Sarah Caballero – 2015

Spin-dependent cyclotron total widths in strong magnetic fields

Research Advisor: Dr. Peter Gonthier

We are developing compact, analytical expressions of the cyclotron total spin-dependent widths, or lifetimes, of excited Landau states.  These widths are required in our recent Compton scattering developments for strong-field magnetar magnetospheres.  We begin with the analytics for the spin-dependent widths presented in Sina (1996), which has been our source for the development of Compton scattering, and establish equivalency with the differential widths developed by Latal (1986).  Both approaches implement correct Sokolov & Ternov wave functions.  From here, we make further improvements to develop an integral for the total differential widths with the integration variable only present in the square of the associated Laguerre functions.  With the use of several series representations, we obtain an integral in the form previously used in our efforts.  The result is an expression that includes Legendre functions of the second kind with an argument >1, for which we seek an algorithm for efficient evaluation.  Exploring two possibilities, recursion relations, and hypergeometric functions, we find that the former breaks down for large values of the magnetic field (B) and the initial Landau state (n) due to the argument being >1.  Taking the hypergeometric approach, we achieved numerical values that agree with Latal (1986) and Sina (1996) results for small n and B, but the analytics break down for large n and B.  This problem encourages the development of asymptotic approaches similar to those developed in the past but for spin-averaged widths.  These processes will be applied to future work.

This work is made possible by the generous support of the National Science Foundation (Grant No. AST-1009731), the NASA Astrophysics Theory and Fundamental Program (NNX13AO12G / 12-ATP12-0169), the American Physical Society, and the Michigan Space Grant Consortium.

Richard Huizen – 2015

 

Temperature Dependence of Signal Intermodulation in a YBCO Resonator

Research Advisor: Dr. Stephen Remillard

Temperature and magnetic field dependence of second and third order intermodulation distortion levels (IMD2 and IMD3 respectively) were investigated for a type II superconducting resonator circuit.  The circuit was comprised of a thin film of YBCO deposited on a substrate of LaAlO2.  A carrier wave at a frequency equal to the resonant frequency of the circuit was introduced into the circuit via an electric coupling element.  Two probe signals were injected into the circuit via a separate magnetic coupling element.  The combination of these three signals locally excites second and third order IMD.  A static magnetic field on the order of 238 Gauss was temporarily applied perpendicularly to the sample.  Upon removal of the static magnetic field, both IMD2 and IMD3 levels were found to exponentially decay with time over a temperature range below 87.5K.  Above 87.5K, however, IMD2 continued to exhibit exponential decay while IMD3 exhibited bounded exponential growth.  The mechanism that causes IMD3 to change its decay mode is unclear at this time.

Funding for this project was provided by Award number DMR-1206149 from the National Science Foundation.

Braden Marks – 2015

Population of 13Be in Nucleon-Exchange Reactions

Research Advisor: Dr. Paul DeYoung

Neutron-unbound nuclei are traditionally formed by the removal of one or more nucleons from a fast beam of ions. This method often results in a background, which is difficult to separate from the particle of interest. Nucleon-removal entrance-channels also require the ion beam to be more massive than the particle of interest, which presents the additional challenges of the beam being unstable, difficult to make, and low in intensity. In an effort to avoid these obstacles, the present work was done with a more unorthodox entrance-channel called nucleon-exchange. At the National Superconducting Cyclotron Laboratory (NSCL), a 71 MeV/u 13B beam impinged on a 47 mg/cm2 thick target of 9Be. As a result numerous reactions occurred, including the population of 13Be through the nucleon-exchange entrance-channel. The 13Be nuclei decayed to 12Be and one neutron in approximately 10-21 seconds. The resulting neutrons were detected by either the Modular Neutron Array (MoNA) or the Large multi-Institution Scintillator Array (LISA), while the 12Be nuclei were directed through an array of charged particle detectors by a 4T superconducting sweeper magnet. The four-momentum vectors of the fragment nucleus and the neutron were calculated to determine the decay energy of 13Be. Monte-Carlo simulations consistent with results from previous analyses of 13Be were satisfactorily fit to the decay-energy spectrum. Additionally, the cross-section for the nucleon-exchange entrance-channel was determined to be consistent with the theoretical prediction.

This material is based upon work supported by the National Science Foundation under grant No. PHY-1306074.

Daniel Clark – 2015

WEBClarkCharacterization of the Pseudocapacitive Nature of Surface Bound Prussian Blue Analogues

Research Advisor: Dr. Jennifer Hampton

With the increased use of intermittent renewable energy sources, more efficient methods of energy storage must be explored. Electrochemical capacitors provide a larger volumetric charge density than physical capacitors while maintaining fast charge and discharge rates. Prussian Blue analogues (nickel and cobalt hexacyanoferrate) are ideal pseudocapacitors for frequent charge and discharge cycles since the crystalline structure does not physically change during switching, causing less stress on the film. This project examines the charge transfer and diffusion coefficients for nickel and nickel-cobalt thin films modified with potassium hexacyanoferrate. The films were examined using a scanning electron microscope, an atomic force microscope and an electrochemical workstation to determine their composition, topography and psuedocapacitive nature. Preliminary data suggest that nickel-cobalt films have a larger quantity of charge and have a lower diffusion coefficient per charge than nickel films.

This work is supported by the Hope College Jacob E. Nyenhuis Faculty Development Fund, the Hope College Department of Physics Guess Research Fund, and the National Science Foundation under NSF-RUI Grant No. DMR-1104725, NSF-MRI Grant No. CHE-0959282, and NSF-MRI Grant No. CHE-1126462.

Benjamin Peecher – 2015

WEBPeecherDealloying NiCo and NiCoCu Alloy Thin Films Using Linear Sweep Voltammetry

Research Advisor: Dr. Jennifer Hampton

When electrodeposited into thin films, metals have unique and well-known electrochemical potentials at which they will be removed from the film. Theoretically, these potential differences can be utilized to re-oxidize only certain metals in an alloy, thus altering the film’s structure and composition. This dealloying process is understood relatively poorly in the case of nickel-cobalt and nickel-cobalt-copper thin films. Here we discuss these films’ response to linear sweep voltammetry as a means of electrochemical dealloying. A three-electrode electrochemical cell was used for both deposition and dealloying. To perform linear sweep voltammetry on a sample, it was immersed in a sodium sulfate solution in the electrochemical cell and a steadily increasing potential was placed between the working and reference electrodes. For each of four different metal ratios, films were dealloyed to various potentials in order to gain insight into the evolution of the film over the course of the linear sweep. Capacitance, topography, and composition were examined for each sample before and after linear sweep voltammetry was performed. For nickel-cobalt films with high percentages of nickel, dealloying resulted in almost no change in composition, but did result in an increased capacitance, with greater increases occurring at higher linear sweep potentials. Dealloying also resulted in the appearance of large (100-1000 nm) pores on the surface of the film. For nickel-cobalt-copper films with high percentages of nickel, copper was almost completely removed from the film at linear sweep potentials greater than 500 mV. Preliminary data suggested that the linear sweep first removes larger copper-rich dendrites from the film’s surface before creating numerous nano-pores, resulting in a net increase in area.

This research was made possible by an award to Hope College from the Howard Hughes Medical Institute through the Undergraduate Science Education Program, the Hope College Department of Physics Frissel Research Fund, and the National Science Foundation under NSF-RUI Grant No. DMR-1104725 and NSF-MRI Grant No. CHE-0959282.