161 research outputs found
Spin Effects in a Quantum Ring
Recent experiments are reviewed that explore the spin states of a ring-shaped
many-electron quantum dot. Coulomb-blockade spectroscopy is used to access the
spin degree of freedom. The Zeeman effect observed for states with successive
electron number allows to select possible sequences of spin ground states of
the ring. Spin-paired orbital levels can be identified by probing their
response to magnetic fields normal to the plane of the ring and electric fields
caused by suitable gate voltages. This narrows down the choice of ground-state
spin sequences. A gate-controlled singlet--triplet transition is identified and
the size of the exchange interaction matrix element is determined.Comment: 13 pages, 3 figures, Proceedings of the QD2004 conference in Banf
Spin-blockade spectroscopy of a two-level artificial molecule
Coulomb and spin blockade spectroscopy investigations have been performed on
an electrostatically defined ``artificial molecule'' connected to spin
polarized leads. The molecule is first effectively reduced to a two-level
system by placing both constituent atoms at a specific location of the level
spectrum. The spin sensitivity of the conductance enables us to identify the
electronic spin-states of the two-level molecule. We find in addition that the
magnetic field induces variations in the tunnel coupling between the two atoms.
The lateral nature of the device is evoked to explain this behavior.Comment: 4 pages, 4 figures; revised version with a minor change in Fig.2 and
additional inset in Fig.3.;accepted by PR
High fidelity measurement of singlet-triplet states in a quantum dot
We demonstrate experimentally a read-out method that distinguishes between
two-electron spin states in a quantum dot. This scheme combines the advantages
of the two existing mechanisms for spin-to-charge conversion with single-shot
charge detection: a large difference in energy between the two states and a
large difference in tunnel rate between the states and a reservoir. As a
result, a spin measurement fidelity of 97% was achieved, which is much higher
than previously reported fidelities.Comment: 4 pages, 1 figur
Tunneling Anisotropic Spin Polarization in lateral (Ga,Mn)As/GaAs spin Esaki diode devices
We report here on anisotropy of spin polarization obtained in lateral
all-semiconductor all-electrical spin injection devices, employing
(Ga,Mn)As/GaAs Esaki diode structures as spin aligning
contacts, resulting from the dependence of the efficiency of spin tunneling on
the orientation of spins with respect to different crystallographic directions.
We observed an in-plane anisotropy of in case of spins oriented either
along or directions and anisotropy between
in-plane and perpendicular-to-plane orientation of spins.Comment: 9 pages, 3 figure
Tunable Negative Differential Resistance controlled by Spin Blockade in Single Electron Transistors
We demonstrate a tunable negative differential resistance controlled by spin
blockade in single electron transistors. The single electron transistors
containing a few electrons and spin polarized source and drain contacts were
formed in GaAs/GaAlAs heterojunctions using metallic gates. Coulomb blockade
measurements performed as a function of applied source-drain bias, electron
number and magnetic field reveal well defined regimes where a decrease in the
current is observed with increasing bias. We establish that the origin of the
negative differential regime is the spin-polarized detection of electrons
combined with a long spin relaxation time in the dot. These results indicate
new functionalities that may be utilized in nano-spintronic devices in which
the spin state is electro-statically controlled via the electron occupation
number.Comment: 8 pages, 4 figure
Local spin valve effect in lateral (Ga,Mn)As/GaAs spin Esaki diode devices
We report on a local spin valve effect observed unambiguously in lateral
all-semiconductor all-electrical spin injection devices, employing
p+-(Ga,Mn)As/n+-GaAs Esaki diode structures as spin aligning contacts. We
discuss the observed local spin-valve signal as a result of interplay between
spin-transport-related contribution and tunneling anisotropic magnetoresistance
of magnetic contacts. The magnitude of the spin-related magnetoresistance
change is equal to 30 Ohm which is twice the magnitude of the measured
non-local signal.Comment: submitted to Appl. Phys. Let
Spin splitting in open quantum dots
We present results from a theoretical and experimental study of
spin-splitting in small open lateral quantum dots (i.e. in the regime when the
dot is connected to the reservoirs via leads that support one or more
propagating modes). We demonstrate that the magnetoconductance shows a
pronounced splitting of the conductance peaks (or dips) which persists over a
wide range of magnetic fields (from zero field to the edge-state regime) and is
virtually independent of magnetic field. A numerical analysis of the
conductance and the dot eigenspectrum indicates that this feature is related to
a lifting of the spin degeneracy in the corresponding closed dot associated
with the interaction between electrons of opposite spin.Comment: 4 pages, 4 figures 1 misdirected figure reference corrected mismatch
between spin-up/spin-down notation in figure 3-4 and discussion corrected,
clarifications in text adde
Coulomb and Spin blockade of two few-electrons quantum dots in series in the co-tunneling regime
We present Coulomb Blockade measurements of two few-electron quantum dots in
series which are configured such that the electrochemical potential of one of
the two dots is aligned with spin-selective leads. The charge transfer through
the system requires co-tunneling through the second dot which is in
resonance with the leads. The observed amplitude modulation of the resulting
current is found to reflect spin blockade events occurring through either of
the two dots. We also confirm that charge redistribution events occurring in
the off-resonance dot are detected indirectly via changes in the
electrochemical potential of the aligned dot.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
All-electrical measurements of direct spin Hall effect in GaAs with Esaki diode electrodes
We report on measurements of direct spin Hall effect in a lightly n-doped
GaAs channel. As spin detecting contacts we employed highly efficient
ferromagnetic Fe/(Ga,Mn)As/GaAs Esaki diode structures. We investigate bias and
temperature dependence of the measured spin Hall signal and evaluate the value
of total spin Hall conductivity and its dependence on channel conductivity and
temperature. From the results we determine skew scattering and side jump
contribution to the total spin hall conductivity and compare it with the
results of experiments on higher conductive n-GaAs channels[Phys. Rev. Lett.
105,156602(2010)]. As a result we conclude that both skewness and side jump
contribution cannot be fully independent on the conductivity of the channel.Comment: 14 pages, 4 figure
The Collapse of the Spin-Singlet Phase in Quantum Dots
We present experimental and theoretical results on a new regime in quantum
dots in which the filling factor 2 singlet state is replaced by new spin
polarized phases. We make use of spin blockade spectroscopy to identify the
transition to this new regime as a function of the number of electrons. The key
experimental observation is a reversal of the phase in the systematic
oscillation of the amplitude of Coulomb blockade peaks as the number of
electrons is increased above a critical number. It is found theoretically that
correlations are crucial to the existence of the new phases.Comment: REVTeX4, 4 pages, 4 figures, to appear in PR
- …