Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L33: Focus Session: Spins in Quantum Dots |
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Sponsoring Units: GMAG FIAP DMP Chair: Alexander Efros, Naval Reseach Laboratory Room: Morial Convention Center 224 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L33.00001: Ultrafast coherent optical manipulation of a single electron spin in a quantum dot M.H. Mikkelsen, J. Berezovsky, N.G. Stoltz, L.A. Coldren, D.D. Awschalom A key ingredient for spin-based quantum information processing is the coherent rotation of a spin state on timescales much faster than the spin coherence time. By applying off-resonant, picosecond-scale optical pulses, we demonstrate the coherent rotation of a single electron spin in a GaAs quantum dot (QD) through arbitrary angles up to $\pi$ rad.\footnote{ J. Berezovsky, M.H. Mikkelsen, N.G. Stoltz, L.A. Coldren, D.D. Awschalom, {\em Submitted}, (2007)} We directly observe this spin manipulation using time-resolved Kerr rotation spectroscopy\footnote{ M.H. Mikkelsen, J. Berezovsky, N.G. Stoltz, L.A. Coldren, D.D. Awschalom, {\em Nature Physics} \textbf{3}, 770 (2007)} at T=10K. Measurements of the spin rotation as a function of laser detuning and intensity confirm that the optical Stark effect is the operative mechanism and the results are well-predicted by a model including the electron-nuclear spin interaction. Using short tipping pulses and QDs with long spin coherence times, this technique enables one to perform a large number of operations within the coherence time. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L33.00002: Coherent Population Trapping of an Electron Spin in a Singly-Charged Quantum Dot Xiaodong Xu, Bo Sun, Paul R. Berman, Duncan G. Steel, Allan S. Bracker, Dan Gammon, Lu J. Sham When two radiation fields drive coupled transitions in a three-level lambda system, a steady-state coherent superposition of the ground states can be formed in which the system is totally decoupled from the applied fields, a process that is sometimes referred to as coherent population trapping (CPT). Here we report the demonstration of the CPT of an electron spin in a singly-charged quantum dot. By applying a magnetic field in the Voigt geometry, we create a three-level lambda system, formed by two Zeeman sublevels of an electron spin and an intermediate trion state. As we tune the driving and probe fields to the two-photon Raman resonance, we observe a pronounced dip in the probe absorption spectrum, indicating the CPT of the electron spin. An arbitrary superposition of the electron spin states can be prepared by varying the ratio of the Rabi frequencies between the driving and probe fields. This work shows that spin based semiconductor quantum dot systems can exhibit the same interesting quantum behavior that has been found in simple atom-field systems. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L33.00003: Spin polarized current generation from quantum dots without magnetic fields Jacob J. Krich, Bertrand I. Halperin An unpolarized charge current passing through a chaotic quantum dot with strong spin-orbit coupling can produce a spin polarized exit current without magnetic fields or ferromagnets. If there is only one channel in the output lead, no spin polarization can be produced. We use random matrix theory to estimate the typical spin polarization as a function of the number of channels in each lead, finding rms spin polarizations up to 45\% with one input channel and two output channels. Finite temperature and dephasing both suppress the effect, and we include dephasing effects using a new variation of the third lead model. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L33.00004: The role of electron-hole exchange in the spin mixing of excited-shell excitons in quantum dots Stefan C. Badescu, Thomas L. Reinecke In semiconductor quantum dots (QDs) the long-range electron-hole exchange is known to lift the spin degeneracy of ground orbital state excitons for small lateral asymmetries. In the context of solid-state quantum computing recent developments in optical manipulation of spin involve the excited shells of charged excitons in single and coupled QDs. We demonstrate here that this electron-hole exchange is important in the mixing of spin states of excitons in higher shells in QDs, e.g., the p-shell exciton. In contrast to the ground states, no lateral asymmetry is necessary for simultaneous reversal of an electron and a hole spin in these excited shells. For charged excitons, this interaction competes with the asymmetric exchange between electrons or between holes. We discuss the optical selection rules and the role of decoherence from phonons for the dynamics of excited excitons. We conclude that the e-h exchange is an important factor in designing logical gates with excited states. