406 research outputs found
Transient thermoelectricity in a vibrating quantum dot in Kondo regime
We investigate the time evolution of the thermopower in a vibrating quantum
dot suddenly shifted into the Kondo regime via a gate voltage by adopting the
time-dependent non-crossing approximation and linear response Onsager
relations. Behaviour of the instantaneous thermopower is studied for a range of
temperatures both in zero and strong electron-phonon coupling. We argue that
inverse of the saturation value of decay time of thermopower to its steady
state value might be an alternative tool in determination of the Kondo
temperature and the value of the electron-phonon coupling strength.Comment: 5 pages, 4 figures, to appear in Physics Letters
Kondo resonance in an ac driven quantum dot subjected to finite bias
We employ the time-dependent non-crossing approximation to study the time
averaged conductance for a single electron transistor in the Kondo regime when
the dot level is sinusoidally driven from its equilibrium position by means of
a gate voltage in finite bias. We find that the average conductance exhibits
significant deviation from the monotonous reduction when the applied bias is
equal to the driving frequency of the dot level. We investigate the effect of
the temperature and the driving frequency on the observed enhancement. We
attribute this behaviour to the overlap of the satellite Kondo peaks with the
split Kondo resonances formed at each lead's Fermi level. We display the
spectral function to put our interpretation into more rigorous footing.Comment: 5 pages, 4 figure
Transient currents and universal timescales for a fully time-dependent quantum dot in the Kondo regime
Using the time-dependent non-crossing approximation, we calculate the
transient response of the current through a quantum dot subject to a finite
bias when the dot level is moved suddenly into a regime where the Kondo effect
is present. After an initial small but rapid response, the time-dependent
conductance is a universal function of the temperature, bias, and inverse time,
all expressed in units of the Kondo temperature. Two timescales emerge: the
first is the time to reach a quasi-metastable point where the Kondo resonance
is formed as a broad structure of half-width of the order of the bias; the
second is the longer time required for the narrower split peak structure to
emerge from the previous structure and to become fully formed. The first time
can be measured by the gross rise time of the conductance, which does not
substantially change later while the split peaks are forming. The second time
characterizes the decay rate of the small split Kondo peak (SKP) oscillations
in the conductance, which may provide a method of experimental access to it.
This latter timescale is accessible via linear response from the steady
stateand appears to be related to the scale identified in that manner [A.
Rosch, J. Kroha, and P. Wolfle, Phys. Rev. Lett. 87, 156802 (2001)].Comment: Revtex with 15 eps figures. Compiles to 11 page
Electronic damping of molecular motion at metal surfaces
A method for the calculation of the damping rate due to electron-hole pair
excitation for atomic and molecular motion at metal surfaces is presented. The
theoretical basis is provided by Time Dependent Density Functional Theory
(TDDFT) in the quasi-static limit and calculations are performed within a
standard plane-wave, pseudopotential framework. The artificial periodicity
introduced by using a super-cell geometry is removed to derive results for the
motion of an isolated atom or molecule, rather than for the coherent motion of
an ordered over-layer. The algorithm is implemented in parallel, distributed
across both and space, and in a form compatible with the
CASTEP code. Test results for the damping of the motion of hydrogen atoms above
the Cu(111) surface are presented.Comment: 10 pages, 3 figure
Spin waves in ultrathin ferromagnetic overlayers
The influence of a non-magnetic metallic substrate on the spin wave
excitations in ultrathin ferromagnetic overlayers is investigated for different
crystalline orientations. We show that spin wave dumping in these systems occur
due to the tunneling of holes from the substrate into the overlayer, and that
the spin wave energies may be considerably affected by the exchange coupling
mediated by the substrate.Comment: RevTeX 4, 7 pages, 5 figures; submitted to Phys. Rev.
Scanning tunneling microscopy of monoatomic gold chains on vicinal Si(335) surface: experimental and theoretical study
We study electronic and topographic properties of the Si(335) surface,
containing Au wires parallel to the steps. We use scanning tunneling microscopy
(STM) supplemented by reflection of high energy electron diffraction (RHEED)
technique. The STM data show the space and voltage dependent oscillations of
the distance between STM tip and the surface which can be explained within one
band tight binding Hubbard model. We calculate the STM current using
nonequilibrium Keldysh Green function formalism.Comment: 6 pages, 4 figure
On the interpretation of spin-polarized electron energy loss spectra
We study the origin of the structure in the spin-polarized electron energy
loss spectroscopy (SPEELS) spectra of ferromagnetic crystals. Our study is
based on a 3d tight-binding Fe model, with constant onsite Coulomb repulsion U
between electrons of opposite spin. We find it is not the total density of
Stoner states as a function of energy loss which determines the response of the
system in the Stoner region, as usually thought, but the densities of Stoner
states for only a few interband transitions. Which transitions are important
depends ultimately on how strongly umklapp processes couple the corresponding
bands. This allows us to show, in particular, that the Stoner peak in SPEELS
spectra does not necessarily indicate the value of the exchange splitting
energy. Thus, the common assumption that this peak allows us to estimate the
magnetic moment through its correlation with exchange splitting should be
reconsidered, both in bulk and surface studies. Furthermore, we are able to
show that the above mechanism is one of the main causes for the typical
broadness of experimental spectra. Finally, our model predicts that optical
spin waves should be excited in SPEELS experiments.Comment: 11 pages, 7 eps figures, REVTeX fil
Electrical conductance at initial stage in epitaxial growth of Pb on modified Si(111) surface
The electrical conductance and RHEED intensities as a function of the
coverage have been measured during Pb depositions at 105 K on Si(111)-(6x6)Au
with up to 4.2 ML of annealed Pb. The experiments show the strong influence of
used substrates on the behavior of the conductance during the epitaxy of Pb
atoms, especially for very initial stage of growth. Oscillations of the
conductance during the layer-by-layer growth are correlated with RHEED
intensity oscillations. The analysis of the conductance behavior is made
according to the theory described by Trivedi and Aschcroft (Phys.Rev.B 38,12298
(1988)).Comment: 5 pages, 3 figures. Surf. Sci. - accepte
Simple model for scanning tunneling spectroscopy of noble metal surfaces with adsorbed Kondo impurities
A simple model is introduced to describe conductance measurements between a
scanning tunneling microscope (STM) tip and a noble metal surface with adsorbed
transition metal atoms which display the Kondo effect. The model assumes a
realistic parameterization of the potential created by the surface and a
d3z2-r2 orbital for the description of the adsorbate. Fano lineshapes
associated with the Kondo resonance are found to be sensitive to details of the
adsorbate-substrate interaction. For instance, bringing the adsorbate closer to
the surface leads to more asymmetric lineshapes while their dependence on the
tip distance is weak. We find that it is important to use a realistic surface
potential, to properly include the tunnelling matrix elements to the tip and to
use substrate states which are orthogonal to the adsorbate and tip states. An
application of our model to Co adsorbed on Cu explains the difference in the
lineshapes observed between Cu(100) and Cu(111) surfaces.Comment: 11 pages, 8 eps figure
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