Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V14: Focus Session: Transport Properties of Nanostructures VI: Inorganic Nanostructures and Nanomechanics |
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Sponsoring Units: DMP Chair: Pierre Darancet, Lawrence Berkeley National Laboratory Room: B113 |
Thursday, March 18, 2010 8:00AM - 8:12AM |
V14.00001: Ambipolar Ballistic Electron Emission Microscopy (BEEM) Studies of Gate-field Modified Schottky Barriers (SBs) Y. Che, J.P. Pelz Gate-field modified SBs are important for ``SB FETs'' [1], and could be used to control spin and charge injection into other semiconductor device structures. We report \textit{ambipolar} BEEM measurements on Au/Si and Cu/Si SBs (on 35nm-Si/150nm-SiO$_{2}$/p-Si silicon-on-insulator substrates) that can be changed from effective $n$-type to $p$-type by applying a positive or negative back-gate bias, respectively. This allows the conduction and valence band energies to be directly measured at the same location, to allow effects of local electric fields and local composition and defects to be directly measured. After correcting for image force lowering, the average intrinsic barrier heights (over a 200 x 200 nm$^{2}$ region) at 80K on a uniform Si film for electrons and holes respectively was 0.840 eV and 0.358 eV for Au/Si, and 0.617 eV and 0.597 eV (with statistical uncertainty $\sim $2 -- 5 meV) for Cu/Si, which sum close to the $\sim $1.17eV Si bandgap at 80K. We will discuss on-going measurements of spatial variations of SB height and BEEM current amplitude on locally thinned Si film regions and in ``mixed'' Au/Cu bilayers, as well as efforts to improve n- and p-type ohmic contacts and to understand the dependence of lateral transport on temperature and back-bias. Work supported by National Science Foundation Grants No. DMR-0505165 and DMR-0805237. \\[4pt] [1] S. Heinze, et al., Phys. Rev. Lett. 89, 106801 (2002). [Preview Abstract] |
Thursday, March 18, 2010 8:12AM - 8:24AM |
V14.00002: Conductivity and Structural Properties of Nanostructures Fabricated by E-Beam Exposure of Nanoparticle Resists Stefan Dickert, Myoung-Hwan Park, Yuval Ofir, Qijun Xiao, Tianyu Yang, Bappaditya Samanta, Vincent Rotello, Mark Tuominen Spin or drop coated solutions of ligand-coated nanoparticles act as a novel type of ebeam resist into which functional nanostructures can be produced in a direct-write process (Y. Ofir et al, Adv. Mater. 20, 2561-2566 (2008)). Thin films were produced by this method from a set of resists in which the size of the Au nanoparticles and ligand length were varied systematically. Small angle x-ray scattering (SAXS) experiments were used to characterize film structure and the role played by ligand length and particle size. Films were characterized at different stages in processing: directly after drop coating, after ebeam exposure and after annealing. Further, 4-point resistance measurements were performed on thin Au nanoparticle films to ascertain the resistance-temperature behavior in the 2-350K range. These results suggest a Mott-type hopping conduction behavior that can be manipulated by fabrication conditions. [Preview Abstract] |
Thursday, March 18, 2010 8:24AM - 8:36AM |
V14.00003: Piezoresistive readout and electromechanical response of GaN nanowire resonators Jason Gray, Kris Bertness, Norman Sanford, Charles Rogers We report on the fabrication, piezoresistive readout, and frequency response of doubly-clamped c-axis GaN nanowire resonators. The nanowires are single crystal, grown by catalyst- free molecular beam epitaxy, from 150-350 nm in diameter, and 15-18 microns in length. The devices are fabricated using a combination of lithographic patterning and dielectrophoresis to bridge the wires across 10 micron gaps, with an electrostatic gate nearby. The gate induces nanowire vibration, which is electronically read out by measuring current through the nanowire. Wires show resonances from 9-14 MHz with mechanical quality factors, Q, as large as 2200. Intrinsic Q is likely larger than this, however, as we have discovered processing steps that can improve Q, as well as parameter noise in the resonator that artificially broadens the peak. These parameters and their noise provide information about the wire's environment, such as surface and clamping effects. We will discuss these observations, along with fabrication steps and behavior of the resonance under different drive conditions. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V14.00004: Correlating structural and resistive changes in Ti:NiO resistive memory elements Amanda Petford-Long, Olle Heinonen, Markus Siegert, Andreas Roelofs, Martin Holt, Wei Li Structural and resistive changes in Ti-doped NiO resistive random access memory structures that occur upon electroforming have been investigated using hard X-ray microscopy with a spatial resolution of 50 nm. Analysis of 2D scans of the NiO (111) diffraction intensity across a 10 $\mu $m $\times $ 10 $\mu $m patterned Pt/NiO:Ti/Pt structure show that electroforming leads to structural changes in regions of size up to about one micrometer, which is much larger than the grain size of the structure (of the order of 15 nm). Such changes are consistent with a migration of ionic species or defects during electroforming over regions containing many crystalline grains. [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V14.00005: How important are vertex corrections in the longitudinal dc transport through multilayers of strongly correlated materials? Simon Hale, Jim Freericks In the bulk, dynamical mean-field theory has no vertex corrections to dc transport, as proved by Khurana in 1990. The proof does not hold for inhomogeneous systems like multilayers with current flow perpendicular to the layers. We examine the effect of vertex corrections on the transport for multilayered inhomogeneous devices composed of semi-infinite metallic leads coupled via a strongly correlated material barrier region. The barrier region can be adjusted from a metallic regime to a Mott insulator through adjusting the interaction strength. We use the Falicov-Kimball model because the exact expression for the vertex corrections is known and it displays a Mott like metal to insulator transition. The resistance is calculated and we find the effects of the vertex correction are relatively small manifesting in a small reduction in the resistance-area product. This as expected this reduction saturates as the barrier layer grows towards the bulk limit. Overall, the effect of vertex corrections is smaller than about 5\% of the total resistance and relatively decreases. [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:12AM |
