43,296 research outputs found
A Flexible Implementation of a Matrix Laurent Series-Based 16-Point Fast Fourier and Hartley Transforms
This paper describes a flexible architecture for implementing a new fast
computation of the discrete Fourier and Hartley transforms, which is based on a
matrix Laurent series. The device calculates the transforms based on a single
bit selection operator. The hardware structure and synthesis are presented,
which handled a 16-point fast transform in 65 nsec, with a Xilinx SPARTAN 3E
device.Comment: 4 pages, 4 figures. IEEE VI Southern Programmable Logic Conference
201
Dimensional-scaling estimate of the energy of a large system from that of its building blocks: Hubbard model and Fermi liquid
A simple, physically motivated, scaling hypothesis, which becomes exact in
important limits, yields estimates for the ground-state energy of large,
composed, systems in terms of the ground-state energy of its building blocks.
The concept is illustrated for the electron liquid, and the Hubbard model. By
means of this scaling argument the energy of the one-dimensional half-filled
Hubbard model is estimated from that of a 2-site Hubbard dimer, obtaining
quantitative agreement with the exact one-dimensional Bethe-Ansatz solution,
and the energies of the two- and three-dimensional half-filled Hubbard models
are estimated from the one-dimensional energy, recovering exact results for
and and coming close to Quantum Monte Carlo data for
intermediate .Comment: 3 figure
Magnetism and Electronic Correlations in Quasi-One-Dimensional Compounds
In this contribution on the celebration of the 80th birthday anniversary of
Prof. Ricardo Ferreira, we present a brief survey on the magnetism of
quasi-one-dimensional compounds. This has been a research area of intense
activity particularly since the first experimental announcements of magnetism
in organic and organometallic polymers in the mid 80s. We review experimental
and theoretical achievements on the field, featuring chain systems of
correlated electrons in a special AB2 unit cell structure present in inorganic
and organic compounds
Thermal Effects on Photon-Induced Quantum Transport
We theoretically investigate laser induced quantum transport in a two-level
quantum dot attached to electric contacts. Our approach, based on
nonequilibrium Green function technique, allows to include thermal effects on
the photon-induced quantum transport and excitonic coherent dynamics. By
solving a set of coupled integrodifferential equations, involving correlation
and propagator functions, we obtain the photocurrent and the dot occupations as
a function of time. The characteristic coherent Rabi oscillations are found in
both occupations and photocurrent, with two distinct sources of decoherence:
incoherent tunneling and thermal fluctuations. In particular, for increasing
temperature the dot becomes more thermally occupied which shrinks the amplitude
of the Rabi oscillations, due to Pauli blockade. Finally, due to the interplay
between photon and thermal induced electron populations, the photocurrent can
switch sign as time evolves and its stationary value can be maximized by
tunning the laser intensity.Comment: 5 pages, 4 figure
FINITE SIZE SCALING FOR FIRST ORDER TRANSITIONS: POTTS MODEL
The finite-size scaling algorithm based on bulk and surface renormalization
of de Oliveira (1992) is tested on q-state Potts models in dimensions D = 2 and
3. Our Monte Carlo data clearly distinguish between first- and second-order
phase transitions. Continuous-q analytic calculations performed for small
lattices show a clear tendency of the magnetic exponent Y = D - beta/nu to
reach a plateau for increasing values of q, which is consistent with the
first-order transition value Y = D. Monte Carlo data confirm this trend.Comment: 5 pages, plain tex, 5 EPS figures, in file POTTS.UU (uufiles
Irreversibility and the arrow of time in a quenched quantum system
Irreversibility is one of the most intriguing concepts in physics. While
microscopic physical laws are perfectly reversible, macroscopic average
behavior has a preferred direction of time. According to the second law of
thermodynamics, this arrow of time is associated with a positive mean entropy
production. Using a nuclear magnetic resonance setup, we measure the
nonequilibrium entropy produced in an isolated spin-1/2 system following fast
quenches of an external magnetic field and experimentally demonstrate that it
is equal to the entropic distance, expressed by the Kullback-Leibler
divergence, between a microscopic process and its time-reverse. Our result
addresses the concept of irreversibility from a microscopic quantum standpoint.Comment: 8 pages, 7 figures, RevTeX4-1; Accepted for publication Phys. Rev.
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