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L33.00005: Theoretical Study of Dynamical Nuclear Spin polarization in Semiconductor Quantum Dots Chia-Wei Huang, Xuedong Hu Dynamical nuclear spin polarization (DNSP) can be achieved optically in a semiconductor quantum dot (QD) via the hyperfine interaction between the optical oriented electron and nuclear spins. Here we explore several mechanisms, combined with the hyperfine interaction, to investigate how efficiently they can transfer the electron spin polarization to nuclei. Specifically, to ensure energy conservation during the spin transfer, we consider both a combination of electron spin-orbit and electron-phonon interactions, and cotunneling processes. Based on these interactions we evaluate the buildup time for DNSP in a semiconductor QD. Our results show that the DNSP buildup time of spin-orbit associated with the hyperfine interaction, accompanied by phonon emission, is of the order of seconds. This is much longer than that of the recent experimental findings. [1] The calculated nuclear spin polarization rate also shows a different Zeeman-energy-dependence from what is observed experimentally. [1] We thus conclude that hyperfine interaction combined with electron level broadening due to cotunneling processes between the QD and the nearby reservoir is more likely to be responsible for the fast buildup of nuclear polarization in experiments. [1] P.Maletinsky, A.Badolato, and A. Imamoglu, Phys. Rev. Lett. 99, 056804 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L33.00006: Emission spectra from InAs quantum dots as a function of carrier spin polarization; experiment and theory. I. Khan, T. Ali, M. Yasar, A. Petrou, G. Kioseoglou, C.H. Li, A.T. Hanbicki, B.T. Jonker, M. Korkusinski, P. Hawrylak We have studied the emission spectra from InAs quantum dots (QDs) under the following conditions: a) recombination of spin-polarized electrons with unpolarized holes; b) recombination of spin-polarized electrons with spin-polarized holes. In the first experiment (a), we recorded the electroluminescence from Fe/GaAs n-i-p spin-LEDs which incorporate a single layer of InAs QDs at the center of the intrinsic region of the device. In the second experiment (b), we studied the photoluminescence spectra from a similar undoped heterostructure using optical pumping. In both experiments, in addition to the typical shell structure of the emission spectra, new polarization maxima are observed on the high and low energy sides of the shell emission features as a result of the imbalance between the two spin populations of the carriers. These results are compared with a calculation for the case of two and six electron-hole occupancy of the QDs. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L33.00007: Determination of spin polarization in InAs/GaAs self-assembled quantum dots F. G. G. Hernandez, T. P. Mayer Alegre, G. Medeiros-Ribeiro The electronic spin in a quantum dot (QD) has been proposed as a candidate two-level system for qubit implementation in a quantum information processing scheme (QIP). For semiconductor quantum dots, the optical transition selection rules provide a natural tool for a direct and quantitative measurement of the electronic spin polarization. In contrast to the optical scheme, electrical readout of the electronic spins orientation do not require any knowledge of hole polarization. Here we perform magneto-capacitance measurements of QDs embedded in a Metal-Insulator-Semiconductor (MIS) capacitor structure. A statistical approach for the population of the spin levels allows one to study the spin orientation in the limit of comparable magnetic and thermal energies. The experimental data are analyzed in terms of the addition energies as measured by magneto-capacitance spectroscopy. The amount of polarization was inferred by measuring the addition energies of electrons sequentially loaded in QDs. In this experiment, we found an electron spin polarization higher than 50\% for $B_{[001]} = 4 $T. Finally, by including the g-tensor anisotropy the angular dependence of spin polarization with the magnetic field B orientation and strength could be explained. