218 research outputs found
Dielectric resonances of ordered passive arrays
The electrical and optical properties of ordered passive arrays, constituted
of inductive and capacitive components, are usually deduced from Kirchhoff's
rules. Under the assumption of periodic boundary conditions, comparable results
may be obtained via an approach employing transfer matrices. In particular,
resonances in the dielectric spectrum are demonstrated to occur if all
eigenvalues of the transfer matrix of the entire array are unity. The latter
condition, which is shown to be equivalent to the habitual definition of a
resonance in impedance for an array between electrodes, allows for a convenient
and accurate determination of the resonance frequencies, and may thus be used
as a tool for the design of materials with a specific dielectric response. For
the opposite case of linear arrays in a large network, where periodic boundary
condition do not apply, several asymptotic properties are derived. Throughout
the article, the derived analytic results are compared to numerical models,
based on either Exact Numerical Renormalisation or the spectral method.Comment: 12 pages, 12 figure
Low-temperature thermochronology of central and northwestern Pamir gneiss domes
Abstract HKT-ISTP 2013
A
The exhumation of the westernmost Tien Shan (Tajikistan, Uzbekistan)
Abstract HKT-ISTP 2013
A
Colored delta-T noise in Fractional Quantum Hall liquids
Photons are emitted or absorbed by a nano-circuit under both equilibrium and
non-equilibrium situations. Here, we focus on the non-equilibrium situation
arising due to a temperature difference between the leads of a quantum point
contact, and study the finite frequency (colored) noise. We explore this
delta- noise in the finite frequency regime for two systems: conventional
conductors described by Fermi liquid scattering theory and the fractional
quantum Hall system at Laughlin filling fractions, described by the chiral
Luttinger liquid formalism. We study the emission noise, its expansion in the
temperature difference (focusing on the quadratic component) as well as the
excess emission noise defined with respect to a properly chosen equilibrium
situation. The behavior of these quantities are markedly different for the
fractional quantum Hall system compared to Fermi liquids, signalling the role
of strong correlations. We briefly treat the strong backscattering regime of
the fractional quantum Hall liquid, where a behavior closer to the Fermi liquid
case is observed
Quartet currents in a biased three-terminal diffusive Josephson junction
Biasing a three-terminal Josephson junction (TTJ) with symmetrical voltages
leads to new kinds of DC currents, namely quartet Josephson currents
and phase-dependent multiple Andreev reflection (MAR) currents. We study these
currents in a system where a normal diffusive metallic node is connected to
three terminals by barriers of arbitrary transparency. We use the
quantum circuit theory to calculate the current in each terminal, including
decoherence. In addition to the stationary combination
of the terminal phases ,
the bias voltage appears as a new and unusual control variable for a DC
Josephson current. A general feature is the sign changes of the current-phase
characteristics, manifesting in minima of the quartet ``critical current".
Those sign changes can be triggered by the voltage, by the junction
transparency or by decoherence. We study the possible separation of quartet
currents from MAR currents in different regimes of parameters, including an
"funnel" regime with very asymmetric couplings to . In the regime of
low transparency and asymetric couplings, we provide an analytic perturbative
expression for the currents which shows an excellent agreement with the full
numerical results
Intra-continental, two-stage shortening along the Alai valley, Pamir–Tien Shan, Central Asia
Abstract HKT-ISTP 2013
A
Scattering Theory of Non-Equilibrium Noise and Delta current fluctuations through a quantum dot
We consider the non-equilibrium zero frequency noise generated by a
temperature gradient applied on a device composed of two normal leads separated
by a quantum dot. We recall the derivation of the scattering theory for
non-equilibrium noise for a general situation where both a bias voltage and a
temperature gradient can coexist and put it in a historical perspective. We
provide a microscopic derivation of zero frequency noise through a quantum dot
based on a tight binding Hamiltonian, which constitutes a generalization of the
pioneering work of Caroli et al. for the current obtained in the context of the
Keldysh formalism. For a single level quantum dot, the obtained transmission
coefficient entering the scattering formula for the non-equilibrium noise
corresponds to a Breit-Wigner resonance. We compute the delta- noise as a
function of the dot level position, and of the dot level width, in the
Breit-Wigner case, for two relevant situations which were considered recently
in two separate experiments. In the regime where the two reservoir temperatures
are comparable, our gradient expansion shows that the delta- noise is
dominated by its quadratic contribution, and is minimal close to resonance. In
the opposite regime where one reservoir is much colder, the gradient expansion
fails and we find the noise to be typically linear in temperature before
saturating. In both situations, we conclude with a short discussion of the case
where both a voltage bias and a temperature gradient are present, in order to
address the potential competition with thermoelectric effects.Comment: 19 pages, 9 figure
Current and shot noise in a spin dependent driven normal metal -- BCS superconductor junction
Andreev reflection is a fundamental transport process occurring at the
junction between a normal metal and a superconductor (a N-S junction), when an
incident electron from the normal side can only be transmitted in the
superconductor as a Cooper pair, with the reflection of a hole in the normal
metal. As a consequence of the spin singlet nature of the BCS Cooper pairs, the
current due to Andreev reflection at a N-S junction is always symmetric in
spin. Using a Keldysh Nambu Floquet approach, combining analytical and
numerical calculations, we study in details the AC transport at a N-S junction,
when the two spin components in the normal metal are driven by different
periodic drives. We show that, in the Andreev regime, i.e. when the
superconducting gap is much larger than the frequency of the drives, the
spin-resolved photo-assisted currents are always equal even if the two drives
are different. In addition, we show that in this regime the excess noise
depends only on the sum of the periodic drives, and we consider in particular
the case of Lorentzian pulses (Levitons). We also show how these properties get
modified when going beyond the Andreev regime. Finally we give a simple
analytical proof of the special properties of the Andreev regime using an exact
mapping to a particular N-N junction.Comment: 13 pages, 4 figure
Theory of non-equilibrium noise in general multi-terminal superconducting hydrid devices: application to multiple Cooper pair resonances
We consider the out-of-equilibrium behavior of a general class of mesoscopic
devices composed of several superconducting or/and normal metal leads separated
by quantum dots. Starting from a microscopic Hamiltonian description, we
provide a non-perturbative approach to quantum electronic transport in the
tunneling amplitudes between dots and leads: using the equivalent of a path
integral formulation, the lead degrees of freedom are integrated out in order
to compute both the current and the current correlations (noise) in this class
of systems, in terms of the dressed Green's function matrix of the quantum
dots. In order to illustrate the efficiency of this formalism, we apply our
results to the "all superconducting Cooper pair beam splitter", a device
composed of three superconducting leads connected via two quantum dots, where
crossed Andreev reflection operates Cooper pair splitting. Commensurate voltage
differences between the three leads allow to obtain expressions for the current
and noise as a function of the Keldysh Nambu Floquet dressed Green's function
of the dot system. This voltage configuration allows the occurrence of
non-local processes involving multiple Cooper pairs which ultimately lead to
the presence of non-zero DC currents in an out-of-equilibrium situation. We
investigate in details the results for the noise obtained numerically in the
specific case of opposite voltages, where the transport properties are
dominated by the so called "quartet processes", involving the coherent exchange
of two Cooper pairs among all three superconducting terminals. We show that
these processes are noiseless in the non-resonant case, and that this property
is also observed for other voltage configurations. When the dots are in a
resonant regime, the noise characteristics change qualitatively, with the
appearance of giant Fano factors.Comment: 18 pages, 12 figure
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