Session A46: SPS Undergraduate I

Electron microscopy of sillenites

In this undergraduate project, the student performed transmission and scanning electron microscopy measurements on two sillenite compounds: Bi$_{12}$SiO$_{20}$ and Bi$_{25}$InO$_{39}$. To our knowledge, the electron diffraction patterns of sillenites have not been reported in the literature before. Our preliminary results show that both the tetravalent and trivalent compound have the sillenite structure. Using concepts from undergraduate solid state physics, the student will explain how the electron diffraction patterns were analyzed.   

Inverted Pyramid Texturing of Si by Single Exposure Three-beam Interference Lithography

Increasing energy demands combined with environmental concerns prompts the need for cost-efficient solar cells. One way in which this can be achieved is by etching an inverted nano-pyramid texture into the silicon substrate thereby reducing the requisite amount of material. This is due to the ability of altering the pyramid size such that it corresponds to specific wavelengths, which results in higher light trapping efficiency. These inverted pyramids can be fabricated using three-beam lithography to create the desired hole/dot photoresist pattern in order to etch the substrate. The process can be done as a single exposure by aligning two dielectric mirrors and the sample at specific angles with respect to one another and the incoming laser beam. Using this method, nanostructures of Si and wide bandgap oxide semiconductors such as ZnO and NiO will be fabricated. Detailed results will be discussed in this presentation. This work is partially supported by National Science Foundation (DMR- 0907037).   

Effects of Sputtering Energy on Surface Defect Formation on Ge(110)

Pyramid-shaped defects were observed in STM images to form on clean Ge(110) surfaces as a result of argon ion sputtering. By periodically imaging the samples after various numbers of sputtering and annealing cycles, we systematically studied the formation of these defects as a function of the Ar$^{\mathrm{+}}$ ion sputtering energy. Although the number and size of pyramids increased with sputtering energy from 100 to 200eV, the sample sputtered with 300eV ions showed a very flat surface with very few pyramids. The sample sputtered with 400eV ions appears to have mountain ranges of highly stepped regions with numerous pyramids on the edges, separated by flat valleys of reconstructed c(8x10) surface. Many pyramids are capped by a cluster of atoms, probably carbon, which may have served as the nucleation site. To explain the dependence of defect formation on sputtering energy, we present a mechanism involving competition between uncovering parts of new pyramids and breaking down older pyramids. Using different sputtering energies for controlled defect formation could be an effective tool for controlling island growth at defects on substrates.   

Studies of electron spin in GaAs quantum dots

We have studied electron spins in GaAs quantum dots with a pump-probe technique that normally yields the T1 spin lifetime, the time required for initially polarized electrons to relax and randomize. Using a circularly polarized laser tuned to the wavelength response of the quantum dot we can ``pump'' the spins into alignment. After aligning the spins we can detect them using a second, linearly polarized ``probe'' laser. By changing the delay between the two lasers we can trace out the spin response over time. In contrast with other samples (bulk GaAs and a GaAs quantum well), where the spin response decayed exponentially with time, initial data on the quantum dots has shown an unexpected, oscillating behavior which dies out on the order of 700 ns, independent of both temperature and magnetic field.   

Surface plasmon enhanced F\"{o}rster resonance energy transfer in fluorescent molecules using metal wire gratings

Forster resonance energy transfer (FRET) is a powerful tool used to study spatial relationships in biological systems. FRET relies on a nonradiative energy transfer between a donor (D) and acceptor (A) fluorophore. The D-A pair must be located within their Forster radius for an efficient transfer of energy. Surface plasmon (SP) excitations increase the emission of fluorescent molecules by two mechanisms. SPs excited at the fluorophore absorption wavelength increase the excitation rate of the fluorophores. SP modes at the fluorophore emission wavelength provide an additional decay channel for the fluorophores to return to the ground state, increasing the quantum yield and the photostability of the fluorophore. In this study, metal wire gratings were chosen because gratings support SP resonances over a wide wavelength range, allowing overlap for both absorption and emission wavelengths. This research seeks to develop methods for using metal grating SPs to increase the Forster radius for D-A pairs. For this project, gold gratings with a period of 500 nm were fabricated using a nanotransfer printing method. Fluorescence was measured as a function of angle to determine the enhancement. These outcomes will increase the number of physical systems that can utilize FRET.   

Remotely Tunable Nonlinear Metamaterial at Microwave Frequency

We demonstrate a remotely tunable metamaterial at microwave frequency. The metamaterial consists of a two-gap split ring resonator with varactor diodes integrated in to one of the gaps. By varying a microwave pump signal remotely, the capacitance of the varactor diodes can be controlled. Thus we can tune the working frequency of the metamaterial. Our metamaterials enable an easily-applicable approach to realize tunable frequency without an external bias circuit compared to other tunable metamaterials.   

Determination of the surface spin-polarization of perovskite oxides using point-contact Andreev reflection spectroscopy

Materials with surface spin-polarization are invaluable for incorporation into devices that utilize spin-polarized currents. Point-contact Andreev reflection spectroscopy is currently one of the few techniques capable of direct measurement of surface spin-polarization. Niobium wire was electrochemically etched in a potassium hydroxide solution to form sharp tips which were used to form point-contacts with perovskite oxides in single crystal and thin film forms. Surface spin-polarization values were determined at 4.2 K for several materials including La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, which is a material with purported 100{\%} spin polarization. The results show that surface spin polarization of perovskites is smaller than theoretically predicted.   

