AVS2001 Session ELThM: Quantum Electronics
Thursday, November 1, 2001 8:20 AM in Room 124
Thursday Morning
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8:20 AM 
ELThM1 Fabrication of a Siliconbased Solid State Quantum Computer
R.G. Clark, M.Y. Simmons, A.S. Dzurak, A.R. Hamilton, S. Prawer, D.N. Jamieson, G. Milburn (University of New South Wales, Australia) The fabrication of a scalable siliconbased quantum computer, in which the qubits are nuclear spin states of single phosphorus atoms embedded in isotopically pure silicon registered to surface control gates^{1}, is a significant technological challenge. The Australian program is approaching this in two ways. In our ‘bottom up’ program the embedded phosphorus array is fabricated using advanced STM lithography techniques followed by Si MBE overgrowth. In our ‘top down’ strategy, a detailed process has been developed in which single phosphorus atoms are implanted (with onchip verification) selfaligned to the surface control gates and fast single electron transistor readout devices. The fabrication pathways each have their list of associated problems. An outline will be given of the practical issues that have to be overcome, together with a view on how this might be achieved including progress to date. In our bottomup program we have recently reported^{2} that it is possible to fabricate an atomicallyprecise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the QC. Our recent work has focused on the next step of implementing strategies for incorporating the P atoms substitutionally into the silicon surface with enhanced bonding and without disturbing the P array prior to encapsulation by subsequent silicon overgrowth. Our strategy in the top down program is to concentrate on fabricating the simplest fewqubit test structures that will enable us to access the critical physics. However we have approached this from the viewpoint of developing a reliable, reproduceable process which, for linear phosphorus arrays, can then be readily scaled up to multiqubit devices. An overview will be given of key details of the top down fabrication scheme and measurements on the first test structures.


9:00 AM 
ELThM3 Ratchets, Heat Pumps and Maxwell's Demon: Quantum Transport in the Nonlinear Regime
H. Linke (Univ. of Oregon, Eugene); T.E. Humphrey (Univ. of New South Wales, Australia); P.E. Lindelof (NielsBohr Inst., Denmark); A. Lofgren (Lund Univ., Sweden); R. Newbury (Univ. of New South Wales, Australia); P. Omling, W.D. Sheng (Lund Univ., Sweden); A.O. Sushkov (Univ. of New South Wales, Australia); A. Svensson (Lund Univ., Sweden); R.P. Taylor (Univ. of Oregon, Eugene); H.Q. Xu (Lund Univ., Sweden) Ratchets are nonequilibrium systems in which directed particle motion is generated using spatial or temporal asymmetry, in the absence of timeaveraged macroscopic forces or gradients. After introducing general examples for ratchets and their application s, an overview will be given on a series of recent experiments on socalled quantum ratchets for electrons. These devices are based on GaAs/AlGaAs heterostructures containing a twodimensional electron gas. The nonlinear response of a spatially asymmetri c nanostructure (such as a triangular quantum dot) to an applied voltage is used to partially rectify a symmetric AC voltage. The required nonlinear behaviour is generated using quantum effects, such as electron interference or tunneling through an asymm etric energy barrier. A particularly interesting observation is that the direction of the current generated in tunneling ratchets depends on energy, that is, the net flow of electrons at low energy is in a direction opposite to that of electrons at highe r energy. This observation implies that quantum ratchets perform an energysorting task similar to that assigned to Maxwell's demon  that is, they may act as heat pumps or even as heat engines. We will discuss the properties of such quantum heat pumps, focusing the discussion on the thermodynamic limits to their efficiency. 

9:40 AM 
ELThM5 Two Dimensional Electronic Properties of a Disordered Three Dimensional Conductor in the Extreme Quantum Limit
D. Haude, M. Morgenstern, I. Meinel, R. Wiesendanger (Hamburg University, Germany) Scanning tunneling spectroscopy images of nInAs(110)are recorded in magnetic fields corresponding to the extreme quantum limit. From the results it is concluded that the appearence of the so called Hall dip in magnetotransport corresponds to an appearance of a contrast pattern in the local density of states. The energy and magnetic field dependence of the contrast pattern is very similar to drift states usually expected in two dimensional systems exhibiting the quantum Hall effect. The appearance of a pseudogap at the Fermi level evidences that localization is involved in the change of the local density of states. From the results a simplified but straightforward explanation of the Hall dip controversially discussed since 1956 can be given. 

