Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D26: Focus Session: Semiconductor Qubits - Silicon |
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Sponsoring Units: GQI Chair: Charles Tahan, LPS and University of Maryland Room: D136 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D26.00001: Atomistic simulations of energy levels in silicon quantum dots for spin filling experiments Rajib Rahman, Richard Muller, Malcolm Carroll Unknown valley splitting magnitudes in enhancement-mode metal-insulator-oxide (MIS) quantum dots combined with the potential of valley-orbit coupling due to imperfect interfaces leads to complex single and multi-electron energy level dependence on magnetic and electric field strengths. Comparable energy spacing between orbital energies and the upper bands in unstrained silicon, split off because of differences in effective mass, introduce additional potential complexity. Addition energy and spin filling experiments provide a fingerprint of these energy levels for which accurate models are invaluable. Using atomistic simulations, we obtain theoretical valley-orbit splittings in silicon dots with different barrier materials, electric and magnetic fields. The multi-valley structure of Si gives rise to degenerate orbital states, which can be split by the valley-orbit interaction. The results illuminate the role of Hund's rules and valley degrees of freedom on the few electron states of the dots. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D26.00002: Coulomb Blockade in Double Top Gated Si MOS Nano-Structures with Integrated Charge Sensing E.P. Nordberg, G.A. Ten Eyck, H.L. Stalford, R.P. Muller, R.W. Young, K. Eng, L.A. Tracy, K.D. Childs, J.R. Wendt, R.K. Grubbs, J. Stevens, M.P. Lilly, M.A. Eriksson, M.S. Carroll Measurements of Si-MOS quantum dots with an open lateral non-collinear geometry are discussed. Periodic, single-period Coulomb blockade is observed. The measured gate-to-dot capacitances are consistent with 3D finite element calculations of the capacitance matrix for a lithographically defined quantum dot, indicating that the quantum dot confinement potential is dominated by the lithographically patterned gates and not by disorder. We report characterization of some of the critical process steps, and we correlate low disorder behavior with a quantitative defect density. We also present charge-sensing measurements of the quantum dot using a nearby disordered constriction. The sensitivity of the charge sensing is as high as a 3{\%} change in current due the change of one electron occupation in the sensed quantum dot. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D26.00003: A Novel Spin-Flip Co-Tunneling Process in the Effective Three-Electron Regime of a Si/SiGe Double Dot Teck Seng Koh, C.B. Simmons, Mark Friesen, S.N. Coppersmith, M.A. Eriksson We study the transport current of a Si/SiGe double quantum dot in the effective three-electron regime, commonly referred to as ``hole'' transport in the literature. Experimental data is modeled with a Hartree-Fock Hamiltonian. We show that the conventional hole transport picture cannot account for all of the features of the data. We also show that understanding the experimental data requires a novel co-tunneling process involving spin flips. This process is possible partly due to the effect of lifetime-enhanced transport [1].\\[4pt] [1] Shaji, N. \textit{et al.} Nature Physics, \underline{4}, 540 (2008). [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D26.00004: Si/SiGe Quantum Dot with SET Charge Sensor Mingyun Yuan, Zhen Yang, A. J. Rimberg, M. A. Eriksson, D. E. Savage Si/SiGe quantum dots promise a long spin coherence time due to reduced electron-nuclear spin interaction. Based on robust fabrication process we have developed, reliable ohmic contacts and non-leaky Schottky gates are repeatedly produced. As a result, stable quantum dots have been successfully formed, with sufficiently high yield to allow subsequent fabrication steps. Representative Coulomb blockade data will be shown. Efforts to couple a single electron transistor (SET) to a Si/SiGe dot in order to monitor its charge state are in progress. Recent experimental results will be discussed. