Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L2: Single Molecule Transistors and Graphene Quantum Dots |
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Sponsoring Units: DCMP Chair: Daniel Ralph, Cornell University Room: Ballroom A2 |
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L2.00001: Spectroscopy and read-out of STM-patterned donor based qubits Invited Speaker: We report low temperature transport measurements of few-to single P donor based quantum dots in silicon. Dots with a high donor number (approx. 7) show a surprisingly dense spectrum of excited states with an average energy spacing of 100 micro eV. The energy spacing of these features is much too low to be accounted for by the nm-scale lateral confinement of either the dot or the leads and can be explained by lifting of valley degeneracy of the dot orbital states [1]. The use of all epitaxial in plane P:Si gates allow us to tune both the electron number in the dot and modulate the transparency of the tunnel barriers [2]. We also present transport through a deterministic single donor device, where we observe both the signature of a single donor directly through STM imaging and demonstrate that the charging energy and excited state spectrum is consistent with the orbital states of a single P-donor. Finally we present our latest results of spin read-out in STM-patterned donor based devices. \\[4pt] [1] M. Fuchsle et al, Spectroscopy of few electron single crystal silicon quantum dots, Nature Nanotechnology 5, 502 (2010). \\[0pt] [2] A. Fuhrer et al, Atomic-Scale, All Epitaxial In-Plane Gated Donor Quantum Dot in Silicon, Nano Letters 9, 707 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L2.00002: Single-donor transport in silicon: Atomic physics in restricted momentum space Invited Speaker: Technology reached a level of miniaturization where we can realize transport through a single dopant atom in a transistor. Such transport spectroscopy can probe the atomic orbitals and the interaction of the atom with the environment. This interaction with the environment in a nano-device leads alters the dopants properties, such as the level spectrum and the charging energy, from those of the bulk. The system discussed here is a gated arsenic donor in a silicon field effect transistor. Electronic control over the wavefunction of dopants is one of the key elements of quantum electronics. This talk focuses on the role of the restricted momentum space which has a severe impact on the charge and spin configuration of a donor atom in a nano-device. The combined experimental and theoretical study of the gated two-electron state of the donor led to the realization of the pseudo spin nature of the valleys. We observe a blocked electronic relaxation due to combined spin and valley selection rules. Time averaged transport measurements put a lower bound of 50 ns on the rate of the blocked transition, 1000 times slower than a bulk transition. For the low lying excited states Hund's rule is violated due to vanishing exchange in orthogonal valleys. Furthermore, we observe reduced charging energies and bound singlet and triplet excited states for this negatively charged donor that can be explained in the self consistent tight binding model. Finally, experiments demonstrating coherent coupling between two donors and between a donor and the leads will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L2.00003: Orbital Gating of Single Molecule Transistors Invited Speaker: Electron devices containing molecules as the active region have been an active area of research over the last few years. In molecular-scale devices, a longstanding challenge has been to create a true three-terminal device; e.g., one that operates by modifying the internal energy structure of the molecule, analogous to conventional FETs. Here we report the observation of such a solid-state molecular device, in which transport current is directly modulated by an external gate voltage. We have realized a molecular transistor made from the prototype molecular junction, benzene dithiol, and have used a combination of spectroscopies to determine the internal energetic structure of the molecular junction. Resonance-enhanced coupling to the nearest molecular orbital is revealed by electron tunneling spectroscopy, demonstrating for the first time direct molecular orbital gating in a molecular electronic device. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L2.00004: A single-molecule optical transistor Invited Speaker: This abstract not available. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L2.00005: Optical, magnetic and electronic properties of graphene quantum dots Invited Speaker: We present a theory of optical, magnetic and electronic properties of graphene quantum dots. We demonstrate that there exists a class of triangular graphene quantum dots with zigzag edges [1-8] which combines magnetic, optical and transport properties in a single-material structure. These dots exhibit robust magnetic moment and optical transitions simultaneously in the THz, visible and UV spectral ranges due to the existence of a band of degenerate states lying at the Fermi level in the middle of the energy gap [1-6]. The magnetic and optical properties[5,7] are determined by strong electron-electron and excitonic interactions in the degenerate band, treated exactly using numerical techniques combining tight-binding, DFT, Hartree-Fock and configuration interactions methods. We show that the spin polarized degenerate band leads to quenching of the absorption spectrum at half-filling, while addition of a single electron fully depolarizes all electron spins and turns the absorption on. It is thus possible to design gate and size tunable graphene quantum dots with desired optical and magnetic properties for optoelectronic and photo-voltaic applications. Collaborators: P. Potasz, O. Voznyy, M. Korkusinski, and P. Hawrylak.\\[4pt] [1] J. Fernandez-Rossier, and J. J. Palacios, Phys. Rev. Lett. 99, 177204 (2007).\\[0pt] [2] W. L. Wang, S. Meng, E. Kaxiras, Nano Lett. 8, 241 (2008).\\[0pt] [3] M. Ezawa, Phys. Rev. B 76, 245415 (2007).\\[0pt] [4] J. Akola, H. P. Heiskanen, and M. Manninen, Phys. Rev. B 77, 193410 (2008).\\[0pt] [5] A. D. G\"u\c{c}l\"u, P. Potasz, O. Voznyy, M. Korkusinski, and P. Hawrylak, Phys. Rev. Lett. 103, 246805 (2009).\\[0pt] [6] P. Potasz, A. D. G\"u\c{c}l\"u, P. Hawrylak, Phys. Rev. B 81, 033403 (2010).\\[0pt] [7] A. D. G\"u\c{c}l\"u, P. Potasz, and P. Hawrylak, Phys. Rev. B 82, 155445 (2010).\\[0pt] [8] O.Voznyy, A. D. G\"u\c{c}l\"u, P. Potasz and P. Hawrylak, arXiv:1011.0369. [Preview Abstract] |
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