10:00 AM 
ELThM6 Firstprinciples Study of Conduction Channels of Atomic Wires
N. Kobayashi (National Institute of Advanced Industrial Science and Technology (AIST), Japan); M. Aono (Osaka University, Japan); M. Tsukada (University of Tokyo, Japan) Electron transport through nanoscale structures has been investigated from the viewpoint of nanoscale physics and technology. A number of studies have been performed for the transport of atomic wires and molecular bridges. One of the theoretical approaches to the transport is analysis of individual conduction channels. We report an analysis of the conduction channels of atom wires using the density functional theory with nonlocal pseudopotentials. Electronic states are calculated using the Green function technique, and are decomposed into individual channel components using the eigenchannel decomposition. We elucidate the channel transmission, the channel local density of states, and the channel current density, and clarify the characteristics of the channel for material kind. Furthermore, we show how the channels open or close for finite bias voltage, and discuss the IV characteristics. 

10:20 AM 
ELThM7 Quantum Transport through One Dimensional Aluminum Wires
I.P. Batra, P. Sen, S. Ciraci (University of Illinois at Chicago) Quantum conductance and quantized Hall resistance in narrow channels have been well understood by using the twodimensional electron gas (2DEG), a model system which has been realized in semiconductor heterojunctions. An essential property of the 2DEG is its ability to produce a constriction of width comparable to the Fermi wavelength, a property not shared by even thin metal films. But the advent of scanning tunneling microscopy (STM) has enabled scientists to fabricate wires of "atomic" dimensions. This has led to an explosion of interest in the quantum transport properties of nanostructures. Here we consider the specific case of a one dimensional (1D) wire consisting of Aluminum atoms. First we have to find the optimal structural arrangement of the 1D system. This was done using the firstprinciples density functional method combined with molecular dynamics. It is found that aluminum can form stable zigzag structures similar to those found for Au. In addition, we find, other novel structures, which have not been reported for any other material. We present our understanding of the bonding as derived from charge density analysis for aluminum wires. With the calculated atomic and electronic structure in hand we proceed to discuss the quantum ballistic transport through these nanowires. Our calculations are based on channel capacity arguments that can be motivated using the Heisenberg's uncertainty principle. Our results are compared with the numerical calculations by Lang, who has performed careful analysis of conductance as a function of Al nanowire length in atomic domain. We finally comment on the thermal conductance and WiedemannFranz law in the nanodomain. 

10:40 AM 
ELThM8 Probing a One Dimensional Conductor Confined Below a Charged Step Edge
Chr. Meyer, M. Morgenstern, J. Klijn, R. Wiesendanger (Hamburg University, Germany) Although one dimensional conductors exhibit unique properties, spatially resolved investigations are still rare. On InAs(110) we found a special type of charged step edge inducing a one dimensional electron system (1DES) below that step edge. We investigated this quantum wire with scanning tunneling spectroscopy (STS) over a length of 800 nm in various magnetic fields between 0 T and 6 T. The STScurves on the step edge show two subbands of the 1DES with ground state energies of 66 meV for the first and 20 meV for the second subband giving an electron density of 1.2x10^{8}/cm. Spatially resolved images of the local density of states (LDOS) reveal that the first and second subband have a width of 20 nm and 50 nm respectively. Along the step edge the LDOS of the 1DES shows an energy dependent fluctuation length in the range from 15 nm to 70 nm corresponding to the dispersion of InAs. We suggest that electron scattering on impurities in the quantum wire is responsible for the observed fluctuations in the LDOS. We compare our measurements with theoretical calculations of a random field disordered 1DES, that predict a broadening of the ideal 1D DOS. 