8 research outputs found
Effect of annealing on the hyperfine interaction in InAs/GaAs quantum dots
The hyperfine interaction of an electron with nuclei in the annealed
self-assembled InAs/GaAs quantum dots is theoretically analyzed. For this
purpose, the annealing process, and energy structure of the quantum dots are
numerically modeled. The modeling is verified by comparison of the calculated
optical transitions and of the experimental data on photoluminescence for set
of the annealed quantum dots. The localization volume of the electron in the
ground state and the partial contributions of In, Ga, and As nuclei to the
hyperfine interaction are calculated as functions of the annealing temperature.
It is established that the contribution of indium nuclei into the hyperfine
interaction becomes predominant up to high annealing temperatures (T = 980 C)
when the In content in the quantum dots does not exceed 25%. Effect of the
nuclear spin fluctuations on the electron spin polarization is numerically
modeled. Effective field of the fluctuations is found to be in good agreement
with experimental data available
Degradation modeling of high temperature proton exchange membrane fuel cells using dual time scale simulation
\u3cp\u3eHT-PEM fuel cells suffer from performance losses due to degradation effects. Therefore, the durability of HT-PEM is currently an important factor of research and development. In this paper a novel approach is presented for an integrated short term and long term simulation of HT-PEM accelerated lifetime testing. The physical phenomena of short term and long term effects are commonly modeled separately due to the different time scales. However, in accelerated lifetime testing, long term degradation effects have a crucial impact on the short term dynamics. Our approach addresses this problem by applying a novel method for dual time scale simulation. A transient system simulation is performed for an open voltage cycle test on a HT-PEM fuel cell for a physical time of 35 days. The analysis describes the system dynamics by numerical electrochemical impedance spectroscopy. Furthermore, a performance assessment is performed in order to demonstrate the efficiency of the approach. The presented approach reduces the simulation time by approximately 73% compared to conventional simulation approach without losing too much accuracy. The approach promises a comprehensive perspective considering short term dynamic behavior and long term degradation effects.\u3c/p\u3
Degradation modeling of high temperature proton exchange membrane fuel cells using dual time scale simulation
Measurement of the Knight field and local nuclear dipole-dipole field in an InGaAs/GaAs quantum dot ensemble
We present a comprehensive investigation of the electron-nuclear system of
negatively charged InGaAs/GaAs self-assembled quantum dots under the influence
of weak external magnetic fields (up to 2 mT). We demonstrate that, in contrast
to conventional semiconductor systems, these small fields have a profound
influence on the electron spin dynamics, via the hyperfine interaction. Quantum
dots, with their comparatively limited number of nuclei, present
electron-nuclear behavior that is unique to low-dimensional systems. We show
that the conventional Hanle effect used to measure electron spin relaxation
times, for example, cannot be used in these systems when the spin lifetimes are
long. An individual nucleus in the QD is subject to milli-Tesla effective
fields, arising from the interaction with its nearest-neighbors and with the
electronic Knight field. The alignment of each nucleus is influenced by
application of external fields of the same magnitude. A polarized nuclear
system, which may have an effective field strength of several Tesla, may easily
be influenced by these milli-Tesla fields. This in turn has a dramatic effect
on the electron spin dynamics, and we use this technique to gain a measure of
both the dipole-dipole field and the maximum Knight field in our system, thus
allowing us to estimate the maximum Overhauser field that may be generated at
zero external magnetic field. We also show that one may fine-tune the angle
which the Overhauser field makes with the optical axis.Comment: 13 pages, 7 figure