectrum is studied across the
metal to Mott insulator transition using dynamical mean-field theory and
Wilson's numerical renormalization group. An asymmetric power-law divergence is
obtained in the metallic phase with an exponent alpha(U,Q)-1 which depends on
the strength of both the Hubbard interaction U and the core-hole potential Q.
For Q <~ U_c/2 alpha decreases with increasing U and vanishes at the transition
(U -> U_c) leading to a symmetric peak in the insulating phase. For Q >~ U_c/2,
alpha remains finite close to the transition, but the integrated intensity of
the power-law vanishes and there is no associated peak in the insulator. The
weight and position of the remaining peaks in the spectra can be understood
within a molecular orbital approach.Comment: 5 pages, 6 figure
Localized Spins on Graphene
The problem of a magnetic impurity, atomic or molecular, absorbed on top of a
carbon atom in otherwise clean graphene is studied using the numerical
renormalization group. The spectral, thermodynamic, and scattering properties
of the impurity are described in detail. In the presence of a small magnetic
field, the low energy electronic features of graphene make possible to inject
spin polarized currents through the impurity using a scanning tunneling
microscope (STM). Furthermore, the impurity scattering becomes strongly spin
dependent and for a finite impurity concentration it leads to spin polarized
bulk currents and a large magnetoresistance. In gated graphene the impurity
spin is Kondo screened at low temperatures. However, at temperatures larger
than the Kondo temperature, the anomalous magnetotransport properties are
recovered.Comment: 4+ pages, 4 figures. Added reference
Magnetoconductance through a vibrating molecule in the Kondo regime
The effect of a magnetic field on the equilibrium spectral and transport
properties of a single-molecule junction is studied using the numerical
renormalization group method. The molecule is described by the
Anderson-Holstein model in which a single vibrational mode is coupled to the
electron density. The effect of an applied magnetic field on the conductance in
the Kondo regime is qualitatively different in the weak and strong
electron-phonon coupling regimes. In the former case, the Kondo resonance is
split and the conductance is strongly suppressed by a magnetic field , with the Kondo temperature. In the strong
electron-phonon coupling regime a charge analog of the Kondo effect develops.
In this case the Kondo resonance is not split by the field and the conductance
in the Kondo regime is enhanced in a broad range of values of .Comment: 6 pages, 4 figure
Electronic Transport through Magnetic Molecules with Soft Vibrating Modes
The low-temperature transport properties of a molecule are studied in the
field-effect transitor geometry. The molecule has an internal mechanical mode
that modulates its electronic levels and renormalizes both the interactions and
the coupling to the electrodes. For a soft mechanical mode the spin
fluctuations in the molecule are dominated by the bare couplings while the
valence changes are determined by the dressed energies. In this case, the
transport properties present an anomalous behavior and the Kondo temperature
has a weak gate voltage dependence. These observations are in agreement with
recent experimental data.Comment: 4 pages, 3 figures, accepted in PRB R
Transport through quantum dots in mesoscopic circuits
We study the transport through a quantum dot, in the Kondo Coulomb blockade
valley, embedded in a mesoscopic device with finite wires. The quantization of
states in the circuit that hosts the quantum dot gives rise to finite size
effects. These effects make the conductance sensitive to the ratio of the Kondo
screening length to the wires length and provide a way of measuring the Kondo
cloud. We present results obtained with the numerical renormalization group for
a wide range of physically accessible parameters.Comment: 4 pages, 5 figure
Quantum Transport Through a Stretched Spin--1 Molecule
We analyze the electronic transport through a model spin-1 molecule as a
function of temperature, magnetic field and bias voltage. We consider the
effect of magnetic anisotropy, which can be generated experimentally by
stretching the molecule. In the experimentally relevant regime the conductance
of the unstretched molecule reaches the unitary limit of the underscreened
spin- 1 Kondo effect at low temperatures. The magnetic anisotropy generates an
antiferromagnetic coupling between the remaining spin 1/2 and a singular
density of quasiparticles, producing a second Kondo effect and a reduced
conductance. The results explain recent measurements in spin-1 molecules
[Science 328 1370 (2010)].Comment: 5 pages, 3 figures, minor changes, accepted for publication in EP
Thermopower of an SU(4) Kondo resonance under an SU(2) symmetry-breaking field
We calculate the thermopower of a quantum dot described by two doublets
hybridized with two degenerate bands of two conducting leads, conserving
orbital (band) and spin quantum numbers, as a function of the temperature
and a splitting of the quantum dot levels which breaks the SU(4)
symmetry. The splitting can be regarded as a Zeeman (spin) or valley (orbital)
splitting. We use the non-crossing approximation (NCA), the slave bosons in the
mean-field approximation (SBMFA) and also the numerical renormalization group
(NRG) for large . The model describes transport through clean C
nanotubes %with weak disorder and in Si fin-type field effect transistors,
under an applied magnetic field. The thermopower as a function of temperature
displays two dips that correspond to the energy scales given by the
Kondo temperature and and one peak when reaches the
charge-transfer energy. These features are much more pronounced than the
corresponding ones in the conductance, indicating that the thermopower is a
more sensitive probe of the electronic structure at intermediate or high
energies. At low temperatures () is a constant that
increases strongly near the degeneracy point . We find that the SBMFA
fails to provide an accurate description of the thermopower for large .
Instead, a combination of Fermi liquid relations with the quantum-dot
occupations calculated within the NCA gives reliable results for .Comment: 8 pages, 7 figure
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