4,229 research outputs found
Coupling Matrix Representation of Nonreciprocal Filters Based on Time Modulated Resonators
This paper addresses the analysis and design of non-reciprocal filters based
on time modulated resonators. We analytically show that time modulating a
resonator leads to a set of harmonic resonators composed of the unmodulated
lumped elements plus a frequency invariant element that accounts for
differences in the resonant frequencies. We then demonstrate that harmonic
resonators of different order are coupled through non-reciprocal admittance
inverters whereas harmonic resonators of the same order couple with the
admittance inverter coming from the unmodulated filter network. This coupling
topology provides useful insights to understand and quickly design
non-reciprocal filters and permits their characterization using an
asynchronously tuned coupled resonators network together with the coupling
matrix formalism. Two designed filters, of orders three and four, are
experimentally demonstrated using quarter wavelength resonators implemented in
microstrip technology and terminated by a varactor on one side. The varactors
are biased using coplanar waveguides integrated in the ground plane of the
device. Measured results are found to be in good agreement with numerical
results, validating the proposed theory
Enhancement of Friction between Carbon Nanotubes: An Efficient Strategy to Strengthen Fibers
Interfacial friction plays a crucial role in the mechanical properties of
carbon nanotube based fibers, composites, and devices. Here we use molecular
dynamics simulation to investigate the pressure effect on the friction within
carbon nanotube bundles. It reveals that the intertube frictional force can be
increased by a factor of 1.5 ~ 4, depending on tube chirality and radius, when
all tubes collapse above a critical pressure and when the bundle remains
collapsed with unloading down to atmospheric pressure. Furthermore, the overall
cross-sectional area also decreases significantly for the collapsed structure,
making the bundle stronger. Our study suggests a new and efficient way to
reinforce nanotube fibers, possibly stronger than carbon fibers, for usage at
ambient conditions.Comment: revtex, 5 pages, accepted by ACS Nano 10 Dec 200
Transport Properties of the One Dimensional Ferromagnetic Kondo Lattice Model : A Qualitative Approach to Oxide Manganites
The transport properties of the ferromagnetic Kondo lattice model in one
dimension are studied via bosonization methods. The antiferromagnetic
fluctuations, which normally appear because of the RKKY interactions, are
explicitly taken into account as a direct exchange between the ``core'' spins.
It is shown that in the paramagnetic regime with the local antiferromagnetic
fluctuations, the resistivity decays exponentially as the temperature increases
while in the ferromagnetic regime the system is an almost perfect conductor. %A
non-perturbative description of localized spin polarons %in the paramagnetic
region is obtained.
The effect of a weak applied field is discussed to be reduced to the case of
the ferromagnetic state leading to band splitting. The qualitative relevance of
the results for the problem of the Oxide Manganites is emphasized.Comment: 4 pages, REVTe
Structure of the Magneto-Exciton and Optical Properties in Fractional Quantum Hall Systems
We report calculated dependence of magneto-exciton energy spectrum upon
electron-hole separation in Fractional Quantum Hall systems. We calculated
the dependence of photoluminescence upon , and we obtained the doublet
structure observed recently. The Raman scattering spectrum around resonance is
calculated: a robust resonance peak at around gap value is reported.Comment: 13 pages, REVTEX, compressed postscript file (3 figures included
Seeing the magnetic monopole through the mirror of topological surface states
Existence of the magnetic monopole is compatible with the fundamental laws of
nature, however, this illusive particle has yet to be detected experimentally.
In this work, we show that an electric charge near the topological surface
state induces an image magnetic monopole charge due to the topological
magneto-electric effect. The magnetic field generated by the image magnetic
monopole can be experimentally measured, and the inverse square law of the
field dependence can be determined quantitatively. We propose that this effect
can be used to experimentally realize a gas of quantum particles carrying
fractional statistics, consisting of the bound states of the electric charge
and the image magnetic monopole charge.Comment: 5 pages, 4 figure
Molecular-field approach to the spin-Peierls transition in CuGeO_3
We present a theory for the spin-Peierls transition in CuGeO_3. We map the
elementary excitations of the dimerized chain (solitons) on an effective Ising
model. Inter-chain coupling (or phonons) then introduce a linear binding
potential between a pair of soliton and anti-soliton, leading to a finite
transition temperature. We evaluate, as a function of temperature, the order
parameter, the singlet-triplet gap, the specific heat, and the susceptibility
and compare with experimental data on CuGeO_3. We find that CuGeO_3 is close to
a first-order phase transition. We point out, that the famous scaling law
\sim\delta^{2/3} of the triplet gap is a simple consequence of the linear
binding potential between pairs of solitons and anti-solitons in dimerized spin
chains.Comment: 7.1 pages, figures include
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