Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P40: Focus Session: Transport Properties of Nanostructures IV: Wires |
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Sponsoring Units: DMP DCP Chair: D. Natelson, Rice University Room: LACC 408A |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P40.00001: Spin-flip scattering times from weak localization studies of Cu$_{93}$Ge$_4$Au$_3$ thin films J.J. Lin, S.M. Huang We have fabricated a series of Cu$_{93}$Ge$_{4}$Au$_{3}$ thin films by sputtering deposition technique. The Ge atoms were introduced to enhance the impurity scattering while the Au atoms were introduced to enhance spin-orbit scattering. The films were either 150 or 200 angstroms thick. The residual resistivities were tuned by adjusting the deposition rate and varied between 14 and 59 $\mu \Omega $ cm. Resistance measurements revealed ln$T$ behavior below 10 K or so, which could be ascribed to Kondo effect in addition to 2D electron-electron interaction effects. The electron dephasing times have been measured through the weak-localization-induced magnetoresistances. In particular, the spin-flip scattering times have been extracted from the total dephasing times. We will present the temperature and disorder behavior of the spin-flip scattering times and compare them with the Nagaoka-Suhl and recent theoretical calculations. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P40.00002: Electrical Transport and 1/f Noise in Au Nanoparticle Films C. Kurdak, J. Kim, J. J. Lucido, A. Kuo, L. A. Farina, X. Bai, M. P Rowe, K. E. Plass, A. J. Matzger We studied the transport properties of Au nanoparticle films deposited on interdigitated electrodes with electrode spacings ranging from 0.1 $\mu $m to 1 $\mu $m. $I-V$ characteristics are found to be nonlinear and strongly dependent on both the coating and size of the nanoparticles. Current is thermally activated at low bias voltages, exhibits a threshold behavior, and scales as $I\propto (V-V_{th} )^\zeta $ at low temperatures. To complement dc transport measurements, we have performed noise measurements on some of the films. All the films that were studied exhibit 1/f type noise at low frequencies. The magnitude of the 1/f noise is smaller in devices with a larger device area, indicating that the 1/f noise is caused by intrinsic processes. The noise amplitude is found to be strongly temperature dependent between 40-300 K, with a local peak at around 100 K, and weakly dependent below 40 K. The noise data could not be fit by a single activated process, which would have led to an Arrhenius type temperature dependence. At low temperatures, the normalized noise spectra scaled as ${S_I } \mathord{\left/ {\vphantom {{S_I } {I^2}}} \right. \kern-\nulldelimiterspace} {I^2}\propto (V-V_{th} )^\gamma $. The relationship between the scaling exponents \textit{$\zeta $} and \textit{$\gamma $} is consistent with our prediction of $\gamma =1-\zeta $. [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P40.00003: Quantum Coherence and Local/Nonlocal Resistance Measurements Sungbae Lee, Aaron Trionfi, Douglas Natelson Low temperature electrical properties of ferromagnetic nanowires are influenced by the interplay between disorder, quantum coherence, and magnetic correlations. Quantum coherence corrections to the conductance are of particular interest, and can be difficult to characterize experimentally. One of the ways of understanding these effects is by measuring the electronic coherence length of the system. Field dependences of local and nonlocal resistance fluctuations were measured for this purpose. Permalloy (Ni$_{0.8}$Fe$_{0.2})$ nanowires were made using standard electron beam lithography along with silver leads with various lead to lead distances. With these nanowires, local and nonlocal length dependent magnetic field correlations were measured and compared. Silver wire samples were also tested for the purpose of comparison. We present initial magnetic field correlation data. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P40.00004: Role of surface anisotropy for magnetic impurities in electron dephasing and energy relaxation and their size effect Alfred Zawadowski, Antal Jakovac, Orsolya Ujsaghy Recently the electron dephasing and energy relaxation due to different magnetic impurities have been extensively investigated experimentally in thin wires. It was shown earlier [1] that a magnetic impurity in a metallic host with strong spin-orbit interaction experiences a surface anisotropy of the form $H=K_d ({\bf n}{\bf S})^2$ which causes size effects for impurities with integer spin. The dephasing and the energy relaxation are influenced by the surface anisotropy in very different ways for integer spin having a singlet and for half-integer spin with a Kramers doublet ground state [2]. Thus for $S=1$ the dephasing is frozen out at low temperatures. That must also result in strong size effects and may resolve the puzzle between the impurity concentrations estimated from the measured electron dephasing and energy relaxation [3,4]. [1] O.~\'Ujs\'aghy, A.~Zawadowski, and B.~L.~Gyorffy, Phys.\ Rev. Lett. {\bf 76}, 2378 (1996). [2] O.~\'Ujs\'aghy, A.~Jakov\'ac, and A.~Zawadowski, to be published in Phys.\ Rev.\ Lett. [3] F.~Pierre, H.~Pothier, D.~Esteve, M.~H.~Devoret, A.~B.~ Gougam, N.~O.~Birge, cond-mat/0012038 and references therein. [4] G.~G\"oppert, Y.