239,570 research outputs found
Input filter compensation for switching regulators
The problems caused by the interaction between the input filter, output filter, and the control loop are discussed. The input filter design is made more complicated because of the need to avoid performance degradation and also stay within the weight and loss limitations. Conventional input filter design techniques are then dicussed. The concept of pole zero cancellation is reviewed; this concept is the basis for an approach to control the peaking of the output impedance of the input filter and thus mitigate some of the problems caused by the input filter. The proposed approach for control of the peaking of the output impedance of the input filter is to use a feedforward loop working in conjunction with feedback loops, thus forming a total state control scheme. The design of the feedforward loop for a buck regulator is described. A possible implementation of the feedforward loop design is suggested
Specific heat of single crystal MgB_2: a two-band superconductor with two different anisotropies
Heat-capacity measurements of a 39 microgramm MgB_2 single crystal in fields
up to 14 T and below 3 K allow the determination of the low-temperature linear
term of the specific heat, its field dependence and its anisotropy. Our results
are compatible with two-band superconductivity, the band carrying the small gap
being isotropic, that carrying the large gap having an anisotropy of ~ 5. Three
different upper critical fields are thus needed to describe the superconducting
state of MgB2.Comment: 4 pages, 4 figures - V2: Bibliography updated and some typo
corrected. One reference added - V3: version accepted for publication in PRL,
changes made in the tex
Computer-Aided Modeling and Analysis of Power Processing Systems (CAMAPPS), phase 1
The large-signal behaviors of a regulator depend largely on the type of power circuit topology and control. Thus, for maximum flexibility, it is best to develop models for each functional block a independent modules. A regulator can then be configured by collecting appropriate pre-defined modules for each functional block. In order to complete the component model generation for a comprehensive spacecraft power system, the following modules were developed: solar array switching unit and control; shunt regulators; and battery discharger. The capability of each module is demonstrated using a simplified Direct Energy Transfer (DET) system. Large-signal behaviors of solar array power systems were analyzed. Stability of the solar array system operating points with a nonlinear load is analyzed. The state-plane analysis illustrates trajectories of the system operating point under various conditions. Stability and transient responses of the system operating near the solar array's maximum power point are also analyzed. The solar array system mode of operation is described using the DET spacecraft power system. The DET system is simulated for various operating conditions. Transfer of the software program CAMAPPS (Computer Aided Modeling and Analysis of Power Processing Systems) to NASA/GSFC (Goddard Space Flight Center) was accomplished
Heavy quarkonium correlators at finite temperature: QCD sum rule approach
We investigate the properties of heavy quarkonia at finite temperature in
detail using QCD sum rules. Extending previous analyses, we take into account a
temperature dependent effective continuum threshold and derive constraints on
the mass, the width, and the varying effective continuum threshold. We find
that at least one of these quantities of a charmonium changes abruptly in the
vicinity of the phase transition. We also calculate the ratio of the imaginary
time correlator to its reconstructed one, , by constructing a
model spectral function and compare it to the corresponding lattice QCD
results. We demonstrate that the almost constant unity of
can be obtained from the destructive interplay of the changes in each part of
the spectral modification which are extracted from QCD sum rules.Comment: Revised version to appear in PRD. 31 pages, 31 figures. Title is
change
Quantifying mixed-state quantum entanglement by optimal entanglement witness
We develop an approach of quantifying entanglement in mixed quantum states by
the optimal entanglement witness operator. We identify the convex set of mixed
states for which a single witness provides the exact value of an entanglement
measure, and show that the convexity, properties, and symmetries of
entanglement or of a target state considerably fix the form of the optimal
witness. This greatly reduces difficulty in computing and experimentally
determining entanglement measures. As an example, we show how to experimentally
quantify bound entanglement in four-qubit noisy Smolin states and three-qubit
Greenberger-Horne-Zeilinger (GHZ) entanglement under white noise. For general
measures and states, we provide a numerical method to efficiently optimize
witness.Comment: Supplemental material is include
Simulation of Transport and Gain in Quantum Cascade Lasers
Quantum cascade lasers can be modeled within a hierarchy of different
approaches: Standard rate equations for the electron densities in the levels,
semiclassical Boltzmann equation for the microscopic distribution functions,
and quantum kinetics including the coherent evolution between the states. Here
we present a quantum transport approach based on nonequilibrium Green
functions. This allows for quantitative simulations of the transport and
optical gain of the device. The division of the current density in two terms
shows that semiclassical transitions are likely to dominate the transport for
the prototype device of Sirtori et al. but not for a recent THz-laser with only
a few layers per period. The many particle effects are extremely dependent on
the design of the heterostructure, and for the case considered here, inclusion
of electron-electron interaction at the Hartree Fock level, provides a sizable
change in absorption but imparts only a minor shift of the gain peak.Comment: 12 pages, 5 figures included, to appear in in "Advances in Solid
State Physics", ed. by B. Kramer (Springer 2003
Self-Consistent Theory of the Gain Linewidth for Quantum Cascade Lasers
The linewidth in intersubband transitions can be significantly reduced below
the sum of the lifetime broadening for the involved states, if the scattering
environment is similar for both states. This is studied within a nonequilibrium
Green function approach here. We find that the effect is of particular
relevance for a recent, relatively low doped, THz quantum cascade laser.Comment: 3 pages, figures include
Dynamics of thermalisation in small Hubbard-model systems
We study numerically the thermalisation and temporal evolution of the reduced
density matrix for a two-site subsystem of a fermionic Hubbard model prepared
far from equilibrium at a definite energy. Even for very small systems near
quantum degeneracy, the subsystem can reach a steady state resembling
equilibrium. This occurs for a non-perturbative coupling between the subsystem
and the rest of the lattice where relaxation to equilibrium is Gaussian in
time, in sharp contrast to perturbative results. We find similar results for
random couplings, suggesting such behaviour is generic for small systems.Comment: 4 pages, 5 figure
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