590 research outputs found
Phase diffusion and charging effects in Josephson junctions
The supercurrent of a Josephson junction is reduced by phase diffusion. For
ultrasmall capacitance junctions the current may be further decreased by
Coulomb blockade effects. We calculate the Cooper pair current by means of
time-dependent perturbation theory to all orders in the Josephson coupling
energy and obtain the current-voltage characteristic in closed form in a range
of parameters of experimental interest. The results comprehend phase diffusion
of the coherent Josephson current in the classical regime as well as the
supercurrent peak due to incoherent Cooper pair tunneling in the strong Coulomb
blockade regime.Comment: 4 pages, 3 figures, RevTe
Direct measurement of the maximum tunnel rate in a radio frequency single electron transistor operated as a microwave mixer
By operating the radio frequency single electron transistor (rf-SET) as a
mixer we present measurements in which the RC roll-off of the tunnel junctions
is observed at high frequencies. Our technique makes use of the non-linear
rf-SET transconductance to mix high frequency gate signals and produce
difference-frequency components that fall within the bandwidth of the rf-SET.
At gate frequencies >15GHz the induced charge on the rf-SET island is altered
on time-scales faster than the inverse tunnel rate, preventing mixer operation.
We suggest the possibility of utilizing this technique to sense high frequency
signals beyond the usual rf-SET bandwidth.Comment: Submitted to Applied Physics Letters. Comments always very welcome,
email:[email protected] (New version contains extra data and new figs
Is the dynamics of open quantum systems always linear?
We study the influence of the preparation of an open quantum system on its
reduced time evolution. In contrast to the frequently considered case of an
initial preparation where the total density matrix factorizes into a product of
a system density matrix and a bath density matrix the time evolution generally
is no longer governed by a linear map nor is this map affine. Put differently,
the evolution is truly nonlinear and cannot be cast into the form of a linear
map plus a term that is independent of the initial density matrix of the open
quantum system. As a consequence, the inhomogeneity that emerges in formally
exact generalized master equations is in fact a nonlinear term that vanishes
for a factorizing initial state. The general results are elucidated with the
example of two interacting spins prepared at thermal equilibrium with one spin
subjected to an external field. The second spin represents the environment. The
field allows the preparation of mixed density matrices of the first spin that
can be represented as a convex combination of two limiting pure states, i.e.
the preparable reduced density matrices make up a convex set. Moreover, the map
from these reduced density matrices onto the corresponding density matrices of
the total system is affine only for vanishing coupling between the spins. In
general, the set of the accessible total density matrices is nonconvex.Comment: 19 pages, 3 figures, minor changes to improve readability, discussion
on Mori's linear regime and references adde
Many body effects in finite metallic carbon nanotubes
The non homogeneity of the charge distribution in a carbon nanotube leads to
the formation of an excitonic resonance, in a similar way to the one observed
in X-ray absorption in metals. As a result, a positive anomaly at low bias
appears in the tunnelling density of states. This effect depends on the
screening of the electron--electron interactions by metallic gates, and it
modifies the coupling of the nanotube to normal and superconducting electrodes.Comment: 5 page
Quantum confinement corrections to the capacitance of gated one-dimensional nanostructures
With the help of a multi-configurational Green's function approach we
simulate single-electron Coulomb charging effects in gated ultimately scaled
nanostructures which are beyond the scope of a selfconsistent mean-field
description. From the simulated Coulomb-blockade characteristics we derive
effective system capacitances and demonstrate how quantum confinement effects
give rise to corrections. Such deviations are crucial for the interpretation of
experimentally determined capacitances and the extraction of
application-relevant system parameters
Coulomb Blockade due to Quantum Phase-Slips Illustrated with Devices
In order to illustrate the emergence of Coulomb blockade from coherent
quantum phase-slip processes in thin superconducting wires, we propose and
theoretically investigate two elementary setups, or "devices". The setups are
derived from Cooper-pair box and Cooper-pair transistor, so we refer to them as
QPS-box and QPS-transistor, respectively. We demonstrate that the devices
exhibit sensitivity to a charge induced by a gate electrode, this being the
main signature of Coulomb blockade. Experimental realization of these devices
