7,096 research outputs found
Spin noise spectroscopy in GaAs
We observe the noise spectrum of electron spins in bulk GaAs by Faraday
rotation noise spectroscopy. The experimental technique enables the undisturbed
measurement of the electron spin dynamics in semiconductors. We measure
exemplarily the electron spin relaxation time and the electron Lande g-factor
in n-doped GaAs at low temperatures and find good agreement of the measured
noise spectrum with an unpretentious theory based on Poisson distribution
probability.Comment: 4 pages, 4 figure
Interaction-Induced Spin Polarization in Quantum Dots
The electronic states of lateral many electron quantum dots in high magnetic
fields are analyzed in terms of energy and spin. In a regime with two Landau
levels in the dot, several Coulomb blockade peaks are measured. A zig-zag
pattern is found as it is known from the Fock-Darwin spectrum. However, only
data from Landau level 0 show the typical spin-induced bimodality, whereas
features from Landau level 1 cannot be explained with the Fock-Darwin picture.
Instead, by including the interaction effects within spin-density-functional
theory a good agreement between experiment and theory is obtained. The absence
of bimodality on Landau level 1 is found to be due to strong spin polarization.Comment: 4 pages, 5 figure
Multiple transitions of the spin configuration in quantum dots
Single electron tunneling is studied in a many electron quantum dot in high
magnetic fields. For such a system multiple transitions of the spin
configuration are theoretically predicted. With a combination of spin blockade
and Kondo effect we are able to detect five regions with different spin
configurations. Transitions are induced with changing electron numbers.Comment: 4 pages, 5 figure
Non-invasive detection of molecular bonds in quantum dots
We performed charge detection on a lateral triple quantum dot with star-like
geometry. The setup allows us to interpret the results in terms of two double
dots with one common dot. One double dot features weak tunnel coupling and can
be understood with atom-like electronic states, the other one is strongly
coupled forming molecule-like states. In nonlinear measurements we identified
patterns that can be analyzed in terms of the symmetry of tunneling rates.
Those patterns strongly depend on the strength of interdot tunnel coupling and
are completely different for atomic- or molecule-like coupled quantum dots
allowing the non-invasive detection of molecular bonds.Comment: 4 pages, 4 figure
Channel Blockade in a Two-Path Triple-Quantum-Dot System
Electronic transport through a two-path triple-quantum-dot system with two
source leads and one drain is studied. By separating the conductance of the two
double dot paths, we are able to observe double dot and triple dot physics in
transport and study the interaction between the paths. We observe channel
blockade as a result of inter-channel Coulomb interaction. The experimental
results are understood with the help of a theoretical model which calculates
the parameters of the system, the stability regions of each state and the full
dynamical transport in the triple dot resonances.Comment: 6 pages, 6 figure
Probing a Kondo correlated quantum dot with spin spectroscopy
We investigate Kondo effect and spin blockade observed on a many-electron
quantum dot and study the magnetic field dependence. At lower fields a
pronounced Kondo effect is found which is replaced by spin blockade at higher
fields. In an intermediate regime both effects are visible. We make use of this
combined effect to gain information about the internal spin configuration of
our quantum dot. We find that the data cannot be explained assuming regular
filling of electronic orbitals. Instead spin polarized filling seems to be
probable.Comment: 4 pages, 5 figure
Giant anisotropy of Zeeman splitting of quantum confined acceptors in Si/Ge
Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are
characterized by resonant tunneling spectroscopy in the presence of high
magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman
splitting of the acceptor levels. In an in-plane field, on the other hand, the
Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is
shown to be a consequence of the huge light hole-heavy hole splitting caused by
a large biaxial strain and a strong quantum confinement in the Ge quantum well.Comment: 5 figures, 4 page
Spin Blockade in Capacitively Coupled Quantum Dots
We present transport measurements on a lateral double dot produced by
combining local anodic oxidation and electron beam lithography. We investigate
the tunability of our device and demonstrate, that we can switch between
capacitive and tunnel coupling. In the regime of capacitive coupling we observe
the phenomenon of spin blockade in a magnetic field and analyze the influence
of capacitive interdot coupling on this effect.Comment: 4 pages, 3 figure
Optical properties of current carrying molecular wires
We consider several fundamental optical phenomena involving single molecules
in biased metal-molecule-metal junctions. The molecule is represented by its
highest occupied and lowest unoccupied molecular orbitals, and the analysis
involves the simultaneous consideration of three coupled fluxes: the electronic
current through the molecule, energy flow between the molecule and
electron-hole excitations in the leads and the incident and/or emitted photon
flux. Using a unified theoretical approach based on the non-equilibrium Green
function method we derive expressions for the absorption lineshape (not an
observable but a ueful reference for considering yields of other optical
processes) and for the current induced molecular emission in such junctions. We
also consider conditions under which resonance radiation can induce electronic
current in an unbiased junction. We find that current driven molecular emission
and resonant light induced electronic currents in single molecule junctions can
be of observable magnitude under appropriate realizable conditions. In
particular, light induced current should be observed in junctions involving
molecular bridges that are characterized by strong charge transfer optical
transitions. For observing current induced molecular emission we find that in
addition to the familiar need to control the damping of molecular excitations
into the metal substrate the phenomenon is also sensitive to the way in which
the potential bias si distributed on the junction.Comment: 56 pages, 8 figures; submitted to JC
Analytical approach to semiconductor Bloch equations
Although semiconductor Bloch equations have been widely used for decades to
address ultrafast optical phenomena in semiconductors, they have a few
important drawbacks: (i) Coulomb terms between free electron-hole pairs require
Hartree-Fock treatment which, in its usual form, preserves excitonic poles but
loses biexcitonic resonances. (ii) Solving the resulting coupled differential
equations imposes heavy numerics which completely hide the physics. This can be
completely avoided if, instead of free electron-hole pairs, we use correlated
pairs, i.e., excitons. Their interactions are easy to handle through the
recently constructed composite-exciton many-body theory, which allows us to
\emph{analytically} obtain the time evolution of the polarization induced by a
laser pulse. This polarization comes from Coulomb interactions between virtual
excitons, but also from Coulomb-free fermion exchanges, which are dominant at
large detuning
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