31 research outputs found

### Decoherence and Full Counting Statistics in a Mach-Zehnder Interferometer

We investigate the Full Counting Statistics of an electrical Mach-Zehnder
interferometer penetrated by an Aharonov-Bohm flux, and in the presence of a
classical fluctuating potential. Of interest is the suppression of the
Aharonov-Bohm oscillations in the distribution function of the transmitted
charge. For a Gaussian fluctuating field we calculate the first three
cumulants. The fluctuating potential causes a modulation of the conductance
leading in the third cumulant to a term cubic in voltage and to a contribution
correlating modulation of current and noise. In the high voltage regime we
present an approximation of the generating function.Comment: 10 pages, 6 figure

### Quantum point contacts as heat engines

The efficiency of macroscopic heat engines is restricted by the second law of
thermodynamics. They can reach at most the efficiency of a Carnot engine. In
contrast, heat currents in mesoscopic heat engines show fluctuations. Thus,
there is a small probability that a mesoscopic heat engine exceeds Carnot's
maximum value during a short measurement time. We illustrate this effect using
a quantum point contact as a heat engine. When a temperature difference is
applied to a quantum point contact, the system may be utilized as a source of
electrical power under steady state conditions. We first discuss the optimal
working point of such a heat engine that maximizes the generated electrical
power and subsequently calculate the statistics for deviations of the
efficiency from its most likely value. We find that deviations surpassing the
Carnot limit are possible, but unlikely.Comment: 9 pages, 2 figures. Contribution to the Physica E special issue on
"Frontiers in quantum electronic transport" in memory of Markus Buttiker.
Published versio

### Decoherence in ballistic mesoscopic interferometers

We provide a theoretical explanation for two recent experiments on
decoherence of Aharonov-Bohm oscillations in two- and multi-terminal ballistic
rings. We consider decoherence due to charge fluctuations and emphasize the
role of charge exchange between the system and the reservoir or nearby gates. A
time-dependent scattering matrix approach is shown to be a convenient tool for
the discussion of decoherence in ballistic conductors.Comment: 11 pages, 3 figures. To appear in a special issue on "Quantum
Computation at the Atomic Scale" in the Turkish Journal of Physic

### Stochastic Field Theory for Transport Statistics in Diffusive Systems

We present a field theory for the statistics of charge and current
fluctuations in diffusive systems. The cumulant generating function is given by
the saddle-point solution for the action of this field theory. The action
depends on two parameters only: the local diffusion and noise coefficients,
which naturally leads to the universality of the transport statistics for a
wide class of multi-dimensional diffusive models. Our theory can be applied to
semi-classical mesoscopic systems, as well as beyond mesoscopic physics.Comment: Submitted to the proceedings of the XXXIXth Rencontres de Moriond (La
Thuile, 2004) "Quantum information and decoherence in nanosystems

### Fluctuation Statistics in Networks: a Stochastic Path Integral Approach

We investigate the statistics of fluctuations in a classical stochastic
network of nodes joined by connectors. The nodes carry generalized charge that
may be randomly transferred from one node to another. Our goal is to find the
time evolution of the probability distribution of charges in the network. The
building blocks of our theoretical approach are (1) known probability
distributions for the connector currents, (2) physical constraints such as
local charge conservation, and (3) a time-scale separation between the slow
charge dynamics of the nodes and the fast current fluctuations of the
connectors. We derive a stochastic path integral representation of the
evolution operator for the slow charges. Once the probability distributions on
the discrete network have been studied, the continuum limit is taken to obtain
a statistical field theory. We find a correspondence between the diffusive
field theory and a Langevin equation with Gaussian noise sources, leading
nevertheless to non-trivial fluctuation statistics. To complete our theory, we
demonstrate that the cascade diagrammatics, recently introduced by Nagaev,
naturally follows from the stochastic path integral. We extend the
diagrammatics to calculate current correlation functions for an arbitrary
network. One primary application of this formalism is that of full counting
statistics (FCS). We stress however, that the formalism is suitable for general
classical stochastic problems as an alternative to the traditional master
equation or Doi-Peliti technique. The formalism is illustrated with several
examples: both instantaneous and time averaged charge fluctuation statistics in
a mesoscopic chaotic cavity, as well as the FCS and new results for a
generalized diffusive wire.Comment: Final version accepted in J. Math. Phys. Discussion of conservation
laws, Refs., 1 Fig., and minor extensions added. 23 pages, 9 figs.,
double-column forma

### Aspects of metallic low-temperature transport in Mott-insulator/ band-insulator superlattices: optical conductivity and thermoelectricity

We investigate the low-temperature electrical and thermal transport
properties in atomically precise metallic heterostructures involving
strongly-correlated electron systems. The model of the Mott-insulator/
band-insulator superlattice was discussed in the framework of the slave-boson
mean-field approximation and transport quantities were derived by use of the
Boltzmann transport equation in the relaxation-time approximation. The results
for the optical conductivity are in good agreement with recently published
experimental data on (LaTiO$_3)_N$/(SrTiO$_3)_M$ superlattices and allow us to
estimate the values of key parameters of the model. Furthermore, predictions
for the thermoelectric response were made and the dependence of the Seebeck
coefficient on model parameters was studied in detail. The width of the
Mott-insulating material was identified as the most relevant parameter, in
particular, this parameter provides a way to optimize the thermoelectric power
factor at low temperatures

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### Statistics of Charge Fluctuations in Chaotic Cavities

We consider the zero frequency fluctuations of charge inside a mesoscopic conductor in the large capacitance limit. In analogy to current counting statistics we derive the characteristic function of charge fluctuations in terms of the scattering matrix of the conductor. Using random matrix theory we evaluate the characteristic function semi-analytically for chaotic cavities. Our result is universal in the sense that it describes not only the fluctuations of charge, but of any observable quantity inside the cavity. We discuss equilibrium and non-equilibrium fluctuations and extend our theory to the case of contacts with arbitrary transparency. Finally we investigate the suppression of fluctuations in the small capacitance limit due to charge screening

### Voltage and dephasing probes in mesoscopic conductors : a study of full-counting statistics

Voltage and dephasing probes introduce incoherent inelastic and incoherent quasielastic scattering into a coherent mesoscopic conductor. We discuss in detail the concepts of voltage and dephasing probes and develop a full-counting statistics approach to investigate their effect on the transport statistics. The formalism is applied to several experimentally relevant examples. A comparison of different probe models and with procedures like phase averaging over an appropriate phase distribution shows that there is a perfect equivalence between the models for the case of one single-channel probe. Interestingly, the appropriate phase-distribution function is found to be uniform. A uniform distribution is provided by a chaotic cavity with a long dwell time. The dwell time of a chaotic cavity plays a role similar to the charge response time of a voltage or dephasing probe. For multichannel or multiple probes the transport statistics of voltage and dephasing probes differs and the equivalence with phase averaging is similarly lost