360 research outputs found
Collective excitations in a magnetically doped quantized Hall ferromagnet
A theory of collective states in a magnetically quantized two-dimensional
electron gas (2DEG) with half-filled Landau level (quantized Hall ferromagnet)
in the presence of magnetic 3d impurities is developed. The spectrum of bound
and delocalized spin-excitons as well as the renormalization of Zeeman
splitting of the impurity 3d levels due to the indirect exchange interaction
with the 2DEG are studied for the specific case of n-type GaAs doped with Mn
where the Lande` g-factors of impurity and 2DEG have opposite signs. If the
sign of the 2DEG g-factor is changed due to external influences, then impurity
related transitions to new ground state phases, presenting various spin-flip
and skyrmion-like textures, are possible. Conditions for existence of these
phases are discussed. PACS: 73.43.Lp, 73.21.Fg, 72.15.RnComment: 32 pages including 7 figures. To be published in Phys. Rev.
Investigation on particle formation during laser ablation process with high brilliant radiation
AbstractNanometer sized particles are formed within the vapor plume during the ablation of metal with laser radiation. Thereby, the particle formation rate depends strongly on the used intensity of the laser source. High brilliant laser sources have the ability to generate intensities higher than 1ā
108W/cm2 during cw operation. Due to the widespread use of high brilliant laser sources in research and industrial applications, it is important to investigate the influence of particle formation on the ablation process. Therefore, the presented work is focused on the particle formation during the ablation process of stainless steel with a single-mode fiber laser. Results of experimental work are shown and analyzed. In the experimental work a probe laser beam is directed through the ablation plume and the scattered intensity is analyzed. TEM images of particles show an average particle size of 9 nm at an intensity of 1.92ā
108Wcm2
Sensor Concept for Controlled Laser Cleaning via Photodiode
Abstract. In the field of laser cleaning of artworks the effect of āover-cleaningā is a commonly well known problem. The detection of laser induced plasma is one possibility in order to identify the kind of material just being irradiated by the laser beam. LIBS is a powerful method for the extraction of spectral information. Instead the detection of the plasma intensity contains much less information. However, this can be realised by using a fast photodiode. It has turned out that for several applications in laser cleaning of artworks a reliable identification of layers during the cleaning process is possible. In cooperation with restorers we proved that this low-cost method may be used for online monitoring as well as automated closed loop cleaning.
Activation Energy in a Quantum Hall Ferromagnet and Non-Hartree-Fock Skyrmions
The energy of Skyrmions is calculated with the help of a technique based on
the excitonic representation: the basic set of one-exciton states is used for
the perturbation-theory formalism instead of the basic set of one-particle
states. We use the approach, at which a skyrmion-type excitation (at zero Lande
factor) is considered as a smooth non-uniform rotation in the 3D spin space.
The result within the framework of an excitonically diagonalized part of the
Coulomb Hamiltonian can be obtained by any ratio [where is the typical Coulomb
energy ( being the magnetic length); is the cyclotron
frequency], and the Landau-level mixing is thereby taken into account. In
parallel with this, the result is also found exactly, to second order in terms
of the (if supposing to be small) with use of the
total Hamiltonian. When extrapolated to the region , our
calculations show that the skyrmion gap becomes substantially reduced in
comparison with the Hartree-Fock calculations. This fact brings the theory
essentially closer to the available experimental data.Comment: 14 pages, 1 figure. to appear in Phys. Rev. B, Vol. 65 (Numbers ~
19-22), 200
Der Einfluss von Schaleneffekten auf die kinetische Energie und die Anregungsenergie von Spaltfragmenten
Reduction of heat sink common-mode currents in switching mode power supply circuits
In this paper, a new filter design for a heat sink is presented. The
parasitic couplings between electric power devices and the heat sink are
responsible for common-mode currents. The main focus is on the reduction of
these currents to reduce the heat sink radiation. For this purpose a new
filter design is proposed. In addition, experimental results are shown to
validate the proposed filter
The Cyclotron Spin-Flip Mode as the Lowest-Energy Excitation of Unpolarized Integer Quantum Hall States
The cyclotron spin-flip modes of spin unpolarized integer quantum Hall states
() have been studied with inelastic light scattering. The energy of
these modes is significantly smaller compared to the bare cyclotron gap. Second
order exchange corrections are held responsible for a negative energy
contribution and render these modes the lowest energy excitations of
unpolarized integer quantum Hall states.Comment: Published: Phys. Rev. B 72, 073304 (2005
Influence of an Electrically Non-Conducting Heat Sink for Power Semiconductors on Radiated Interferences
Power semiconductors are used in a growing number of applications.
Furthermore, faster switching transients and therefore higher switching
frequencies can be realised. This leads to higher radiated interferences at
higher frequencies. In power electronic applications, often a cooling concept
for the semiconductors is required to comply with the allowed temperature
range. Typically, heat sinks are made of aluminium and their conductive
behaviour can cause EMC problems. Via capacitive coupling, voltage transients
are transferred from the power semiconductor to the heat sink, so that the
heat sink behaves as an antenna and radiated interferences occur.
Furthermore, a common mode current will appear if the heat sink is grounded.
In this paper, a promising approach to reduce radiated interferences from the
heat sink and solve the common mode issue by using an electrically isolating
ceramic heat sink is presented. The influence of an aluminium nitride ceramic
heat sink on cooling performance and EMC behaviour has been investigated. For
this purpose, two geometrical identically heat sinks made from aluminium and
aluminium nitride have been compared regarding thermal performance and
radiated interferences characterised by S-parameters.</p
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