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D26.00005: Spectroscopy of a few-electron single-crystal silicon quantum dot Mark Friesen, Martin Fuechsle, S. Mahapatra, F. A. Zwanenburg, M. A. Eriksson, Michelle Y. Simmons We report measurements and theoretical simulations of a few-electron quantum dot formed by atomically patterned doping in a P:Si $\delta$-layer. The device is embedded entirely within epitaxial Si, including source-drain tunnel leads and capacitive side-gates. Coulomb blockade and excited state resonances are observed, and we discuss these excited states in the context of calculated multi-electron levels on the dot and van Hove singularities in the leads. The atomically abrupt confinement potential causes a large valley splitting of states within the $\Gamma$ band, as appropriate for spin quantum computing. The sharp lateral confinement in the dot, with radius $<2$~nm, leads to novel effects associated with valley splitting in the $\Delta$ band. [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D26.00006: Towards a single P donor in Si Suddhasatta Mahapatra, Tang Wei, Hoon Ryu, Gerhard Klimeck, Michelle Simmons Individual P donors in Si form the basis of several schemes for realizing solid-state qubits. Technologically, all such schemes rely on the precise positioning of P donors in the Si crystal and fabrication of local gates, only a few tens of nanometers wide. Towards this goal, we have demonstrated that the spatial resolution of STM-lithography allows precise positioning of donors on the sub-nm length scale and also fabrication of \textit{all-epitaxial, planar }quantum dot (QD) architectures, with source, drain, and gate patterns of precisely defined dimensions. Here, we report the STM-lithography fabrication of an ultra-small QD consisting, in the extreme limit, of a single P donor. Transport spectroscopy of the QD-device at mK temperatures shows stable Coulomb oscillations with an addition energy (around zero gate bias) of 44$\pm $2 meV. This value corresponds to the difference in the binding energies of the 1-electron (D$^{0})$ and the 2 --electron (D$^{-})$ states of a P donor in Si. The first two D$^{0 }$excited states have also been identified. [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D26.00007: Spin Relaxation of P Dimer Electrons in Silicon Massoud Borhani, Xuedong Hu We study the transition rates from different energy states of two P donor electrons to the ground state singlet. By considering the full set of the states of the two donor electrons within the Heitler-London approximation, we classify them according to their hyperfine interaction with P nuclei. Here the electron spin-flip is assisted by hyperfine interaction with the phosphorus nuclei and phonon emissions. This study is crucial in order to find an upper bound for the relaxation time of the silicon-based qubits, and finding various regimes to decelerate the spin decay rate. [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D26.00008: Coherence of Donor Electron Spins in Isotopically Enriched Silicon Wayne Witzel, Richard Muller, Malcolm Carroll, Andrea Morello, Lukasz Cywinski, Sankar Das Sarma Silicon is promising for spin-based quantum computing because nuclear spins, a source of magnetic noise, may be nearly eliminated through enrichment. However, any Si device is expected to contain some phosphorus donor impurities. These donors generate magnetic noise through spin dynamics, induced by dipolar interactions, that conserve Zeeman and hyperfine energies. Ironically, increasing the number of nuclear spins will suppress this decoherence mechanism by effecting more hyperfine energy variation (i.e., Overhauser shifts). We study spin coherence decay as a function of donor concentration, $^{29}$Si concentration, and temperature using cluster expansion techniques specifically adapted to the problem of a sparse electron spin bath. [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D26.00009: Spin coherence and relaxation of natural quantum dots at the Si/SiO$_2$ interface S. Shankar, A. M. Tyryshkin, Jianhua He, S. A. Lyon While electron spins confined in quantum