26 research outputs found
Charge fluctuations in open chaotic cavities
We present a discussion of the charge response and the charge fluctuations of
mesoscopic chaotic cavities in terms of a generalized Wigner-Smith matrix. The
Wigner-Smith matrix is well known in investigations of time-delay of quantum
scattering. It is expressed in terms of the scattering matrix and its
derivatives with energy. We consider a similar matrix but instead of an energy
derivative we investigate the derivative with regard to the electric potential.
The resulting matrix is then the operator of charge. If this charge operator is
combined with a self-consistent treatment of Coulomb interaction, the charge
operator determines the capacitance of the system, the non-dissipative
ac-linear response, the RC-time with a novel charge relaxation resistance, and
in the presence of transport a resistance that governs the displacement
currents induced into a nearby conductor. In particular these capacitances and
resistances determine the relaxation rate and dephasing rate of a nearby qubit
(a double quantum dot). We discuss the role of screening of mesoscopic chaotic
detectors. Coulomb interaction effects in quantum pumping and in photon
assisted electron-hole shot noise are treated similarly. For the latter we
present novel results for chaotic cavities with non-ideal leads.Comment: 29 pages, 13 figures;v.2--minor changes; contribution for the special
issue of J. Phys. A on "Trends in Quantum Chaotic Scattering
Stub model for dephasing in a quantum dot
As an alternative to Buttiker's dephasing lead model, we examine a dephasing
stub. Both models are phenomenological ways to introduce decoherence in chaotic
scattering by a quantum dot. The difference is that the dephasing lead opens up
the quantum dot by connecting it to an electron reservoir, while the dephasing
stub is closed at one end. Voltage fluctuations in the stub take over the
dephasing role from the reservoir. Because the quantum dot with dephasing lead
is an open system, only expectation values of the current can be forced to
vanish at low frequencies, while the outcome of an individual measurement is
not so constrained. The quantum dot with dephasing stub, in contrast, remains a
closed system with a vanishing low-frequency current at each and every
measurement. This difference is a crucial one in the context of quantum
algorithms, which are based on the outcome of individual measurements rather
than on expectation values. We demonstrate that the dephasing stub model has a
parameter range in which the voltage fluctuations are sufficiently strong to
suppress quantum interference effects, while still being sufficiently weak that
classical current fluctuations can be neglected relative to the nonequilibrium
shot noise.Comment: 8 pages with 1 figure; contribution for the special issue of J.Phys.A
on "Trends in Quantum Chaotic Scattering
Pumping in an interacting quantum wire
We study charge and spin pumping in an interacting one-dimensional wire. We
show that a spatially periodic potential modulated in space and time acts as a
quantum pump inducing a dc-current component at zero bias. The current
generated by the pump is strongly affected by the interactions. It has a power
law dependence on the frequency or temperature with the exponent determined by
the interaction in the wire, while the coupling to the pump affects the
amplitudes only. We also show that pure spin-pumping can be achieved, without
the presence of a magnetic field.Comment: 13 pages,2 figure
Efficient, highly selective laser isotope separation of carbon-13
We recently demonstrated an original approach to highly selective laser isotope separation of carbon-13 that employs vibrational overtone pre-excitation of CF3 H together with infrared multiphoton dissociation [O.V. Boyarkin, M. Kowalczyk, T.R. Rizzo, J. Chem. Phys. 118, 93 (2003)]. The practical implementation of this approach was complicated by the long absorption path length needed for the overtone excitation laser beam. In the present work, we employ a low overtone level for the pre-excitation that shortens this pathway, facilitating engineering of the process. We propose an optimal configuration of the isotope separation scheme and consider a realistic example of a separation unit for isotopic enrichment of carbon-13 to 94–98%. The photon energy expenditure of 97 eV per separated atom is much lower than that of the current commercial laser technology, making this process economically feasible
Optimal quantum pump in the presence of a superconducting lead
We investigate the parametric pumping of a hybrid structure consisting of a
normal quantum dot, a normal lead and a superconducting lead. Using the time
dependent scattering matrix theory, we have derived a general expression for
the pumped electric current and heat current. We have also derived the
