250 research outputs found
The visibility study of S-T Landau-Zener-St\"uckelberg oscillations without applied initialization
Probabilities deduced from quantum information studies are usually based on
averaging many identical experiments separated by an initialization step. Such
initialization steps become experimentally more challenging to implement as the
complexity of quantum circuits increases. To better understand the consequences
of imperfect initialization on the deduced probabilities, we study the effect
of not initializing the system between measurements. For this we utilize
Landau-Zener-St\"uckelberg oscillations in a double quantum dot circuit.
Experimental results are successfully compared to theoretical simulations.Comment: 8 pages, 5 figure
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
From laterally modulated two-dimensional electron gas towards artificial graphene
Cyclotron resonance has been measured in far-infrared transmission of
GaAs/AlGaAs heterostructures with an etched hexagonal lateral
superlattice. Non-linear dependence of the resonance position on magnetic field
was observed as well as its splitting into several modes. Our explanation,
based on a perturbative calculation, describes the observed phenomena as a weak
effect of the lateral potential on the two-dimensional electron gas. Using this
approach, we found a correlation between parameters of the lateral patterning
and the created effective potential and obtain thus insights on how the
electronic miniband structure has been tuned. The miniband dispersion was
calculated using a simplified model and allowed us to formulate four basic
criteria that have to be satisfied to reach graphene-like physics in such
systems
Enhanced charge detection of spin qubit readout via an intermediate state
We employ an intermediate excited charge state of a lateral quantum dot
device to increase the charge detection contrast during the qubit state readout
procedure, allowing us to increase the visibility of coherent qubit
oscillations. This approach amplifies the coherent oscillation magnitude but
has no effect on the detector noise resulting in an increase in the signal to
noise ratio. In this letter we apply this scheme to demonstrate a significant
enhancement of the fringe contrast of coherent Landau-Zener-Stuckleberg
oscillations between singlet S and triplet T+ two-spin states.Comment: 3 pages, 3 figure
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 Μ=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
Bipolar spin blockade and coherent state superpositions in a triple quantum dot
Spin qubits based on interacting spins in double quantum dots have been
successfully demonstrated. Readout of the qubit state involves a conversion of
spin to charge information, universally achieved by taking advantage of a spin
blockade phenomenon resulting from Pauli's exclusion principle. The archetypal
spin blockade transport signature in double quantum dots takes the form of a
rectified current. Currently more complex spin qubit circuits including triple
quantum dots are being developed. Here we show both experimentally and
theoretically (a) that in a linear triple quantum dot circuit, the spin
blockade becomes bipolar with current strongly suppressed in both bias
directions and (b) that a new quantum coherent mechanism becomes relevant.
Within this mechanism charge is transferred non-intuitively via coherent states
from one end of the linear triple dot circuit to the other without involving
the centre site. Our results have implications in future complex
nano-spintronic circuits.Comment: 21 pages, 7 figure
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
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.
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
Dispersive line shape in the vicinity of the {\nu} = 1 quantum Hall state: Coexistence of Knight shifted and unshifted resistively detected NMR responses
The frequency splitting between the dip and the peak of the resistively
detected nuclear magnetic resonance (RDNMR) dispersive line shape (DLS) has
been measured in the quantum Hall effect regime as a function of filling
factor, carrier density and nuclear isotope. The splitting increases as the
filling factor tends to {\nu} = 1 and is proportional to the hyperfine
coupling, similar to the usual Knight shift versus {\nu}-dependence. The peak
frequency shifts linearly with magnetic field throughout the studied filling
factor range and matches the unshifted substrate signal, detected by classical
NMR. Thus, the evolution of the splitting is entirely due to the changing
Knight shift of the dip feature. The nuclear spin relaxation time, T1, is
extremely long (hours) at precisely the peak frequency. These results are
consistent with the local formation of a {\nu} = 2 phase due to the existence
of spin singlet D complexes.Comment: to be published in Rapid Communication PR
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