442 research outputs found
A Generic Model for Current Collapse in Spin Blockaded Transport
A decrease in current with increasing voltage, often referred to as negative
differential resistance (NDR), has been observed in many electronic devices and
can usually be understood within a one-electron picture. However, NDR has
recently been reported in nanoscale devices with large single-electron charging
energies which require a many-electron picture in Fock space. This paper
presents a generic model in this transport regime leading to a simple criterion
for the conditions required to observe NDR and shows that this model describes
the recent observation of multiple NDR's in Spin Blockaded transport through
weakly coupled-double quantum dots quite well. This model shows clearly how a
delicate interplay of orbital energy offset, delocalization and Coulomb
interaction lead to the observed NDR under the right conditions, and also aids
in obtaining a good match with experimentally observed features. We believe the
basic model could be useful in understanding other experiments in this
transport regime as well.Comment: 10 pages, 10 figures. to appear in Phys Rev
Single-molecule study for a graphene-based nano-position sensor
In this study we lay the groundwork for a graphene-based fundamental ruler at
the nanoscale. It relies on the efficient energy-transfer mechanism between
single quantum emitters and low-doped graphene monolayers. Our experiments,
conducted with dibenzoterrylene (DBT) molecules, allow going beyond ensemble
analysis due to the emitter photo-stability and brightness. A quantitative
characterization of the fluorescence decay-rate modification is presented and
compared to a simple model, showing agreement with the dependence, a
genuine manifestation of a dipole interacting with a 2D material. With DBT
molecules, we can estimate a potential uncertainty in position measurements as
low as 5nm in the range below 30nm
Nuclear Spin Dynamics in Double Quantum Dots: Fixed Points, Transients, and Intermittency
Transport through spin-blockaded quantum dots provides a means for electrical
control and detection of nuclear spin dynamics in the host material. Although
such experiments have become increasingly popular in recent years,
interpretation of their results in terms of the underlying nuclear spin
dynamics remains challenging. Here we point out a fundamental process in which
nuclear spin dynamics can be driven by electron shot noise; fast electric
current fluctuations generate much slower nuclear polarization dynamics, which
in turn affect electron dynamics via the Overhauser field. The resulting
extremely slow intermittent current fluctuations account for a variety of
observed phenomena that were not previously understood.Comment: version accepted for publication in Physical Review B, figure
repaire
Universal phase shift and non-exponential decay of driven single-spin oscillations
We study, both theoretically and experimentally, driven Rabi oscillations of
a single electron spin coupled to a nuclear spin bath. Due to the long
correlation time of the bath, two unusual features are observed in the
oscillations. The decay follows a power law, and the oscillations are shifted
in phase by a universal value of ~pi/4. These properties are well understood
from a theoretical expression that we derive here in the static limit for the
nuclear bath. This improved understanding of the coupled electron-nuclear
system is important for future experiments using the electron spin as a qubit.Comment: Main text: 4 pages, 3 figures, Supplementary material: 2 pages, 3
figure
Spin-echo of a single electron spin in a quantum dot
We report a measurement of the spin-echo decay of a single electron spin
confined in a semiconductor quantum dot. When we tip the spin in the transverse
plane via a magnetic field burst, it dephases in 37 ns due to the Larmor
precession around a random effective field from the nuclear spins in the host
material. We reverse this dephasing to a large extent via a spin-echo pulse,
and find a spin-echo decay time of about 0.5 microseconds at 70 mT. These
results are in the range of theoretical predictions of the electron spin
coherence time governed by the dynamics of the electron-nuclear system.Comment: 5 pages, 4 figure
Detection of single electron spin resonance in a double quantum dot
Spin-dependent transport measurements through a double quantum dot are a
valuable tool for detecting both the coherent evolution of the spin state of a
single electron as well as the hybridization of two-electron spin states. In
this paper, we discuss a model that describes the transport cycle in this
regime, including the effects of an oscillating magnetic field (causing
electron spin resonance) and the effective nuclear fields on the spin states in
the two dots. We numerically calculate the current flow due to the induced spin
flips via electron spin resonance and we study the detector efficiency for a
range of parameters. The experimental data are compared with the model and we
find a reasonable agreement.Comment: 7 pages, 5 figures. To be published in Journal of Applied Physics,
proceedings ICPS 200
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