5,309 research outputs found
Double Exchange model for nanoscopic clusters
We solve the double exchange model on nanoscopic clusters exactly, and
specifically consider a six-site benzene-like nanocluster. This simple model is
an ideal testbed for studying magnetism in nanoclusters and for validating
approximations such as the dynamical mean field theory (DMFT). Non-local
correlations arise between neighboring localized spins due to the Hund's rule
coupling, favoring a short-range magnetic order of ferro- or antiferromagnetic
type. For a geometry with more neighboring sites or a sufficiently strong
hybridization between leads and the nanocluster, these non-local correlations
are less relevant, and DMFT can be applied reliably.Comment: 9 pages, 9 figures, 1 tabl
Whispering gallery modes in open quantum billiards
The poles of the S-matrix and the wave functions of open 2D quantum billiards
with convex boundary of different shape are calculated by the method of complex
scaling. Two leads are attached to the cavities. The conductance of the
cavities is calculated at energies with one, two and three open channels in
each lead. Bands of overlapping resonance states appear which are localized
along the convex boundary of the cavities and contribute coherently to the
conductance. These bands correspond to the whispering gallery modes appearing
in the classical calculations.Comment: 9 pages, 3 figures in jpg and gif forma
Influence of branch points in the complex plane on the transmission through double quantum dots
We consider single-channel transmission through a double quantum dot system
consisting of two single dots that are connected by a wire and coupled each to
one lead. The system is described in the framework of the S-matrix theory by
using the effective Hamiltonian of the open quantum system. It consists of the
Hamiltonian of the closed system (without attached leads) and a term that
accounts for the coupling of the states via the continuum of propagating modes
in the leads. This model allows to study the physical meaning of branch points
in the complex plane. They are points of coalesced eigenvalues and separate the
two scenarios with avoided level crossings and without any crossings in the
complex plane. They influence strongly the features of transmission through
double quantum dots.Comment: 30 pages, 14 figure
Decreasing excitation gap in Andreev billiards by disorder scattering
We investigate the distribution of the lowest-lying energy states in a
disordered Andreev billiard by solving the Bogoliubov-de Gennes equation
numerically. Contrary to conventional predictions we find a decrease rather
than an increase of the excitation gap relative to its clean ballistic limit.
We relate this finding to the eigenvalue spectrum of the Wigner-Smith time
delay matrix between successive Andreev reflections. We show that the longest
rather than the mean time delay determines the size of the excitation gap. With
increasing disorder strength the values of the longest delay times increase,
thereby, in turn, reducing the excitation gap.Comment: 6 pages, 5 figures, submitted to EP
Constant-pressure sound waves in non-Hermitian disordered media
When waves impinge on a disordered material they are back-scattered and form
a highly complex interference pattern. Suppressing any such distortions in the
free propagation of a wave is a challenging task with many applications in a
number of different disciplines. In a recent theoretical proposal, it was
pointed out that both perfect transmission through disorder as well as a
complete suppression of any variation in a wave intensity can be achieved by
adding a continuous gain-loss distribution to the disorder. Here we show that
this abstract concept can be implemented in a realistic acoustic system. Our
prototype consists of an acoustic waveguide containing several inclusions that
scatter the incoming wave in a passive configuration and provide the gain or
loss when being actively controlled. Our measurements on this non-Hermitian
acoustic metamaterial demonstrate unambiguously the creation of a
reflectionless scattering wave state that features a unique form of discrete
constant-amplitude pressure waves. In addition to demonstrating that gain-loss
additions can turn localised systems into transparent ones, we expect our
proof-of-principle demonstration to trigger interesting new developments not
only in sound engineering, but also in other related fields such as in
non-Hermitian photonics
New PbSnTe heterojunction laser diode structures with improved performance
Several recent advances in the state-of-the-art of lead tin telluride double heterojunction laser diodes are summarized. Continuous Wave operation to 120 K and pulsed operation to 166 K with single, lowest order transverse mode emission to in excess of four times threshold at 80 K were achieved in buried stripe lasers fabricated by liquid phase epitaxy in the lattice-matched system, lead-tin telluride-lead telluride selenide. At the same time, liquid phase epitaxy was used to produce PbSnTe distributed feedback lasers with much broader continuous single mode tuning ranges than are available from Fabry-Perot lasers. The physics and philosophy behind these advances is as important as the structures and performance of the specific devices embodying the advances, particularly since structures are continually being evolved and the performance continues to be improved
Commensurate Itinerant Antiferromagnetism in BaFe2As2: 75As-NMR Studies on a Self-Flux Grown Single Crystal
We report results of 75As nuclear magnetic resonance (NMR) experiments on a
self-flux grown single crystal of BaFe2As2. A first-order antiferromagnetic
(AF) transition near 135 K was detected by the splitting of NMR lines, which is
accompanied by simultaneous structural transition as evidenced by a sudden
large change of the electric field gradient tensor at the As site. The NMR
results lead almost uniquely to the stripe spin structure in the AF phase. The
data of spin-lattice relaxation rate indicate development of anisotropic spin
fluctuations of the stripe-type with decreasing temperature in the paramagnetic
phase.Comment: 7 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp
Breaking of PT-symmetry in bounded and unbounded scattering systems
PT-symmetric scattering systems with balanced gain and loss can undergo a
symmetry-breaking transition in which the eigenvalues of the non-unitary
scattering matrix change their phase shifts from real to complex values. We
relate the PT-symmetry breaking points of such an unbounded scattering system
to those of underlying bounded systems. In particular, we show how the
PT-thresholds in the scattering matrix of the unbounded system translate into
analogous transitions in the Robin boundary conditions of the corresponding
bounded systems. Based on this relation, we argue and then confirm that the
PT-transitions in the scattering matrix are, under very general conditions,
entirely insensitive to a variable coupling strength between the bounded region
and the unbounded asymptotic region, a result that can be tested experimentally
and visualized using the concept of Smith charts.Comment: 9 pages, 6 figures (final version, including newly added connection
to the concept of "Smith charts"
The brachistochrone problem in open quantum systems
Recently, the quantum brachistochrone problem is discussed in the literature
by using non-Hermitian Hamilton operators of different type. Here, it is
demonstrated that the passage time is tunable in realistic open quantum systems
due to the biorthogonality of the eigenfunctions of the non-Hermitian Hamilton
operator. As an example, the numerical results obtained by Bulgakov et al. for
the transmission through microwave cavities of different shape are analyzed
from the point of view of the brachistochrone problem. The passage time is
shortened in the crossover from the weak-coupling to the strong-coupling regime
where the resonance states overlap and many branch points (exceptional points)
in the complex plane exist. The effect can {\it not} be described in the
framework of standard quantum mechanics with Hermitian Hamilton operator and
consideration of matrix poles.Comment: 18 page
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