dots in silicon heterostructures are good candidates to make qubits, little is known about the coherence of electrons at the Si/SiO$_2$ interface. We perform pulsed electron spin resonance on a Metal-Oxide-Silicon transistor and report the spin relaxation (T$_{1}$) and coherence (T$_{2}$) times for mobile two-dimensional electrons as well as electrons isolated in natural quantum dots at the Si/SiO$_2$ interface. Mobile electrons have short T$_1$ and T$_2$ of around 0.3~$\mu$s at 5~K. Upon confining electrons into isolated dots with a few meV binding energy, T$_1$ rises dramatically as temperature is decreased, reaching 1.1~ms at 350~mK. Simultaneously, T$_2$ rises and saturates at 10~$\mu$s below 1~K. The long T$_1$ is consistent with a reduced efficiency of Rashba fluctuations in causing spin relaxation in a quantum dot. However T$_2$ is not controlled by T$_1$ below 1~K, but is instead caused by an unknown extrinsic mechanism. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:30PM |
D26.00010: Spin incoherence of donor electrons near c-Si(111)/SiO2 interface defects Seoyoung Paik, Sang-Yun Lee, William Baker, Dane McCamey, Christoph Boehme Electron and nuclear spins of phosphorous ($^{31}$P) donors in crystalline silicon have been investigated extensively in recent years as they both have extremely long quantum mechanical coherence times which makes them extremely interesting candidates for quantum information and spin-electronics applications [1]. Existing silicon quantum computer concepts [2] propose to use $^{31}$P qubits close to the silicon surface. We present here a study of how microscopic defects at the oxide layer of the silicon surface influence the spin coherence times (T$_{1}$ and T$_{2}$ times) of the $^{31}$P qubits. Using pulsed electrically detected magnetic resonance spectroscopy [3], we show that due to the interaction of the $^{31}$P qubits with the interface states, the previously known, extremely long bulk coherence times are drastically shortened [4]. \\[4pt] [1] J. J. L.Morton, et al., Nature \textbf{455}, 1085 (2008). \\[0pt] [2] B. E. Kane, Nature 393, 133 (1998). \\[0pt] [3] A. R. Stegner, et al., \textit{Nature Physics} 2, 835 (2006). \\[0pt] [4] S.-Y. Paik, et al., arXiv:cond-mat/0905.0416 (2009). [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 4:42PM |
D26.00011: Spectral diffusion of P donors in isotopically-enriched silicon with low $^{29}$Si concentrations A.M. Tyryshkin, S.A. Lyon, Shinichi Tojo, K.M. Itoh, J.J.L. Morton, J.W. Ager, M.L.W. Thewalt, H. Riemann, N.V. Abrosimov, P. Becker, H.-J. Pohl Spectral diffusion caused by fluctuating magnetic nuclei has been recognized as one of the most severe decohering processes for electrons spins in semiconductor devices. Aiming to answer how spectral diffusion scales with concentration of magnetic nuclei in the environment we report X-band pulsed ESR measurements of spectral diffusion for phosphorus donors in silicon at different $^{29}$Si fractional concentrations from natural abundance 4.7\% down to 50 ppm. At all (but 50 ppm) $^ {29}$Si concentrations we found a non-exponential Hahn echo decay which can be best fit using exp(-(t/T$_{SD}$)$^n$); at 50 ppm the spectral diffusion was unmeasurably long. The spectral diffusion parameters, T$_{SD}$ and n, both show a pronounced dependence on orientation of the magnetic field with respect to silicon crystal axes. T$_{SD}$ changes from 0.6 ms at 4.7\% $^ {29}$Si to 18 ms at 800 ppm, while n always stays in the range 2- 3. We find our experimental results in good agreement with the existing theories. The work is funded by DOE and LPS. [Preview Abstract] |
Monday, March 15, 2010 4:42PM - 4:54PM |