V14.00006: Transport through two-dimensional metallic nanoparticle arrays Virginia Estevez, Elena Bascones The electronic transport through a metallic nanoparticle array is blocked up to a finite threshold voltage which depends on disorder and size of the array. Here we analyze the tranport properties of two- dimensional metallic nanoparticle arrays for different lattice geometries and type of disorder (charge and resistance disorder or vacancies). We show that, contrary to general believe, the current depends linearly on voltage close to threshold. At higher voltages a range of superlinear behavior is found. In general, this superlinear behavior cannot be described in terms of a power-law. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:24AM |
V14.00007: Acoustic transport and electron phonon interaction in nanorod array Masashi Yamaguchi, Jianxun Liu, Pei-I Wang, Dexian Ye, Toh-Ming Lu Coherent acoustic transport through vertically grown nanorod array on substrate and electron phonon interactions are experimentally studied by using femtosecond laser spectroscopy and glancing angle deposition technique. We have designed a model structure to study the phonon transport along the long axis of nanorods with the use of acoustic spectroscopy. We have experimentally observed the transport of the acoustic pulse with comparable wave length to the diameter of the nanorod, and damping due to the coupling of propagating acoustic phonon to the eigen mode of nanorod (bending motion). Electron-phonon interaction in copper nanorod arrays was studied using ultrafast transient reflectivity spectroscopy with both resonant and off-resonant probe to d-band to Fermi-level transition. Slanted nanorod arrays 10 nm - 50 nm in diameter were fabricated by newly developed deposition technique. The use of a variable probe wavelength over the transition energy range suggested the modification of electronic structure in slanted nanorod arrays with relatively large diameter. [Preview Abstract] |
Thursday, March 18, 2010 9:24AM - 9:36AM |
V14.00008: Measurement of stochastic current switching in semiconductor superlattices Yuriy Bomze, Huidong Xu, Stephen W. Teitsworth, Rudolf Hey, Holger T. Grahn We report on the experimental measurement of stochastic current switching in doped, weakly coupled GaAs/AlAs superlattices with large contact conductance.~ Static current-voltage ($I-V)$ curves exhibit multiple current branches with the number of branches approximately equal to the number of superlattice periods. Some current branches are split, indicating injection from more than one subband in the contact layer. High bandwidth current switching data are collected in response to steps in applied voltage to final voltages $V_{f}$ near the voltage corresponding to a particular current jump. For a certain range of $V_{f}$ values, switching times reveal large stochastic fluctuations driven by shot noise, and switching time distributions show exponential tails indicative of a first passage process from an initial metastable state. The mean switching time \textit{$\tau $} and its standard deviation are plotted versus final voltage $V_{f}$. The \textit{$\tau $} -$ V_{f }$plot reveals an exponential dependence on $V_{f}$, in qualitative agreement with predictions based on a discrete drift-diffusion transport model.~~ [Preview Abstract] |
Thursday, March 18, 2010 9:36AM - 9:48AM |
V14.00009: Scaling behavior of stochastically varying current switching times in semiconductor superlattices Huidong Xu, Scott Schmidler, Stephen Teitsworth The stochastic switching process from a metastable state of electronic transport in a semiconductor superlattice with $N$ periods ($N\gg 1)$ is simulated using a discrete drift-diffusion model that also includes shot noise in the tunneling currents. Sequential resonant tunneling between quantum wells is the primary conduction mechanism and noise terms are treated as delta-correlated in space and time. This is a high-dimensional, non-gradient system; furthermore, the metastable state possesses stability eigenvalues with non-zero imaginary part. The distribution of metastable lifetimes is studied as a function of bias voltage $V$, in a regime for which the current-voltage characteristics exhibit bistability. The mean lifetime \textit{$\tau $} is fitted to an expression of the form $\ln \tau \propto \left| {V-V_{th} } \right|^\alpha $, where $V_{th}$ denotes the voltage for which the metastable state disappears in a saddle-node bifurcation. We find that the exponent \textit{$\alpha $} is sensitive to the initial state preparation. Starting from the exact metastable state, the exponent is $\alpha =1.67\pm 0.06$. In contrast, a pulsed initial condition, of the type that is readily achievable in experimental measurements, yields larger \textit{$\alpha $} values. In both cases, the determined \textit{$\alpha $} values exceed 3/2, which is the exponent value for a typical one-dimensional system. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V14.00010: The spectrum of an oscillator with fluctuating mass and nanomechanical mass sensing J. Portman, M. Khasin, S.W Shaw, M.I. Dykman We study resonant response of an underdamped oscillator with shot-noise type mass fluctuations. The model describes a nano-mechanical resonator which at random adsorbs and desorbs molecules. We derive an analytical expression for the spectrum of the oscillator. It applies for an arbitrary interrelation between the essential parameters: the oscillator damping rate, the adsorption-desorption rate, and the oscillator frequency shift due to a single adsorption event. Depending on this interrelation the spectrum may have fine structure or display a single peak. The fine structure emerges if the frequency shift exceeds damping which in turn exceeds the adsorption-desorption rate. However, even in this case the spectrum differs, generally, from a superposition of individual spectral lines for different numbers of adsorbed molecules. With decreasing frequency shift or increasing adsorption-desorption rate the fine structure disappears, and the spectrum becomes a single asymmetric peak. The results can be used for high-precision fast measurements of molecular mass with nano-mechanical resonators. [Preview Abstract] |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V14.00011: Fluctuation spectrums of a noise-driven micromachined oscillator with tunable nonlinearity K. Ninios, H.B. Chan We measure the spectrum of fluctuations of a nonlinear underdamped micromechanical oscillator, whose nonlinearity can be electrostatically tuned. In the linear regime where the eigenfrequency is independent of the energy of the oscillator, the spectral peaks are well-characterized by a lorentzian lineshape, the width of which is determined by the relaxation rate. In the presence of cubic nonlinearity in the restoring force, the eigenfrequency depends monotonically on the energy. As a result, the energy straggling due to fluctuations gives rise to frequency straggling. For sufficiently large fluctuation intensity the frequency straggling exceeds the frequency uncertainty due to relaxation and broadening of the spectral peaks with fluctuation intensity is observed. We also measure the fluctuation spectrum when the dependence of the eigenfrequency of the oscillator on energy is not monotonic due to higher order nonlinearities. Our measurements indicate that for a certain range of parameters, it is possible for the width of the spectral peak to decrease as the fluctuation intensity increases. [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V14.00012: Non-Markovian dynamics of a nanomechanical resonator measured by a quantum point contact P. Chen, C.-C. Jian, H.-S. Goan We study the dynamics of a nanomechanical resonator (NR) subjected to a measurement by a low-transparency quantum point contact (QPC) or tunneling junction in the non-Markovian domain. By partially taking trace over the microscopic degree of freedom of the QPC reservoir and keeping track of the number $n$ of electrons that have tunneled through the QPC during the time period (0,t), we obtain the non-Markovian $n$-resolved (conditional ) master equation valid to second order in the tunneling Hamiltonian. In our derivation, we do not make the rotating-wave approximation and the Markovian approximation, generally made in quantum optics systems. Our non-Markovian master equation reduces, in appropriate limits, to various Markovian versions of master equations in the literature. We find considerable difference in dynamics betweem the non-Markovian case and its Markovian counterpart. We also calculate the time-dependent transport current through the QPC which contains information about the measured NR system. We find an extra transient current term proportional to the expectation value of the symmetrized product of the position and momentum operators of the NR. This extra term, with a coefficient coming from the combination of the imaginary parts of the QPC reservoir correlation functions, was generally ignored in the study of the same problem in the literature. But we find that it has a substantial contribution to the total transient current in both the Markovian and Non-Markovian cases. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V14.00013: Detecting quantum-coherent nanomechanical oscillations using the current-noise spectrum of a double quantum dot Neill Lambert, Franco Nori We consider a nanomechanical resonator coupled to a double quantum dot. We demonstrate how the finite-frequency current-noise spectrum through the double quantum dot can be used to distinguish classical and quantum behavior in the nearby nanoelectromechanical resonator. We also show how the full-frequency current-noise spectrum gives important information on the combined double quantum dot-resonator energy spectrum. Finally, we point out regimes where the quantum state of the resonator becomes squeezed and also examine the cross-correlated electron-phonon current noise. [Preview Abstract] |
Thursday, March 18, 2010 10:36AM - 10:48AM |
V14.00014: Detection of oscillator-qubit entanglement in nanomechanical systems Kjetil Borkje, Thomas Schmidt, Bjoern Trauzettel, Christoph Bruder In recent years, various proposals have been made on how to create entanglement between a quantum oscillator and a qubit. We propose a nanoelectromechanical device which would allow the detection of such entanglement by coupling the oscillator and the qubit to a quantum point contact. We demonstrate that measurements of the current and the symmetrized current noise of the quantum point contact reveal information about a bipartite expectation value matrix of the oscillator-qubit system. This matrix allows the evaluation of a specific entanglement witness. This proposal is a step towards the detection of entanglement between a discrete and a continuous variable in nanomechanical systems. [Preview Abstract] |
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