673 research outputs found
Quantum Graphs via Exercises
Studying the spectral theory of Schroedinger operator on metric graphs (also
known as quantum graphs) is advantageous on its own as well as to demonstrate
key concepts of general spectral theory. There are some excellent references
for this study such as a mathematically oriented book by Berkolaiko and
Kuchment, a review with applications to theoretical physicsby Gnutzmann and
Smilansky, and elementary lecture notes by Berkolaiko. Here, we provide a set
of questions and exercises which can accompany the reading of these references
or an elementary course on quantum graphs. The exercises are taken from courses
on quantum graphs which were taught by the authors
Conductance and absolutely continuous spectrum of 1D samples
We characterize the absolutely continuous spectrum of the one-dimensional
Schr\"odinger operators acting on in terms
of the limiting behavior of the Landauer-B\"uttiker and Thouless conductances
of the associated finite samples. The finite sample is defined by restricting
to a finite interval and the conductance refers to
the charge current across the sample in the open quantum system obtained by
attaching independent electronic reservoirs to the sample ends. Our main result
is that the conductances associated to an energy interval are non-vanishing
in the limit iff . We also
discuss the relationship between this result and the Schr\"odinger Conjecture
The Berry-Keating operator on L^2(\rz_>, x) and on compact quantum graphs with general self-adjoint realizations
The Berry-Keating operator H_{\mathrm{BK}}:=
-\ui\hbar(x\frac{
\phantom{x}}{
x}+{1/2}) [M. V. Berry and J. P. Keating,
SIAM Rev. 41 (1999) 236] governing the Schr\"odinger dynamics is discussed in
the Hilbert space L^2(\rz_>,
x) and on compact quantum graphs. It is
proved that the spectrum of defined on L^2(\rz_>,
x) is
purely continuous and thus this quantization of cannot yield
the hypothetical Hilbert-Polya operator possessing as eigenvalues the
nontrivial zeros of the Riemann zeta function. A complete classification of all
self-adjoint extensions of acting on compact quantum graphs
is given together with the corresponding secular equation in form of a
determinant whose zeros determine the discrete spectrum of .
In addition, an exact trace formula and the Weyl asymptotics of the eigenvalue
counting function are derived. Furthermore, we introduce the "squared"
Berry-Keating operator which is a special case of the
Black-Scholes operator used in financial theory of option pricing. Again, all
self-adjoint extensions, the corresponding secular equation, the trace formula
and the Weyl asymptotics are derived for on compact quantum
graphs. While the spectra of both and on
any compact quantum graph are discrete, their Weyl asymptotics demonstrate that
neither nor can yield as eigenvalues the
nontrivial Riemann zeros. Some simple examples are worked out in detail.Comment: 33p
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