Person: Stopa, Michael P
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Stopa
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Michael P
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Stopa, Michael P
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Publication Measurement of the Absolute Raman Cross Section of the Optical Phonon in Silicon(Elsevier BV, 2011) Aggarwal, R. L.; Farrar, L. W.; Saikin, S. K.; Aspuru-Guzik, Alan; Stopa, Michael P; Polla, D. L.The absolute Raman cross section \(\sigma_{RS}\) of the first-order \(519 cm^{−1}\) optical phonon in silicon was measured using a small temperature-controlled blackbody for the signal calibration of the Raman system. Measurements were made with a 25-mil thick (001) silicon sample located in the focal plane of a 20-mm effective focal length (EFL) lens using 785-, 1064-, and 1535-nm CW pump lasers for the excitation of Raman scattering. The pump beam was polarized along the [100] axis of the silicon sample. Values of \(1.0±0.2×10^{−27}\), \(3.6±0.7×10^{−28}\), and \(1.1±0.2×10^{−29} cm^2\) were determined for \(\sigma_{RS}\) for 785-, 1064-, and 1535-nm excitation, respectively. The corresponding values of the Raman scattering efficiency S are \(4.0±0.8×10^{−6}\), \(1.4±0.3×10^{−6}\), and \(4.4±0.8×10^{−8} cm^{−1} sr^{−1}\).The values of the Raman polarizability |d| for 785-, 1064-, and 1535-nm excitation are \(4.4±0.4×10^{−15}\), \(5.1±0.5×10^{−15}\), and \(1.9±0.2×10^{−15} cm^2\), respectively. The values of \(4.4±0.4×10^{−15}\) and \(5.1±0.5×10^{−15} cm^2\) for |d| for 785- and 1064-nm excitation, respectively, are 1.3 and 2.0 times larger than the values of \(3.5×10^{−15}\) and \(2.5×10^{−15} cm^2\) calculated by Wendel. The Raman polarizability |d| computed using the density functional theory in the long-wavelength limit is consistent with the general trend of the measured data and Wendel’s model.Publication Dynamic Nuclear Polarization in Double Quantum Dots(American Physical Society, 2010) Gullans, Michael John; Krich, Jacob Jonathan; Taylor, Jacob; Bluhm, Hendrik; Halperin, Bertrand; Marcus, C; Stopa, Michael P; Yacoby, Amir; Lukin, MikhailWe theoretically investigate the controlled dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons. Three regimes of long-term dynamics are identified, including the buildup of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with formation of so-called ‘‘dark states’’, and the elimination of the difference field. We show that in the case of unequal dots, buildup of difference fields generally accompanies the nuclear polarization process, whereas for nearly identical dots, buildup of difference fields competes with polarization saturation in dark states. The elimination of the difference field does not, in general, correspond to a stable steady state of the polarization process.Publication Anion Stabilization in Electrostatic Environments(American Chemical Society, 2011) Olivares-Amaya, Roberto; Stopa, Michael P; Andrade, Xavier; Watson, Mark A.; Aspuru-Guzik, AlanExcess charge stabilization of molecules in metallic environments is of particular importance for fields such as molecular electronics and surface chemistry. We study the energetics of benzene and its anion between two metallic plates. We observe that orientational effects are important at small inter-plate separation. This leads to benzene oriented perpendicular to the gates being more stable than the parallel case due to induced dipole effects. We find that the benzene anion, known for being unstable in the gas-phase, is stabilized by the plates at zero bias and an inter-plate distance of 21 Å. We also observe the effect of benzene under a voltage bias generated by the plates; under a negative bias, the anion becomes destabilized. We use the electron localization function to analyze the changes in electron density due to the bias. These findings suggest that image effects such as those present in nanoscale devices, are able to stabilize excess charge and should be important to consider when modeling molecular transport junctions and charge-transfer effects.Publication Fast Sensing of Double-Dot Charge Arrangement and Spin State with a Radio-Frequency Sensor Quantum Dot(American Physical Society, 2010) Barthel, C; Kjærgaard, M.; Medford, James Redding; Stopa, Michael P; Marcus, C; Hanson, M. P.; Gossard, Arthur C.Single-shot measurement of the charge arrangement and spin state of a double quantum dot are reported with measurement times down to 100 ns. Sensing uses radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade regime. The sensor quantum dot is up to 30 times more sensitive than a comparable quantum point-contact sensor and yields three times greater signal to noise in rf single-shot measurements. Numerical modeling is qualitatively consistent with experiment and shows that the improved sensitivity of the sensor quantum dot results from reduced lifetime broadening and screening.