972 research outputs found
Improvements of the Variable Thermal Resistance
A flat mounting unit with electronically variable thermal resistance [1] has
been presented in the last year [2]. The design was based on a Peltier cell and
the appropriate control electronics and software. The device is devoted
especially to the thermal characterization of packages, e.g. in dual cold plate
arrangements. Although this design meets the requirements of the static
measurement we are intended to improve its parameters as the settling time and
dynamic thermal impedance and the range of realized thermal resistance. The new
design applies the heat flux sensor developed by our team as well [3], making
easier the control of the device. This development allows even the realization
of negative thermal resistances.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Exact real-time dynamics of the quantum Rabi model
We use the analytical solution of the quantum Rabi model to obtain absolutely
convergent series expressions of the exact eigenstates and their scalar
products with Fock states. This enables us to calculate the numerically exact
time evolution of and for all regimes of the
coupling strength, without truncation of the Hilbert space. We find a
qualitatively different behavior of both observables which can be related to
their representations in the invariant parity subspaces.Comment: 8 pages, 7 figures, published versio
Inverse Design of Blade Shapes for Vertical Axis Wind Turbines
An inverse design process is applied to determine blade shapes used in vertical axis wind turbines (VAWTs). The method is based on the modified Garabedian-McFadden technique that uses the deviation of the pressure distribution or the velocity distribution on the surface from a target pressure or velocity distribution, and modifies the blade surface in order to reduce this deviation. The method was originally developed for wing design and applied for such aerofoil shapes as those of horizontal axis wind turbine blades. The procedure is employed here successfully to find a target shape that is used in a VAWT operating with blades of constant thickness rather than of aerofoil shape. The method is presently applicable to determine blade shapes in a two dimensional section, and considers some performance criteria of VAWTs; thus, it contributes to finding blade shapes that most closely corresponds to the efficient operation of such turbines
Hopping on the Bethe lattice: Exact results for densities of states and dynamical mean-field theory
We derive an operator identity which relates tight-binding Hamiltonians with
arbitrary hopping on the Bethe lattice to the Hamiltonian with nearest-neighbor
hopping. This provides an exact expression for the density of states (DOS) of a
non-interacting quantum-mechanical particle for any hopping. We present
analytic results for the DOS corresponding to hopping between nearest and
next-nearest neighbors, and also for exponentially decreasing hopping
amplitudes. Conversely it is possible to construct a hopping Hamiltonian on the
Bethe lattice for any given DOS. These methods are based only on the so-called
distance regularity of the infinite Bethe lattice, and not on the absence of
loops. Results are also obtained for the triangular Husimi cactus, a recursive
lattice with loops. Furthermore we derive the exact self-consistency equations
arising in the context of dynamical mean-field theory, which serve as a
starting point for studies of Hubbard-type models with frustration.Comment: 14 pages, 9 figures; introduction expanded, references added;
published versio
Exact analytic results for the Gutzwiller wave function with finite magnetization
We present analytic results for ground-state properties of Hubbard-type
models in terms of the Gutzwiller variational wave function with non-zero
values of the magnetization m. In dimension D=1 approximation-free evaluations
are made possible by appropriate canonical transformations and an analysis of
Umklapp processes. We calculate the double occupation and the momentum
distribution, as well as its discontinuity at the Fermi surface, for arbitrary
values of the interaction parameter g, density n, and magnetization m. These
quantities determine the expectation value of the one-dimensional Hubbard
Hamiltonian for any symmetric, monotonically increasing dispersion epsilon_k.
In particular for nearest-neighbor hopping and densities away from half filling
the Gutzwiller wave function is found to predict ferromagnetic behavior for
sufficiently large interaction U.Comment: REVTeX 4, 32 pages, 8 figure
Viking X-band telemetry experiment
In order to uncover operational and design problems in the use of X-band by the 1977 Mariner Jupiter-Saturn mission and future spacecraft using the Deep Space Network, the Viking X-band telemetry experiment was conducted. The experiment was conducted during the months of December 1975 and January 1976. During each of the five successful passes, a periodic sequence (in lieu of ranging) was transmitted to the spacecraft and returned by the spacecraft transponder on both S- and X-bands. These telemetry-like signals were received, demodulated, and detected. From a variety of measurements at the station, four independent measurements were made of the received signal-to-noise ratio (SNR). These four SNRs were later compared with each other and the predicted SNR. The principal result of the experiment is that X-band telemetry works as expected. That is, the measured SNRs were consistent relative to each other and to the predicted values within the accuracy of the experiment
Ferromagnetic Luttinger Liquids
We study weak itinerant ferromagnetism in one-dimensional Fermi systems using
perturbation theory and bosonization. We find that longitudinal spin
fluctuations propagate ballistically with velocity v_m << v_F, where v_F is the
Fermi velocity. This leads to a large anomalous dimension in the spin-channel
and strong algebraic singularities in the single-particle spectral function and
in the transverse structure factor for momentum transfers q ~ 2 Delta/v_F,
where 2 Delta is the exchange splitting.Comment: 4 pages, 3 figure
Ferromagnetism in the two dimensional t-t' Hubbard model at the Van Hove density
Using an improved version of the projection quantum Monte Carlo technique, we
study the square-lattice Hubbard model with nearest-neighbor hopping t and
next-nearest-neighbor hopping t', by simulation of lattices with up to 20 X 20
sites. For a given R=2t'/t, we consider that filling which leads to a singular
density of states of the noninteracting problem. For repulsive interactions, we
find an itinerant ferromagnet (antiferromagnet) for R=0.94 (R=0.2). This is
consistent with the prediction of the T-matrix approximation, which sums the
most singular set of diagrams.Comment: 10 pages, RevTeX 3.0 + a single postscript file with all figure
Momentum-resolved spectral functions of SrVO calculated by LDA+DMFT
LDA+DMFT, the merger of density functional theory in the local density
approximation and dynamical mean-field theory, has been mostly employed to
calculate k-integrated spectra accessible by photoemission spectroscopy. In
this paper, we calculate k-resolved spectral functions by LDA+DMFT. To this
end, we employ the Nth order muffin-tin (NMTO) downfolding to set up an
effective low-energy Hamiltonian with three t_2g orbitals. This downfolded
Hamiltonian is solved by DMFT yielding k-dependent spectra. Our results show
renormalized quasiparticle bands over a broad energy range from -0.7 eV to +0.9
eV with small ``kinks'', discernible in the dispersion below the Fermi energy.Comment: 21 pages, 8 figure
- …