~M.~Galperin, B.~L.~Altshuler, and H.~Grabert, Phys.\ Rev.\ {\bf B66}, 195328 (2002). [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P40.00005: Time dependent universal conductance fluctuations in AuPd, Ag, and Au wires A. Trionfi, S. Lee, D. Natelson Quantum transport phenomena allow experimental determinations of the phase coherence information in metals. We report quantitative comparisons of inferred coherence lengths from weak localization magnetoresistance measurements and time-dependent universal conductance fluctuation data. A detailed explanation of how these two measurements are performed and analyzed will be given. Strong agreement is observed in both quasi-2D and quasi-1D AuPd samples, a metal known to have high spin-orbit scattering. However, quantitative agreement is not seen in quasi-1D Ag wires below 10 K, a material with intermediate spin-orbit scattering. Possible explanations for this disagreement are discussed. An enhancement of the conductance fluctuations in Au has also been observed by depositing a thin layer of Al$_{2}$O$_{x}$, an oxide known to have a large number of mobile two level systems, over the sample. Preliminary results will be reported. [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P40.00006: Superconducting ultra narrow Al nanowires Fabio Altomare, Albert M. Chang, Michael R. Melloch, Yuguang Hong, Charles W. Tu We have successfully developed a technique\footnote{F. Altomare \textit{et al.},\textit{March Meeting 2001}}$^,$\footnote{F. Altomare \textit{et al.},\textit{March Meeting 2004}} for the fabrication of nanowires, of width comparable or smaller to 10~nm, using the (1$\bar{1}0 $) plane of a narrow MBE-grown ridge as a template. These wires are formed with direct connections to 4 terminal measurement pads. The versatility and reliability (yield exceeding 75\%) in the fabrication process, together with the small lateral size achievable, makes this technique uniquely suited for the study of diverse physical phenomena in nanowires composed of a variety of materials. In particular we have characterized AuPd wires as long as 20~$\mu$m and Al wires with length exceeding 10~$\mu$m. The Al wires superconduct at low temperature despite having a normal state resistance much greater than the superconducting quantum resistance($=\hbar/4e^2$). We will discuss the observed behavior of the normal-superconducting transition in applied magnetic field and the Current-Voltage characteristics of these wires. This work has been supported by NSF DMR 0135931 and 0401648. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 1:03PM |
P40.00007: End States in One-Dimensional Chains Invited Speaker: As a consequence of the lower dimensionality, a new kind of state is observed at the boundaries of one-dimensional atomic chains that are self-assembled by depositing gold on the vicinal Si(553) surface. Such “end states” can be thought of as zero- dimensional analogs to two-dimensional states that occur at a bulk surface. Scanning tunneling microscopy images taken at positive and negative polarities reveal contrast reversal at the end atoms that provides evidence for the formation of end states. To confirm this attribution, spatially resolved scanning tunneling spectroscopy along finite chains maps the density of states revealing the formation of quantized states. Further, we observe a transfer of spectral weight from the empty to the filled states over the end atoms. These end states lead to a breakdown in the simple particle in a box model for states along the chains. A comparison to a tight-binding model demonstrates how the formation of end electronic states transforms the density of states and the quantized levels within the chains. As a confirmation of the tight-binding model and the end electronic effects, calculated STM topography profiles at positive and negative biases reproduce the experimentally observed contrast at the end atoms. This work is done in collaboration with Daniel T. Pierce and is supported in part by the office of naval research. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P40.00008: Gate Dependent One-dimensional Transport in In2O3 Nanowires Fei Liu, Mingqiang Bao, Kang L Wang, Chao Li, Bo Lei, Chongwu Zhou The gate-dependent one-dimensional transport of single- crystalline semiconducting In$_{2}$O$_{3}$ nanowire field effect transistors is studied at low temperature by measuring I-V and differential conductance. The In$_{2}$O$_{3}$ nanowires were synthesized by a laser ablation process to have a diameter of 10 nm and a length of 2 $\mu $m. Back gate was formed using a highly-doped silicon substrate with a gate oxide thickness of 0.5 $\mu $m. At a smaller positive gate bias, gaps at near zero source-drain bias were observed for both current and differential conductance spectra due to the absence of the density of states in the source-drain energy window. The transport can be explained by Fermi-liquid theory. On the other hand, when the Fermi energy of the nanowire moves up into the conduction band, the differential conductance of the semiconducting In$_{2}$O$_{3}$ nanowire exhibits zero-bias anomalies, following a power-law behavior similar to one-dimensional Luttinger-liquid. These results suggest that electron-electron interaction must be taken into consideration for the understanding of transport of nanowires at low temperature under a large gate bias. