1,092 research outputs found
Approximations of singular vertex couplings in quantum graphs
We discuss approximations of the vertex coupling on a star-shaped quantum
graph of edges in the singular case when the wave functions are not
continuous at the vertex and no edge-permutation symmetry is present. It is
shown that the Cheon-Shigehara technique using interactions with
nonlinearly scaled couplings yields a -parameter family of boundary
conditions in the sense of norm resolvent topology. Moreover, using graphs with
additional edges one can approximate the -parameter family of
all time-reversal invariant couplings.Comment: LaTeX source file, 33 pages, with 3 eps figure
Investigation and evaluation of the aging behaviour of technical materials as a selection criterion for use in zinc-air flow batteries
Zinc-air secondary batteries have the potential to act as electrochemical energy storage devices in broad industrial applications. The main arguments for developing marketable systems are the good commercial availability and environmental compatibility of zinc [1]. A consortium of different companies and scientific institutions is engaged in the
development of a scalable zinc-air secondary battery. For the establishment of the system, the concept and all components, such as the gas diffusion electrode as well as the zinc electrode, are being investigated and optimised. In order to achieve a certain marketability of the battery after the end of the project, the plastic-based housing, sealing and current-conducting components are also being examined for their long-term stability and suitability. The system concept has high demands on the chemical resistance of the components due to the alkaline electrolyte in use. The plastics in question are typical housing materials with good chemical resistance, soft sealing materials based on thermoplastic elastomers and compounds highly filled with graphite for current conduction within the battery. To evaluate the materials, comparative studies are carried out with regard to the material properties, such as mechanical stability and electrical conductivity, and the combustion behaviour to assess the aging between newly produced and aged parts. In particular, the compounds highly filled with graphite presumably exhibit side reactions in contact with the active materials used in the system due to unavoidable impurities. This behaviour is also integrated in the evaluation of the raw material selection
Doping driven magnetic instabilities and quantum criticality of NbFe
Using density functional theory we investigate the evolution of the magnetic
ground state of NbFe due to doping by Nb-excess and Fe-excess. We find
that non-rigid-band effects, due to the contribution of Fe-\textit{d} states to
the density of states at the Fermi level are crucial to the evolution of the
magnetic phase diagram. Furthermore, the influence of disorder is important to
the development of ferromagnetism upon Nb doping. These findings give a
framework in which to understand the evolution of the magnetic ground state in
the temperature-doping phase diagram. We investigate the magnetic instabilities
in NbFe. We find that explicit calculation of the Lindhard function,
, indicates that the primary instability is to finite
antiferromagnetism driven by Fermi surface nesting. Total energy
calculations indicate that antiferromagnetism is the ground
state. We discuss the influence of competing and finite
instabilities on the presence of the non-Fermi liquid behavior in
this material.Comment: 8 pages, 7 figure
First-principles calculations of magnetization relaxation in pure Fe, Co, and Ni with frozen thermal lattice disorder
The effect of the electron-phonon interaction on magnetization relaxation is
studied within the framework of first-principles scattering theory for Fe, Co,
and Ni by displacing atoms in the scattering region randomly with a thermal
distribution. This "frozen thermal lattice disorder" approach reproduces the
non-monotonic damping behaviour observed in ferromagnetic resonance
measurements and yields reasonable quantitative agreement between calculated
and experimental values. It can be readily applied to alloys and easily
extended by determining the atomic displacements from ab initio phonon spectra
Writing and Reading antiferromagnetic MnAu: N\'eel spin-orbit torques and large anisotropic magnetoresistance
Antiferromagnets are magnetically ordered materials which exhibit no net
moment and thus are insensitive to magnetic fields. Antiferromagnetic
spintronics aims to take advantage of this insensitivity for enhanced
stability, while at the same time active manipulation up to the natural THz
dynamic speeds of antiferromagnets is possible, thus combining exceptional
storage density and ultra-fast switching. However, the active manipulation and
read-out of the N\'eel vector (staggered moment) orientation is challenging.
Recent predictions have opened up a path based on a new spin-orbit torque,
which couples directly to the N\'eel order parameter. This N\'eel spin-orbit
torque was first experimentally demonstrated in a pioneering work using
semimetallic CuMnAs. Here we demonstrate for MnAu, a good conductor with a
high ordering temperature suitable for applications, reliable and reproducible
switching using current pulses and readout by magnetoresistance measurements.
The symmetry of the torques agrees with theoretical predictions and a large
read-out magnetoresistance effect of more than ~ is reproduced by
ab initio transport calculations.Comment: 5 pages, 4 figure
Thermopower of Kondo Effect in Single Quantum Dot Systems with Orbital at Finite Temperatures
We investigate the thermopower due to the orbital Kondo effect in a single
quantum dot system by means of the noncrossing approximation. It is elucidated
how the asymmetry of tunneling resonance due to the orbital Kondo effect
affects the thermopower under gate-voltage and magnetic-field control.Comment: 4 pages, 4 figures, proceeding of Second International Symposium on
Nanometer-Scale Quantum Physic
On the spectrum of a bent chain graph
We study Schr\"odinger operators on an infinite quantum graph of a chain form
which consists of identical rings connected at the touching points by
-couplings with a parameter . If the graph is "straight",
i.e. periodic with respect to ring shifts, its Hamiltonian has a band spectrum
with all the gaps open whenever . We consider a "bending"
deformation of the chain consisting of changing one position at a single ring
and show that it gives rise to eigenvalues in the open spectral gaps. We
analyze dependence of these eigenvalues on the coupling and the
"bending angle" as well as resonances of the system coming from the bending. We
also discuss the behaviour of the eigenvalues and resonances at the edges of
the spectral bands.Comment: LaTeX, 23 pages with 7 figures; minor changes, references added; to
appear in J. Phys. A: Math. Theo
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
Induced Magnetic Ordering by Proton Irradiation in Graphite
We provide evidence that proton irradiation of energy 2.25 MeV on
highly-oriented pyrolytic graphite samples triggers ferro- or ferrimagnetism.
Measurements performed with a superconducting quantum interferometer device
(SQUID) and magnetic force microscopy (MFM) reveal that the magnetic ordering
is stable at room temperature.Comment: 3 Figure
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