185 research outputs found
Observation of Fano-Resonances in Single-Wall Carbon Nanotubes
We have explored the low-temperature linear and non-linear electrical
conductance of metallic carbon nanotubes (CNTs), which were grown by the
chemical-vapor deposition method. The high transparency of the contacts allows
to study these two-terminal devices in the high conductance regime. We observe
the expected four-fold shell pattern together with Kondo physics at
intermediate transparency {G\alt 2e^2/h} and a transition to the open regime
in which the maximum conductance is doubled and bound by . In
the high- regime, at the transition from a quantum dot to a weak link, the
CNT levels are strongly broadened. Nonetheless, sharp resonances appear
superimposed on the background which varies slowly with gate voltage. The
resonances are identified by their lineshape as Fano resonances. The origin of
Fano resonances is discussed along the modelling.Comment: pdf including figures, see:
http://www.unibas.ch/phys-meso/Research/Papers/2004/Fano-CVD-SWNT.pd
Resonators coupled to voltage-biased Josephson junctions: From linear response to strongly driven nonlinear oscillations
Motivated by recent experiments, where a voltage biased Josephson junction is
placed in series with a resonator, the classical dynamics of the circuit is
studied in various domains of parameter space. This problem can be mapped onto
the dissipative motion of a single degree of freedom in a nonlinear
time-dependent potential, where in contrast to conventional settings the
nonlinearity appears in the driving while the static potential is purely
harmonic. For long times the system approaches steady states which are analyzed
in the underdamped regime over the full range of driving parameters including
the fundamental resonance as well as higher and sub-harmonics. Observables such
as the dc-Josephson current and the radiated microwave power give direct
information about the underlying dynamics covering phenomena as bifurcations,
irregular motion, up- and down conversion. Due to their tunability, present and
future set-ups provide versatile platforms to explore the changeover from
linear response to strongly nonlinear behavior in driven dissipative systems
under well defined conditions.Comment: 12 pages, 11 figure
Resonant tunneling and Fano resonance in quantum dots with electron-phonon interaction
We theoretically study the resonant tunneling and Fano resonance in quantum
dots with electron-phonon (e-ph) interaction. We examine the bias-voltage ()
dependence of the decoherence, using Keldysh Green function method and
perturbation with respect to the e-ph interaction. With optical phonons of
energy , only the elastic process takes place when , in
which electrons emit and absorb phonons virtually. The process suppresses the
resonant amplitude. When , the inelastic process is possible which
is accompanied by real emission of phonons. It results in the dephasing and
broadens the resonant width. The bias-voltage dependence of the decoherence
cannot be obtained by the canonical transformation method to consider the e-ph
interaction if its effect on the tunnel coupling is neglected. With acoustic
phonons, the asymmetric shape of the Fano resonance grows like a symmetric one
as the bias voltage increases, in qualitative accordance with experimental
results.Comment: 28 pages, 11 figure
Interference through quantum dots
We discuss the effect of quantum interference on transport through a quantum
dot system. We introduce an indirect coherent coupling parameter alpha, which
provides constructive/destructive interference in the transport current
depending on its phase and the magnetic flux. We estimate the current through
the quantum dot system using the non-equilibrium Green's function method as
well as the master equation method in the sequential tunneling regime. The
visibility of the Aharonov-Bohm oscillation is evaluated. For a large inter-dot
Coulomb interaction, the current is strongly suppressed by the quantum
interference effect, while the current is restored by applying an oscillating
resonance field with the frequency of twice the inter-dot tunneling energy.Comment: 10 pages, 3 figure
Antibunched photons emitted by a dc-biased Josephson junction
We show experimentally that a dc biased Josephson junction in series with a high-enough-impedance microwave resonator emits antibunched photons. Our resonator is made of a simple microfabricated spiral coil that resonates at 4.4 GHz and reaches a 1.97kΩ characteristic impedance. The second order correlation function of the power leaking out of the resonator drops down to 0.3 at zero delay, which demonstrates the antibunching of the photons emitted by the circuit at a rate of 6×10^7 photons per second. Results are found in quantitative agreement with our theoretical predictions. This simple scheme could offer an efficient and bright single-photon source in the microwave domain
Effects of Menopause in Women With Multiple Sclerosis: An Evidence-Based Review