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P40.00009: Nanowires and Nanoribbons of Charge-Density-Wave Conductor NbSe$_{3}$ Yew San Hor, Zhili Xiao, Ulrich Welp, John F. Mitchell, Russell E. Cook, Wai-Kwong Kwok, George W. Crabtree, Yasuo Ito We report synthesis of nanowires and nanoribbons of the charge- density-wave conductor NbSe$_{3}$ through direct reaction of Nb and Se powders under careful temperature control. The thickness of these nanostructures range from tens of nanometers to a few hundreds nanometers. Their morphologies and crystal structures were studied with scanning electron microscopy, x- ray diffraction and transmission electron microscopy. Four- probe resistivity measurements were conducted to characterize their electronic properties. Significant enhancement in depinning threshold fields was also observed in the nanowires in comparison to the values of the bulk crystals. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P40.00010: Effects of Morphology on the Electronic Properties of Nano-lithographic Thin Pb Wires U. Kemiktarak, K.C. Schwab, Kamil Ekinci Miniaturization of superconducting wires is being pursued aggressively for a number of applications as well as for studying the fundamentals of superconductivity in 1-dimensional systems. In this work, we have investigated the conductance of Pb wires with submicron widths and thicknesses as a function of temperature and wire morphology. Our wires are fabricated by electron beam lithography and thermal Pb deposition. In an effort to vary the morphology, we have deposited Pb upon substrates held at room temperature and at 77 K. We shall present from our latest measurements and discuss the effects of morphology upon the electronic properties of these wires, with particular emphasis on superconducting behavior. The authors gratefully acknowledge support from the NSF under grant~No. 0315662. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P40.00011: Transport Measurements on Individual Branched Nanostructures Yong Sun, Jeong-Il Oh, Dezhi Wang, K. Kempa, Z.F. Ren, Wenzhi Li, M.J. Naughton We have made electrical measurements on individual branched (``Y-junction") carbon nanotubes. After isolation on silicon substrates and identification via electron microscopy, photo and e-beam lithography were used to deposit metal electrodes ($e.g.$ Au/Ti) onto individual branches of the nanostructures, including 4-probe configurations across a branch point (Y-junction). Various post-processing procedures, such as rapid thermal annealing and electron beam welding, were employed in attempts to improve contact resistances. Four-probe I-V measurements at room temperature yield varied intrinsic conductivity in these nanostructures (resistances between $10^4$ and $10^7\Omega$). Transmission microscopy reveals a fishbone internal structure, which could be responsible for the low conductance. We also report on the construction of ``divining rod'' cantilevers out of these branched nanotubes, using an etch-well technique, toward potential SPM applications, and on similar attempts using inorganic ($e.g.$ ZnO$_2$) nanowires. [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P40.00012: Scattering Mechanisms in 3D Quantum Mechanical Simulations of a Silicon Quantum Wire Matthew Gilbert, Richard Akis, David Ferry We examine the inclusion of scattering processes in MOS quantum wires at room temperature, where phonon effects are prevalent. In a manner similar to Green's function approaches, but more amenable to the site representation used in these methods, scattering is computed on a mode basis and then transformed to the site basis. Beginning with the corresponding matrix elements, we derive mode to mode scattering rates for the different phonon processes and then perform a basis transformation to switch from mode space to real space. The resultant transformation gives us a complex matrix with terms that represent both the elastic and inelastic contributions resulting from the inclusion of different phonon processes. This matrix is then added into the pre-existing tight-binding Hamiltonian and the relevant transport quantities are calculated. Here, we present results of the first implementation of separable phonon scattering rates in a three-dimensional, fully quantum mechanical, self-consistent device simulation. The processes included are acoustic deformation potential scattering, intervalley absorption and emission containing both $f$ and $g$ type processes. [Preview Abstract] |
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P40.00013: Electronic structure of single fullerene molecules in C60 nanocrystals grown on thin NaBr film Gareguin Mikaelian, Xiuwen Tu, Wilson Ho Since the discovery of C60 much attention has been devoted to utilizing its interesting electronic properties for the advancement of the controllable single molecule technologies. Here we report scanning tunneling microscopy and spectroscopy of C60 nanocrystals grown on thin sodium-bromide films supported on the NiAl(110) surface. STM allows us to study the electronic properties of the C60 crystal with submolecular resolution. Differential conduction spectroscopy reveals a variety of features corresponding to the vibronic excitation and charging of individual C60 molecules in these nanocrystals. [Preview Abstract] |
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