85 research outputs found
Field-Enhanced Kondo Correlations in a Half-Filling Nanotube Dot: Evolution of an SU(N) Fermi-Liquid Fixed Point
We theoretically study an emergent SU(2) symmetry which is suggested by
recent magneto-transport measurements, carried out near two electrons filling
of a carbon nanotube quantum dot. It emerges in the case where the spin and
orbital Zeeman splittings cancel each other out for two of the one-particle dot
levels among four. Using the Wilson numerical renormalization group, we show
that a crossover from the SU(4) to SU(2) Fermi-liquid behavior occurs at two
impurity-electrons filling as magnetic field increases. We also find that the
quasiparticles are significantly renormalized as the remaining two one-particle
levels move away from the Fermi level and are frozen at high magnetic fields.
In order to clarify how the ground state evolves during such a crossover, we
also reexamine the SU(N) Kondo singlet state for M impurity-electrons filling
in the limit of strong exchange interactions. We show that the nondegenerate
Fermi-liquid fixed point of Nozi\`{e}es and Blandin can be described as a
bosonic Perron-Frobenius vector for M hard-core bosons, each of which consists
of one impurity-electron and one conduction hole. This interpretation can also
be applied to the Fermi-liquid fixed-point without the SU(N) symmetry.Comment: 20 pages, 10 figures, Sec.III B. has been revised. J.Phys.Soc.Jpn.in
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Observation of finite excess noise in the voltage-biased quantum Hall regime as a precursor for breakdown
We performed noise measurements in a two-dimensional electron gas to
investigate the nonequilibrium quantum Hall effect (QHE) state. While excess
noise is perfectly suppressed around the zero-biased QHE state reflecting the
dissipationless electron transport of the QHE state, considerable finite excess
noise is observed in the breakdown regime of the QHE. The noise temperature
deduced from the excess noise is found to be of the same order as the energy
gap between the highest occupied Landau level and the lowest empty one.
Moreover, unexpected finite excess noise is observed at a finite source-drain
bias voltagesmaller than the onset voltage of the QHE breakdown, which
indicates finite dissipation in the QHE state and may be related to the
prebreakdown of the QHE.Comment: 8 pages, 8 figure
Enhanced shot noise of multiple Andreev reflections in a carbon nanotube quantum dot in SU(2) and SU(4) Kondo regimes
The sensitivity of shot noise to the interplay between Kondo correlations and
superconductivity is investigated in a carbon nanotube quantum dot connected to
superconducting electrodes. Depending on the gate voltage, the SU(2) and SU(4)
Kondo unitary regimes can be clearly identified. We observe enhancement of the
shot noise via the Fano factor in the superconducting state. Its divergence at
low bias voltage, which is more pronounced in the SU(4) regime than in the
SU(2) one, is larger than what is expected from proliferation of multiple
Andreev reflections predicted by the existing theories. Our result suggests
that Kondo effect is responsible for this strong enhancement
Unraveling a concealed resonance by multiple Kondo transitions in a quantum dot
Kondo correlations are responsible for the emergence of a zero-bias peak in the low temperature differential conductance of Coulomb blockaded quantum dots. In the presence of a global SU(2) circle times SU(2) symmetry, which can be realized in carbon nanotubes, they also inhibit inelastic transitions which preserve the Kramers pseudospins associated to the symmetry. We report on magnetotransport experiments on a Kondo correlated carbon nanotube where resonant features at the bias corresponding to the pseudospin-preserving transitions are observed. We attribute this effect to a simultaneous enhancement of pseudospin-nonpreserving transitions occurring at that bias. This process is boosted by asymmetric tunneling couplings of the two Kramers doublets to the leads and by asymmetries in the potential drops at the leads. Hence, the present work discloses a fundamental microscopic mechanisms ruling transport in Kondo systems far from equilibrium
Synchronization of uncoupled oscillators by common gamma impulses: from phase locking to noise-induced synchronization
Nonlinear oscillators can mutually synchronize when they are driven by common
external impulses. Two important scenarios are (i) synchronization resulting
from phase locking of each oscillator to regular periodic impulses and (ii)
noise-induced synchronization caused by Poisson random impulses, but their
difference has not been fully quantified. Here we analyze a pair of uncoupled
oscillators subject to common random impulses with gamma-distributed intervals,
which can be smoothly interpolated between regular periodic and random Poisson
impulses. Their dynamics are charac- terized by phase distributions, frequency
detuning, Lyapunov exponents, and information-theoretic measures, which clearly
reveal the differences between the two synchronization scenarios.Comment: 18 page
Kondo Temperature Evaluated from Linear Conductance in Magnetic Fields
We theoretically and experimentally study the universal scaling property of
the spin-1/2 Kondo state in the magnetic field dependence of bias-voltage
linear conductance through a quantum dot at low temperatures. We discuss an
efficient and reliable procedure to evaluate the Kondo temperature defined at
the ground state from experimental or numerical data sets of the magnetic field
dependence of the linear conductance or the magnetization of the quantum dot.
This procedure is helpful for quantitative comparison of the theory and the
experiment, and useful in Kondo-correlated systems where temperature control
over a wide range is difficult, such as for cold atoms. We demonstrate its
application to experimentally measured electric current through a carbon
nanotube quantum dot.Comment: 10 pages, 4 figure
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