606 research outputs found
Goldstone Mode Relaxation in a Quantum Hall Ferromagnet due to Hyperfine Interaction with Nuclei
Spin relaxation in quantum Hall ferromagnet regimes is studied. As the
initial non-equilibrium state, a coherent deviation of the spin system from the
direction is considered and the breakdown of this Goldstone-mode
state due to hyperfine coupling to nuclei is analyzed. The relaxation occurring
non-exponentially with time is studied in terms of annihilation processes in
the "Goldstone condensate" formed by "zero spin excitons". The relaxation rate
is calculated analytically even if the initial deviation is not small. This
relaxation channel competes with the relaxation mechanisms due to spin-orbit
coupling, and at strong magnetic fields it becomes dominating.Comment: 8 page
Key signal contributions in photothermal deflection spectroscopy
We report on key signal contributions in photothermal deflection spectroscopy
(PDS) of semiconductors at photon energies below the bandgap energy and show
how to extract the actual absorption properties from the measurement data. To
this end, we establish a rigorous computation scheme for the deflection signal
including semi-analytic raytracing to analyze the underlying physical effects.
The computation takes into account linear and nonlinear absorption processes
affecting the refractive index and thus leading to a deflection of the probe
beam. We find that beside the linear mirage effect, nonlinear absorption
mechanisms make a substantial contribution to the signal for strongly focussed
pump beams and sample materials with high two-photon absorption coefficients.
For example, the measured quadratic absorption contribution exceeds 5% at a
pump beam intensity of about in Si and at
in GaAs. In addition, our method also
includes thermal expansion effects as well as spatial gradients of the
attenuation properties. We demonstrate that these effects result in an
additional deflection contribution which substantially depends on the distance
of the photodetector from the readout point. This distance dependent
contribution enhances the surface related PDS signal up to two orders of
magnitude and may be misinterpreted as surface absorption if not corrected in
the analysis of the measurement data. We verify these findings by PDS
measurements on crystalline silicon at a wavelength of 1550 nm and provide
guidelines how to extract the actual attenuation coefficient from the PDS
signal.Comment: 10 pages, 16 figures, submitted to Journal of Applied Physiv
Long-range morphogen gradient formation by cell-to-cell signal propagation
Morphogen gradients are a central concept in developmental biology. Their formation often involves the secretion of morphogens from a local source, that spread by diffusion in the cell field, where molecules eventually get degraded. This implies limits to both the time and length scales over which morphogen gradients can form which are set by diffusion coefficients and degradationrates. Towards the goal of identifying plausible mechanisms capable of extending the gradient range, we here use theory to explore properties of a cell-to-cell signaling relay. Inspired by the millimeter-scale wnt-expression and signaling gradients in flatworms, we consider morphogen-mediated morphogen production in the cell field. We show that such a relay cangenerate stable morphogen and signaling gradients that are oriented by a local,morphogen-independent source of morphogen at a boundary. This gradient formation can be related to an effective diffusion and an effective degradation that result from morphogen production due to signaling relay. If the secretion of morphogen produced in response to the relay is polarized, it further gives rise to an effective drift. We find that signaling relay can generate long-range gradients in relevant times without relying on extreme choices of diffusion coefficients or degradation rates, thus exceeding the limits set by physiological diffusion coefficients and degradation rates. A signaling relay is hence an attractive principle to conceptualize long-range gradient formation by slowly diffusing morphogens that are relevant for patterning in adult contexts such as regeneration and tissue turn-over
Multistable alignment states in nematic liquid crystal filled wells
Two distinct, stable alignment states have been observed for a nematic liquid crystal confined in a layer with thickness of 12 μm and in square wells with sides of length between 20 and 80 μm. The director lies in the plane of the layer and line defects occur in two corners of the squares. The positions of the defects determine whether the director orientation is across the diagonal or is parallel to two opposite edges of the square. The device is multistable because both the diagonal and parallel states are stable when rotated by multiples of 90° in plane
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
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
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
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