182 research outputs found
Magnetization of ballistic quantum dots induced by a linear-polarized microwave field
On a basis of extensive analytical and numerical studies we show that a
linear-polarized microwave field creates a stationary magnetization in
mesoscopic ballistic quantum dots with two-dimensional electron gas being at a
thermal equilibrium. The magnetization is proportional to a number of electrons
in a dot and to a microwave power. Microwave fields of moderate strength create
in a one dot of few micron size a magnetization which is by few orders of
magnitude larger than a magnetization produced by persistent currents. The
effect is weakly dependent on temperature and can be observed with existing
experimental techniques. The parallels between this effect and ratchets in
asymmetric nanostructures are also discussed.Comment: 10 pages, 11 figs, research at http://www.quantware.ups-tlse.f
0- quantum transition in a carbon nanotube Josephson junction: universal phase dependence and orbital degeneracy
We investigate experimentally the supercurrent in a clean carbon nanotube
quantum dot, close to orbital degeneracy, connected to superconducting leads in
a regime of strong competition between local electronic correlations and
superconducting proximity effect. For an odd occupancy of the dot and
intermediate coupling to the reservoir, the Kondo effect can develop in the
normal state and screen the local magnetic moment of the dot. This leads to
singlet-doublet transitions that strongly affect the Josephson effect in a
single-level quantum dot: the sign of the supercurrent changes from positive to
negative (0 to -junction). In the regime of strongest competition between
the Kondo effect and proximity effect, meaning that the Kondo temperature
equals the superconducting gap, the magnetic state of the dot undergoes a first
order quantum transition induced by the superconducting phase difference across
the junction. This is revealed experimentally by anharmonic current-phase
relations. In addition, the very specific electronic configuration of clean
carbon nanotubes, with two nearly orbitally degenerated states, leads to
different physics depending whether only one or both quasi-degenerate upper
levels of the dots participate to transport, which is determined by their
occupancy and relative widths. When the transport of Cooper pairs takes place
through only one of these levels, we find that the phase diagram of the
phase-dependent 0- transition is a universal characteristic of a
discontinuous level-crossing quantum transition at zero temperature. In the
case were two levels participate to transport, the nanotube Josephson current
exhibits a continuous 0- transition, independent of the superconducting
phase, revealing a different physical mechanism of the transition.Comment: 14 pages, 12 figure
Manipulating the magnetic state of a carbon nanotube Josephson junction using the superconducting phase
The magnetic state of a quantum dot attached to superconducting leads is
experimentally shown to be controlled by the superconducting phase difference
across the dot. This is done by probing the relation between the Josephson
current and the superconducting phase difference of a carbon nanotube junction
whose Kondo energy and superconducting gap are of comparable size. It exhibits
distinctively anharmonic behavior, revealing a phase mediated singlet to
doublet transition. We obtain an excellent quantitative agreement with
numerically exact quantum Monte Carlo calculations. This provides strong
support that we indeed observed the finite temperature signatures of the phase
controlled zero temperature level-crossing transition originating from strong
local electronic correlations.Comment: 5 pages, 4 figures + supp. material
Quantum Noise Measurement of a Carbon Nanotube Quantum Dot in the Kondo Regime
The current emission noise of a carbon nanotube quantum dot in the Kondo
regime is measured at frequencies of the order or higher than the
frequency associated with the Kondo effect , with the Kondo
temperature. The carbon nanotube is coupled via an on-chip resonant circuit to
a quantum noise detector, a superconductor-insulator-superconductor junction.
We find for a Kondo effect related singularity at a
voltage bias , and a strong reduction of this singularity
for , in good agreement with theory. Our experiment
constitutes a new original tool for the investigation of the non-equilibrium
dynamics of many-body phenomena in nanoscale devices.Comment: 6 pages, 4 figure
High Frequency Quantum Admittance and Noise Measurement with an On-chip Resonant Circuit
By coupling a quantum detector, a superconductor-insulator-superconductor
junction, to a Josephson junction \textit{via} a resonant circuit we probe the
high frequency properties, namely the ac complex admittance and the current
fluctuations of the Josephson junction at the resonant frequencies. The
admittance components show frequency dependent singularities related to the
superconducting density of state while the noise exhibits a strong frequency
dependence, consistent with theoretical predictions. The circuit also allows to
probe separately the emission and absorption noise in the quantum regime of the
superconducting resonant circuit at equilibrium. At low temperature the
resonant circuit exhibits only absorption noise related to zero point
fluctuations, whereas at higher temperature emission noise is also present.Comment: 15 pages, 15 figure
Tuning the Josephson current in carbon nanotubes with the Kondo effect
We investigate the Josephson current in a single wall carbon nanotube
connected to superconducting electrodes. We focus on the parameter regime in
which transport is dominated by Kondo physics. A sizeable supercurrent is
observed for odd number of electrons on the nanotube when the Kondo temperature
Tk is sufficiently large compared to the superconducting gap. On the other hand
when, in the center of the Kondo ridge, Tk is slightly smaller than the
superconducting gap, the supercurrent is found to be extremely sensitive to the
gate voltage Vbg. Whereas it is largely suppressed at the center of the ridge,
it shows a sharp increase at a finite value of Vbg. This increase can be
attributed to a doublet-singlet transition of the spin state of the nanotube
island leading to a pi shift in the current phase relation. This transition is
very sensitive to the asymmetry of the contacts and is in good agreement with
theoretical predictions.Comment: 5 pages, 4 figure
Mesoscopic Cavity Quantum Electrodynamics with Quantum Dots
We describe an electrodynamic mechanism for coherent, quantum mechanical
coupling between spacially separated quantum dots on a microchip. The technique
is based on capacitive interactions between the electron charge and a
superconducting transmission line resonator, and is closely related to atomic
cavity quantum electrodynamics. We investigate several potential applications
of this technique which have varying degrees of complexity. In particular, we
demonstrate that this mechanism allows design and investigation of an on-chip
double-dot microscopic maser. Moreover, the interaction may be extended to
couple spatially separated electron spin states while only virtually populating
fast-decaying superpositions of charge states. This represents an effective,
controllable long-range interaction, which may facilitate implementation of
quantum information processing with electron spin qubits and potentially allow
coupling to other quantum systems such as atomic or superconducting qubits.Comment: 8 pages, 5 figure
Phonon assisted dynamical Coulomb blockade in a thin suspended graphite sheet
The differential conductance in a suspended few layered graphene sample is
fou nd to exhibit a series of quasi-periodic sharp dips as a function of bias
at l ow temperature. We show that they can be understood within a simple model
of dyn amical Coulomb blockade where energy exchanges take place between the
charge carriers transmitted trough the sample and a dissipative electromagnetic
envir onment with a resonant phonon mode strongly coupled to the electrons
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