118 research outputs found
Fractionalization of minimal excitations in integer quantum Hall edge channels
A theoretical study of the single electron coherence properties of Lorentzian
and rectangular pulses is presented. By combining bosonization and the Floquet
scattering approach, the effect of interactions on a periodic source of voltage
pulses is computed exactly. When such excitations are injected into one of the
channels of a system of two copropagating quantum Hall edge channels, they
fractionalize into pulses whose charge and shape reflects the properties of
interactions. We show that the dependence of fractionalization induced
electron/hole pair production in the pulses amplitude contains clear signatures
of the fractionalization of the individual excitations. We propose an
experimental setup combining a source of Lorentzian pulses and an Hanbury Brown
and Twiss interferometer to measure interaction induced electron/hole pair
production and more generally to reconstruct single electron coherence of these
excitations before and after their fractionalization.Comment: 18 pages, 10 figures, 1 tabl
Noise dephasing in the edge states of the Integer Quantum Hall regime
An electronic Mach Zehnder interferometer is used in the integer quantum hall
regime at filling factor 2, to study the dephasing of the interferences. This
is found to be induced by the electrical noise existing in the edge states
capacitively coupled to each others. Electrical shot noise created in one
channel leads to phase randomization in the other, which destroys the
interference pattern. These findings are extended to the dephasing induced by
thermal noise instead of shot noise: it explains the underlying mechanism
responsible for the finite temperature coherence time of the
edge states at filling factor 2, measured in a recent experiment. Finally, we
present here a theory of the dephasing based on Gaussian noise, which is found
in excellent agreement with our experimental results.Comment: ~4 pages, 4 figure
Quantum capacitance and density of states of graphene
We report on measurements of the quantum capacitance in graphene as a
function of charge carrier density. A resonant LC-circuit giving high
sensitivity to small capacitance changes is employed. The density of states,
which is directly proportional to the quantum capacitance, is found to be
significantly larger than zero at and around the charge neutrality point. This
finding is interpreted to be a result of potential fluctuations with amplitudes
of the order of 100 meV in good agreement with scanning single-electron
transistor measurements on bulk graphene and transport studies on nanoribbons
Photon-assisted shot noise in graphene in the Terahertz range
When subjected to electromagnetic radiation, the fluctuation of the
electronic current across a quantum conductor increases. This additional noise,
called photon-assisted shot noise, arises from the generation and subsequent
partition of electron-hole pairs in the conductor. The physics of
photon-assisted shot noise has been thoroughly investigated at microwave
frequencies up to 20 GHz, and its robustness suggests that it could be extended
to the Terahertz (THz) range. Here, we present measurements of the quantum shot
noise generated in a graphene nanoribbon subjected to a THz radiation. Our
results show signatures of photon-assisted shot noise, further demonstrating
that hallmark time-dependant quantum transport phenomena can be transposed to
the THz range.Comment: includes supplemental materia
Tuning decoherence with a voltage probe
We present an experiment where we tune the decoherence in a quantum
interferometer using one of the simplest object available in the physic of
quantum conductors : an ohmic contact. For that purpose, we designed an
electronic Mach-Zehnder interferometer which has one of its two arms connected
to an ohmic contact through a quantum point contact. At low temperature, we
observe quantum interference patterns with a visibility up to 57%. Increasing
the connection between one arm of the interferometer to the floating ohmic
contact, the voltage probe, reduces quantum interferences as it probes the
electron trajectory. This unique experimental realization of a voltage probe
works as a trivial which-path detector whose efficiency can be simply tuned by
a gate voltage
Robust quantum coherence above the Fermi sea
In this paper we present an experiment where we measured the quantum
coherence of a quasiparticle injected at a well-defined energy above the Fermi
sea into the edge states of the integer quantum Hall regime. Electrons are
introduced in an electronic Mach-Zehnder interferometer after passing through a
quantum dot that plays the role of an energy filter. Measurements show that
above a threshold injection energy, the visibility of the quantum interferences
is almost independent of the energy. This is true even for high energies, up to
130~eV, well above the thermal energy of the measured sample. This result
is in strong contradiction with our theoretical predictions, which instead
predict a continuous decrease of the interference visibility with increasing
energy. This experiment raises serious questions concerning the understanding
of excitations in the integer quantum Hall regime
Intrinsic and extrinsic decay of edge magnetoplasmons in graphene
We investigate intrinsic and extrinsic decay of edge magnetoplasmons (EMPs)
in graphene quantum Hall (QH) systems by high-frequency electronic
measurements. From EMP resonances in disk shaped graphene, we show that the
dispersion relation of EMPs is nonlinear due to interactions, giving rise to
intrinsic decay of EMP wavepacket. We also identify extrinsic dissipation
mechanisms due to interaction with localized states in bulk graphene from the
decay time of EMP wavepackets. We indicate that, owing to the unique linear and
gapless band structure, EMP dissipation in graphene can be lower than that in
GaAs systems.Comment: 5 page
Finite bias visibility of the electronic Mach-Zehnder interferometer
We present an original statistical method to measure the visibility of
interferences in an electronic Mach-Zehnder interferometer in the presence of
low frequency fluctuations. The visibility presents a single side lobe
structure shown to result from a gaussian phase averaging whose variance is
quadratic with the bias. To reinforce our approach and validate our statistical
method, the same experiment is also realized with a stable sample. It exhibits
the same visibility behavior as the fluctuating one, indicating the intrinsic
character of finite bias phase averaging. In both samples, the dilution of the
impinging current reduces the variance of the gaussian distribution.Comment: 4 pages, 5 figure
A Josephson relation for fractionally charged anyons.
Anyons occur in two-dimensional electron systems as excitations with fractional charge in the topologically ordered states of the fractional quantum Hall effect (FQHE). Their dynamics are of utmost importance for topological quantum phases and possible decoherence-free quantum information approaches, but observing these dynamics experimentally is challenging. Here, we report on a dynamical property of anyons: the long-predicted Josephson relation f J = e*V/h for charges e* = e/3 and e/5, where e is the charge of the electron and h is Planck's constant. The relation manifests itself as marked signatures in the dependence of photo-assisted shot noise (PASN) on voltage V when irradiating contacts at microwaves frequency f J The validation of FQHE PASN models indicates a path toward realizing time-resolved anyon sources based on levitons
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