will unambiguously prove the Coulomb blockade as an effect of coherence of
phase-slip processes. We analyze the emergence of discrete charging in the
limit strong phase-slips. We have found and investigated six distinct regimes
that are realized depending on the relation between three characteristic energy
scales: inductive and charging energy, and phase-slip amplitude. For
completeness, we include a brief discussion of dual Josephson-junction devices
Dissipative Quantum Systems with Potential Barrier. General Theory and Parabolic Barrier
We study the real time dynamics of a quantum system with potential barrier
coupled to a heat-bath environment. Employing the path integral approach an
evolution equation for the time dependent density matrix is derived. The time
evolution is evaluated explicitly near the barrier top in the temperature
region where quantum effects become important. It is shown that there exists a
quasi-stationary state with a constant flux across the potential barrier. This
state generalizes the Kramers flux solution of the classical Fokker-Planck
equation to the quantum regime. In the temperature range explored the quantum
flux state depends only on the parabolic approximation of the anharmonic
barrier potential near the top. The parameter range within which the solution
is valid is investigated in detail. In particular, by matching the flux state
onto the equilibrium state on one side of the barrier we gain a condition on
the minimal damping strength. For very high temperatures this condition reduces
to a known result from classical rate theory. Within the specified parameter
range the decay rate out of a metastable state is calculated from the flux
solution. The rate is shown to coincide with the result of purely thermodynamic
methods. The real time approach presented can be extended to lower temperatures
and smaller damping.Comment: 29 pages + 1 figure as compressed ps-file (uufiles) to appear in
Phys. Rev.
Radio-frequency operation of a double-island single-electron transistor
We present results on a double-island single-electron transistor (DISET)
operated at radio-frequency (rf) for fast and highly sensitive detection of
charge motion in the solid state. Using an intuitive definition for the charge
sensitivity, we compare a DISET to a conventional single-electron transistor
(SET). We find that a DISET can be more sensitive than a SET for identical,
minimum device resistances in the Coulomb blockade regime. This is of
particular importance for rf operation where ideal impedance matching to 50 Ohm
transmission lines is only possible for a limited range of device resistances.
We report a charge sensitivity of 5.6E-6 e/sqrt(Hz) for a rf-DISET, together
with a demonstration of single-shot detection of small (<=0.1e) charge signals
on microsecond timescales.Comment: 6 pages, 6 figure
Quantum Brownian Motion With Large Friction
Quantum Brownian motion in the strong friction limit is studied based on the
exact path integral formulation of dissipative systems. In this limit the
time-nonlocal reduced dynamics can be cast into an effective equation of
motion, the quantum Smoluchowski equation. For strongly condensed phase
environments it plays a similar role as master equations in the weak coupling
range. Applications for chemical, mesoscopic, and soft matter systems are
discussed and reveal the substantial role of quantum fluctuations.Comment: 11 pages, 6 figures, to appear in: Chaos: "100 years of Brownian
motion
Electron tunneling into a quantum wire in the Fabry-Perot regime
We study a gated quantum wire contacted to source and drain electrodes in the
Fabry-Perot regime. The wire is also coupled to a third terminal (tip), and we
allow for an asymmetry of the tip tunneling amplitudes of right and left moving
electrons. We analyze configurations where the tip acts as an electron injector
or as a voltage-probe, and show that the transport properties of this
three-terminal set-up exhibit very rich physical behavior. For a
non-interacting wire we find that a tip in the voltage-probe configuration
affects the source-drain transport in different ways, namely by suppressing the
conductance, by modulating the Fabry-Perot oscillations, and by reducing their
visibility. The combined effect of electron electron interaction and finite
length of the wire, accounted for by the inhomogeneous Luttinger liquid model,
leads to significantly modified predictions as compared to models based on
infinite wires. We show that when the tip injects electrons asymmetrically the
charge fractionalization induced by interaction cannot be inferred from the
asymmetry of the currents flowing in source and drain. Nevertheless interaction
effects are visible as oscillations in the non-linear tip-source and tip-drain
conductances. Important differences with respect to a two-terminal set-up
emerge, suggesting new strategies for the experimental investigation of
Luttinger liquid behavior.Comment: 27 pages, 10 figure
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