87 research outputs found
Measurement back-action on adiabatic coherent electron transport
We study the back-action of a nearby measurement device on electrons
undergoing coherent transfer via adiabatic passage (CTAP) in a triple-well
system. The measurement is provided by a quantum point contact capacitively
coupled to the middle well, thus acting as a detector sensitive to the charge
configuration of the triple-well system. We account for this continuous
measurement by treating the whole {triple-well + detector} as a closed quantum
system. This leads to a set of coupled differential equations for the density
matrix of the enlarged system which we solve numerically. This approach allows
to study a single realization of the measurement process while keeping track of
the detector output, which is especially relevant for experiments. In
particular, we find the emergence of a new peak in the distribution of
electrons that passed through the point contact. As one increases the coupling
between the middle potential well and the detector, this feature becomes more
prominent and is accompanied by a substantial drop in the fidelity of the CTAP
scheme
Transport properties of partially equilibrated quantum wires
We study the effect of thermal equilibration on the transport properties of a
weakly interacting one-dimensional electron system. Although equilibration is
severely suppressed due to phase-space restrictions and conservation laws, it
can lead to intriguing signatures in partially equilibrated quantum wires. We
consider an ideal homogeneous quantum wire. We find a finite temperature
correction to the quantized conductance, which for a short wire scales with its
length, but saturates to a length-independent value once the wire becomes
exponentially long. We also discuss thermoelectric properties of long quantum
wires. We show that the uniform quantum wire is a perfect thermoelectric
refrigerator, approaching Carnot efficiency with increasing wire length.Comment: 20 pages, 6 figure
Resistivity of inhomogeneous quantum wires
We study the effect of electron-electron interactions on the transport in an
inhomogeneous quantum wire. We show that contrary to the well-known Luttinger
liquid result, non-uniform interactions contribute substantially to the
resistance of the wire. In the regime of weakly interacting electrons and
moderately low temperatures we find a linear in T resistivity induced by the
interactions. We then use the bosonization technique to generalize this result
to the case of arbitrarily strong interactions.Comment: 4 pages, 1 figur
Interactions and charge fractionalization in an electronic Hong-Ou-Mandel interferometer
We consider an electronic analog of the Hong-Ou-Mandel (HOM) interferometer,
where two single electrons travel along opposite chiral edge states and collide
at a Quantum Point Contact. Studying the current noise, we show that because of
interactions between co-propagating edge states, the degree of
indistinguishability between the two electron wavepackets is dramatically
reduced, leading to reduced contrast for the HOM signal. This decoherence
phenomenon strongly depends on the energy resolution of the packets. Insofar as
interactions cause charge fractionalization, we show that charge and neutral
modes interfere with each other, leading to satellite dips or peaks in the
current noise. Our calculations explain recent experimental results [E.
Bocquillon, et al., Science 339, 1054(2013)] where an electronic HOM signal
with reduced contrast was observed.Comment: 5 pages, 2 figure
Poissonian tunneling through an extended impurity in the quantum Hall effect
We consider transport in the Poissonian regime between edge states in the
quantum Hall effect. The backscattering potential is assumed to be arbitrary,
as it allows for multiple tunneling paths. We show that the Schottky relation
between the backscattering current and noise can be established in full
generality: the Fano factor corresponds to the electron charge (the
quasiparticle charge) in the integer (fractional) quantum Hall effect, as in
the case of purely local tunneling. We derive an analytical expression for the
backscattering current, which can be written as that of a local tunneling
current, albeit with a renormalized tunneling amplitude which depends on the
voltage bias. We apply our results to a separable tunneling amplitude which can
represent an extended point contact in the integer or in the fractional quantum
Hall effect. We show that the differential conductance of an extended quantum
point contact is suppressed by the interference between tunneling paths, and it
has an anomalous dependence with respect to the bias voltage
Crystallization of Levitons in the fractional quantum Hall regime
Using a periodic train of Lorentzian voltage pulses, which generates
soliton-like electronic excitations called Levitons, we investigate the charge
density backscattered off a quantum point contact in the fractional quantum
Hall regime. We find a regular pattern of peaks and valleys, reminiscent of
analogous self-organization recently observed for optical solitons in
