114 research outputs found
A Coherent RC Circuit
We review the first experiment on dynamic transport in a phase-coherent
quantum conductor. In our discussion, we highlight the use of time-dependent
transport as a means of gaining insight into charge relaxation on a mesoscopic
scale. For this purpose, we studied the ac conductance of a model quantum
conductor, i.e. the quantum RC circuit. Prior to our experimental work, M.
B\"{u}ttiker, H. Thomas and A. Pr\^{e}tre first worked on dynamic mesoscopic
transport in the 1990s. They predicted that the mesoscopic RC circuit can be
described by a quantum capacitance related to the density of states in the
capacitor and a constant charge relaxation resistance value equal to half of
the resistance quantum h/2e^2, when a single mode is transmitted between the
capacitance and a reservoir. By applying a microwave excitation to a gate
located on top of a coherent submicronic quantum dot that is coupled to a
reservoir, we validate this theoretical prediction on the ac conductance of the
quantum RC circuit. Our study demonstrates that the ac conductance is directly
related to the dwell time of electrons in the capacitor. Thereby, we observed a
counterintuitive behavior of a quantum origin: as the transmission of the
single conducting mode decreases, the resistance of the quantum RC circuit
remains constant while the capacitance oscillates.Comment: 30 page
Violation of Kirchhoff's Laws for a Coherent RC Circuit
What is the complex impedance of a fully coherent quantum
resistance-capacitance (RC) circuit at GHz frequencies in which a resistor and
a capacitor are connected in series? While Kirchhoff's laws predict addition of
capacitor and resistor impedances, we report on observation of a different
behavior. The resistance, here associated with charge relaxation, differs from
the usual transport resistance given by the Landauer formula. In particular,
for a single mode conductor, the charge relaxation resistance is half the
resistance quantum, regardless of the transmission of the mode. The new
mesoscopic effect reported here is relevant for the dynamical regime of all
quantum devices
The relaxation time of a chiral quantum R-L circuit
We report on the GHz complex admittance of a chiral one dimensional ballistic
conductor formed by edge states in the quantum Hall regime. The circuit
consists of a wide Hall bar (the inductor L) in series with a tunable resistor
(R) formed by a quantum point contact. Electron interactions between edges are
screened by a pair of side gates. Conductance steps are observed on both real
and imaginary parts of the admittance. Remarkably, the phase of the admittance
is transmission-independent. This shows that the relaxation time of a chiral
R-L circuit is resistance independent. A current and charge conserving
scattering theory is presented that accounts for this observation with a
relaxation time given by the electronic transit time in the c cuit
Dirac fermion reflector by ballistic graphene sawtooth-shaped npn junctions
We have realized a Dirac fermion reflector in graphene by controlling the
ballistic carrier trajectory in a sawtooth-shaped npn junction. When the
carrier density in the inner p-region is much larger than that in the outer
n-regions, the first straight np interface works as a collimator and the
collimated ballistic carriers can be totally reflected at the second zigzag pn
interface. We observed clear resistance enhancement around the np+n regime,
which is in good agreement with the numerical simulation. The tunable
reflectance of ballistic carriers could be an elementary and important step for
realizing ultrahigh-mobility graphene field effect transistors utilizing Dirac
fermion optics in the near future
An On-Demand Coherent Single Electron Source
We report on the electron analog of the single photon gun. On demand single
electron injection in a quantum conductor was obtained using a quantum dot
connected to the conductor via a tunnel barrier. Electron emission is triggered
by application of a potential step which compensates the dot charging energy.
Depending on the barrier transparency the quantum emission time ranges from 0.1
to 10 nanoseconds. The single electron source should prove useful for the
implementation of quantum bits in ballistic conductors. Additionally periodic
sequences of single electron emission and absorption generate a quantized
AC-current
A high sensitivity ultra-low temperature RF conductance and noise measurement setup
We report on the realization of a high sensitivity RF noise measurement
scheme to study small current fluctuations of mesoscopic systems at milliKelvin
temperatures. The setup relies on the combination of an interferometric ampli-
fication scheme and a quarter-wave impedance transformer, allowing the mea-
surement of noise power spectral densities with GHz bandwith up to five orders
of magnitude below the amplifier noise floor. We simultaneously measure the
high frequency conductance of the sample by derivating a portion of the signal
to a microwave homodyne detection. We describe the principle of the setup, as
well as its implementation and calibration. Finally, we show that our setup
allows to fully characterize a subnanosecond on-demand single electron source.
More generally, its sensitivity and bandwith make it suitable for applications
manipulating single charges at GHz frequencies.Comment: The following article has been submitted to Review of Scientific
Instrument
Excitonic recombinations in hBN: from bulk to exfoliated layers
Hexagonal boron nitride (h-BN) and graphite are structurally similar but with
very different properties. Their combination in graphene-based devices meets
now a huge research focus, and it becomes particularly important to evaluate
the role played by crystalline defects in them. In this work, the
cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are
reported and compared to those of nanosheets mechanically exfoliated from them.
First the link between the presence of structural defects and the recombination
intensity of bound-excitons, the so-called D series, is confirmed. Low
defective h-BN regions are further evidenced by CL spectral mapping
(hyperspectral imaging), allowing us to observe new features in the
near-band-edge region, tentatively attributed to phonon replica of exciton
recombinations. Second the h-BN thickness was reduced down to six atomic
layers, using mechanical exfoliation, as evidenced by atomic force microscopy.
Even at these low thicknesses, the luminescence remains intense and exciton
recombination energies are not strongly modified with respect to the bulk, as
expected from theoretical calculations indicating extremely compact excitons in
h-BN
Coherence and Indistinguishability of Single Electrons Emitted by Independent Sources
The on-demand emission of coherent and indistinguishable electrons by
independent synchronized sources is a challenging task of quantum electronics,
in particular regarding its application for quantum information processing.
Using two independent on-demand electron sources, we trigger the emission of
two single-electron wavepackets at different inputs of an electronic
beamsplitter. Whereas classical particles would be randomly partitioned by the
splitter, we observe two-particle interferences resulting from quantum
exchange. Both electrons, emitted in indistinguishable wavepackets with
synchronized arrival time on the splitter, exit in different outputs as
recorded by the low frequency current noise. The demonstration of two-electron
interference provides the possibility to manipulate coherent and
indistinguishable single-electron wavepackets in quantum conductors.Comment: Science Express of January 24 201
Klein-tunneling transistor with ballistic graphene
Today the availability of high mobility graphene up to room temperature makes
ballistic transport in nanodevices achievable. In particular, p-n-p transistor
in the ballistic regime gives access to the Klein tunneling physics and allows
the realization of devices exploiting the optics-like behavior of Dirac
Fermions (DF) as in the Vesalego lens or the Fabry P\'erot cavity. Here we
propose a Klein tunneling transistor based on geometrical optics of DF. We
consider the case of a prismatic active region delimited by a triangular gate,
where total internal reflection may occur, which leads to the tunable
suppression of the transistor transmission. We calculate the transmission and
the current by means of scattering theory and the finite bias properties using
Non Equilibrium Green's Function(NEGF) simulation.Comment: 4 pages, 5 figure
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