281 research outputs found
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
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
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
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
Infrared magneto-optical properties of (III,Mn)V ferromagetic semiconductors
We present a theoretical study of the infrared magneto-optical properties of
ferromagnetic (III,Mn)V semiconductors. Our analysis combines the kinetic
exchange model for (III,Mn)V ferromagnetism with Kubo linear response theory
and Born approximation estimates for the effect of disorder on the valence band
quasiparticles. We predict a prominent feature in the ac-Hall conductivity at a
frequency that varies over the range from 200 to 400 meV, depending on Mn and
carrier densities, and is associated with transitions between heavy-hole and
light-hole bands. In its zero frequency limit, our Hall conductivity reduces to
the -space Berry's phase value predicted by a recent theory of the
anomalous Hall effect that is able to account quantitatively for experiment. We
compute theoretical estimates for magnetic circular dichroism, Faraday
rotation, and Kerr effect parameters as a function of Mn concentration and free
carrier density. The mid-infrared response feature is present in each of these
magneto-optical effects.Comment: 11 pages, 5 figure
Non-Drude Optical Conductivity of (III,Mn)V Ferromagnetic Semiconductors
We present a numerical model study of the zero-temperature infrared optical
properties of (III,Mn)V diluted magnetic semiconductors. Our calculations
demonstrate the importance of treating disorder and interaction effects
simultaneously in modelling these materials. We find that the conductivity has
no clear Drude peak, that it has a broadened inter-band peak near 220 meV, and
that oscillator weight is shifted to higher frequencies by stronger disorder.
These results are in good qualitative agreement with recent thin film
absorption measurements. We use our numerical findings to discuss the use of
f-sum rules evaluated by integrating optical absorption data for accurate
carrier-density estimates.Comment: 7 pages, 3 figure
Magnetotransport properties of (Ga,Mn)As investigated at low temperature and high magnetic field
Magnetotransport properties of ferromagnetic semiconductor (Ga,Mn)As have
been investigated. Measurements at low temperature (50 mK) and high magnetic
field (<= 27 T) have been employed in order to determine the hole concentration
p = 3.5x10^20 cm ^-3 of a metallic (Ga0.947Mn0.053)As layer. The analysis of
the temperature and magnetic field dependencies of the resistivity in the
paramagnetic region was performed with the use of the above value of p, which
gave the magnitude of p-d exchange energy |N0beta | ~ 1.5 eV.Comment: PDF file, 8 pages, 4 figure
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