Bulletin of the American Physical Society
74th Annual Meeting of the Southeastern Section
Volume 52, Number 13
Thursday–Saturday, November 8–10, 2007; Nashville, Tennessee
Session NB: Condensed Matter |
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Chair: Daniel Boye, Davidson College Room: Scarritt-Bennett Center Laskey C |
Saturday, November 10, 2007 10:45AM - 10:57AM |
NB.00001: Design and Optimization of Force-Reduced Superconducting Magnets Szabolcs Rembeczki Superconducting magnet design and development has particular significance in physics not only in the area of research, but also due to the extensive interest in civil and military applications. Due to the complexity of the superconducting magnets, detailed analytical methods are needed during their design and construction. One of the main problems in high-field superconducting magnets is the presence of huge electromagnetic stresses that must be counteracted by heavy support structures. Different force-free magnet concepts have been studied both numerically and analytically to solve this problem. In this research I propose to design and optimize a high-field, low weight superconducting magnet using force-reduced coil configuration. [Preview Abstract] |
Saturday, November 10, 2007 10:57AM - 11:09AM |
NB.00002: High-current hunt for Bardeen-Stephen flux motion in A15 superconductor V$_{3}$Si at high fields Rajendra Khadka, Albert Gapud, Lloyd Lumata, Arneil Reyes, Philip Kuhns, David Christen The motion of \textit{flux lines} -- quantized supercurrent vortices -- is observed in a high-quality superconducting single crystal of V$_{3}$Si with very little flux ``pinning''. Consequently flux lines are more free to move under a Lorentz force when a transport current is applied. This opens up the possibility of Bardeen-Stephen flux flow (BSFF), wherein flux lines move in an orderly fashion. BSFF is clearly recognizable by a linear dependence of its dissipation resistivity on applied field $H$ and can be observed in voltage vs. current (VI) curves. Since this requires both pinning-free samples and currents in the tens to hundreds of amperes, BSFF is difficult to attain especially because of current-induced heating. In this study, heating is significantly reduced via ultrasonically soldered contacts, pulsed currents, and submerging the sample in liquid helium. Measuring from fields of 6 T up to 20 T, dissipation levels characteristic of ordered flux flow are clearly distinguishable, along with other interesting features such as the ``peak'' effect in critical current $J_{c}(H)$ seen only when pinning energy density is comparable to the elasticity of the flux medium. The data and their interesting ramifications will be discussed. [Preview Abstract] |
Saturday, November 10, 2007 11:09AM - 11:21AM |
NB.00003: On narrowed coated conductors: granular field-hysteresis effect on transport critical current A.A. Gapud, D.K. Christen, F.A. List III, R. Feenstra Superconducting film coated onto flexible metallic tapes -- so-called coated conductors -- which are being developed for second-generation power transmission lines, are long polycrystals subject to effects of the film's granularity. Although intergranular misorientations have been effectively minimized, local higher-angle misorientations remain. One earmark of such weak links is the hysteresis of the critical current density $J_{c}$ with respect to applied field $H$, brought about when large circulating currents trapped within adjacent grains produce a focused field within the grain \textit{boundaries }(GB's) which can partially cancel out $H$ when applied field is \textit{decreasing}. This shifts the maximum $J_{c}$ from zero to a finite $H$ where the local field at the GB is at minimum. This effect has been seen in measurements of \textit{induced }(magnetization) currents, but has not been documented using \textit{transport }(applied) currents that can be percolative. However, in applications where the coated conductor is \textit{patterned} into conduits 100 $\mu $m wide or less, it is possible to effectively channel the percolation across a single GB; the field-focusing effect is well known in transport currents across \textit{single }GB's. This study shows that, indeed, the effect is clearly manifested in coated conductors narrowed to a few grains wide. Systematic measurements, analyses, and ramifications will be discussed. [Preview Abstract] |
Saturday, November 10, 2007 11:21AM - 11:33AM |
NB.00004: Ultrafast dynamics of one-dimensional excitons in metals and semiconductors. Tobias Herel Exciton resonances in metallic and semiconducting carbon nanotubes (CNTs) allow insights into the unique nature and peculiarities of excited states with one-dimensional character. We discuss recent advances and discoveries in this exciting field of materials research with particular focus on the ultrafast dynamics of exciton decay in structurally and electronically sorted carbon nanotube samples. Among others we discuss changes in the photophysical properties of one-dimensionally localized excitons in individual single-wall carbon nanotubes and of more delocalized states in semiconducting nanotube-crystals and how these changes relate to fundamental aspects of carrier interactions in confined systems. In addition we explore the detailed mechanisms and the importance of electronic and vibrational degrees of freedom for energy redistribution in CNTs. Lastly, we will clarify the relevance of these findings for the potential implementation of carbon nanotubes in new technologies such as flexible electronics, sensing-, photovoltaics and biomedical applications. [Preview Abstract] |
Saturday, November 10, 2007 11:33AM - 11:45AM |
