1,352 research outputs found
Full Counting Statistics of Spin Currents
We discuss how to detect fluctuating spin currents and derive full counting
statistics of electron spin transfers. It is interesting to consider several
detectors in series that simultaneously monitor different components of the
spins transferred. We have found that in general the statistics of the
measurement outcomes cannot be explained with the projection postulate and
essentially depends on the quantum dynamics of the detectors.Comment: twocolumns, 4 pages, 2 figure
Feedback of the electromagnetic environment on current and voltage fluctuations out of equilibrium
A theory is presented for low-frequency current and voltage correlators of a
mesoscopic conductor embedded in a macroscopic electromagnetic environment.
This Keldysh field theory evaluated at its saddle-point provides the
microscopic justification for our earlier phenomenological calculation (using
the cascaded Langevin approach). The nonlinear feedback from the environment
mixes correlators of different orders, which explains the unexpected
temperature dependence of the third moment of tunneling noise observed in a
recent experiment. At non-zero temperature, current and voltage correlators of
order three and higher are no longer linearly related. We show that a Hall bar
measures voltage correlators in the longitudinal voltage and current
correlators in the Hall voltage. We go beyond the saddle-point approximation to
consider the environmental Coulomb blockade. We derive that the leading order
Coulomb blockade correction to the n-th cumulant of current fluctuations is
proportional to the voltage derivative of the (n+1)-th cumulant, generalizing
to any n the earlier results for n=1,2.Comment: 12 pages, 8 figure
Temperature dependent third cumulant of tunneling noise
Poisson statistics predicts that the shot noise in a tunnel junction has a
temperature independent third cumulant e^2\I, determined solely by the mean
current I. Experimental data, however, show a puzzling temperature dependence.
We demonstrate theoretically that the third cumulant becomes strongly
temperature dependent and may even change sign as a result of feedback from the
electromagnetic environment. In the limit of a noninvasive (zero-impedance)
measurement circuit in thermal equilibrium with the junction, we find that the
third cumulant crosses over from e^2/I at low temperatures to -e^2/I at high
temperatures.Comment: 4 pages including 2 figure
Spin-transport in multi-terminal normal metal - ferromagnet systems with non-collinear magnetizations
A theory of spin-transport in hybrid normal metal - ferromagnetic electronic
circuits is developed, taking into account non-collinear spin-accumulation.
Spin-transport through resistive elements is described by 4 conductance
parameters. Microscopic expression for these conductances are derived in terms
of scattering matrices and explicitly calculated for simple models. The circuit
theory is applied to 2-terminal and 3-terminal devices attached to
ferromagnetic reservoirs.Comment: 12 pages, 6 figures, to appear in Eur. Phys. J.
Secondary "Smile"-gap in the density of states of a diffusive Josephson junction for a wide range of contact types
The superconducting proximity effect leads to strong modifications of the
local density of states in diffusive or chaotic cavity Josephson junctions,
which displays a phase-dependent energy gap around the Fermi energy. The
so-called minigap of the order of the Thouless energy is
related to the inverse dwell time in the diffusive region in the limit
, where is the superconducting energy gap.
In the opposite limit of a large Thouless energy , a
small new feature has recently attracted attention, namely, the appearance of a
further secondary gap, which is around two orders of magnitude smaller compared
to the usual superconducting gap. It appears in a chaotic cavity just below the
superconducting gap edge and vanishes for some value of the phase
difference between the superconductors. We extend previous theory restricted to
a normal cavity connected to two superconductors through ballistic contacts to
a wider range of contact types. We show that the existence of the secondary gap
is not limited to ballistic contacts, but is a more general property of such
systems. Furthermore, we derive a criterion which directly relates the
existence of a secondary gap to the presence of small transmission eigenvalues
of the contacts. For generic continuous distributions of transmission
eigenvalues of the contacts, no secondary gap exists, although we observe a
singular behavior of the density of states at . Finally, we provide a
simple one-dimensional scattering model which is able to explain the
characteristic "smile" shape of the secondary gap.Comment: 12 pages, 12 figure
"Smile"-gap in the density of states of a cavity between superconductors
The density of Andreev levels in a normal metal () in contact with two
superconductors () is known to exhibit an induced minigap related to the
inverse dwell time. We predict a small secondary gap just below the
superconducting gap edge---a feature that has been overlooked so far in
numerous studies of the density of states in structures. In a generic
structure with being a chaotic cavity, the secondary gap is the widest at
zero phase bias. It closes at some finite phase bias, forming the shape of a
"smile". Asymmetric couplings give even richer gap structures near the phase
difference \pi. All the features found should be amendable to experimental
detection in high-resolution low-temperature tunneling spectroscopy.Comment: 5 pages, 4 figure
Non-equilibrium spin accumulation in ferromagnetic single-electron transistors
We study transport in ferromagnetic single-electron transistors. The non-
equilibrium spin accumulation on the island caused by a finite current through
the system is described by a generalized theory of the Coulomb blockade. It
enhances the tunnel magnetoresistance and has a drastic effect on the time-
dependent transport properties. A transient decay of the spin accumulation may
reverse the electric current on time scales of the order of the spin-flip
relaxation time. This can be used as an experimental signature of the non-
equilibrium spin accumulation.Comment: 9 postscript figures, to appear in The European Physical Journal
Elementary events of electron transfer in a voltage-driven quantum point contact
We show that the statistics of electron transfer in a coherent quantum point
contact driven by an arbitrary time-dependent voltage is composed of elementary
events of two kinds: unidirectional one-electron transfers determining the
average current and bidirectional two-electron processes contributing to the
noise only. This result pertains at vanishing temperature while the extended
Keldysh-Green's function formalism in use also enables the systematic
calculation of the higher-order current correlators at finite temperatures.Comment: 4 pages, 2 figures; to appear in PRL. Version 1 contains additional
results for noise and the third cumulant at finite temperatures [Eqs. (19)
and (20)
Full Counting Statistics of Superconductor--Normal-Metal Heterostructures
The article develops a powerful theoretical tool to obtain the full counting
statistics. By a slight extension of the standard Keldysh method we can access
immediately all correlation functions of the current operator. Embedded in a
quantum generalization of the circuit theory of electronic transport, we are
able to study the full counting statistics of a large class of two-terminal
contacts and multi-terminal structures, containing superconductors and normal
metals as elements. The practical use of the method is demonstrated in many
examples.Comment: 35 pages, contribution to "Quantum Noise", ed. by Yu.V. Nazarov and
Ya.M. Blanter, minor changes in text, references adde
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