321 research outputs found

### Reversing the sign of current-current correlations

Current-correlations are a very sensitive probe of the fluctuations of small
conductors. For non-interacting particles injected from thermal sources there
is a simple connection between the sign of correlations and statistics:
current-current correlations of Fermions are negative, intensity-intensity
correlations of Bosons can be positive. In contrast to photons, electrons are
interacting entities, and we can expect the simple connection between
statistics and the sign of current-current correlations to be broken, if
interactions play a crucial role. We present a number of examples in which
interactions are important. At a voltage probe the potential fluctuates to
maintain zero current. It is shown that there are geometries for which these
fluctuations lead to positive correlations. Displacement currents at
capacitively coupled contacts are also positively correlated if both contacts
contribute to screening of the same excess charge fluctuation. Hybrid normal
superconducting systems provide another example which permits positive
correlations. The conditions for positive correlations differ strongly
depending on whether the normal conductor is open and well coupled to the
superconductor or is only weakly coupled via a barrier to the superconductor.
In latter case, positive correlations result if the partition noise generated
by Cooper pairs is overcome by pairs which are broken up and emit one electron
into the contacts of interest.Comment: 30 pages, 9 figures, for "Quantum Noise", edited by Yu. V. Nazarov
and Ya. M. Blanter (Kluwer

### Time-Dependent Transport in Mesoscopic Structures

A discussion of recent work on time-dependent transport in mesoscopic
structures is presented. The discussion emphasizes the use of time-dependent
transport to gain information on the charge distribution and its collective
dynamics. We discuss the RC-time of mesoscopic capacitors, the dynamic
conductance of quantum point contacts and dynamic weak localization effects in
chaotic cavities. We review work on adiabatic quantum pumping and
photon-assisted transport, and conclude with a list which demonstrates the wide
range of problems which are of interest

### The Local Larmor Clock, Partial Densities of States, and Mesoscopic Physics

The local Larmor clock is used to derive a hierarchy of local densities of
states. At the bottom of this hierarchy are the partial density of states for
which represent the contribution to the local density of states if both the
incident and outgoing scattering channel are prescribed. On the next higher
level is the injectivity which represents the contribution to the local density
of states if only the incident channel is prescribed regardless of the final
scattering channel. The injectivity is related by reciprocity to the emissivity
of a point into a quantum channel. The sum of all partial density of states or
the sum of all injectivities or the sum of all emissivities is equal to the
local density of states. The use of the partial density of states is
illustrated for a number of different electron transport problems in mesoscopic
physics: The transmission from a tunneling tip into a mesoscopic conductor, the
discussion of inelastic or phase breaking scattering with a voltage probe, and
the ac-conductance of mesoscopic conductors. The transition from a capacitive
response (positive time-delay) to an inductive response (negative time-delay)
for a quantum point contact is used to illustrate the difficulty in associating
time-scales with a linear response analysis. A brief discussion of the
off-diagonal elements of a partial density of states matrix is presented. The
off-diagonal elements permit to investigate carrier fluctuations away from the
average carrier density. The work concludes with a discussion of the relation
between the partial density of states matrix and the Wigner-Smith delay time
matrix

### Charge Relaxation Resistances and Charge Fluctuations in Mesoscopic Conductors

A brief overview is presented of recent work which investigates the
time-dependent relaxation of charge and its spontaneous fluctuations on
mesoscopic conductors in the proximity of gates. The leading terms of the low
frequency conductance are determined by a capacitive or inductive emittance and
a dissipative charge relaxation resistance. The charge relaxation resistance is
determined by the ratio of the mean square dwell time of the carriers in the
conductor and the square of the mean dwell time. The contribution of each
scattering channel is proportional to half a resistance quantum. We discuss the
charge relaxation resistance for mesoscopic capacitors, quantum point contacts,
chaotic cavities, ballistic wires and for transport along edge channels in the
quantized Hall regime. At equilibrium the charge relaxation resistance also
determines via the fluctuation-dissipation theorem the spontaneous fluctuations
of charge on the conductor. Of particular interest are the charge fluctuations
in the presence of transport in a regime where the conductor exhibits shot
noise. At low frequencies and voltages charge relaxation is determined by a
nonequilibrium charge relaxation resistance

### Hidden quantum pump effects in quantum coherent rings

Time periodic perturbations of an electron system on a ring are examined. For
small frequencies periodic small amplitude perturbations give rise to side band
currents which in leading order are inversely proportional to the frequency.
These side band currents compensate the current of the central band such that
to leading order no net pumped current is generated. In the non-adiabatic
limit, larger pump frequencies can lead to resonant excitations: as a
consequence a net pumped current arises. We illustrate our results for a one
channel ring with a quantum dot whose barriers are modulated parametrically.Comment: 8 pages, 5 figure

### Quantum capacitance: a microscopic derivation

We start from microscopic approach to many body physics and show the
analytical steps and approximations required to arrive at the concept of
quantum capacitance. These approximations are valid only in the semi-classical
limit and the quantum capacitance in that case is determined by Lindhard
function. The effective capacitance is the geometrical capacitance and the
quantum capacitance in series, and this too is established starting from a
microscopic theory.Comment: 7 fig

### Gauge invariant nonlinear electric transport in mesoscopic conductors

We use the scattering approach to investigate the nonlinear current-voltage
characteristic of mesoscopic conductors. We discuss the leading nonlinearity by
taking into account the self-consistent nonequilibrium potential. We emphasize
conservation of the overall charge and current which are connected to the
invariance under a global voltage shift (gauge invariance). As examples, we
discuss the rectification coefficient of a quantum point contact and the
nonlinear current-voltage characteristic of a resonant level in a double
barrier structure.Comment: (Replaced version, with corrected Eq.(4)); 5 pages, RevTeX, 1 figure,
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### Chaotic dot-superconductor analog of the Hanbury Brown Twiss effect

As an electrical analog of the optical Hanbury Brown Twiss effect, we study
current cross-correlations in a chaotic quantum dot-superconductor junction.
One superconducting and two normal reservoirs are connected via point contacts
to a chaotic quantum dot. For a wide range of contact widths and
transparencies, we find large positive current correlations. The positive
correlations are generally enhanced by normal backscattering in the contacts.
Moreover, for normal backscattering in the contacts, the positive correlations
survive when suppressing the proximity effect in the dot with a weak magnetic
field.Comment: 4 pages, 3 figure

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