Physics Commons^™

Transition From Static To Dynamic Macroscopic Friction In The Framework Of The Frenkel-Kontorova Model, Naum I. Gershenzon, Gust Bambakidis

Physics Faculty Publications

A new generation of experiments on dry macroscopic friction has revealed that the transition from static to dynamic friction is essentially a spatially and temporally non-uniform process, initiated by a rupture-like detachment front. We show the suitability of the Frenkel-Kontorova model for describing this transition. The model predicts the existence of two types of detachment fronts, explaining both the variability and abrupt change of velocity observed in experiments. The quantitative relation obtained between the velocity of the detachment front and the ratio of shear to normal stress is consistent with experiments. The model provides a functional dependence between slip velocity …

Terahertz Spectroscopic Reflection And Scattering Measurements Of Aligned Cnt Arrays As A Function Of Carbon Nanotube Length, Satya Ganti, Lindsay Owens, Stanley Smith Iv, Jason A. Deibel

Physics Faculty Publications

No abstract provided.

Crucible Coating Analysis By Thz Spectroscopy, Ryan Shaver, Hannah R. Jones, Jason A. Deibel

Physics Faculty Publications

No abstract provided.

Characterization Of Metameterial Devices Using Terahertz Time-Domain Spectroscopy, Hannah R. Jones, Satya Ganti, Jason A. Deibel, Ronald A. Coutu Jr.

Physics Faculty Publications

No abstract provided.

Self-Compensation In Semiconductors: The Zn Vacancy In Ga-Doped Zno, David C. Look, K. D. Leedy, L. Vines, B. G. Svensson, A. Zubiaga, F. Tuomisto, Daniel R. Doutt, L. J. Brillson

Physics Faculty Publications

Self-compensation, the tendency of a crystal to lower its energy by forming point defects to counter the effects of a dopant, is here quantitatively proven. Based on a new theoretical formalism and several different experimental techniques, we demonstrate that the addition of 1.4 × 10²¹-cm⁻³ Ga donors in ZnO causes the lattice to form 1.7 × 10²⁰-cm⁻³ Zn-vacancy acceptors. The calculated V_Zn formation energy of 0.2 eV is consistent with predictions from density functional theory. Our formalism is of general validity and can be used to investigate self-compensation in any degenerate semiconductor material.

Thermal Annealing Effect On Spin Coherence In Zno Single Crystals, Z. Yang, David C. Look, H. M. Zhou, W. V. Kawakami, R. K. Kawakami, P. K. L. Yu, J. L. Liu

Physics Faculty Publications

The spin coherence time (T₂^*) in ZnO single crystals at 8.5 K decreases significantly from ∼11.2 ns to ∼2.3 ns after annealing at 500 °C, as indicated by time-resolved Kerr-rotation pump-probe magneto-optical spectroscopy. The annealing-induced spin coherence degradation in ZnO arises neither from crystallinity degradation during the annealing process, as confirmed by x-ray rocking curves; nor from reflection variations of the probe laser beam induced by surface roughness changes during the annealing process, as confirmed by atomic force microscopy. Temperature-dependent Hall-effect studies indicate that decreased mobility and increased shallow-donor concentration in the annealing-induced surface conducting layer …

Dynamics Of Fingering Convection Ii: The Formation Of Thermohaline Staircases, Stephan Stellmach, Adrienne L. Traxler, Pascale Garaud, N. Brummell, T. Radko

Physics Faculty Publications

Regions of the ocean's thermocline unstable to salt fingering are often observed to host thermohaline staircases, stacks of deep well-mixed convective layers separated by thin stably stratified interfaces. Decades after their discovery, however, their origin remains controversial. In this paper we use three-dimensional direct numerical simulations to shed light on the problem. We study the evolution of an analogous double-diffusive system, starting from an initial statistically homogeneous fingering state, and find that it spontaneously transforms into a layered state. By analysing our results in the light of the mean-field theory developed in Part 1 (Traxler et al., J. Fluid …

Growth Of Low-Dimensional Carbon Nanomaterials, John J. Boeckl, Weijie Lu, William Mitchel

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Nano-Based Optical Chemical Sensors, Aleksandra Lobnik

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Understanding Irreducible And Reducible Oxides As Catalysts For Carbon Nanotubes And Graphene Formation, Alicja Bachmatiuk

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

High Ammonia Yield Using Magnetic Induction Method, Noorhana Yahya

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Ultrasonics And Carbon Nanotubes, Marian Nowak, Krzysztof Koziol, Gregory Kozlowski, Jan Berdowski, Jacek Kasperczyk, Ivan V. Kityk, Krystian Mistewicz, Marcin Jesionek

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Nonlinear Optical Properties Of Monolayer Films Deposited On Ir Transparent Substrates, Ivan V. Kityk, Marek Matusiewicz, Gregory Kozlowski, Jacek Kasperczyk, Krzysztof Koziol

Special Session 5: Carbon and Oxide Based Nanostructured Materials (2011)

No abstract provided.

