1,381 research outputs found
Josephson Effect through an isotropic magnetic molecule
We investigate the Josephson effect through a molecular quantum dot magnet
connected to superconducting leads. The molecule contains a magnetic atom,
whose spin is assumed to be isotropic. It is coupled to the electron spin on
the dot via exchange coupling. Using the numerical renormalization group method
we calculate the Andreev levels and the supercurrent and examine intertwined
effect of the exchange coupling, Kondo correlation, and superconductivity on
the current. Exchange coupling typically suppresses the Kondo correlation so
that the system undergoes a phase transition from 0 to state as the
modulus of exchange coupling increases. Antiferromagnetic coupling is found to
drive exotic transitions: the reentrance to the state for a small
superconducting gap and the restoration of 0 state for large antiferromagnetic
exchange coupling. We suggest that the asymmetric dependence of supercurrent on
the exchange coupling could be used as to detect its sign in experiments
Nanomechanical effects in an Andreev quantum dot
We consider a quantum dot with mechanical degrees of freedom which is coupled
to superconducting electrodes. A Josephson current is generated by applying a
phase difference. In the absence of coupling to vibrations, this setup was
previously proposed as a detector of magnetic flux and we wish here to address
the effect of the phonon coupling to this detection scheme. We compute the
charge on the quantum dot and determine its dependence on the phase difference
in the presence of phonon coupling and Coulomb interaction. This allows to
identify regions in parameter space with the highest charge to phase
sensitivity, which are relevant for flux detection. Further insight about the
interplay of such couplings and subsequent entanglement properties between
electron and phonon degrees of freedom are gained by computing the von Neuman
entropy.Comment: 9 pages, 7 figures; minor corretion
Vijf dagen op zoek naar levende exemplaren van <i>Nassarius reticulatus</i> (L., 1758) op de stranden van de Westkust
Quantum phase transition of dynamical resistance in a mesoscopic capacitor
We study theoretically dynamic response of a mesoscopic capacitor, which
consists of a quantum dot connected to an electron reservoir via a point
contact and capacitively coupled to a gate voltage. A quantum Hall edge state
with a filling factor nu is realized in a strong magnetic field applied
perpendicular to the two-dimensional electron gas. We discuss a noise-driven
quantum phase transition of the transport property of the edge state by taking
into account an ohmic bath connected to the gate voltage. Without the noise,
the charge relaxation for nu>1/2 is universally quantized at R_q=h/(2e^2),
while for nu<1/2, the system undergoes the Kosterlitz-Thouless transtion, which
drastically changes the nature of the dynamical resistance. The phase
transition is facilitated by the noisy gate voltage, and we see that it can
occur even for an integer quantum Hall edge at nu=1. When the dissipation by
the noise is sufficiently small, the quantized value of R_q is shifted by the
bath impedance.Comment: 5 pages, 2 figures, proceeding of the 19th International Conference
on the Application of High Magnetic Fields in Semiconductor Physics and
Nanotechnology (HMF-19
Andreev quantum dot with several conducting channels
We study an Andreev quantum dot, that is a quantum dot inserted in a
superconducting ring, with several levels or conducting channels. We analyze
the degeneracy of the ground state as a function of the phase difference and of
the gate voltage and find its dependence on the Coulomb interaction within and
between channels. We compute a (non integer) charge of the dot region and
Josephson current. The charge-to-phase and current-to-gate voltage
sensitivities are studied. We find that, even in the presence of Coulomb
interaction between the channels, the sensitivity increases with the number of
channels, although it does not scale linearly as in the case with no
interactions. The Andreev quantum dot may therefore be used as a sensitive
detector of magnetic flux or as a Josephson transistor.Comment: 13 pages, 10 figures, minor correction
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