272 research outputs found
Electrical coherent control of nuclear spins in a breakdown regime of quantum Hall effect
Using a conventional Hall-bar geometry with a micro-metal strip on top of the
surface, we demonstrate an electrical coherent control of nuclear spins in an
AlGaAs/GaAs semiconductor heterostructure. A breakdown of integer quantum Hall
(QH) effect is utilized to dynamically polarize nuclear spins. By applying a
pulse rf magnetic field with the metal strip, the quantum state of the nuclear
spins shows Rabi oscillations, which is detected by measuring longitudinal
voltage of the QH conductor.Comment: 3 pages, 4 figure
Observation of the Fano-Kondo Anti-Resonance in a Quantum Wire with a Side-Coupled Quantum Dot
We have observed the Fano-Kondo anti-resonance in a quantum wire with a
side-coupled quantum dot. In a weak coupling regime, dips due to the Fano
effect appeared. As the coupling strength increased, conductance in the regions
between the dips decreased alternately. From the temperature dependence and the
response to the magnetic field, we conclude that the conductance reduction is
due to the Fano-Kondo anti-resonance. At a Kondo valley with the Fano parameter
, the phase shift is locked to against the gate voltage
when the system is close to the unitary limit in agreement with theoretical
predictions by Gerland {\it et al.} [Phys. Rev. Lett. {\bf 84}, 3710 (2000)].Comment: 4 pages, 4 figure
Dynamic nuclear polarization induced by breakdown of fractional quantum Hall effect
We study dynamic nuclear polarization (DNP) induced by breakdown of the
fractional quantum Hall (FQH) effect. We find that voltage-current
characteristics depend on current sweep rates at the quantum Hall states of
Landau level filling factors = 1, 2/3, and 1/3. The sweep rate dependence
is attributed to DNP occurring in the breakdown regime of FQH states. Results
of a pump and probe experiment show that the polarities of the DNP induced in
the breakdown regimes of the FQH states is opposite to that of the DNP induced
in the breakdown regimes of odd-integer quantum Hall states.Comment: 4 pages, 4 figure
Fano Resonance in a Quantum Wire with a Side-coupled Quantum Dot
We report a transport experiment on the Fano effect in a quantum connecting
wire (QW) with a side-coupled quantum dot (QD). The Fano resonance occurs
between the QD and the "T-shaped" junction in the wire, and the transport
detects anti-resonance or forward scattered part of the wavefunction. While in
this geometry it is more difficult to tune the shape of the resonance than in
the previously reported Aharonov-Bohm-ring type interferometer, the resonance
purely consists of the coherent part of transport. By utilizing this advantage,
we have qualitatively explained the temperature dependence of the Fano effect
by including the thermal broadening and the decoherence. We have also proven
that this geometry can be a useful interferometer to measure the phase
evolution of electrons at a QD.Comment: REVTEX, 6 pages including 5 figures, final versio
Evidence for a quantum-spin-Hall phase in graphene decorated with Bi2Te3 nanoparticles
Realization of the quantum-spin-Hall effect in graphene devices has remained
an outstanding challenge dating back to the inception of the field of
topological insulators. Graphene's exceptionally weak spin-orbit coupling
-stemming from carbon's low mass- poses the primary obstacle. We experimentally
and theoretically study artificially enhanced spin-orbit coupling in graphene
via random decoration with dilute Bi2Te3 nanoparticles. Remarkably,
multi-terminal resistance measurements suggest the presence of helical edge
states characteristic of a quantum-spin-Hall phase; the magnetic-field and
temperature dependence of the resistance peaks, X-ray photoelectron spectra,
scanning tunneling spectroscopy, and first-principles calculations further
support this scenario. These observations highlight a pathway to spintronics
and quantum-information applications in graphene-based quantum-spin-Hall
platforms
Evidence for a quantum-spin-Hall phase in graphene decorated with Bi_2Te_3 nanoparticles
Realization of the quantum spin Hall effect in graphene devices has remained an outstanding challenge dating back to the inception of the field of topological insulators. Graphene’s exceptionally weak spin-orbit coupling—stemming from carbon’s low mass—poses the primary obstacle. We experimentally and theoretically study artificially enhanced spin-orbit coupling in graphene via random decoration with dilute Bi_2Te_3 nanoparticles. Multiterminal resistance measurements suggest the presence of helical edge states characteristic of a quantum spin Hall phase; the magnetic field and temperature dependence of the resistance peaks, x-ray photoelectron spectra, scanning tunneling spectroscopy, and first-principles calculations further support this scenario. These observations highlight a pathway to spintronics and quantum information applications in graphene-based quantum spin Hall platforms
Mesoscopic Fano Effect in a Quantum Dot Embedded in an Aharonov-Bohm Ring
The Fano effect, which occurs through the quantum-mechanical cooperation
between resonance and interference, can be observed in electron transport
through a hybrid system of a quantum dot and an Aharonov-Bohm ring. While a
clear correlation appears between the height of the Coulomb peak and the real
asymmetric parameter for the corresponding Fano lineshape, we need to
introduce a complex to describe the variation of the lineshape by the
magnetic and electrostatic fields. The present analysis demonstrates that the
Fano effect with complex asymmetric parameters provides a good probe to detect
a quantum-mechanical phase of traversing electrons.Comment: REVTEX, 9 pages including 8 figure
Mechanism of carrier-induced ferromagnetism in magnetic semiconductors
Taking into account both random impurity distribution and thermal
fluctuations of localized spins, we have performed a model calculation for the
carrier (hole) state in GaMnAs by using the coherent potential
approximation (CPA). The result reveals that a {\it p}-hole in the band tail of
GaMnAs is not like a free carrier but is rather virtually bounded
to impurity sites. The carrier spin strongly couples to the localized {\it d}
spins on Mn ions. The hopping of the carrier among Mn sites causes the
ferromagnetic ordering of the localized spins through the double-exchange
mechanism. The Curie temperature obtained by using conventional parameters
agrees well with the experimental result.Comment: 7 pages, 4 figure
Measurement of diffusion thermopower in the quantum Hall systems
We have measured diffusion thermopower in a two-dimensional electron gas at
low temperature (=40 mK) in the field range 0 3.4 T, by employing the
current heating technique. A Hall bar device is designed for this purpose,
which contains two crossing Hall bars, one for the measurement and the other
used as a heater, and is equipped with a metallic front gate to control the
resistivity of the areas to be heated. In the low magnetic field regime
( 1 T), we obtain the transverse thermopower that
quantitatively agrees with the calculated from resistivities using the
generalized Mott formula. In the quantum Hall regime ( 1T), we find that
signal appears only when both the measured and the heater area are in
the resistive (inter-quantum Hall transition) region. Anomalous gate-voltage
dependence is observed above 1.8 T, where spin-splitting in the measured
area becomes apparent.Comment: 4 pages, 4 figures, EP2DS-1
- …