2,509 research outputs found
Theoretical study of spin-orbit coupling constants for O+2 (A 2Π3/2,1/2u, v+=0–17 and a 4Π5/2,3/2,1/2,−1/2u, v+=0–25)
The spin-orbit coupling constants (Av+) for O+2(A 2Πu,v+=0–17) and O+2(a 4Πu,v+=0–25) were computed based on the Pauli–Breit Hamiltonian with one and two electron terms for comparison with experimental measurements. In the present theoretical study, the vibrational wave functions are obtained using the potential energy curve calculated at the multireference configuration interaction (MRCI) level of theory, with single and double excitations from the complete active space self-consistent field (CASSCF) reference wave function. The electronic wave functions and spin-orbit coupling constants are obtained at the CASSCF and restricted MRCI levels. The effect on Av+ for O+2(A 2Πu,v+) and O+2(a 4Πu,v+) due to interactions of the O+2(A 2Πu,v+), O+2(a 4Πu,v+), and O+2(2Σ+u)states is examined. The theoretical Av+ predictions for O+2(A 2Πu,v+) are found to be consistent with the experimental finding that O+2(A 2Πu) is an inverted spin-orbit state at low v+ levels and becomes a regular spin-orbit state at higher v+ levels. Good accord between theoretical predictions and experimental results for O+2(A 2Πu,v+=0–12) is observed with discrepancies in the range of 2–10 cm−1. In the case of O+2(a 4Πu,v+), excellent agreement between theoretical ab initio and experimental results is found with a discrepancy of 2–5 cm−1. Our effort to theoretically reproduce experimental fine structure in the Av+ curve for O+2(a 4Πu,v+) based on interstate vibrational interactions has met with limited success
An Experimental and Theoretical Study of the Spin–Orbit Interaction for CO+(A 2Π3/2,1/2, v+=0–41) and O+2(X 2Π3/2,1/2g, v+=0–38)
Accurate spin–orbit splitting constants (Av+) for the vibrational levels v+=0–41 of CO+(A 2Π3/2,1/2) have been determined in a rotationally resolved pulsed field ionization photoelectron study. A change in slope is observed in the v+ dependence for Av+ at v+≈19–20. This observation is attributed to perturbation of the CO+(A 2Π) potential by the CO+(B 2Σ+) state. Theoretical Av+ values for CO+(A 2Π3/2,1/2, v+=0–41) have also been obtained using a newly developed ab initio computational routine for spin–orbit coupling calculations. The theoretical Av+ predictions computed using this routine are found to be in agreement with the experimental Av+ values for CO+(A 2Π3/2,1/2, v+=0–41). Similar Av+calculations obtained for O+2(X 2Π3/2,1/2g, v+=0–38) are also in accord with the recent experimental Av+ values reported by Song et al. [J. Chem. Phys. 111, 1905 (1999)]
A new perturbation treatment applied to the transport through a quantum dot
Resonant tunnelling through an Anderson impurity is investigated by employing
a new perturbation scheme at nonequilibrium. This new approach gives the
correct weak and strong coupling limit in by introducing adjustable
parameters in the self-energy and imposing self-consistency of the occupation
number of the impurity. We have found that the zero-temperature linear response
conductance agrees well with that obtained from the exact sum rule. At finite
temperature the conductance shows a nonzero minimum at the Kondo valley, as
shown in recent experiments. The effects of an applied bias voltage on the
single-particle density of states and on the differential conductances are
discussed for Kondo and non-Kondo systems.Comment: 4 pages, 4 figures, submitted to PRB-Rapid Comm. Email addresses
[email protected], [email protected]
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
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Metabolic reprogramming of human CD8+ memory T cells through loss of SIRT1.
