16,320 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
Addressing culture in the EFL classroom: A dialogic proposal built up through dialogism
Language teaching has gone from a linguistic centered approach towards a lingocultural experience in which learning a language goes hand in hand with the understanding of, not only the target culture but the learner’s own culture. This paper intends to describe and reflect upon a collaborative and dialogical experience carried out between two teachers of the Languages Program of Universidad de la Salle. The bilateral enrichment of such a pedagogical experience not only helped the teachers to improve their language teaching contexts but also prompted the construction of a theoretical proposal to enhance intercultural awareness and develop critical intercultural competence in FL learners
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Nonreciprocal Wavefront Engineering with Time-Modulated Gradient Metasurfaces
We propose a paradigm to realize nonreciprocal wavefront engineering using time-modulated gradient metasurfaces. The essential building block of these surfaces is a subwavelength unit cell whose reflection coefficient oscillates at low frequency. We demonstrate theoretically and experimentally that such modulation permits tailoring the phase and amplitude of any desired nonlinear harmonic and determines the behavior of all other emerging fields. By appropriately adjusting the phase delay applied to the modulation of each unit cell, we realize time-modulated gradient metasurfaces that provide efficient conversion between two desired frequencies and enable nonreciprocity by (i) imposing drastically different phase gradients during the up/down conversion processes and (ii) exploiting the interplay between the generation of certain nonlinear surface and propagative waves. To demonstrate the performance and broad reach of the proposed platform, we design and analyze metasurfaces able to implement various functionalities, including beam steering and focusing, while exhibiting strong and angle-insensitive nonreciprocal responses. Our findings open an alternative direction in the field of gradient metasurfaces, in which wavefront control and magnetic-free nonreciprocity are locally merged to manipulate the scattered fields
Non-dissipative Thermal Transport and Magnetothermal Effect for the Spin-1/2 Heisenberg Chain
Anomalous magnetothermal effects are discussed in the spin-1/2 Heisenberg
chain. The energy current is related to one of the non-trivial conserved
quantities underlying integrability and therefore both the diagonal and off
diagonal dynamical correlations of spin and energy current diverge. The
energy-energy and spin-energy current correlations at finite temperatures are
exactly calculated by a lattice path integral formulation. The low-temperature
behavior of the thermomagnetic (magnetic Seebeck) coefficient is also
discussed. Due to effects of strong correlations, we observe the magnetic
Seebeck coefficient changes sign at certain interaction strengths and magnetic
fields.Comment: 4 pages, references added, typos corrected, Conference proceedings of
SPQS 2004, Sendai, Japa
Target Space Duality between Simple Compact Lie Groups and Lie Algebras under the Hamiltonian Formalism: I. Remnants of Duality at the Classical Level
It has been suggested that a possible classical remnant of the phenomenon of
target-space duality (T-duality) would be the equivalence of the classical
string Hamiltonian systems. Given a simple compact Lie group with a
bi-invariant metric and a generating function suggested in the physics
literature, we follow the above line of thought and work out the canonical
transformation generated by together with an \Ad-invariant
metric and a B-field on the associated Lie algebra of so that
and form a string target-space dual pair at the classical level under
the Hamiltonian formalism. In this article, some general features of this
Hamiltonian setting are discussed. We study properties of the canonical
transformation including a careful analysis of its domain and image. The
geometry of the T-dual structure on is lightly touched.Comment: Two references and related comments added, also some typos corrected.
LaTeX and epsf.tex, 36 pages, 4 EPS figures included in a uuencoded fil
Low-temperature transport in Heisenberg chains
A technique to determine accurately transport properties of integrable and
non-integrable quantum-spin chains at finite temperatures by Quantum
Monte-Carlo is presented. The reduction of the Drude weight by interactions in
the integrable gapless regime is evaluated. Evidence for the absence of a Drude
weight in the gapless regime of a non-integrable system with longer-ranged
interactions is presented. We estimate the effect of the non-integrability on
the transport properties and compare with recent experiments on one-dimensional
quantum-spin chains.Comment: accepted for publication (PRL
Asymmetric Heat Flow in Mesoscopic Magnetic System
The characteristics of heat flow in a coupled magnetic system are studied.
The coupled system is composed of a gapped chain and a gapless chain. The
system size is assumed to be quite small so that the mean free path is
comparable to it. When the parameter set of the temperatures of reservoirs is
exchanged, the characteristics of heat flow are studied with the Keldysh Green
function technique. The asymmetry of current is found in the presence of a
local equilibrium process at the contact between the magnetic systems. The
present setup is realistic and such an effect will be observed in real
experiments. We also discuss the simple phenomenological explanation to obtain
the asymmetry.Comment: 13 pages, 3 figure
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