272 research outputs found

    Electrical coherent control of nuclear spins in a breakdown regime of quantum Hall effect

    Full text link
    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

    Full text link
    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 q≈0q\approx 0, the phase shift is locked to π/2\pi/2 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

    Full text link
    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 ν\nu = 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

    Full text link
    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

    Full text link
    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

    Get PDF
    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

    Full text link
    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 qq for the corresponding Fano lineshape, we need to introduce a complex qq 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

    Full text link
    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 Ga1−x_{1-x}Mnx_xAs by using the coherent potential approximation (CPA). The result reveals that a {\it p}-hole in the band tail of Ga1−x_{1-x}Mnx_xAs 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

    Full text link
    We have measured diffusion thermopower in a two-dimensional electron gas at low temperature (TT=40 mK) in the field range 0 <B<<B< 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 (B≤B\leq 1 T), we obtain the transverse thermopower SyxS_{yx} that quantitatively agrees with the SyxS_{yx} calculated from resistivities using the generalized Mott formula. In the quantum Hall regime (B≥B\geq 1T), we find that SyxS_{yx} 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 ∼\sim1.8 T, where spin-splitting in the measured area becomes apparent.Comment: 4 pages, 4 figures, EP2DS-1
    • …
    corecore