D26.00012: Electron Spin Resonance on Mobile and Confined States in Gated Modulation Doped Si/SiGe Heterostructures Jianhua He, H. Malissa, Tzu-Ming Lu, S. Shankar, A.M. Tyryshkin, S.A. Lyon, Hung-Ming Chen, Chieh-Hsiung Kuan Electron spins in quantum dots in Si/SiGe heterostructures are promising qubits but controlling and measuring spins in gated dots is challenging. Fortunately, electrons confined into natural quantum dots by interface disorder can capture the spin physics with minimal processing, exhibiting long T$_{1}$ and T$_{2}$ at the Si/SiO$_{2}$ interface$^{1}$. Natural quantum dots in the Si/SiGe system may be similarly useful. As a first step, we have fabricated a 2.2 x 13mm$^{2}$ Hall bar on a Si/SiGe substrate gated with an Al gate above an Al$_{2}$O$_{3}$ insulator, and performed electron spin resonance (ESR) at gate voltages above and below threshold. The ESR signal arising from the Si quantum well evolves with gate voltage, and its intensity (spin susceptibility) is measured as a function of temperature down to 0.4K. The susceptibility follows a Pauli dependence when the gate is biased above threshold, while it is Curie-like below threshold, indicating an evolution from a mobile 2D system towards localized states confined in natural dots by the intrinsic disorder in the quantum well. This work is supported by LPS and ARO. [1] S. Shankar, et al., \textit{Physica E}, 40, 1659-1661 (2008). [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:06PM |
D26.00013: Physical mechanisms for interface-mediated intervalley coupling in Si Andre Saraiva, Maria Calderon, Xuedong Hu, Sankar Das Sarma, Belita Koiller The conduction band degeneracy in Si is detrimental to quantum computing based on spin qubits, for which a nondegenerate ground orbital state is desirable. This degeneracy is lifted at an interface with an insulator as the spatially abrupt change in the conduction band minimum leads to intervalley scattering. We present a theoretical study of the interface-induced valley splitting in Si that provides simple criteria for optimal fabrication parameters to maximize this splitting. Our work emphasizes the relevance of different interface-related properties to the valley splitting. [Preview Abstract] |
Monday, March 15, 2010 5:06PM - 5:18PM |
D26.00014: Interface roughness and valley manipulation in quantum dots Dimitrie Culcer, Xuedong Hu, S. Das Sarma We present a systematic study of interface roughness and its role in enabling intervalley tunneling in coherent dynamical processes in quantum dots. The interface potential lifts the degeneracy of the lowest energy valleys and yields a set of valley eigenstates. Transitions between these valley eigenstates can take place in dynamics involving two or more dots. We demonstrate that interface roughness provides a mechanism to mediate interdot intervalley transitions and analyze the way this occurs in the dynamics of one and two electrons in a double quantum dot. Based on a simple theoretical model of the interface we derive an expression for the intervalley tunneling matrix element, and discuss the principal factors affecting its magnitude. We demonstrate that experimentally interdot intervalley transitions can be induced by appropriate manipulation of the bias between two dots and can be detected by charge sensing. We discuss further a method involving resonant tunneling for extracting additional information on intervalley tunneling and identifying valley-split states. This work was supported by LPS-NSA-CMTC. [Preview Abstract] |
Monday, March 15, 2010 5:18PM - 5:30PM |
D26.00015: Detection of Spin Dependent Scattering at High Magnetic Fields C.C. Lo, J. Bokor, V. Lang, R.E. George, J.J.L. Morton, S. Zvyagin, A.M. Tyryshkin, S.A. Lyon, Arun Persaud, T. Schenkel Neutral donor spin-dependent scattering of conduction electrons in silicon field-effect transistors can be used as a spin-state readout mechanism for donor qubits in silicon[1]. Experimentally, the effect is measured by electrically detected magnetic resonance, usually in a low magnetic field regime ($\approx0.35T$)[2]. In such measurements, the resonance signal amplitude is limited by the conduction electron spin polarization, which is typically less than 5\% at those fields. We report recent progress in the measurement of spin-dependent scattering at high magnetic fields ($3.5T-11T$), where the conduction electron polarization is much higher and results in much stronger resonance signals. \newline [1] Sarovar et al, PRB, 78, 245302 (2008), de Sousa et al, PRB,80, 045320 (2009) \newline [2] Lo et al, APL, 91, 242106 (2007) [Preview Abstract] |
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