1,523 research outputs found
Non-equilibrium supercurrent through a quantum dot: current harmonics and proximity effect due to a normal metal lead
We consider a Hamiltonian model for a quantum dot which is placed between two
superconducting leads with a constant bias imposed between these leads. Using
the non-equilibrium Keldysh technique, we focus on the subgap current, where it
is known that multiple Andreev reflections (MAR) are responsible for charge
transfer through the dot. Attention is put on the DC current and on the first
harmonics of the supercurrent. Varying the energy and width of the resonant
level on the dot, we first investigate a cross-over from a quantum dot regime
to a quantum point contact regime when there is zero coupling to the normal
probe. We then study the effect on the supercurrent of the normal probe which
is attached to the dot. This normal probe is understood to lead to dephasing,
or alternatively to induce reverse proximity effect. We describe the full
crossover from zero dephasing to the incoherent case. We also compute the
Josephson current in the presence of the normal lead, and find it in excellent
agreement with the values of the non-equlibrium current extrapolated at zero
voltage
Quantum phase transition of dynamical resistance in a mesoscopic capacitor
We study theoretically dynamic response of a mesoscopic capacitor, which
consists of a quantum dot connected to an electron reservoir via a point
contact and capacitively coupled to a gate voltage. A quantum Hall edge state
with a filling factor nu is realized in a strong magnetic field applied
perpendicular to the two-dimensional electron gas. We discuss a noise-driven
quantum phase transition of the transport property of the edge state by taking
into account an ohmic bath connected to the gate voltage. Without the noise,
the charge relaxation for nu>1/2 is universally quantized at R_q=h/(2e^2),
while for nu<1/2, the system undergoes the Kosterlitz-Thouless transtion, which
drastically changes the nature of the dynamical resistance. The phase
transition is facilitated by the noisy gate voltage, and we see that it can
occur even for an integer quantum Hall edge at nu=1. When the dissipation by
the noise is sufficiently small, the quantized value of R_q is shifted by the
bath impedance.Comment: 5 pages, 2 figures, proceeding of the 19th International Conference
on the Application of High Magnetic Fields in Semiconductor Physics and
Nanotechnology (HMF-19
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
Observation of coherent backscattering of light by cold atoms
Coherent backscattering (CBS) of light waves by a random medium is a
signature of interference effects in multiple scattering. This effect has been
studied in many systems ranging from white paint to biological tissues.
Recently, we have observed CBS from a sample of laser-cooled atoms, a
scattering medium with interesting new properties. In this paper we discuss
various effects, which have to be taken into account for a quantitative study
of coherent backscattering of light by cold atoms.Comment: 25 pages LaTex2e, 17 figures, submitted to J. Opt. B: Quant. Semicl.
Op
Photo-assisted Andreev reflection as a probe of quantum noise
19 pages, 11 figuresAndreev reflection, which corresponds to the tunneling of two electrons from a metallic lead to a superconductor lead as a Cooper pair (or vice versa), can be exploited to measure high frequency noise. A detector is proposed, which consists of a normal lead--superconductor circuit, which is capacitively coupled to a mesoscopic circuit where noise is to be measured. We discuss two detector circuits: a single normal metal -- superconductor tunnel junction and a normal metal separated from a superconductor by a quantum dot operating in the Coulomb blockade regime. A substantial DC current flows in the detector circuit when an appropriate photon is provided or absorbed by the mesoscopic circuit, which plays the role of an environment for the junction to which it couples. Results for the current can be cast in all cases in the form of a frequency integral of the excess noise of the environment weighted by a kernel which is specific to the transport process (quasiparticle tunneling, Andreev reflection,...) which is considered. We apply these ideas to the measurement of the excess noise of a quantum point contact and we provide numerical estimates of the detector current
Fermionic Mach-Zehnder interferometer subject to a quantum bath
We study fermions in a Mach-Zehnder interferometer, subject to a
quantum-mechanical environment leading to inelastic scattering, decoherence,
renormalization effects, and time-dependent conductance fluctuations. Both the
loss of interference contrast as well as the shot noise are calculated, using
equations of motion and leading order perturbation theory. The full dependence
of the shot-noise correction on setup parameters, voltage, temperature and the
bath spectrum is presented. We find an interesting contribution due to
correlations between the fluctuating renormalized phase shift and the output
current, discuss the limiting behaviours at low and high voltages, and compare
with simpler models of dephasing.Comment: 5 pages, 3 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
Conductivity in quasi two-dimensional systems
The conductivity in quasi two-dimensional systems is calculated using the
quantum kinetic equation. Linearizing the Lenard-Balescu collision integral
with the extension to include external field dependences allows one to
calculate the conductivity with diagrams beyond the GW approximation including
maximally crossed lines. Consequently the weak localization correction as an
interference effect appears here from the field dependence of the collision
integral (the latter dependence sometimes called intra-collisional field
effect). It is shown that this weak localization correction has the same origin
as the Debye-Onsager relaxation effect in plasma physics. The approximation is
applied to a system of quasi two-dimensional electrons in hetero-junctions
which interact with charged and neutral impurities and the low temperature
correction to the conductivity is calculated analytically. It turns out that
the dynamical screening due to charged impurities leads to a linear temperature
dependence, while the scattering from neutral impurities leads to the usual
Fermi-liquid behavior. By considering an appropriate mass action law to
determine the ratio of charged to neutral impurities we can describe the
experimental metal-insulator transition at low temperatures as a Mott-Hubbard
transition.Comment: 7 pages 7 pages appendix 11 figure
Statistics of level spacing of geometric resonances in random binary composites
We study the statistics of level spacing of geometric resonances in the
disordered binary networks. For a definite concentration within the
interval , numerical calculations indicate that the unfolded level
spacing distribution and level number variance have the
general features. It is also shown that the short-range fluctuation and
long-range spectral correlation lie between the profiles of the
Poisson ensemble and Gaussion orthogonal ensemble (GOE). At the percolation
threshold , crossover behavior of functions and is
obtained, giving the finite size scaling of mean level spacing and
mean level number , which obey the scaling laws, and .Comment: 11 pages, 7 figures,submitted to Phys. Rev.
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