Dynamics Of Fingering Convection I: Small-Scale Fluxes And Large-Scale Instabilities, Adrienne L. Traxler, Stephan Stellmach, Pascale Garaud, T. Radko, N. Brummell

Physics Faculty Publications

Double-diffusive instabilities are often invoked to explain enhanced transport in stably stratified fluids. The most-studied natural manifestation of this process, fingering convection, commonly occurs in the ocean's thermocline and typically increases diapycnal mixing by 2 orders of magnitude over molecular diffusion. Fingering convection is also often associated with structures on much larger scales, such as thermohaline intrusions, gravity waves and thermohaline staircases. In this paper, we present an exhaustive study of the phenomenon from small to large scales. We perform the first three-dimensional simulations of the process at realistic values of the heat and salt diffusivities and provide accurate estimates …

Propagation Dynamics Of Controlled Cross-Talk Via Interplay Between Chi((1)) And Chi((3)) Processes, Paul S. Hsu, George R. Welch, James R. Gord, Anil K. Patnaik

Physics Faculty Publications

We investigate theoretically and experimentally the propagation dynamics of a nonlinear cross-talk effect between two probe channels in a double-ladder system and show that an interplay between χ⁽¹⁾ and χ⁽³⁾ processes leads to the control of cross-talk. We derive analytical solutions to describe the propagation dynamics of the probe fields with the cross-talk effect built in. From the analytical results we identify and examine the regimes of interest where contributions of either χ⁽¹⁾ or χ⁽³⁾ or both are significant. The control of cross-talk is demonstrated experimentally, and good quantitative agreement is found between the analytical solutions …

Ga-Doped Zno Grown By Pulsed Laser Deposition In H-2: The Roles Of Ga And H, David C. Look, T. C. Droubay, J. S. Mccloy, Z. H. Zhu, S. A. Chambers

Physics Faculty Publications

Highly conductive thin films of ZnO doped with Ga were grown by pulsed laser deposition with 10 mTorr of H₂ in the growth chamber. Compared with a more conventional method of producing conductive films of ZnO, i.e., growth in O₂ followed by annealing in forming gas (5% H₂ in Ar), the H₂ method requires no postgrowth anneal and also produces higher carrier concentrations and lower resistivities with better depth uniformity. As an example, a 65-nm-thick sample had a room-temperature mobility of 32 cm²/V s, a concentration of 6.8×10²⁰ cm⁻³, and a …

Turbulent Mixing And Layer Formation In Double-Diffusive Convection: 3d Numerical Simulations And Theory, Erica Rosenblum, Pascale Garaud, Adrienne L. Traxler, Stephan Stellmach

Physics Faculty Publications

Double-diffusive convection, often referred to as semi-convection in astrophysics, occurs in thermally and compositionally stratified systems which are stable according to the Ledoux criterion but unstable according to the Schwarzschild criterion. This process has been given relatively little attention so far, and its properties remain poorly constrained. In this paper, we present and analyze a set of three-dimensional simulations of this phenomenon in a Cartesian domain under the Boussinesq approximation. We find that in some cases the double-diffusive convection saturates into a state of homogeneous turbulence, but with turbulent fluxes several orders of magnitude smaller than those expected from direct …

Episodic Tremors And Slip In Cascadia In The Framework Of The Frenkel-Kontorova Model, Naum I. Gershenzon, Gust Bambakidis, Ernest Hauser, Abhijit Ghosh, Kenneth C. Creager

Physics Faculty Publications

The seismic moment for regular earthquakes is proportional to the cube of rupture time. A second class of phenomena, collectively called slow earthquakes, has very different scaling. We propose a model, inspired from the phenomenology of dislocation dynamics in crystals, that is consistent with the scaling relations observed in the Cascadia episodic tremor and slip (ETS) events. Two fundamental features of ETS are periodicity and migration. In the northern Cascadia subduction zone, ETS events appear every 14.5 months or so. During these events, tremors migrate along-strike with a velocity of 10 km/day and simultaneously zip back and forth in the …

Numerically Determined Transport Laws For Fingering ("Thermohaline") Convection In Astrophysics, Adrienne L. Traxler, Pascale Garaud, Stephan Stellmach

Physics Faculty Publications

We present the first three-dimensional simulations of fingering convection performed in a parameter regime close to the one relevant for astrophysics, and reveal the existence of simple asymptotic scaling laws for turbulent heat and compositional transport. These laws can straightforwardly be extrapolated to the true astrophysical regime. Our investigation also indicates that thermocompositional "staircases," a key consequence of fingering convection in the ocean, cannot form spontaneously in the fingering regime in stellar interiors. Our proposed empirically-determined transport laws thus provide simple prescriptions for mixing by fingering convection in a variety of astrophysical situations, and should, from here on, be used …

Computed Tomography Reconstruction: Investigating The Effect Of Varying Circle Diameter, William F. Sanders Iv

Browse all Theses and Dissertations

In Computed Tomography (CT) reconstruction, several methods for determining the intensity of individual pixels from the back-projection of the scanned profiles exist. The standard reconstruction method uses linear interpolation between ray values to determine pixel intensity. This study quantifies the effects of varying circle diameter on the accuracy of an alternative method where the pixel is approximated as a circle and the area contributions calculated.

A library of 104 scans in 3 image families was created by a synthetic CT scanner and reconstructed with circle radii from 0.1 to 1.0 pixel in 0.1 pixel steps. Images were compared against a …

Growth And Characterization Of Carbon Nanomaterials, Jay Patel

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In this thesis, single and multi-layered graphene films were epitaxially grown on either Si-face or C-face of SiC single crystal substrates. The film growth conditions, such as decomposition temperatures and pressures, and their surface morphologies were optimized. These films were then characterized by using surface analysis tools including SEM, TEM, AFM evanescent wave microscopy and electron educed spectroscopy. In addition to studying graphene decomposed from SiC crystals, carbon nanotube material was fabricated using a floating catalyst technique. These carbon nanotube material was then studied for potential cathode applications in this thesis. Field emission properties of these cathodes was measured and …