Spin Propagation Through Antiferromagnetic Bulk Structure in Exchange Biased Magnetic Trilayers

When an exchange bias is induced in materials with a ferromagnetic (FM) -- antiferromagnetic (AF) interface, the interfacial coupling between the antiferromagnet and FM manifests itself as a shift in the magnetic hysteresis loop. It has been an unresolved issue as to the role the bulk spin of the antiferromagnet plays in exchange bias and whether or not exchange bias is entirely an interfacial effect. We fabricated several FM/AF/FM trilayer structures of Py(100{\AA})/FeMn(x)/Ni$_{69}$Cu$_{31}$(200{\AA}) with varying antiferromagnet thicknesses and used a field cool procedure to induce an exchange bias. A Magneto-Optical Kerr Effect magnetometer was used to investigate the propagation of spin information through the antiferromagnet by examining the hysteresis loops at different angles of applied field with respect to the magnetization. It was observed that there was no induced exchange bias in the NiCu probe layer for any of the antiferromagnet thicknesses, and we conclude that the patterning of the antiferromagnetic layer transmits no spin information for thicknesses greater than 100{\AA}.   

Systematic Investigation of Magnetostriction in Composite Magnetorheological Elastomers: the Effect of Particle Shape, Alignment, and Volume Fraction

We have completed a study of the magnetoelastic properties of several types of magnetorheological elastomers (MREs), composites consisting of magnetic particles cured in an elastic matrix. We have made a number of samples with different particle arrangements (pseudo-random and aligned), volume fraction, and particle shape (rods, spheres, and disks) and measured the field dependent strain in order to determine the magnetostriction. We found that the magnetostriction in these samples is highly dependent on the sample particle shape (aspect ratio) and volume fraction and ordering to a lesser extent. While much of the past work has focused on spherical particles, our results indicate that both rods and disks can yield enhanced results. We discuss our findings in terms of magnetic energy of the particles and elastic energy of the matrix. We then consider the issue of optimization. This work was supported in part by NSF Grant CMMI - 0927326.   

Magnetic-Field Dependence of the Spinon Velocity in the $S=$1/2 Linear-Chain Heisenberg Antiferromagnet Copper Pyrazine Dinitrate

We have measured the specific heat of fully deuterated copper pyrazine dinitrate (CuPzN), a spin-1/2 antiferromagnetic chain compound, at temperatures down to 0.12 K in magnetic fields up to 14 T. This was done to reduce nuclear heat contributions by using deuterated CuPzN and to better define the magnetic heat capacity by taking measurements beyond the saturation field. The results are in good agreement with previous data taken by Hammar \textit{et al.} in fields up to 9 T. The spinon velocity obtained from the specific heat is compared to theoretical predictions as a function of magnetic field.   

Bulk Growth of YBa$_2$Cu$_3$O$_{7-\delta}$ Superconductors with Enhanced Flux Pinning

We present our work on the bulk growth of YBa$_2$Cu$_3$O$_{7-\delta}$ (Y-123) superconductors with enhanced flux pinning abilities grown using the melt textured growth method. Polycrystalline precursor materials of superconducting Y-123 and insulating Y$_2$BaCuO$_5$ (Y-211) are synthesized by sintering commercially available Y$_2$O$_3$, CuO, and BaCO$_3$. This process is repeated multiple times to improve the purity and crystal structure of the precursors. In order to make a superconductor with enhanced flux-pinning, it is necessary to add insulating Y-211 impurities to act as pinning centers to the bulk Y-123 superconductor, heat the mixture to temperatures that liquefy the superconducting phase, then cool the mixture slowly to crystallize the superconducting phase. Afterwards we anneal the enhanced flux-pinning superconductor in oxygen to restore oxygen content that was removed during the firing process. We present data on the crystal structure of the precursor materials (Y-123 and Y-211)) and the superconducting transition temperature of the precursor Y-123. In addition, we present data on the transition temperatures and the flux pinning forces of the enhanced flux-pinning superconductors.   

Growth and Characterization of Na-doped KFeAs

We grew mulitple dopings of Na-doped KFeAs, with a goal of observing an upward shift in the Tc from the KFeAs parent compound and a sharpening of the transistion phase curve. Using a VSM and PPMS to charecterize the magnetic transport, resistivity, and heat capacity, we have come to a conclusion on the sucess of Na-doping into the KFeAs family of FeAs superconductors.   

Synthesis and Characterization of Ytterbium-filled CoGe$_{1.5}$Se$_{1.5}$ compositions

Polycrystalline skutterudite-related compounds with nominal composition YbCo$_{4}$Ge$_{\mathrm{6+x}}$Se$_{\mathrm{6-x}}$(0 \textless x \textless 1) were prepared by melting of the constituent elements followed by annealing, and subsequent hot-pressing for densification. Structural and phase characterized was achieved by X-ray diffraction and electron microscopy. The crystal structure of skutterudites allows for voids within the crystal lattice that can be filled by ``guest atoms'' such as ytterbium. It is well known that this guest-atom-filling of the voids can result in significant phonon scattering, although these materials possess relatively good electrical properties, and are therefore thought of as PGEC (Phonon Glass Electron Crystal) materials. The goal of this research was to synthesize these skutterudite-related compounds and examine their thermoelectric properties. Their composition and properties will be discussed.   

Designing Drops, Loops, and Hills: The Physics behind Roller Coaster Design

Almost everyone has seen a roller coaster at one time in their life. They range in type from old wooden coasters from decades passes to modern machines made of steel that allow you to stand up while riding. The basic physics behind these machines is relatively simple, but in the modern world we strive to design bigger and better machines that push the human body and the laws of physics to their limits. But how do the designers of these rides maintain the balance between making riders feel like they're on the brink of death while keeping them completely safe? The answer can be found in basic physics and mechanical engineering. This is a part of the honors thesis that focuses on the mechanical principles applied in roller coaster design. The theoretical part of the thesis will be complemented by a full small scale ride design.