Session D28: SPS Undergraduate Research II

 Monday, March 15, 2010 2:30PM - 2:42PM D28.00001: Growth of Superconducting Bulk Single Crystals and their use in Leviation Demonstrations A. Kotlyarevsky , M.C. Sullivan , J. Hunting We present our work on the growth of bulk single-crystal YBa$_2$Cu$_3$O$_7$ and a novel superconducting demonstration using our superconductors. We report the first successful fabrication at the undergraduate level of YBa$_2$Cu$_3$O$_7$ superconducting pucks with enhanced flux-pinning properties. We follow a bulk superconductor growth recipe developed by Dr. Kazumasa Iida at the Institute for Metallic Materials in Dresden, Germany. In order to grow a puck that is largely single crystalline in phase, it is necessary to mix portions of both superconducting (YBa$_2$Cu$_3$O$_7$) and non-superconducting (Y$_2$BaCuO$_5$) phases together and fire them at near liquefying temperatures. This process is known as melt-textured growth. We have also constructed a figure-8 track of strong permanent NdFeB magnets to demonstrate the dramatic effect of flux-pinning. This track is outfitted with an accelerator to keep the puck circling the track. We accelerate the puck to a speed that, without the introduction of enhanced flux-pinning, would cause the puck to be thrown from the track. We will show a video of our puck and novel demonstration. Monday, March 15, 2010 2:42PM - 2:54PM D28.00002: Magnetic properties of BiMnO$_{3}$ thin films Kristen Voigt , Hyoung Jeen Jeen , Guneeta Singh-Bhalla , Sefaattin Tongay , Patrick Mickel , Arthur Hebard , Amlan Biswas The growth conditions for growing BiMnO$_{3,}$ with pulsed laser deposition, are optimized. The optimal oxygen pressure was found to be near 32 mTorr, and the optimal substrate temperature was found to be between 630\r{ } C and 635\r{ } C. The quality of the films was checked using x-ray diffraction, Auger electron spectroscopy, and atomic force microscopy. Magnetic properties of two thin films were measured and showed that there is no clear 2$^{nd}$ order transition in ferromagnetic BiMnO$_{3}$ at the Curie temperature. A pronounced ferroelectric polarization loop was also obtained in these thin films. Monday, March 15, 2010 2:54PM - 3:06PM D28.00003: Magnetic Ordering in Yb$_{4}$LiGe$_{4}$ J.N. Svensson , S. Disseler , R.C. Johnson , M.J. Graf , S. Giblin , P. Carretta , S. Peter R$_{5}$T$_{4}$ compounds (R = rare earth, T = Ge or Si) are interesting because the magnetic properties depend sensitively on changes in the crystalline structure. Yb$_{5}$Ge$_{4}$ such a compound, with (presumed) AFM order occurring at T$_{N}$ = 1.7 K. We are interested in the effects of substituting Li in place of one Yb atom. Previous measurements of the magnetic properties of polycrystalline Yb$_{4}$LiGe$_{4}$ using NMR, specific heat, and resistance measurements at temperatures down to 0.5 K and in magnetic fields up to 4 T were made for comparison with the parent compound. The resistance measurements showed a maximum at 1.1 K, which may indicate the onset of magnetic order. Thus we performed $\mu$SR measurements on Yb$_{4}$LiGe$_{4}$ and Yb$_{5}$Ge$_{4}$, and analysis of the data confirmed magnetic ordering (possibly AFM) at 1.1 K. $\mu$SR also revealed a dependence on the magnetic history of the sample. Currently we are studying the pressure dependence of the (presumed) T$_{N}$ to explore if increased pressure can drive the T$_{N}$ to 0 K, and results will be discussed. Monday, March 15, 2010 3:06PM - 3:18PM D28.00004: Longitudinal Field $\mu$SR Study of Spin Dynamics and Onset of Magnetic Correlations in LiHo$_{x}$Y$_{1-x}$F$_{4}$ K. Chen , R.C. Johnson , S.R. Giblin , J.S. Lord , A. Amato , C. Baines , A. Lascialfari , A.M. Tkachuk , B. Barbara , B.Z. Malkin , M.J. Graf Nanomagnets - non-interacting spins typically imbedded in a solid host material - are intensely studied for applications ranging from high density data storage to quantum computing. We are studying the magnetic ion Ho$^{3+}$ spin dynamics in LiHo$_{x}$Y$_{1-x}$F$_{4}$ in the dilute limit (x $<<$ 1) using the muon spin