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L33.00008: Non-volatile spin bistability based on ferromagnet-semiconductor quantum dot hybrid nanostructure Yuriy Semenov, Hani Enaya, John Zavada, Ki Wook Kim Electrical manipulation of a memory cell based on bistability effect in a nanostructure consisting of a semiconductor quantum dot (QD) adjoining on opposite sides with a dielectric ferromagnetic layer (DFL) and a reservoir of itinerant holes is investigated theoretically. The operating principle is based on the interplay between the exchange field of the holes \textbf{B}$_{h}$ acting on the magnetization vector of the DFL \textbf{M} perpendicular to structure plane and the anisotropy field \textbf{B}$_{a}$ which aligns \textbf{M} along the plane. At low hole population of the QD (B$_{h}<<$B$_{a})$, \textbf{M} is still in plane direction (first stable state ``0''). If an applied bias populates the QD sufficiently (B$_{h}>$B$_{a})$, the subsequent \textbf{M} rotation will decrease the hole energy in the QD; hence the high hole population state is sustained (second stable state ``1'') under a fixed electro-chemical potential set by the reservoir even after bias is removed. The analysis of bit retention time of the proposed memory demonstrates the feasibility of the device with lateral QD size at least 30 nm under room temperature operation. Another advantage is the extremely small dissipative energy for Write/Erase operations. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L33.00009: Method for Full Bloch-Sphere Control of a Localized Spin in a Quantum Dot via a Single Electrical Gate Joseph Pingenot, Craig E. Pryor, Michael E. Flatt\'e Manipulating individual spins in solids requires quickly and coherently reorienting localized spins while leaving neighboring spins unaffected. Difficulties confining oscillating magnetic fields have motivated alternate approaches that use electric fields to change the local magnetic environment, including moving an electron within a hyperfine field gradient or fringe-field gradient. Higher temperatures require spins to be localized in much smaller quantum dots, where these techniques are less effective. In contrast, g-tensor manipulation techniques[1] couple an electric field to the spin via the spin-orbit interaction, and should be scalable to small dots with large confinement. Here we calculate the g-tensor of a single electron in a small quantum dot and show the symmetry of its electric field dependence permits full Bloch sphere control of the spin using an electric field applied in a single direction. We find the spin manipulation frequency of an InAs/GaAs QD in 1 Tesla exceeds 150 MHz. We acknowledge support of an ONR MURI and an NSF NIRT. [1] Kato et al. Nature 299, 1201 (2003) [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L33.00010: Enhancement of spin lifetime in quantum well lasers using interface fluctuation quantum dots Ryan Woodworth, Ari Mizel, Rusko Ruskov, Gerald Mahan Research on semiconductor heterostructures provides many insights into the next generation of optoelectronic devices. In particular, the gain of a microdisk laser seems to be enhanced by the presence of a long-lived optical cavity mode. Here we analyze a recent experiment using GaAs-AlGaAs microdisks with interface fluctuation quantum dots. A numerical simulation shows enhancement of spin dephasing time in the conduction band due to exchange scattering and D'yakonov-Perel' coupling. Possible applications to quantum computing are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L33.00011: Piezomagnetic quantum dots Ramin Abolfath, A.G. Petukhov, Igor Zutic We study the influence of deformations on magnetic ordering in quantum dots doped with magnetic impurities. The reduction of symmetry and the associated deformation from circular to elliptical quantum confinement lead to the formation of piezomagnetic quantum dots \footnote {R. M. Abolfath, A. G. Petukhov, and I. Zutic, arXiv:0707.2805.}. The strength of elliptical deformation can be controlled by the gate voltage to change the magnitude of magnetization, at a fixed number of carriers and in the absence of applied magnetic field. We reveal a reentrant magnetic ordering with the increase of elliptical deformation and suggest that the piezomagnetic quantum dots can be used as nanoscale magnetic switches. Finally, we discuss thermodynamic stability of piezomagnetism in such quantum dots. [Preview Abstract] |
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