Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W2: Imaging Charge and Spin and Semiconductors |
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Sponsoring Units: DCMP Chair: Robert Westervelt, Harvard University Room: Baltimore Convention Center Ballroom III |
Thursday, March 16, 2006 2:30PM - 3:06PM |
W2.00001: Imaging Transport Resonances in the Quantum Hall Effect Invited Speaker: We image charge transport in the quantum Hall effect using a charge accumulation microscope. Scanning a charge sensitive tip just above the surface of a very high mobility AlGaAs/GaAs heterostructure, we measure the charging underneath the tip that results from applying an ac voltage to the 2D electron system (2DES). Applying a dc bias voltage to the tip induces a highly resistive ring-shaped incompressible strip (IS) in the 2D electron system (2DES) that moves along with the tip. This IS acts as a barrier that prevents charging of the region under the tip. At certain tip positions, short-range disorder in the 2DES creates a quantum dot island inside the IS that enables breaching of the IS barrier by means of resonant tunneling through the island. The images that result show striking ring shapes that directly reflect the shape of the IS. Within the ring shaped features, we also observe striations that arise from Coulomb Blockade of the quantum dot island. Varying the magnetic field, the tunneling resistance of the IS varies significantly, and takes on drastically different values at different filling factors. Measuring this tunneling resistance provides a unique {\em microscopic} probe of energy gaps in the quantum Hall system. To better understand the origin of the transport resonances, we have completed a series of simulations that show that the native disorder from remote ionized donors can create islands in the IS. Comparing the simulations with the experimental images provides a direct view of the disorder potential of a very high mobility 2DES. The experiments and simulations reveal the potential importance of single-electron resonant tunneling to bulk transport in the quantum Hall effect. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:42PM |
W2.00002: Terahertz Imaging of cyclotron emission from quantum Hall conductors Invited Speaker: Microscopy of extremely weak terahertz (THz) waves via photon-counting method is reported. A quantum-dot photon detector [1] is incorporated into a scanning terahertz microscope [2]. By using a quantum Hall detector [3] as well, measurements cover the intensity dynamic range more than five orders of magnitude. The minimum intensity reaches as lo as 10\^{}-21$^{ }$watt (one photon per one second). Applying the measurement system to the study of semiconductor quantum Hall (QH) devices, we image cyclotron radiation emitted by non-equilibrium electrons generated in QH electron systems. Owing to the unprecedented sensitivity, a variety of new features of electron kinetics are unveiled [4]. It is stressed that the present approach is in marked contrast to the THz- wave applications recently discussed extensively in a wide variety of fields including clinic, security, and environment. In the vast majority of those applications, room-temperature operation is implicit. The intensity of treated THz radiation is hence well beyond the level of 300K black body radiation (roughly 10\^{}-7 watts or 10\^{}14 photons/s per square centimeter in a 1/10 relative band width). From the scientific viewpoint, however, detecting extremely weak THz waves from an object without external illumination such as applied in the present work is of strong importance, because the microscopic kinetics of an object can be probed only in such a passive method. Besides semiconductor electric devices studied here, we will also discuss possible applications of the present method for molecular dynamics, micro thermography, and cell activities.. \newline \newline [1] S. Komiyama et al., Nature 403, 405 (2000). \newline [2] K. Ikushima et al.,. Rev. Sci. Instrum. 74, 4209 (2003). \newline [3] Y.Kawano et al., J. Appl. Phys. 89, 4037 (2001). \newline [4] K.Ikushima et al., Phys. Rev. Lett. 93, 146804 (2004). [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 4:18PM |
W2.00003: Imaging coherent electron flow in a two-dimensional electron gas Invited Speaker: Images of electron flow through a two-dimensional electron gas can be obtained at liquid He temperatures using scanning probe microscopy. Near a quantum point contact (QPC), the images show angular lobe patterns characteristic of the wavefunctions in the QPC. At distances greater than one micron from the QPC, narrow branches of electron flow are observed due to the cumulative effect of small angle scattering. All of the images are decorated by interference fringes spaced by half the Fermi wavelength demonstrating that the flow is coherent. To determine the origin of the interference fringes, an imaging interferometer is created by adding a circular reflecting gate. The strength and position of the interference fringes can then be controlled by the voltage on this reflecting gate. Using the interferometer, we show that the interference fringes are due to backscattering to the QPC. Both experiments and theory demonstrate that the interference signal is robust against thermal averaging. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:54PM |
W2.00004: Imaging Magnetic Focusing in a Two-Dimensional Electron Gas Invited Speaker: Using a liquid-He cooled scanning probe microscope (SPM), we have directly imaged cyclotron orbits of electrons in a two-dimensional electron gas (2DEG) traveling between two side-by-side quantum point contacts (QPCs). The images show magnetic focusing when the spacing between the QPCs is an integer multiple of twice the cyclotron radius. An image is created by deflecting electrons away from their original trajectories using a capacitively coupled SPM tip, and recording the change in conductance as the tip is raster scanned above the surface.~ The cyclotron orbits are clearly visualized, as well as fringes that demonstrate the coherent nature of the flow.~ Classical and quantum simulations show how electrons are deflected by the tip to produce the image. With an applied magnetic field, the simulated images of magnetic focusing agree very well with the measured images. The simulations also show the effect of small angle scattering due to the ionized donor atoms. Fully quantum simulations show that interference fringes can be produced. Imaging and understanding the motion of electrons in magnetic fields is useful for the development of devices for spintronics and quantum information processing. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:30PM |
W2.00005: STM and AFM; Which is Better for Surface Structural Analysis? Non- contact AFM Studies on Ge/Si(105) Surface Invited Speaker: Scanning tunneling microscopy (STM) has been utilized to determine surface atomic structure with its highly resolved images. Probing surface electronic states near the Fermi energy (E$_{F})$, STM images, however, do not necessarily represent the atomic structure of surfaces. It has been believed that atomic force microscopy (AFM) provides us surface topographic images without being disturbed by the electronic states. In order to prove the surpassing performance, we performed noncontact (nc) AFM on the Ge/Si(105) surface [1], which is a facet plane of the ?hut? clusters formed on Ge-deposited Si(001) surface. It is found that STM images taken on the surface, either filled- or empty-state images, do not show all surface atoms because of the electronic effect; some surface atoms have dangling bond states below E$_{F}$ and other surface atoms have states above E$_{F}$. [2]. In a nc-AFM image, on the other hand, all surface atoms having a dangling bond are observed [3], directly representing an atomic structure of the surface. Electronic information can also be obtained in AFM by using a Kelvin-probe method. From atomically resolved potential profile we obtained, charge transfer among the dangling bond states is directly demonstrated. These results clearly demonstrate that highly-resolved nc-AFM with a Kelvin-probe method is an ideal tool for analysis of atomic structures and electronic properties of surfaces. This work was done in collaboration with T. Eguchi, K. Akiyama, T. An, and M. Ono, ISSP, Univ. Tokyo and JST, Y. Fujikawa and T. Sakurai, IMR. Tohoku Univ. T. Hashimoto, AIST, Y. Morikawa, ISIR, Osaka Univ. K. Terakura, Hokkaido Univ., and M.G. Lagally, University of Wisconsin-Madison. \newline \newline [1] T. Eguchi et al., Phys. Rev. Lett. 93, 266102 (2004). \newline [2] Y. Fujikawa et al., Phys. Rev. Lett. 88, 176101 (2002). \newline [3] T. Eguchi and Y. Hasegawa, Phys. Rev. Lett. 89, 256105 (2002) [Preview Abstract] |
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