704 research outputs found
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
Kondo time scales for quantum dots - response to pulsed bias potentials
The response of a quantum dot in the Kondo regime to rectangular pulsed bias
potentials of various strengths and durations is studied theoretically. It is
found that the rise time is faster than the fall time, and also faster than
time scales normally associated with the Kondo problem. For larger values of
the pulsed bias, one can induce dramatic oscillations in the induced current
with a frequency approximating the splitting between the Kondo peaks that would
be present in steady state. The effect persists in the total charge transported
per pulse, which should facilitate the experimental observation of the
phenomenon.Comment: 5 pages with 4 encapsulated figures which come in separate postscript
files: latex file: text.tex figures: fig1.eps, fig2.eps, fig3.eps, fig4.ep
Electronic structure of small GaAs clusters
The electronic structure of small Ga_xAs_y clusters (x+y≤10) are calculated using the local density method. The calculation shows that even‐numbered clusters tend to be singlets, as opposed to odd‐numbered clusters which are open shell systems. This is in agreement with the experimental observations of even/odd alternations of the electron affinity and ionization potential. In the larger clusters, the atoms prefer an alternating bond arrangement; charge transfers are observed from Ga sites to As sites. This observation is also in agreement with recent chemisorption studies of ammonia on GaAs clusters. The close agreement between theoretical calculations and experimental results, together with the rich variation of electronic properties of GaAs clusters with composition makes GaAs clusters an ideal prototype system for the study of how electronic structure influences chemical reactivity
Resonance Lifetimes from Complex Densities
The ab-initio calculation of resonance lifetimes of metastable anions
challenges modern quantum-chemical methods. The exact lifetime of the
lowest-energy resonance is encoded into a complex "density" that can be
obtained via complex-coordinate scaling. We illustrate this with one-electron
examples and show how the lifetime can be extracted from the complex density in
much the same way as the ground-state energy of bound systems is extracted from
its ground-state density
Fundamental stellar parameters of benchmark stars from CHARA interferometry. I. Metal-poor stars
Benchmark stars are crucial as validating standards for current as well as
future large stellar surveys of the Milky Way. However, the number of suitable
metal-poor benchmarks is currently limited. We aim to construct a new set of
metal-poor benchmarks, based on reliable interferometric effective temperature
() determinations and a homogeneous analysis with a desired
precision of in . We observed ten late-type metal-poor
dwarf and giants: HD2665, HD6755, HD6833, HD103095, HD122563, HD127243,
HD140283, HD175305, HD221170, and HD224930. Only three of the ten stars
(HD103095, HD122563, and HD140283) have previously been used as benchmarks. For
the observations, we used the high angular resolution optical interferometric
instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb
darkening models and determined directly from the
Stefan-Boltzmann relation, with an iterative procedure to interpolate over
tables of bolometric corrections. Surface gravities () were estimated
from comparisons to Dartmouth stellar evolution model tracks. We collected
spectroscopic observations from the ELODIE and FIES spectrographs and estimated
metallicities () from a 1D non-LTE abundance analysis of
unblended lines of neutral and singly ionized iron. We inferred
to better than for five of the stars (HD103095, HD122563, HD127243,
HD140283, and HD224930). The of the other five stars are
reliable to between ; the higher uncertainty on the for
those stars is mainly due to their having a larger uncertainty in the
bolometric fluxes. We also determined and with
median uncertainties of and ,
respectively. These ten stars can, therefore, be adopted as a new, reliable set
of metal-poor benchmarks.Comment: 13 pages, 7 figures, 8 tables + 10 online tables, abstract shortened
to meet arXiv requirements, accepted in A&
Many Body Theory of Charge Transfer in Hyperthermal Atomic Scattering
We use the Newns-Anderson Hamiltonian to describe many-body electronic
processes that occur when hyperthermal alkali atoms scatter off metallic
surfaces. Following Brako and Newns, we expand the electronic many-body
wavefunction in the number of particle-hole pairs (we keep terms up to and
including a single particle-hole pair). We extend their earlier work by
including level crossings, excited neutrals and negative ions. The full set of
equations of motion are integrated numerically, without further approximations,
to obtain the many-body amplitudes as a function of time. The velocity and
work-function dependence of final state quantities such as the distribution of
ion charges and excited atomic occupancies are compared with experiment. In
particular, experiments that scatter alkali ions off clean Cu(001) surfaces in
the energy range 5 to 1600 eV constrain the theory quantitatively. The
neutralization probability of Na ions shows a minimum at intermediate
velocity in agreement with the theory. This behavior contrasts with that of
K, which shows ... (7 figures, not included. Figure requests:
[email protected])Comment: 43 pages, plain TeX, BUP-JBM-
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