Five new publications added to Papers and Publications:
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L. DiCarlo, J. R. Williams, D. T. McClure, Yiming Zhang, C. M. Marcus,
Shot Noise in Graphene, Phys. Rev. Lett. 100, 156801 (2008). PDFWe report measurements of current noise in single- and multi-layer graphene devices. In four single-layer devices, including a p-n junction, the Fano factor remains constant to within +/-10% upon varying carrier type and density, and averages between 0.35 and 0.38. The Fano factor in a multi-layer device is found to decrease from a maximal value of 0.33 at the charge-neutrality point to 0.25 at high carrier density. These results are compared to theoretical predictions for shot noise in ballistic and disordered graphene.
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Quasiparticle Tunneling in the Fractional Quantum Hall State at \nu = 5/2 , arXiv:0803.3530 (2008)Theory predicts that quasiparticle tunneling between the counter-propagating edges in a fractional quantum Hall state can be used to measure the effective quasiparticle charge e* and dimensionless interaction parameter g, and thereby characterize the many-body wavefunction describing the state. We report measurements of quasiparticle tunneling in a high mobility GaAs two dimensional electron system in the fractional quantum Hall state at nu=5/2 using a gate-defined constriction to bring the edges close together. We find the dc-bias peaks in the tunneling conductance at different temperatures collapse onto a single curve when scaled, in agreement with weak tunneling theory. Various models for the \nu=5/2 state predict different values for g. Among these models, the non-abelian states with e*=1/4 and g=1/2 are most consistent with the data.
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Exchange Control of Nuclear Spin Diffusion in a Double Quantum Dot, arXiv:0803.3082 (2008)Coherent two-level systems, or qubits, based on electron spins in GaAs quantum dots are strongly coupled to the nuclear spins of the host lattice via the hyperfine interaction. Realizing nuclear spin control would likely improve electron spin coherence and potentially enable the nuclear environment to be harnessed for the long-term storage of quantum information. Toward this goal, we report experimental control of the relaxation of nuclear spin polarization in a gate-defined two-electron GaAs double quantum dot. A cyclic gate-pulse sequence transfers the spin of an electron pair to the host nuclear system, establishing a local nuclear polarization that relaxes on a time scale of seconds. We find nuclear relaxation depends on magnetic field and gate-controlled two-electron exchange, consistent with a model of electron mediated nuclear spin diffusion.
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Dynamic Nuclear Polarization with Single Electron Spins, Phys. Rev. Lett. 100, 067601 (2008). PDFWe polarize nuclear spins in a GaAs double quantum dot by controlling two-electron spin states near the anti-crossing of the singlet (S) and m_S=+1 triplet (T+) using pulsed gates. An initialized S state is cyclically brought into resonance with the T+ state, where hyperfine fields drive rapid rotations between S and T+, 'flipping' an electron spin and 'flopping' a nuclear spin. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage.
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Measurement of Temporal Correlations of the Overhauser Field in a Double Quantum Dot, arXiv:0712.4033 (2008)Correlation functions and power spectral densities of Overhauser field fluctuations, resulting from the hyperfine coupling of a separated pair of electrons to local nuclear ensembles, are measured in a few-electron GaAs double quantum dot over a bandwidth of 40 mHz to 1 kHz. Measured spectra show broadband content, from milliseconds to the tens of seconds. Experimental spectra are found to be in excellent agreement with a simple model based on nuclear spin diffusion.
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