581 research outputs found
Optical readout of charge and spin in a self-assembled quantum dot in a strong magnetic field
We present a theory and experiment demonstrating optical readout of charge
and spin in a single InAs/GaAs self-assembled quantum dot. By applying a
magnetic field we create the filling factor 2 quantum Hall singlet phase of the
charged exciton. Increasing or decreasing the magnetic field leads to
electronic spin-flip transitions and increasing spin polarization. The
increasing total spin of electrons appears as a manifold of closely spaced
emission lines, while spin flips appear as discontinuities of emission lines.
The number of multiplets and discontinuities measures the number of carriers
and their spin. We present a complete analysis of the emission spectrum of a
single quantum dot with N=4 electrons and a single hole, calculated and
measured in magnetic fields up to 23 Tesla.Comment: 9 pages, 3 figures, submitted to Europhysics Letter
Composite fermions in periodic and random antidot lattices
The longitudinal and Hall magnetoresistance of random and periodic arrays of artificial scatterers, imposed on a high-mobility two-dimensional electron gas, were investigated in the vicinity of Landau level filling factor ν=1/2. In periodic arrays, commensurability effects between the period of the antidot array and the cyclotron radius of composite fermions are observed. In addition, the Hall resistance shows a deviation from the anticipated linear dependence, reminiscent of quenching around zero magnetic field. Both effects are absent for random antidot lattices. The relative amplitude of the geometric resonances for opposite signs of the effective magnetic field and its dependence on illumination illustrate enhanced soft wall effects for composite fermions
Classical percolation fingerprints in the high-temperature regime of the integer quantum Hall effect
We have performed magnetotransport experiments in the high-temperature regime
(up to 50 K) of the integer quantum Hall effect for two-dimensional electron
gases in semiconducting heterostructures. While the magnetic field dependence
of the classical Hall law presents no anomaly at high temperatures, we find a
breakdown of the Drude-Lorentz law for the longitudinal conductance beyond a
crossover magnetic field B_c ~ 1 T, which turns out to be correlated with the
onset of the integer quantum Hall effect at low temperatures. We show that the
high magnetic field regime at B > B_c can be understood in terms of classical
percolative transport in a smooth disordered potential. From the temperature
dependence of the peak longitudinal conductance, we extract scaling exponents
which are in good agreement with the theoretically expected values. We also
prove that inelastic scattering on phonons is responsible for dissipation in a
wide temperature range going from 1 to 50 K at high magnetic fields.Comment: 14 pages + 8 Figure
Quantum interference and phonon-mediated back-action in lateral quantum dot circuits
Spin qubits have been successfully realized in electrostatically defined,
lateral few-electron quantum dot circuits. Qubit readout typically involves
spin to charge information conversion, followed by a charge measurement made
using a nearby biased quantum point contact. It is critical to understand the
back-action disturbances resulting from such a measurement approach. Previous
studies have indicated that quantum point contact detectors emit phonons which
are then absorbed by nearby qubits. We report here the observation of a
pronounced back-action effect in multiple dot circuits where the absorption of
detector-generated phonons is strongly modified by a quantum interference
effect, and show that the phenomenon is well described by a theory
incorporating both the quantum point contact and coherent phonon absorption.
Our combined experimental and theoretical results suggest strategies to
suppress back-action during the qubit readout procedure.Comment: 25 pages, 8 figure
COA6 facilitates cytochrome c oxidase biogenesis as thiol-reductase for copper metallochaperones in mitochondria.