Over two thirds of all individuals who develop multiple sclerosis (MS) will be women prior to the age of menopause. Further, an estimated 30% of the current MS population consists of peri- or postmenopausal women. The presence of MS does not appear to influence age of menopausal onset. In clinical practice, symptoms of MS and menopause can frequently overlap, including disturbances in cognition, mood, sleep, and bladder function, which can create challenges in ascertaining the likely cause of symptoms to be treated. A holistic and comprehensive approach to address these common physical and psychological changes is often suggested to patients during menopause. Although some studies have suggested that women with MS experience reduced relapse rates and increased disability progression post menopause, the data are not consistent enough for firm conclusions to be drawn. Mechanisms through which postmenopausal women with MS may experience disability progression include neuroinflammation and neurodegeneration from age-associated phenomena such as immunosenescence and inflammaging. Additional effects are likely to result from reduced levels of estrogen, which affects MS disease course. Following early retrospective studies of women with MS receiving steroid hormones, more recent interventional trials of exogenous hormone use, albeit as oral contraceptive, have provided some indications of potential benefit on MS outcomes. This review summarizes current research on the effects of menopause in women with MS, including the psychological impact and symptoms of menopause on disease worsening, and the treatment options. Finally, we highlight the need for more inclusion of MS patients from underrepresented racial and geographic groups in clinical trials, including among menopausal women
Aharonov-Bohm interferometry with quantum dots: scattering approach versus tunneling picture
We address the question of how to model electron transport through closed
Aharonov-Bohm interferometers which contain quantum dots. By explicitly
studying interferometers with one and two quantum dots, we establish the
connection between a tunneling-Hamiltonian formulation on the one hand and a
scattering-matrix approach on the other hand. We prove that, under certain
circumstances, both approaches are equivalent, i.e., both types of models can
describe the same experimental setups. Furthermore, we analyze how the
interplay of the Aharonov-Bohm phase and the orbital phase associated with the
lengths of the interferometers' arms affect transport properties.Comment: 8 pages, 8 figures, published versio
Josephson photonics with a two-mode superconducting circuit
We analyze the quantum dynamics of two electromagnetic oscillators coupled in series to a voltage-biased Josephson junction. When the applied voltage leads to a Josephson frequency across the junction which matches the sum of the two mode frequencies, tunneling Cooper pairs excite photons in both modes simultaneously leading to far-from-equilibrium states. These states display highly nonclassical features including strong antibunching, violation of Cauchy-Schwartz inequalities, and number squeezing. We obtain approximate analytic results for both the regimes of low and high photon occupancies which are supported by a full numerical treatment. The impact of asymmetries between the two modes is explored, revealing a pronounced enhancement of number squeezing when the modes are damped at different rates
Aharonov-Bohm Effect for Parallel and T-shaped Double Quantum Dots
We investigate the Aharonov-Bohm (AB) effect for the double quantum dots in
the Kondo regime using the slave-boson mean-field approximation. In contrast to
the non-interacting case, where the AB oscillation generally has the period of
4 when the two-subring structure is formed via the interdot tunneling
, we find that the AB oscillation has the period of 2 in the Kondo
regime. Such effects appear for the double quantum dots close to the T-shaped
geometry even in the charge-fluctuation regime. These results follow from the
fact that the Kondo resonance is always fixed to the Fermi level irrespective
of the detailed structure of the bare dot-levels.Comment: 3 pages, 4 figures; minor change
Crossing of two Coulomb-Blockade Resonances
We investigate theoretically the transport of non--interacting electrons
through an Aharanov--Bohm (AB) interferometer with two quantum dots (QD)
embedded into its arms. In the Coulomb-blockade regime, transport through each
QD proceeds via a single resonance. The resonances are coupled through the arms
of the AB device but may also be coupled directly. In the framework of the
Landauer--Buttiker approach, we present expressions for the scattering matrix
which depend explicitly on the energies of the two resonances and on the AB
phase. We pay particular attention to the crossing of the two resonances.Comment: 15 pages, 1 figur
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