25 research outputs found
Radio frequency pulsed-gate charge spectroscopy on coupled quantum dots
Time-resolved electron dynamics in coupled quantum dots is directly observed
by a pulsed-gate technique. While individual gate voltages are modulated with
periodic pulse trains, average charge occupations are measured with a nearby
quantum point contact as detector. A key component of our setup is a sample
holder optimized for broadband radio frequency applications. Our setup can
detect displacements of single electrons on time scales well below a
nanosecond. Tunneling rates through individual barriers and relaxation times
are obtained by using a rate equation model. We demonstrate the full
characterization of a tunable double quantum dot using this technique, which
could also be used for coherent charge qubit control
Activated Transport in the individual Layers that form the =1 Exciton Condensate
We observe the total filling factor =1 quantum Hall state in a
bilayer two-dimensional electron system with virtually no tunnelling. We find
thermally activated transport in the balanced system with a monotonic increase
of the activation energy with decreasing below 1.65. In the
imbalanced system we find activated transport in each of the layers separately,
yet the activation energies show a striking asymmetry around the balance point.
This implies that the gap to charge-excitations in the {\em individual} layers
is substantially different for positive and negative imbalance.Comment: 4 pages. 4 figure
Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field
We observe mixing between two-electron singlet and triplet states in a double
quantum dot, caused by interactions with nuclear spins in the host
semiconductor. This mixing is suppressed by applying a small magnetic field, or
by increasing the interdot tunnel coupling and thereby the singlet-triplet
splitting. Electron transport involving transitions between triplets and
singlets in turn polarizes the nuclei, resulting in striking bistabilities. We
extract from the fluctuating nuclear field a limitation on the time-averaged
spin coherence time T2* of 25 ns. Control of the electron-nuclear interaction
will therefore be crucial for the coherent manipulation of individual electron
spins.Comment: 4 pages main text, 4 figure
Relaxation of hot electrons in a degenerate two-dimensional electron system: transition to one-dimensional scattering
The energy relaxation channels of hot electrons far from thermal equilibrium
in a degenerate two-dimensional electron system are investigated in transport
experiments in a mesoscopic three-terminal device. We observe a transition from
two dimensions at zero magnetic field to quasi--one-dimensional scattering of
the hot electrons in a strong magnetic field. In the two-dimensional case
electron-electron scattering is the dominant relaxation mechanism, while the
emission of optical phonons becomes more and more important as the magnetic
field is increased. The observation of up to 11 optical phonons emitted per hot
electron allows us to determine the onset energy of LO phonons in GaAs at
cryogenic temperatures with a high precision, \eph=36.0\pm0.1\,meV. Numerical
calculations of electron-electron scattering and the emission of optical
phonons underline our interpretation in terms of a transition to
one-dimensional dynamics.Comment: 15 pages, 9 figure
In situ reduction of charge noise in GaAs/AlGaAs Schottky-gated devices
We show that an insulated electrostatic gate can be used to strongly suppress
ubiquitous background charge noise in Schottky-gated GaAs/AlGaAs devices. Via a
2-D self-consistent simulation of the conduction band profile we show that this
observation can be explained by reduced leakage of electrons from the Schottky
gates into the semiconductor through the Schottky barrier, consistent with the
effect of "bias cooling". Upon noise reduction, the noise power spectrum
generally changes from Lorentzian to type. By comparing wafers with
different Al content, we exclude that DX centers play a dominant role in the
charge noise.Comment: 4 pages, 3 figure
Magneto-Gyrotropic Photogalvanic Effects in Semiconductor Quantum Wells
We show that free-carrier (Drude) absorption of both polarized and
unpolarized terahertz radiation in quantum well (QW) structures causes an
electric photocurrent in the presence of an in-plane magnetic field.
Experimental and theoretical analysis evidences that the observed photocurrents
are spin-dependent and related to the gyrotropy of the QWs. Microscopic models
for the photogalvanic effects in QWs based on asymmetry of photoexcitation and
relaxation processes are proposed. In most of the investigated structures the
observed magneto-induced photocurrents are caused by spin-dependent relaxation
of non-equilibrium carriers
Exciton dephasing and biexciton binding in CdSe/ZnSe islands
The dephasing of excitons and the formation of biexcitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of exciton-exciton scattering efficiencies and dephasing times in the range of 0.5–10 ps are observed. This indicates the presence of differently localized exciton states at comparable transition energies. Polarization-dependent measurements identify the formation of biexcitons with a biexciton binding energy of more than four times the bulk value. With decreasing exciton energy, the binding energy slightly increases from 21.5 to 23 meV, while its broadening decreases from 5.5 to 3 meV. This is attributed to a strong three-dimensional confinement with improving shape uniformity for decreasing exciton energy