relaxation in applied longitudinal magnetic fields as a function of temperature and magnetic field strength. At low concentrations (x $\le$ 0.01) and for all magnetic field values studied, the spin lattice relaxation rate, 1/T$_{1}$, shows a characteristic peak, usually associated with relaxation by spin fluctuations, near T = 10 K; this peak is consistent with the single-ion energy level diagram of Ho$^{3+}$. At larger x this peak is lost in low fields; stronger fields ($>$ 600 G) restore the peak but show qualitatively different low temperature behavior. This suggests that both magnetic disorder and Ho-Ho interactions play an increasingly important role at larger x values. Monday, March 15, 2010 3:18PM - 3:30PM D28.00005: A study of the critical current density in optimally doped and under-doped thin-films of the cuprate superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ E.S. Backus , M.C. Sullivan Scaling analysis of voltage vs.\ current isotherms has often been used to study the normal-superconducting phase transition in cuprate superconductors, and a consensus is now emerging regarding the critical exponents that govern this phase transition. In the past, researchers have measured the critical current and found mean-field exponents, however, sample quality and uniformity has greatly improved since those measurements were taken. With better samples it is possible to measure the critical regime exponents at temperatures very close to the critical temperature and also the mean-field exponents at temperatures that are further from the critical temperature. We conducted reverse polarity measurements sent through meander patterns in thin films of the cuprate superconductor YBa$_2$Cu$_3$O$_{7-\delta}$. We present our results as a plot of the critical current as a function of temperature in zero-field, and discuss both the critical regime and mean-field exponents. Monday, March 15, 2010 3:30PM - 3:42PM D28.00006: Quantum yield of semiconductor nanocrystals in controlled environment Clayton Jackson , Farbod Shafiei , Daniel Ratchford , Xiaoqin Li Semiconductor nanocrystals (SCNC) have potential applications in such fields as photovoltaic cells, biological labeling and solid-state lighting. In order to best utilize SCNC in these applications, one must understand their unique, dynamic properties such as photoluminescence intermittency, spectral wandering and time dependent quantum yield. The focus of this research is to understand the change in quantum yield with respect to time and what various environmental parameters contribute to that effect. Thus far a universal liquid-gas cell has been constructed for inert gas and vacuum studies. Currently, we are conducting a study of the quantum yield of SCNC in solution at various concentrations. Future work includes the study of SCNC time dependent quantum yield on substrates in various atmospheric conditions. Monday, March 15, 2010 3:42PM - 3:54PM D28.00007: Manganese Doping in Lead Sulfide Nanowires Kaitlyn Yoha , Dong Yu , Christopher Miller Lead sulfide nanowires were synthesized using a Vapor-Liquid-Solid method, and a similar procedure with the addition of a manganese doping agent was used to potentially fabricate Mn doped PbS wires. A reaction between PbCl$_2$ and sulfur under nitrogen gas at 635 $^{\circ}$C produced PbS nanowires and and identical synthesis with the addition of the doping agent MnCl$_2$ at a variable temperature between 645 $^{\circ}$C and 665 $^{\circ}$C was used to dope the wires. Various growth formations were fabricated and examined. The wires were analyzed by an X-ray diffractometer, and an electron paramagnetic resonance spectroscopy was performed to determine if the wires were doped with manganese. Once characterized, these wires may have applications in spintronics and solar cells. Monday, March 15, 2010 3:54PM - 4:06PM D28.00008: Molecular dynamics simulations of ablation and spallation of gold irradiated by femtosecond laser Brian Demaske , Vasily Zhakhovsky , Nail Inogamov , Ivan Oleynik The dynamics of material response to irradiation of thin gold foils