27 research outputs found
Thermopower of a superconducting single-electron transistor
We present a linear-response theory for the thermopower of a single-electron
transistor consisting of a superconducting island weakly coupled to two
normal-conducting leads (NSN SET). The thermopower shows oscillations with the
same periodicity as the conductance and is rather sensitive to the size of the
superconducting gap. In particular, the previously studied sawtooth-like shape
of the thermopower for a normal-conducting single-electron device is
qualitatively changed even for small gap energies.Comment: 9 pages, 3 figure
Loss Analysis for Laser Separated Solar Cells
AbstractHalf-cell modules are promising candidates for new innovative module designs as they offer major advantages. Modified connection schemes reduce the serial resistance losses yielding a higher overall module performance. The reduced size of the cells allows a more flexible module design that is needed for special applications such as implementations on curved surface. Furthermore, a better performance under partial shading can be achieved. However, these advantages lead to a benefit only if the losses induced by the cell separation process are negligible. In this work, we study the different sources of power reduction, i.e. increased shunting and recombination, for mono-crystalline and multi-crystalline silicon solar cells separated using different laser process parameters. It is shown that recombination plays the major role for an optimized laser separation process. Additionally we identify the laser scribing process as the major source of losses in comparison to the mechanical breaking
Interplay of bulk and surface properties for steady-state measurements of minority carrier lifetimes
The measurement of the minority carrier lifetime is a powerful tool in the
field of semiconductor material characterization as it is very sensitive to
electrically active defects. Furthermore, it is applicable to a wide range of
samples such as ingots or wafers. In this work, a systematic theoretical
analysis of the steady-state approach is presented. It is shown how the
measured lifetime relates to the intrinsic bulk lifetime for a given material
quality, sample thickness, and surface passivation. This makes the bulk
properties experimentally accessible by separating them from the surface
effects. In particular, closed analytical solutions of the most important
cases, such as passivated and unpassivated wafers and blocks are given. Based
on these results, a criterion for a critical sample thickness is given beyond
which a lifetime measurement allows deducing the bulk properties for a given
surface recombination. These results are of particular interest for
semiconductor material diagnostics especially for photovoltaic applications but
not limited to this field.Comment: 17 pages, 3 figure
Semiclassical form factor for spectral and matrix element fluctuations of multi-dimensional chaotic systems
We present a semiclassical calculation of the generalized form factor which
characterizes the fluctuations of matrix elements of the quantum operators in
the eigenbasis of the Hamiltonian of a chaotic system. Our approach is based on
some recently developed techniques for the spectral form factor of systems with
hyperbolic and ergodic underlying classical dynamics and f=2 degrees of
freedom, that allow us to go beyond the diagonal approximation. First we extend
these techniques to systems with f>2. Then we use these results to calculate
the generalized form factor. We show that the dependence on the rescaled time
in units of the Heisenberg time is universal for both the spectral and the
generalized form factor. Furthermore, we derive a relation between the
generalized form factor and the classical time-correlation function of the Weyl
symbols of the quantum operators.Comment: some typos corrected and few minor changes made; final version in PR
Leading off-diagonal contribution to the spectral form factor of chaotic quantum systems
We semiclassically derive the leading off-diagonal correction to the spectral
form factor of quantum systems with a chaotic classical counterpart. To this
end we present a phase space generalization of a recent approach for uniformly
hyperbolic systems (M. Sieber and K. Richter, Phys. Scr. T90, 128 (2001); M.
Sieber, J. Phys. A: Math. Gen. 35, L613 (2002)). Our results coincide with
corresponding random matrix predictions. Furthermore, we study the transition
from the Gaussian orthogonal to the Gaussian unitary ensemble.Comment: 8 pages, 2 figures; J. Phys. A: Math. Gen. (accepted for publication
Semiclassics beyond the diagonal approximation
The statistical properties of the energy spectrum of classically chaotic closed quantum systems are the central subject of this thesis. It has been conjectured by O.Bohigas, M.-J.Giannoni and C.Schmit that the spectral statistics of chaotic systems is universal and can be described by random-matrix theory.
This conjecture has been confirmed in many experiments and numerical studies but a formal proof is still lacking. In this thesis we present a semiclassical evaluation of the spectral form factor which goes beyond M.V.Berry's diagonal approximation. To this end we extend a method developed by M.Sieber and K.Richter for a specific system: the motion of a particle on a two-dimensional
surface of constant negative curvature. In particular we prove that these semiclassical methods reproduce the random-matrix theory predictions for the next to leading order correction also for a much wider class of systems, namely non-uniformly hyperbolic systems with f>2 degrees of freedom. We achieve
this result by extending the configuration-space approach of M.Sieber and K.Richter to a canonically invariant phase-space approach
All-Electrical Measurement of the Density of States in (Ga,Mn)As
We report on electrical measurements of the effective density of states in the ferromagnetic semiconductor material (Ga,Mn)As. By analyzing the conductivity correction to an enhanced electron-electron interaction the electrical diffusion constant was extracted for (Ga,Mn)As samples of different dimensionality. Using the Einstein relation allows us to deduce the effective density of states of (Ga,Mn)As at the Fermi energy
Localization of a pair of bound particles in a random potential
We study the localization length {\it l} of a pair of two attractively bound particles moving in a one-dimensional random potential. We show in which way it depends on the interaction potential between the constituents of this composite particle. For a pair with many bound states {\it N} the localization length is proportional to {\it N}, independently of the form of the two particle interaction. For the case of two bound states, we present an exact solution for the corresponding Fokker–Planck equation and demonstrate that {\it l} depends sensitively on the shape of the interaction potential and the symmetry of the bound state wave functions