The mitochondrial cytochrome c oxidase, the terminal enzyme of the respiratory chain, contains heme and copper centers for electron transfer. The conserved COX2 subunit contains the CuA site, a binuclear copper center. The copper chaperones SCO1, SCO2, and COA6 are required for CuA center formation. Loss of function of these chaperones and the concomitant cytochrome c oxidase deficiency cause severe human disorders. Here we analyzed the molecular function of COA6 and the consequences of COA6 deficiency for mitochondria. Our analyses show that loss of COA6 causes combined complex I and complex IV deficiency and impacts membrane potential driven protein transport across the inner membrane. We demonstrate that COA6 acts as a thiol-reductase to reduce disulphide bridges of critical cysteine residues in SCO1 and SCO2. Cysteines within the CX3CXNH domain of SCO2 mediate its interaction with COA6 but are dispensable for SCO2-SCO1 interaction. Our analyses define COA6 as thiol-reductase, which is essential for CuA biogenesis
Quantum Hall induced currents and the magnetoresistance of a quantum point contact
We report an investigation of quantum Hall induced currents by simultaneous
measurements of their magnetic moment and their effect on the conductance of a
quantum point contact (QPC). Features in the magnetic moment and QPC resistance
are correlated at Landau-level filling factors nu=1, 2 and 4, which
demonstrates the common origin of the effects. Temperature and non-linear sweep
rate dependences are observed to be similar for the two effects. Furthermore,
features in the noise of the induced currents, caused by breakdown of the
quantum Hall effect, are observed to have clear correlations between the two
measurements. In contrast, there is a distinct difference in the way that the
induced currents decay with time when the sweeping field halts at integer
filling factor. We attribute this difference to the fact that, while both
effects are sensitive to the magnitude of the induced current, the QPC
resistance is also sensitive to the proximity of the current to the QPC
split-gate. Although it is clearly demonstrated that induced currents affect
the electrostatics of a QPC, the reverse effect, the QPC influencing the
induced current, was not observed
Influence of the single-particle Zeeman energy on the quantum Hall ferromagnet at high filling factors
In a recent paper [B. A. Piot et al., Phys. Rev. B 72, 245325 (2005)], we
have shown that the lifting of the electron spin degeneracy in the integer
quantum Hall effect at high filling factors should be interpreted as a
magnetic-field-induced Stoner transition. In this work, we extend the analysis
to investigate the influence of the single-particle Zeeman energy on the
quantum Hall ferromagnet at high filling factors. The single-particle Zeeman
energy is tuned through the application of an additional in-plane magnetic
field. Both the evolution of the spin polarization of the system and the
critical magnetic field for spin splitting are well described as a function of
the tilt angle of the sample in the magnetic field.Comment: Published in Phys. Rev.
Correlated Photon-Pair Emission from a Charged Single Quantum Dot
The optical creation and recombination of charged biexciton and trion
complexes in an (In,Ga)As/GaAs quantum dot is investigated by
micro-photoluminescence spectroscopy. Photon cross-correlation measurements
demonstrate the temporally correlated decay of charged biexciton and trion
states. Our calculations provide strong evidence for radiative decay from the
excited trion state which allows for a deeper insight into the spin
configurations and their dynamics in these systems.Comment: 5 pages, 3 figures, submitted for publicatio
Novel 3D Reciprocal Space Visualization of Strain Relaxation in InSb on GaAs Substrates
This study introduces the Reciprocal Space Polar Visualization (RSPV) method,
a novel approach for visualizing X-ray diffraction-based reciprocal space data.
RSPV allows for the precise separation of tilt and strain, facilitating their
individual analysis. InSb was grown by molecular beam epitaxy (MBE) on two
(001) GaAs substrates \unicode{x2014} one with no misorientation (Sample A)
\unicode{x2014} one with 2{\deg} surface misorientation from the (001) planes
(Sample B). There is a substantial lattice mismatch with the substrate and this
results in the generation of defects within the InSb layer during growth. To
demonstrate RSPV's effectiveness, a comprehensive comparison of surface
morphology, dislocation density, strain, and tilt was conducted. RSPV revealed
previously unobserved features of the (004) InSb Bragg peak, partially
explained by the presence of threading dislocations and oriented abrupt steps
(OASs). Surface morphologies examined by an atomic force microscope (AFM)
revealed that Sample B had significantly lower root mean square (RMS)
roughness. Independent estimates of threading dislocation density (TDD) using
X-ray diffraction (XRD) and electron channelling contrast imaging (ECCI)
confirmed that Sample B exhibited a significantly lower TDD than Sample A. XRD
methods further revealed unequal amounts of and type threading
dislocations in both samples, contributing to an anisotropic Bragg peak. RSPV
is shown to be a robust method for exploring 3D reciprocal space in any
crystal, demonstrating that growing InSb on misoriented GaAs produced a
higher-quality crystal compared to an on-orientation substrate.Comment: 11 pages, 7 figures. This paper will be submitted to Journal of
Vacuum Science and Technology
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