453 research outputs found
Localized Joule heating produced by ion current focusing through micron-size holes
We provide an experimental demonstration that the focusing of ionic currents
in a micron size hole connecting two chambers can produce local temperature
increases of up to C with gradients as large as K. We find a good agreement between the measured temperature profiles and
a finite elements-based numerical calculation. We show how the thermal
gradients can be used to measure the full melting profile of DNA duplexes
within a region of 40 m. The possibility to produce even larger gradients
using sub-micron pores is discussed.Comment: 3 pages, accepted to Appl. Phys. Lett
Electron-Phonon Interacation in Quantum Dots: A Solvable Model
The relaxation of electrons in quantum dots via phonon emission is hindered
by the discrete nature of the dot levels (phonon bottleneck). In order to
clarify the issue theoretically we consider a system of discrete fermionic
states (dot levels) coupled to an unlimited number of bosonic modes with the
same energy (dispersionless phonons). In analogy to the Gram-Schmidt
orthogonalization procedure, we perform a unitary transformation into new
bosonic modes. Since only of them couple to the fermions, a
numerically exact treatment is possible. The formalism is applied to a GaAs
quantum dot with only two electronic levels. If close to resonance with the
phonon energy, the electronic transition shows a splitting due to quantum
mechanical level repulsion. This is driven mainly by one bosonic mode, whereas
the other two provide further polaronic renormalizations. The numerically exact
results for the electron spectral function compare favourably with an analytic
solution based on degenerate perturbation theory in the basis of shifted
oscillator states. In contrast, the widely used selfconsistent first-order Born
approximation proves insufficient in describing the rich spectral features.Comment: 8 pages, 4 figure
Localization of Denaturation Bubbles in Random DNA Sequences
We study the thermodynamic and dynamic behaviors of twist-induced
denaturation bubbles in a long, stretched random sequence of DNA. The small
bubbles associated with weak twist are delocalized. Above a threshold torque,
the bubbles of several tens of bases or larger become preferentially localized
to \AT-rich segments. In the localized regime, the bubbles exhibit ``aging''
and move around sub-diffusively with continuously varying dynamic exponents.
These properties are derived using results of large-deviation theory together
with scaling arguments, and are verified by Monte-Carlo simulations.Comment: TeX file with postscript figure
Carrier relaxation in GaAs v-groove quantum wires and the effects of localization
Carrier relaxation processes have been investigated in GaAs/AlGaAs v-groove
quantum wires (QWRs) with a large subband separation (46 meV). Signatures of
inhibited carrier relaxation mechanisms are seen in temperature-dependent
photoluminescence (PL) and photoluminescence-excitation (PLE) measurements; we
observe strong emission from the first excited state of the QWR below ~50 K.
This is attributed to reduced inter-subband relaxation via phonon scattering
between localized states. Theoretical calculations and experimental results
indicate that the pinch-off regions, which provide additional two-dimensional
confinement for the QWR structure, have a blocking effect on relaxation
mechanisms for certain structures within the v-groove. Time-resolved PL
measurements show that efficient carrier relaxation from excited QWR states
into the ground state, occurs only at temperatures > 30 K. Values for the low
temperature radiative lifetimes of the ground- and first excited-state excitons
have been obtained (340 ps and 160 ps respectively), and their corresponding
localization lengths along the wire estimated.Comment: 9 pages, 8 figures, submitted to Phys. Rev. B Attempted to correct
corrupt figure
Strain-induced quantum dots by self-organized stressors
Novel in situ method to produce quantum dots is reported. Three‐dimensional confinement of carriers to a GaInAs/GaAs quantum welldots is observed by photoluminescence. The confinement potential is induced by stressors, formed by self‐organizing growth of InP nanoscale islands on top barrier GaAssurface. Two transitions arising from the strain‐induced quantum dots produced by two types of InP islands are identified. The luminescence from higher electronic states of the quantum dots having a level splitting of 8 meV is also observed.Peer reviewe
Current Switch by Coherent Trapping of Electrons in Quantum Dots
We propose a new transport mechanism through tunnel-coupled quantum dots
based on the coherent population trapping effect. Coupling to an excited level
by the coherent radiation of two microwaves can lead to an extremely narrow
current antiresonance. The effect can be used to determine interdot dephasing
rates and is a mechanism for a very sensitive, optically controlled current
switch.Comment: to appear in Phys. Rev. Let
Evaluation and Optimization of Underground Thermal Energy Storage Systems of Energy Efficient Buildings (WKSP)- A Project within the new German R&D- Framework EnBop
Until 2003 the research on buildings in operation in Germany focused mainly on demonstration buildings. Starting with the EVA project managed by IGS the attention is shifting towards performance in operation. The paper gives a general review of these research projects and presents detailed results of project WKSP. The performance of buildings with systems for underground thermal energy storage is analysed in this project. As the analyses show several systems work worse than expected. Within the project most of the systems could be significantly improved in operation. The scientific work on building performance in operation will be broadened within the new R&D framework EnBop. IGS will coordinate the framework funded by the German Ministry of Economics and Technology
Multiband theory of multi-exciton complexes in self-assembled quantum dots
We report on a multiband microscopic theory of many-exciton complexes in
self-assembled quantum dots. The single particle states are obtained by three
methods: single-band effective-mass approximation, the multiband
method, and the tight-binding method. The electronic structure calculations are
coupled with strain calculations via Bir-Pikus Hamiltonian. The many-body wave
functions of electrons and valence holes are expanded in the basis of
Slater determinants. The Coulomb matrix elements are evaluated using statically
screened interaction for the three different sets of single particle states and
the correlated -exciton states are obtained by the configuration interaction
method. The theory is applied to the excitonic recombination spectrum in
InAs/GaAs self-assembled quantum dots. The results of the single-band
effective-mass approximation are successfully compared with those obtained by
using the of and tight-binding methods.Comment: 10 pages, 8 figure
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