non-linear environments. This crystallization phenomenon is confirmed by
additional side dips in the Hong-Ou-Mandel noise, a feature that can be
observed in nowadays electron quantum optics experiments.Comment: 9 pages, 4 figure
Minimal excitation states for heat transport in driven quantum Hall systems
We investigate minimal excitation states for heat transport into a fractional
quantum Hall system driven out of equilibrium by means of time-periodic voltage
pulses. A quantum point contact allows for tunneling of fractional
quasi-particles between opposite edge states, thus acting as a beam splitter in
the framework of the electron quantum optics. Excitations are then studied
through heat and mixed noise generated by the random partitioning at the
barrier. It is shown that levitons, the single-particle excitations of a filled
Fermi sea recently observed in experiments, represent the cleanest states for
heat transport, since excess heat and mixed shot noise both vanish only when
Lorentzian voltage pulses carrying integer electric charge are applied to the
conductor. This happens in the integer quantum Hall regime and for Laughlin
fractional states as well, with no influence of fractional physics on the
conditions for clean energy pulses. In addition, we demonstrate the robustness
of such excitations to the overlap of Lorentzian wavepackets. Even though mixed
and heat noise have nonlinear dependence on the voltage bias, and despite the
non-integer power-law behavior arising from the fractional quantum Hall
physics, an arbitrary superposition of levitons always generates minimal
excitation states.Comment: 15 pages, 7 figure
Conductance of fully equilibrated quantum wires
We study the conductance of a quantum wire in the presence of weak
electron-electron scattering. In a sufficiently long wire the scattering leads
to full equilibration of the electron distribution function in the frame moving
with the electric current. At non-zero temperature this equilibrium
distribution differs from the one supplied by the leads. As a result the
contact resistance increases, and the quantized conductance of the wire
acquires a quadratic in temperature correction. The magnitude of the correction
is found by analysis of the conservation laws of the system and does not depend
on the details of the interaction mechanism responsible for equilibration.Comment: 4 pages, 2 figure
Two experimental set-ups designed for investigation of friction stir spot welding process
International audienceThe effects of positioning and clamping conditions of a specimen of friction stir spot welding are investigated in this paper in terms of axial force and torque generated during the process. For this purpose, two special designs of experimental set-ups embedding different positioning and clamping conditions are presented. A four-component mechanical sensor is used for the measurements. First, the effects of the rotational speed of the spindle and the plunge depth of the tool on the axial force and torque are studied. Second, the effects of positioning and clamping conditions are investigated through both set-ups designed, varying the spindle rotation speed. It is shown that the axial force and torque exhibit an important dependence with respect to the rotation speed of the tool and that their maxima depend on positioning and clamping conditions of the specimen
Quantum critical behavior in itinerant electron systems -- Eliashberg theory and instability of a ferromagnetic quantum-critical point
We consider the problem of fermions interacting with gapless long-wavelength
collective bosonic modes. The theory describes, among other cases, a
ferromagnetic quantum-critical point (QCP) and a QCP towards nematic ordering.
We construct a controllable expansion at the QCP in two steps: we first create
a new, non Fermi-liquid ``zero-order'' Eliashberg-type theory, and then
demonstrate that the residual interaction effects are small. We prove that this
approach is justified under two conditions: the interaction should be smaller
than the fermionic bandwidth, and either the band mass should be much
smaller than , or the number of fermionic flavors should be
large. For an SU(2) symmetric ferromagnetic QCP, we find that the Eliashberg
theory itself includes a set of singular renormalizations which can be
understood as a consequence of an effective long-range dynamic interaction
between quasi-particles, generated by the Landau damping term. These singular
renormalizations give rise to a negative non-analytic correction to
the static spin susceptibility, and destroy a ferromagnetic QCP. We demonstrate
that this effect can be understood in the framework of the theory of
quantum criticality. We also show that the non-analytic correction to
the bosonic propagator is specific to the SU(2) symmetric case. For systems
with a scalar order parameter, the contributions from individual
diagrams cancel out in the full expression of the susceptibility, and the QCP
remains stable.Comment: 37 pages, 10 fig
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