relationship among the instantaneous pumped heat current, electric current, and
shot noise. This gives a lower bound for the pumped heat current in the hybrid
system similar to that of the normal case obtained by Avron et al
Adiabatic quantum pump in the presence of external ac voltages
We investigate a quantum pump which in addition to its dynamic pump
parameters is subject to oscillating external potentials applied to the
contacts of the sample. Of interest is the rectification of the ac currents
flowing through the mesoscopic scatterer and their interplay with the quantum
pump effect. We calculate the adiabatic dc current arising under the
simultaneous action of both the quantum pump effect and classical
rectification. In addition to two known terms we find a third novel
contribution which arises from the interference of the ac currents generated by
the external potentials and the ac currents generated by the pump. The
interference contribution renormalizes both the quantum pump effect and the ac
rectification effect. Analysis of this interference effect requires a
calculation of the Floquet scattering matrix beyond the adiabatic approximation
based on the frozen scattering matrix alone. The results permit us to find the
instantaneous current. In addition to the current generated by the oscillating
potentials, and the ac current due to the variation of the charge of the frozen
scatterer, there is a third contribution which represents the ac currents
generated by an oscillating scatterer. We argue that the resulting pump effect
can be viewed as a quantum rectification of the instantaneous ac currents
generated by the oscillating scatterer. These instantaneous currents are an
intrinsic property of a nonstationary scattering process.Comment: 11 pages, 1 figur
Quantum chaotic scattering in time-dependent external fields: random matrix approach
We review the random matrix description of electron transport through open
quantum dots, subject to time-dependent perturbations. All characteristics of
the current linear in the bias can be expressed in terms of the scattering
matrix, calculated for a time-dependent Hamiltonian. Assuming that the
Hamiltonian belongs to a Gaussian ensemble of random matrices, we investigate
various statistical properties of the direct current in the ensemble.
Particularly, even at zero bias the time-dependent perturbation induces
current, called photovoltaic current. We discuss dependence of the photovoltaic
current and its noise on the frequency and the strength of the perturbation. We
also describe the effect of time-dependent perturbation on the weak
localization correction to the conductance and on conductance fluctuations.Comment: 27 pages, 6 figures; contribution for the special issue of J. Phys.
A: "Trends in Quantum Chaotic Scattering
Floquet scattering theory of quantum pumps
We develop the Floquet scattering theory for quantum mechanical pumping in
mesoscopic conductors. The nonequilibrium distribution function, the dc charge
and heat currents are investigated at arbitrary pumping amplitude and
frequency. For mesoscopic samples with discrete spectrum we predict a sign
reversal of the pumped current when the pump frequency is equal to the level
spacing in the sample. This effect allows to measure the phase of the
transmission coefficient through the mesoscopic sample. We discuss the
necessary symmetry conditions (both spatial and temporal) for pumping.Comment: 11 pages, 5 figure
Quantum pumping: Coherent Rings versus Open Conductors
We examine adiabatic quantum pumping generated by an oscillating scatterer
embedded in a one-dimensional ballistic ring and compare it with pumping caused
by the same scatterer connected to external reservoirs. The pumped current for
an open conductor, paradoxically, is non-zero even in the limit of vanishing
transmission. In contrast, for the ring geometry the pumped current vanishes in
the limit of vanishing transmission. We explain this paradoxical result and
demonstrate that the physics underlying adiabatic pumping is the same in open
and in closed systems.Comment: 4 pages, 2 figure
Quantization of adiabatic pumped charge in the presence of superconducting lead
We investigate the parametric electron pumping of a double barrier structure
in the presence of a superconducting lead. The parametric pumping is
facilitated by cyclic variation of the barrier heights and of the
barriers. In the weak coupling regime, there exists a resonance line in the
parameter space so that the energy of the quasi-bound state is in
line with the incoming Fermi energy. Levinson et al found recently that the
pumped charge for each pumping cycle is quantized with for normal
structure when the pumping contour encircles the resonance line. In the
presence of a superconducting lead, we find that the pumped charge is quantized
with the value