256 research outputs found
Quantum ballistic transport in in-plane-gate transistors showing onset of a novel ferromagnetic phase transition
We study one-dimensional transport in focused-ion-beam written in-plane-gate
transistors on III-V heterostructures at moderately low temperatures at zero
bias without any external magnetic field applied. In accordance with a recent
proposal of A. Gold and L. Calmels, Valley- and spin-occupancy instability in
the quasi-one-dimensional electron gas, Phil. Mag. Lett. 74, 33-42 (1996) and
earlier experimental data, we observe plateaux in the source-drain conductivity
considered as a function of the gate voltage, not only at multliples of 2e^2/h
but also clearly at e^2/h, just before the channel closes to zero conductivity.
This may be interpreted as a many electron effect, namely as a novel ballistic
ferromagnetic ground state evading standard descriptions and theorems.Comment: 19 pages, 9 figures, 22 reference
Single-hole transistor in p-type GaAs/AlGaAs heterostructures
A single-hole transistor is patterned in a p-type, C-doped GaAs/AlGaAs
heterostructure by AFM oxidation lithography. Clear Coulomb blockade resonances
have been observed at T=300 mK. A charging energy of ~ 1.5 meV is extracted
from Coulomb diamond measurements, in agreement with the lithographic
dimensions of the dot. The absence of excited states in Coulomb diamond
measurements, as well as the temperature dependence of Coulomb peak heights
indicate that the dot is in the multi-level transport regime. Fluctuations in
peak spacings larger than the estimated mean single-particle level spacing are
observed.Comment: 4 pages, 5 figure
Asymmetry of charge relaxation times in quantum dots: The influence of degeneracy
Using time-resolved transconductance spectroscopy, we study the tunneling
dynamics between a two-dimensional electron gas (2DEG) and self-assembled
quantum dots (QDs), embedded in a field-effect transistor structure. We find
that the tunneling of electrons from the 2DEG into the QDs is governed by a
different time constant than the reverse process, i.e., tunneling from the QDs
to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and
makes it possible to determine the degeneracy of the quantum dot orbitals even
when the individual states cannot be resolved energetically because of
inhomogeneous broadening. Our experimental data can be qualitatively explained
within a master-equation approach
Magneto-optical probing of weak disorder in a two-dimensional hole gas
In two-beam magneto-photoluminescence spectra of a two-dimensional valence
hole gas we identify the three-level energy spectrum of a free positive trion
with a field-induced singlet-triplet transition. The recombination spectrum of
acceptor-bound trions is also detected, including a cyclotron replica
corresponding to the hole shake-up process. The emergence of a shake-up peak at
low temperature is shown to be a sensitive probe of the presence of a small
number of impurities inside the high-mobility quantum well, and its relative
position is directly related to the hole cyclotron mass.Comment: 4 pages, 5 figure
Ergodic vs diffusive decoherence in mesoscopic devices
We report on the measurement of phase coherence length in a high mobility
two-dimensional electron gas patterned in two different geometries, a wire and
a ring. The phase coherence length is extracted both from the weak localization
correction in long wires and from the amplitude of the Aharonov-Bohm
oscillations in a single ring, in a low temperature regime when decoherence is
dominated by electronic interactions. We show that these two measurements lead
to different phase coherence lengths, namely and . This difference
reflects the fact that the electrons winding around the ring necessarily
explore the whole sample (ergodic trajectories), while in a long wire the
electrons lose their phase coherence before reaching the edges of the sample
(diffusive regime).Comment: LaTeX, 5 pages, 4 pdf figures ; v2: revised versio
A few-electron quadruple quantum dot in a closed loop
We report the realization of a quadruple quantum dot device in a square-like
configuration where a single electron can be transferred on a closed path free
of other electrons. By studying the stability diagrams of this system, we
demonstrate that we are able to reach the few-electron regime and to control
the electronic population of each quantum dot with gate voltages. This allows
us to control the transfer of a single electron on a closed path inside the
quadruple dot system. This work opens the route towards electron spin
manipulation using spin-orbit interaction by moving an electron on complex
paths free of electron
Transform-limited single photons from a single quantum dot
A semiconductor quantum dot mimics a two-level atom. Performance as a single
photon source is limited by decoherence and dephasing of the optical
transition. Even with high quality material at low temperature, the optical
linewidths are a factor of two larger than the transform-limit. A major
contributor to the inhomogeneous linewdith is the nuclear spin noise. We show
here that the nuclear spin noise depends on optical excitation, increasing
(decreasing) with increasing resonant laser power for the neutral (charged)
exciton. Based on this observation, we discover regimes where we demonstrate
transform-limited linewidths on both neutral and charged excitons even when the
measurement is performed very slowly
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