by a femtosecond laser pulse is examined by molecular dynamics simulations. The major physical phenomena include ablation - the removal of material from irradiated surface and spallation - the ejection of a thin layer of material from the rear of the film. In order to reproduce the physical processes that occur under experimental conditions, we simulated 1 $\mu m$ thick foils containing up to 170,000,000 atoms. Such thick foils are also needed to prevent the ablation and spallation zones from overlapping. In this presentation, we discuss the major physics of laser ablation and spallation observed in MD simulations: heating of a narrow region beneath the surface of the foil, its transformation to a metastable stress-confined state, and the rapid decomposition of this state into a strong rarefaction and compression wave. At some critical absorbed laser fluence, the rarefaction wave results in nucleation and growth of voids leading to ablation of the frontal surface. At higher absorbed fluences, the compression wave causes rear-side spallation of crystalline gold. Quantitative data such as the absorbed fluence thresholds, crater depths, and cavitation strength of gold are obtained from simulation and compared to experimental data. Monday, March 15, 2010 4:06PM - 4:18PM D28.00009: Synthesis and optical studies of CdSe nanoparticles/ferroelectric thin film hybrid materials Elizabeth Bushong , J.K. Krebs , Katherine E. Plass Ferroelectric thin films have attracted recent research attention due to their promise as thin film photovoltaic devices. Due to the large bandgap of the ferroelectric, these films absorb weakly in the region of the solar spectrum. Meanwhile, semiconductor nanoparticles exhibit tunable absorption which can be tailored through size-selective synthesis techniques. We report on low-temperature chemical synthesis to incorporate CdSe nanoparticles into a ferroelectric thin film. Optical absorption and emission measurements confirm that the CdSe particles in solution have a diameter of 4 nm and retain that size through the film processing. AFM images of the films show that they are crack-free and of uniform thickness. These hybrid materials form a model system for the study of charge separation in semiconductor nanoparticles under the influence of an external field. Monday, March 15, 2010 4:18PM - 4:30PM D28.00010: Spectroscopy Measurements of Magnesium Diboride Josephson Junctions J.T. Mlack , J.G. Lambert , S.A. Carabello , Z.E. Thrailkill , P.T. Galwaduge , R.C. Ramos MgB$_{2}$ has the highest Tc of the conventional superconductors at 39K and exhibits two superconducting energy bands. This material is also inexpensive to produce and has been utilized in new designs for MRI, RF cavities, and Josephson junctions.~We report results of recent spectroscopy and transport measurements~ of Josephson junctions made of MgB$_{2}$ obtained from our collaborators. We investigate its transport characteristics at sub-kelvin temperatures as well as its responses to resonant microwave activation. Monday, March 15, 2010 4:30PM - 4:42PM D28.00011: Multiple-mode grating-coupled enhancement of fluorescence by gold nanowires Iuri Gagnidze , Jennifer Steele We demonstrate directional enhanced fluorescence emission from a gold wire grating. The dominant enhancement mechanism was shown to be excited fluorophores decaying into surface plasmon modes that radiate via the periodicity of the grating. The emission from fluorophores decaying in this way was strongly directional. The fluorophores efficiently coupled to multiple surface plasmon grating modes on both the top and substrate side of the grating, enhancing a broad spectrum of fluorescence wavelengths. This makes periodic systems more flexible than their nanoparticle counterparts. Coupling to multiple modes also allows gratings to enhance fluorescence at wavelengths smaller than the period of the grating, allowing gratings with micron and larger sized features to enhance fluorescence wavelengths in the visible range. This greatly loosens fabrication requirements for potential applications.