Apollo-Soyuz pamphlet no. 2: X-rays, gamma-rays

The nature of high energy radiation and its penetration through earth's atmosphere is examined with emphasis on X-rays, gamma rays, and cosmic radiation and the instruments used in their detection. The history of radio astronomy and the capabilities of the Uhuru satellite are summarized. The ASTP soft X-ray experiment (MA-048) designed to study the spectra in the range from 0.1 to 10 keV and survey the background over a large section of the sky is described, as well as the determination of SMC C-1 as an X-ray pulsar. The crystal activation experiment (MA-151) used to measure the radioactive isotopes created by cosmic rays in crystals used for gamma ray detectors is also discussed

Developing germanium on nothing (GON) nanowire arrays

Advanced crystal growth techniques enable novel devices and circuit designs to further scale and integrate heterogeneous structures for CMOS, MEMS/NEMS, and optoelectronic applications. In particular, nanowires (NW) are among the promising structures derived from these developments. Research has demonstrated the utility of NWs as a channel material for gate-all-around transistors, high sensitivity biological/chemical sensors, photodetectors, as well as a whole spectrum of LEDs and lasers. However, NW based devices are not without their fabrication challenges. Relatively simple structures for CMOS or MEMS/NEMS processes are difficult to reproduce when many NW based devices rely on a dropcast process. This thesis demonstrates a method for producing Germanium on Nothing (GON) NW arrays on a Si substrate that forgoes dropcasting and, instead, creates NWs via selective material removal methods commonly utilized by industry. GON NW arrays are formed through the sequential use of E-beam lithography, selective wet chemical etching, and reactive ion etching. Global oxide thinning in BOE leaves a thin masking layer that protects the underlying Si, preventing etching in a TMAH solution. GON regions are defined by E-beam lithography and are subject to a RIE which creates release points in the remaining SiO2. Unmasked Si is then etched by a TMAH solution, undercutting the Ge lines, leaving an array of suspended Ge wires. NW dimensions are reached by thinning the Ge wire diameter with a H2O2 solution. NWs with ~50 nm diameters and ~ 200 nm lengths, as well as 10 [micron] by 10 [micron] membranes of Ge/SiO2, have been demonstrated in this thesis

Ground States of the Two-Dimensional Electron System at Half-Filling Under Hydrostatic Pressure

A many-body electron system in two dimensions at high magnetic field hosts a diverse set of electron ground states. Many of these ground states have been well understood for years, yet some continue to challenge researchers. The ν =5/2 fractional quantum Hall state at half-filling is perhaps the most mysterious state. It holds the promise of novel physics such as non-Abelian statistics, and it possesses topological order, both properties of great interest due to potential applications for robust quantum computing. However, despite many experiments to this date, questions surround the exact nature of ν =5/2 fractional quantum Hall state. This unsatisfactory state of affairs in the understanding of ν =5/2 calls for new and refined experimental methods. Hydrostatic pressure is a widely-used tool that provides a great deal of insight into condensed matter physics. By shrinking the lattice constant in crystalline systems, pressure changes the Bloch wavefunction and the band structure. As a result, pressure permits us to tune material parameters in ways not possible with other techniques. In particular, we may tune the energy scales of the fractional quantum Hall states and gather information about these states from their response to pressure. Pressure therefore has the potential to provide new insight of the behavior of the ν =5/2 fractional quantum Hall state. In this thesis, I describe experiments in which I applied up to 12 kbar to two dimensional electron systems hosted in gallium arsenide heterostructures. With the application of pressure, we observed an unexpected result: a never-before-seen phase transition at filling factor ν =5/2 from the fractional quantum Hall state to the nematic phase. The nematic phase is a phase characterized by spontaneously broken rotational symmetry and highly anisotropic resistances. This represented the first time such a nematic phase developed spontaneously at ν =5/2, without any external symmetry breaking fields. Probing the temperature dependence of the ν =5/2 fractional quantum Hall state and nematic phase at different pressures allowed us to map a stability diagram of the different phases. Evidence suggests that this transition is a quantum phase transition – a phase transition at zero temperature. There are many examples of quantum phase transitions in condensed matter, but the one we have observed at ν =5/2 is unusual. This is a quantum phase transition which changes topological order, as the quantum Hall state is destroyed, as well as nematic order, a traditional Landau order, as rotational symmetry breaks in the transition. This discovery brings about new questions about the instabilities at ν =5/2, and invites further study, both experimental and theoretical. To gain further insight into the underlying mechanism of the fractional quantum Hall state-to-nematic transition, we also studied the filling factor ν =7/2, the closely-related cousin of ν =5/2, under pressure. The fractional quantum Hall state at ν =7/2 is expected to share the same physics as the ν =5/2 fractional quantum Hall state. Importantly, we find that ν =7/2 also undergoes the fractional quantum Hall state-to-nematic transition. The quantum phase transitions at ν =5/2 and ν =7/2 do not occur at the same pressure, but rather the same magnetic field. Because the magnetic field sets the scale for the electron-electron interactions, this suggests that electron-electron interactions are the dominant factor driving this quantum phase transition. Corroborating this conclusion, a specially-engineered sample studied at ambient pressure also revealed a nematic phase at ν =7/2 at a similar magnitude of electron-electron interactions as the pressurized samples