NB.00005: Control of rehydration in sol-gel glasses Carlos Ortiz, Daniel Boye Optical properties of monoliths synthesized via the sol gel process result in a large number of residual silanol (Si-OH) groups, even after annealing at 900\r{ }C. High silanol content quenches emission from rare earth ions via excitation of vibrations of the silanol groups. Medium density glasses (1.5gcm$^{-3})$ have a highly interconnected porous structure that allows the diffusion of molecules throughout the material. Diffusion of atmospheric water molecules results in chemiadsorption reactions that increase silanol group content, adding to quenching. By monitoring the intensity of terbium (Tb$^{3+})$ emissions from the $^{5}$D$_{3}$ level relative to the $^{5}$D$_{4}$ level, we report an 80{\%} decrease in ratio within 12 hours. Monoliths prepared with of N,N-dimethylformamide, a drying control chemical additive (DCCA), were annealed at 1050\r{ }C and maintained good optical quality with nearly complete densification of the material (2.1gcm$^{-3})$. DCCAs minimize the capillary stresses in the network during drying, even during the onset of viscous flow at the glass transition. Monoliths prepared with DMF and annealed at 1050\r{ }C for 6 hours showed no change in their $^{5}$D$_{3}$: $^{5}$D$_{4}$ intensity ratio upon exposure to the atmosphere. [Preview Abstract] |
Saturday, November 10, 2007 11:45AM - 11:57AM |
NB.00006: EXAFS Studies of the Local Bonding Structures of Ge$_{2}$Sb$_{2}$Te$_{4}$, Ge$_{2}$Sb$_{2}$Te$_{5}$, AND Ge$_{2}$Sb$_{2}$Te$_{7}$ Joseph Washington, Michael Paesler, Dave Baker, Gerald Lucovsky, Craig Taylor Bond constraint theory (BCT) and rigidity theory provide powerful frameworks for understanding the structure and properties of a-materials. Application of these theories to switching in a-chalcogenides holds the promise of finding the ideal a-chalcogenide suited for switching applications. Recently a-chalcogenide switching of Ge$_{2}$Sb$_{2}$Te$_{5 }$(GST) has been applied successfully to programmable memory devices as well as DVD technology - where the quest for the discovery of better-suited materials continues. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is an ideally suited technique to investigate the switching properties of these materials. We analyze films of amorphous Ge$_{2}$Sb$_{2}$Te$_{4}$, Ge$_{2}$Sb$_{2}$Te$_{5}$, and Ge$_{2}$Sb$_{2}$Te$_{7}$ through EXAFS and propose predictions of their aptitude for reversible phase change using bond constraint theory. [Preview Abstract] |
Saturday, November 10, 2007 11:57AM - 12:09PM |
NB.00007: Graphite Superlattices Simulation Software and its Research Applications Reza Rock, Philip W.T. Pong Superlattices are commonly observed on graphite surfaces when imaged using scanning tunneling microscopy. There is, however, currently no reproducible laboratory method to create these superlattices in a controlled fashion for study. A simple model has been used to simulate graphite superlattices using the Moir\'{e} interference pattern assumption. However, it is difficult to perform quantitative analyses on the output data from this simple model because the corrugation amplitude of the atomic lattice is comparable to that of the superlattice. We have developed an averaging algorithm to filter the output to rectify this problem, yielding a more realistic simulated surface. The parameters for this filter were optimized and the simulation results are much more realistic and similar to actual STM measurements. A MATLAB program utilizing this algorithm with graphical user interface to simulate graphite superlattices was developed. Coexisting superlattices, superlattices surrounding a screw dislocation, one-dimensional fringes resulting from Moir\'{e} patterns and other variable-periodicity superlattices were modeled and analyzed. [Preview Abstract] |
Saturday, November 10, 2007 12:09PM - 12:21PM |
NB.00008: The effect of oxide impurities on the performance of 4H-SiC MOSFETs Xingguang Zhu, A.C. Ahyi, J.R. Williams Recent studies for SiC [1] show that oxidation in the presence of metal impurities (especially Na) introduced from an alumina environment yields enhanced thermal oxidation rate, low interface trap density, and high MOSFET channel mobility. In this work, different sodium compounds (NaCl, Na$_{2}$O$_{2})$ and ion implantation were used to introduce Na at different stages during the oxidation process. The effect on oxide growth rate is discussed and interface trap densities are reported for simultaneous hi-lo C-V measurements using n-4H-MOS capacitors. Positive Bias Temperature Stress (1.5MV/cm, 5min, 250$^{o}$C) to move the mobile ions to the SiO$_{2}$/SiC interface produced a significant increase in MOSFET channel mobility, while no such effect was observed for Negative BTS. This indicates the possibility of a shielding mechanism for negatively charged interface traps when positive mobile ions are present at the oxide-semiconductor interface. [1] F. Allerstam, \textit{et al.}, J. Appl. Phys. \textbf{101} (2007) 124502. [Preview Abstract] |
Saturday, November 10, 2007 12:21PM - 12:33PM |
NB.00009: Electrical properties of MOS devices fabricated on the 4H-SiC C-face. Zengjun Chen, A.C. Ahyi, J.R. Williams The electrical characteristics of MOS devices fabricated on the carbon face of 4H-SiC will be described. The C-face has a higher oxidation rate and a higher interface trap density compared to the Si-face. The thermal oxidation rate and the distribution of interface traps under different oxidation conditions will be discussed in this presentation. Sequential post-oxidation anneals in nitric oxide and hydrogen effectively reduces the interface density (D$_{it})$ near the conduction band edge. However, deeper in the band gap, the trap density remains higher compared to the Si-face. Time-dependent dielectric breakdown (TDDB) studies have also been performed to investigate oxide reliability on the C-face, and current-voltage measurements show that a low barrier height against carrier injection likely contributes to oxide degradation. Nevertheless, the effective channel mobility and threshold voltage for n-channel C-face lateral MOSFETs compare favorably with similar Si-face devices. [Preview Abstract] |
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