Physics
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Item Characterizing Unusual Behavior in Pristine Bi2Se3(2015) Syers, Paul James Howard; Paglione, Johnpierre; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bismuth Selenide is a material of great interest to physicists as it is one of the first materials proven to be a strong topological insulator (TI). Despite its promise as an ideal TI, defects in the material that have proven difficult to elminate, keep the material's conductive states at its surface hidden by metallic behavior in the bulk. The work discussed in this thesis focuses primarily on techniques aimed at improving the material quality of pure Bi2Se3 and better understanding the effects of air exposure on both the surface and bulk of the material. The goal is to reliably produce samples in which signatures of the surface states can be seen in simple, bulk measurements. Changes in sample quality were achieved through the manipulation parameters such as Se flux and environmental pressure during the growth process. Through these techniques, the intrinsic carrier concentration of Bi2Se3 samples was lowered to the lowest levels ever reported. Samples showing nonmetallic behavior were also produced and investigated. Furthermore, it was discovered that samples of Bi2Se3 with low carrier concentrations showed strong linear magnetoresistance. The nature of this linear magnetoresistance, its angular dependence, and its evolution over time were also investigated. This thesis will discuss the results of these experiments as examples of how growth techniques influence sample quality, which in turn affects the properties of Bi2Se3.Item Spin injection and detection in copper spin valve structures(2005-01-25) Garzon, Samir Y; Webb, Richard A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We report measurements of spin injection and detection in a mesoscopic copper wire from which the electron spin relaxation time and the spin current polarization in copper can be found. Spin injection is realized by applying a voltage to drive a current from a ferromagnet into the normal metal, while spin detection is done using transport measurements. Precession of the spin of the injected electrons due to an external magnetic field is also studied. The existence of a previously unobserved spin signal which vanishes at low temperatures but increases nonlinearly above 100K is reported and a possible explanation for its origin, based on interfacial spin-flip scattering, is suggested. Multiple cross checks to test the possibility of artifacts as an origin of this signal are discussed. An alternative spin detection method using magnetic force microscopy (MFM) is also studied. This method measures the magnetic field produced by the injected spins directly, so the spin coherence length and the spin current polarization can be extracted directly without the need of a particular transport model, avoiding issues like contact resistance and interface scattering. The MFM method can also be useful for measuring the spin polarization of currents in semiconductors and semiconductor heterostructures, which is important for the development of spintronics.