5,573 research outputs found
Photon heat transport in low-dimensional nanostructures
At low temperatures when the phonon modes are effectively frozen, photon
transport is the dominating mechanism of thermal relaxation in metallic
systems. Starting from a microscopic many-body Hamiltonian, we develop a
nonequilibrium Green's function method to study energy transport by photons in
nanostructures. A formally exact expression for the energy current between a
metallic island and a one-dimensional electromagnetic field is obtained. From
this expression we derive the quantized thermal conductance as well as show how
the results can be generalized to nonequilibrium situations. Generally, the
frequency-dependent current noise of the island electrons determines the energy
transfer rate.Comment: 4 pages, 3 Fig
Mobilities and Scattering Times in Decoupled Graphene Monolayers
Folded single layer graphene forms a system of two decoupled monolayers being
only a few Angstroms apart. Using magnetotransport measurements we investigate
the electronic properties of the two layers conducting in parallel. We show a
method to obtain the mobilities for the individual layers despite them being
jointly contacted. The mobilities in the upper layer are significantly larger
than in the bottom one indicating weaker substrate influence. This is confirmed
by larger transport and quantum scattering times in the top layer. Analyzing
the temperature dependence of the Shubnikov-de Haas oscillations effective
masses and corresponding Fermi velocities are obtained yielding reduced values
down to 66 percent in comparison to monolayers.Comment: 4 pages, 5 figure
Tunable graphene system with two decoupled monolayers
The use of two truly two-dimensional gapless semiconductors, monolayer and bilayer graphene, as current-carrying components in field-effect transistors (FET) gives access to new types of nanoelectronic devices. Here, we report on the development of graphene-based FETs containing two decoupled graphene monolayers manufactured from a single one folded during the exfoliation process. The transport characteristics of these newly-developed devices differ markedly from those manufactured from a single-crystal bilayer. By analyzing Shubnikov-de Haas oscillations, we demonstrate the possibility to independently control the carrier densities in both layers using top and bottom gates, despite there being only a nanometer scale separation between them
The Effect of a Non-Thermal Tail on the Sunyaev-Zeldovich Effect in clusters of galaxies
We study the spectral distortions of the cosmic microwave background
radiation induced by the Sunyaev-Zel'dovich (SZ) effect in clusters of galaxies
when the target electrons have a modified Maxwell-Boltzmann distribution with a
high-energy non-thermal tail. Bremsstrahlung radiation from this type of \\
electron distribution may explain the supra-thermal X-ray emission observed in
some clusters such as the Coma cluster and A2199 and serve as an alternative to
the classical but problematic inverse Compton scattering interpretation. We
show that the SZ effect can be used as a powerful tool to probe the electron
distribution in clusters of galaxies and discriminate among these different
interpretations of the X-ray excess. The existence of a non-thermal tail can
have important consequences for cluster based estimators of cosmological
parameters.Comment: 14 pages, 3 figures, version to be published in ApJ. Let
Polarons in semiconductor quantum-dots and their role in the quantum kinetics of carrier relaxation
While time-dependent perturbation theory shows inefficient carrier-phonon
scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic
description of carrier-phonon interaction predicts fast carrier capture and
relaxation. The considered processes do not fulfill energy conservation in
terms of free-carrier energies because polar coupling of localized quantum-dot
states strongly modifies this picture.Comment: 6 pages, 6 figures, accepted for publication in Phys.Rev.
Hartree-Fock theory of a current-carrying electron gas
State-of-the-art simulation tools for nonequilibrium quantum transport systems typically take the current-carrier occupations to be described in terms of equilibrium distribution functions characterized by two different electrochemical potentials, while for the description of electronic exchange and correlation, the local density approximation (LDA) to density functional theory is generally used. However, this involves an inconsistency because the LDA is based on the homogeneous electron gas in equilibrium, while the system is not in equilibrium and may be far from it. In this paper, we analyze this inconsistency by studying the interplay between nonequilibrium occupancies obtained from a maximum entropy approach and the Hartree-Fock exchange energy, single-particle spectrum and exchange hole, for the case of a two-dimensional homogeneous electron gas. The current dependence of the local exchange potential is also discussed. It is found that the single-particle spectrum and exchange hole have a significant dependence on the current, which has not been taken into account in practical calculations since it is not captured by the commonly used functionals. The exchange energy and the local exchange potential, however, are shown to change very little with respect to their equilibrium counterparts. The weak dependence of these quantities on the current is explained in terms of the symmetries of the exchange hole
Measurement of the energy dependence of phase relaxation by single electron tunneling
Single electron tunneling through a single impurity level is used to probe
the fluctuations of the local density of states in the emitter. The energy
dependence of quasi-particle relaxation in the emitter can be extracted from
the damping of the fluctuations of the local density of states (LDOS). At
larger magnetic fields Zeeman splitting is observed.Comment: 2 pages, 4 figures; 25th International Conference on the Physics of
Semiconductors, Osaka, Japan, September 17-22, 200
Influence of carrier-carrier and carrier-phonon correlations on optical absorption and gain in quantum-dot systems
A microscopic theory is used to study the optical properties of semiconductor
quantum dots. The dephasing of a coherent excitation and line-shifts of the
interband transitions due to carrier-carrier Coulomb interaction and
carrier-phonon interaction are determined from a quantum kinetic treatment of
correlation processes. We investigate the density dependence of both mechanisms
and clarify the importance of various dephasing channels involving the
localized and delocalized states of the system.Comment: 12 pages, 10 figure
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