1,778 research outputs found
On the Kondo effect in carbon nanotubes at half halfing
In a single state of a quantum dot the Kondo effect arises due to the
spin-degeneracy, which is present if the dot is occupied with one electron (N =
1). The eigenstates of a carbon nanotube quantum dot possess an additional
orbital degeneracy leading to a four-fold shell pattern. This additional
degeneracy increases the possibility for the Kondo effect to appear. We revisit
the Kondo problem in metallic carbon nanotubes by linear and non-linear
transport measurement in this regime, in which the four-fold pattern is
present. We have analyzed the ground state of CNTs, which were grown by
chemical vapor deposition, at filling N = 1, N = 2, and N = 3. Of particular
interest is the half-filled shell, i.e. N = 2. In this case, the ground state
is either a paired electron state or a state for which the singlet and triplet
states are effectively degenerate, allowing in the latter case for the
appearance of the Kondo effect. We deduce numbers for the effective missmatch d
of the levels from perfect degeneracy and the exchange energy J. While d ~ 0.1
- 0.2 (in units of level spacing) is in agreement with previous work, the
exchange term is found to be surprisingly small: J < 0.02. In addition we
report on the observation of gaps, which in one case is seen at N = 3 and in
another is present over an extended sequence of levels.Comment: full paper including figures at:
http://www.unibas.ch/phys-meso/Research/Papers/2004/Kondo-4shell-SWNT.pd
Stamping single wall nanotubes for circuit quantum electrodynamics
We report on a dry transfer technique for single wall carbon nanotube devices
which allows to embed them in high finesse microwave cavity. We demonstrate the
ground state charge readout and a quality factor of about 3000 down to the
single photon regime. This technique allows to make devices such as double
quantum dots which could be instrumental for achieving the strong spin photon
coupling. It can easily be extended to generic carbon nanotube based microwave
devices.Comment: Version similar to the one accepte
Electrical Spin Injection in Multi-Wall carbon NanoTubes with transparent ferromagnetic contacts
We report on electrical spin injection measurements on MWNTs . We use a
ferromagnetic alloy PdNi with x 0.7 which allows to
obtain devices with resistances as low as 5.6 at 300 . The yield
of device resistances below 100 , at 300 , is around 50%. We
measure at 2 a hysteretic magneto-resistance due to the magnetization
reversal of the ferromagnetic leads. The relative difference between the
resistance in the antiparallel (AP) orientation and the parallel (P)
orientation is about 2%.Comment: submitted to APL version without figures version with figures
available on http://www.unibas.ch/phys-meso
Kondo resonance in a nanotube quantum dot coupled to a normal and a superconducting lead
We report on electrical transport measurements through a carbon nanotube
quantum dot coupled to a normal and a superconducting lead. The ratio of Kondo
temperature and superconducting gap is identified to govern the
transport properties of the system. In the case of the
conductance resonance splits into two resonances at . For the
opposite scenario the conductance resonance persists, however
the conductance is not enhanced compared to the normal state due to a relative
asymmetry of the lead-dot couplings. Within this limit the data is in agreement
with a simple model of a resonant SN-interface.Comment: 4 pages, 2 figures. submitted to the Proc. Rencontres de Moriond on
Quantum Information and Decoherence in Nanosystems 200
Ferromagnetic 0-pi Junctions as Classical Spins
The ground state of highly damped PdNi based 0-pi ferromagnetic Josephson
junctions shows a spontaneous half quantum vortex, sustained by a supercurrent
of undetermined sign. This supercurrent flows in the electrode of a Josephson
junction used as a detector and produces a phi(0)/4 shift in its magnetic
diffraction pattern. We have measured the statistics of the positive or
negative sign shift occurring at the superconducting transition of such a
junction. The randomness of the shift sign, the reproducibility of its
magnitude and the possibility of achieving exact flux compensation upon field
cooling: all these features show that 0-pi junctions behave as classical spins,
just as magnetic nanoparticles with uniaxial anisotropy.Comment: 4 pages, 4 figure
Controlling spin in an electronic interferometer with spin-active interfaces
We consider electronic current transport through a ballistic one-dimensional
quantum wire connected to two ferromagnetic leads. We study the effects of the
spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon
scattering at the boundaries of the wire. The SDIPS produces a spin splitting
of the wire resonant energies which is tunable with the gate voltage and the
angle between the ferromagnetic polarizations. This property could be used for
manipulating spins. In particular, it leads to a giant magnetoresistance effect
with a sign tunable with the gate voltage and the magnetic field applied to the
wire.Comment: 5 pages, 3 figures. to be published in Europhysics Letter
Shot noise in carbon nanotube based Fabry-Perot interferometers
We report on shot noise measurements in carbon nanotube based Fabry-Perot
electronic interferometers. As a consequence of quantum interferences, the
noise power spectral density oscillates as a function of the voltage applied to
the gate electrode. The quantum shot noise theory accounts for the data
quantitatively. It allows to confirm the existence of two nearly degenerate
orbitals. At resonance, the transmission of the nanotube approaches unity, and
the nanotube becomes noiseless, as observed in quantum point contacts. In this
weak backscattering regime, the dependence of the noise on the backscattering
current is found weaker than expected, pointing either to electron-electron
interactions or to weak decoherence
Photon mediated interaction between distant quantum dot circuits
Engineering the interaction between light and matter is an important goal in
the emerging field of quantum opto-electronics. Thanks to the use of cavity
quantum electrodynamics architectures, one can envision a fully hybrid
multiplexing of quantum conductors. Here, we use such an architecture to couple
two quantum dot circuits . Our quantum dots are separated by 200 times their
own size, with no direct tunnel and electrostatic couplings between them. We
demonstrate their interaction, mediated by the cavity photons. This could be
used to scale up quantum bit architectures based on quantum dot circuits or
simulate on-chip phonon-mediated interactions between strongly correlated
electrons
- âŠ