Calibration and Characterization of the IceCube Photomultiplier Tube

Over 5,000 PMTs are being deployed at the South Pole to compose the IceCube neutrino observatory. Many are placed deep in the ice to detect Cherenkov light emitted by the products of high-energy neutrino interactions, and others are frozen into tanks on the surface to detect particles from atmospheric cosmic ray showers. IceCube is using the 10-inch diameter R7081-02 made by Hamamatsu Photonics. This paper describes the laboratory characterization and calibration of these PMTs before deployment. PMTs were illuminated with pulses ranging from single photons to saturation level. Parameterizations are given for the single photoelectron charge spectrum and the saturation behavior. Time resolution, late pulses and afterpulses are characterized. Because the PMTs are relatively large, the cathode sensitivity uniformity was measured. The absolute photon detection efficiency was calibrated using Rayleigh-scattered photons from a nitrogen laser. Measured characteristics are discussed in the context of their relevance to IceCube event reconstruction and simulation efforts.Comment: 40 pages, 12 figure

Electronic Transport Behavior of Adatom- and Nanoparticle-Decorated Graphene

To induce a non-negligible spin-orbit coupling in monolayer graphene, for the purposes of realizing the Kane-Mele Hamiltonian, transition metal adatoms have been deposited in dilute amounts by thermal evaporation in situ while holding the device temperature near 4K. Electronic transport studies including measurements such as gate voltage dependent conductivity and mobility, weak localization, high field magnetoresistance (Shubnikov de Haas oscillations), quantum Hall, and nonlocal voltage were performed at low temperature before and after sequential evaporations. Studies of tungsten adatoms are consistent with literature regarding other metal adatoms on graphene but were unsuccessful in producing a spin-orbit signature, at least partially due to lithography residue inhibiting the adatoms’ ability to dope the graphene. Osmium adatoms on graphene behave differently from other adatoms in several ways. While all other measured adatoms donate electrons to graphene, osmium is observed to donate holes to graphene. In addition, tungsten and other adatoms directly affect the scattering potentials by causing a dominant Coulomb-like potential from isolated point charges. Osmium, on the other hand, does not obey this simple model. Separately, a claim was made in a recent study of Bi2Te3 nanoparticles on graphene showing tantalizing evidence of quantization in resistance coinciding with predictions for edge channel conduction. Our attempts to reproduce these observations have not been successful so far

Emergent Phenomena in Spatially Confined Manganites

Rare earth manganites exhibit colossal magnetoresistance (CMR). There is evidence that alloyed single crystal materials in this class can display electronic inhomogeneity in which areas with vastly different electronic and magnetic properties can form and coexist in phase separated domains ranging in size from a few nanometers to micrometers. This phase separation (PS) is of particular interest, as it has been suggested that it is the central feature that leads to CMR in manganites, the Mott transition in VO2 and may play a role in high-TC superconductivity in cuprates. However there is debate as to its precise role. The purpose of my research is to answer fundamental questions about the specific role of PS in complex oxides. I reduce single crystal thin films of an electronically phase separated manganite to the scale of their inherent electronic phase domains near the metal-insulator transition. Unlike transport measurements done on bulk or thin films where the electrons follow only the metallic path of least resistance, this configuration forces electrons to travel through both the metallic and insulating regions residing in the material. This has led to observations of several new phenomena such as a reemergent metal-insulator transition, ultra-sharp jumps in resistivity at the metal-insulator transition, and the first high resolution observation of single domain electronic phase transitions in the time domain. While the manganites will be the primary focus throughout this dissertation, the spatial confinement techniques presented here are not limited to only these materials. They can be applied to any phase separated system to probe regions resistively hidden to transport measurements

Virtual Takings: The Coming Fifth Amendment Challenge to Net Neutrality Regulation

“Net neutrality” refers to the principle that broadband providers should not limit the content and applications available over the Internet. Long a rallying cry of techies and academics, it has become one of the central pillars of the Obama Administration’s telecommunications policy. The Federal Communications Commission’s efforts to regulate the “onramp to the Internet” have attracted significant attention from the telecommunications industry and the academic community, which have debated whether the proposed restrictions violate broadband providers’ First Amendment rights. But there is an additional constitutional implication of net neutrality that has not yet been sufficiently addressed in the scholarly literature: the Takings Clause. This article argues that under the Supreme Court’s Takings Clause jurisprudence, the Commission’s proposed net neutrality rules effect a permanent physical occupation of private broadband networks and therefore take broadband providers’ property without just compensation. In essence, net neutrality would grant Internet content providers a permanent virtual easement across privately-owned broadband networks to deliver content to end-users. It thus would deprive broadband providers of the right to exclude others from their networks—a right that the Court’s takings jurisprudence has repeatedly dubbed “one of the most essential sticks in the bundle of rights that are commonly characterized as property.” At the very least, the Takings Clause issue raises a serious constitutional question regarding the Commission’s authority to adopt net neutrality regulations without clear authority from Congress to do so. The Commission should therefore seek explicit Congressional approval before promulgating net neutrality rules, rather than continuing to freelance at the periphery of its regulatory authority