The expansion of CD8+CD28- T cells, a population of terminally differentiated memory T cells, is one of the most consistent immunological changes in humans during aging. CD8+CD28- T cells are highly cytotoxic, and their frequency is linked to many age-related diseases. As they do not accumulate in mice, many of the molecular mechanisms regulating their fate and function remain unclear. In this paper, we find that human CD8+CD28- T cells, under resting conditions, have an enhanced capacity to use glycolysis, a function linked to decreased expression of the NAD+-dependent protein deacetylase SIRT1. Global gene expression profiling identified the transcription factor FoxO1 as a SIRT1 target involved in transcriptional reprogramming of CD8+CD28- T cells. FoxO1 is proteasomally degraded in SIRT1-deficient CD8+CD28- T cells, and inhibiting its activity in resting CD8+CD28+ T cells enhanced glycolytic capacity and granzyme B production as in CD8+CD28- T cells. These data identify the evolutionarily conserved SIRT1-FoxO1 axis as a regulator of resting CD8+ memory T cell metabolism and activity in humans
Transmission Phase Shift of a Quantum Dot with Kondo Correlations
We study the effects of Kondo correlations on the transmission phase shift of
a quantum dot in an Aharonov-Bohm ring. We predict in detail how the
development of a Kondo resonance should affect the dependence of the phase
shift on transport voltage, gate voltage and temperature. This system should
allow the first direct observation of the well-known scattering phase shift of
pi/2 expected (but not directly measurable in bulk systems) at zero temperature
for an electron scattering off a spin-1/2 impurity that is screened into a
singlet.Comment: 4 pages Revtex, 4 figures, final published versio
Nonlinear Response of a Kondo system: Direct and Alternating Tunneling Currents
Non - equilibrium tunneling current of an Anderson impurity system subject to
both constant and alternating electric fields is studied. A time - dependent
Schrieffer - Wolff transformation maps the time - dependent Anderson
Hamiltonian onto a Kondo one. Perturbation expansion in powers of the Kondo
coupling strength is carried out up to third order, yielding a remarkably
simple analytical expression for the tunneling current. It is found that the
zero - bias anomaly is suppressed by an ac - field. Both dc and the first
harmonic are equally enhanced by the Kondo effect, while the higher harmonics
are relatively small. These results are shown to be valid also below the Kondo
temperature.Comment: 7 pages, RevTeX, 3 PS figures attached, the article has been
significantly developed: time - dependent Schrieffer - Wolff transformation
is presented in the full form, the results are applied to the change in the
direct current induced by an alternating field (2 figures are new
Transport through Quantum Dots: Analytic Results from Integrability
Recent experiments have probed quantum dots through transport measurements in
the regime where they are described by a two lead Anderson model. In this paper
we develop a new method to analytically compute for the first time the
corresponding transport properties. This is done by using the exact solvability
of the Anderson Hamiltonian, together with a generalization of the
Landauer-Buttiker approach to integrable systems. The latter requires proper
identification of scattering states, a complex and crucial step in our
approach. In the Kondo regime, our results include the zero-field, finite
temperature linear response conductance, as well as the zero-temperature,
non-equilibrium conductance in an applied Zeeman field.Comment: 5 pages, 3 figure
Excess Kondo resonance in a quantum dot device with normal and superconducting leads: the physics of Andreev-normal co-tunneling
We report on a novel Kondo phenomenon of interacting quantum dots coupled
asymmetrically to a normal and a superconducting lead. The effects of intradot
Coulomb interaction and Andreev tunneling give rise to Andreev bound
resonances. As a result, a new type of co-tunneling process which we term
Andreev-normal co-tunneling, is predicted. At low temperatures, coherent
superposition of these co-tunneling processes induces a Kondo effect in which
Cooper pairs directly participate formation of a spin singlet, leading to four
Kondo resonance peaks in the local density of states, and enhancing the
tunneling current.Comment: 4 pages, 2 figures, Late
Modified Perturbation Theory Applied to Kondo-Type Transport through a Quantum Dot under a Magnetic Field
Linear conductance through a quantum dot is calculated under a finite
magnetic field using the modified perturbation theory. The method is based on
the second-order perturbation theory with respect to the Coulomb repulsion, but
the self-energy is modified to reproduce the correct atomic limit and to
fulfill the Friedel sum rule exactly. Although this method is applicable only
to zero temperature in a strict sense, it is approximately extended to finite
temperatures. It is found that the conductance near electron-hole symmetry is
suppressed by the application of the magnetic field at low temperatures.
Positive magnetoconductance is observed in the case of large electron-hole
asymmetry.Comment: 4pages, 5 figure
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