4,375 research outputs found
Break junctions of the heavy-fermion superconductors
Mechanical-controllable break junctions of the heavy-fermion superconductors
can show Josephson-like superconducting anomalies. But a systematic study on
the contact size demonstrates that these anomalies are mainly due to Maxwell's
resistance being suppressed in the superconducting heavy-fermion phase. Up to
day, we could not find any superconducting features by vacuum-tunnelling
spectroscopy, providing further evidence for the pair-breaking effect of the
heavy-fermion interfaces.Comment: 5 pages, EPS figures included, REVTeX, to be published in Physica B
9
Simultaneous Spin-Charge Relaxation in Double Quantum Dots
We investigate phonon-induced spin and charge relaxation mediated by
spin-orbit and hyperfine interactions for a single electron confined within a
double quantum dot. A simple toy model incorporating both direct decay to the
ground state of the double dot and indirect decay via an intermediate excited
state yields an electron spin relaxation rate that varies non-monotonically
with the detuning between the dots. We confirm this model with experiments
performed on a GaAs double dot, demonstrating that the relaxation rate exhibits
the expected detuning dependence and can be electrically tuned over several
orders of magnitude. Our analysis suggests that spin-orbit mediated relaxation
via phonons serves as the dominant mechanism through which the double-dot
electron spin-flip rate varies with detuning.Comment: 5 pages, 3 figures, Supplemental Material (2 pages, 2 figures
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
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
Resolving Spin-Orbit and Hyperfine Mediated Electric Dipole Spin Resonance in a Quantum Dot
We investigate the electric manipulation of a single electron spin in a
single gate-defined quantum dot. We observe that so-far neglected differences
between the hyperfine and spin-orbit mediated electric dipole spin resonance
conditions have important consequences at high magnetic fields. In experiments
using adiabatic rapid passage to invert the electron spin, we observe an
unusually wide and asymmetric response as a function of magnetic field.
Simulations support the interpretation of the lineshape in terms of four
different resonance conditions. These findings may lead to isotope-selective
control of dynamic nuclear polarization in quantum dots
Nuclear Spin Effects in Semiconductor Quantum Dots
The interaction of an electronic spin with its nuclear environment, an issue known as the central spin problem, has been the subject of considerable attention due to its relevance for spin-based quantum computation using semiconductor quantum dots. Independent control of the nuclear spin bath using nuclear magnetic resonance techniques and dynamic nuclear polarization using the central spin itself offer unique possibilities for manipulating the nuclear bath with significant consequences for the coherence and controlled manipulation of the central spin. Here we review some of the recent optical and transport experiments that have explored this central spin problem using semiconductor quantum dots. We focus on the interaction between nuclear spins and a spin of a single electron or valence-band hole. We also review the experimental techniques as well as the key theoretical ideas and the implications for quantum information science.Physic
Multiple Nuclear Polarization States in a Double Quantum Dot
We observe multiple stable states of nuclear polarization in a double quantum
dot under conditions of electron spin resonance. The stable states can be
understood within an elaborated theoretical rate equation model for the
polarization in each of the dots, in the limit of strong driving. This model
also captures unusual features of the data, such as fast switching and a
`wrong' sign of polarization. The results reported enable applications of this
polarization effect, including manipulation and control of nuclear fields.Comment: 5 pages, 4 figures, 7 pages supplementary materia
Verification and intercomparison of reactive transport codes to describe root-uptake
Several mathematical models have been developed to simulate processes and interactions in the plant rhizosphere. Most of these models are based on a rather simplified description of the soil chemistry and interactions of plant roots in the rhizosphere. In particular the feedback loops between exudation, water and solute uptake are mostly not considered, although their importance in the bioavailability of mineral elements for plants has been demonstrated. The aim of this work was to evaluate three existing coupled speciation-transport tools to model rhizosphere processes. In the field of hydrogeochemistry, such␣computational tools have been developed to␣describe acid-base and redox reactions, complexation and ion exchange, adsorption and precipitation of chemical species in soils and aquifers using thermodynamic and kinetic relationships. We implemented and tested a simple rhizosphere model with three geochemical computational tools (ORCHESTRA, MIN3P, and PHREEQC). The first step was an accuracy analysis of the different solution strategies by comparing the numerical results to the analytical solution of solute uptake (K or Ca) by a single cylindrical root. All models are able to reproduce the concentration profiles as well as the uptake flux. The relative error of the simulated concentration profile decreases with increasing distance from the root. The uptake flux was simulated for all codes with less than 5% error for K and less than 0.4% for Ca. The strength of the codes presented in this paper is that they can also be used to investigate more complex and coupled biogeochemical processes in rhizosphere models. This is shown exemplarily with simulations involving both exudation and uptake and the simultaneous uptake of solute and wate
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