Magneto-Raman Spectroscopy of Graphene and Graphite: Probing Electronic Structure and Electron-Phonon Interaction
Kim, Younghee (author)
Smirnov, Dmitry (professor directing dissertation)
Brooks, James (professor co-directing dissertation)
Stiegman, Albert E. (university representative)
Schlottmann, Pedro (committee member)
Crede, Volker (committee member)
Department of Physics (degree granting department)
Florida State University (degree granting institution)
Single layer graphene (SLG) is a novel 2D system consisting of a single sheet of carbon atoms arranged in a honeycomb lattice, and exhibits a unique, linear low-energy dispersion. Bilayer graphene (BLG), two sheets stacked together, is an equally interesting system displaying a second unique, but hyperbolic, dispersion. Graphite consists of Bernal stacked graphene layers. Graphite band structure at K-point mimics the band structure of BLG, while at H- point it is similar to SLG. Hence, depending on their momentum along the c axis, K-point electrons in bulk graphite behave as massive Dirac fermions in BLG. The carriers at the H-point have a character of massless particles due to the eectively vanishing inter-layer coupling. In this work, we study the inter-Landau level transitions of graphene and grpahite to probe the electron structure and electron-phonon interactions by using magneto-Raman spectroscopy. Raman spectroscopy is used extensively to characterize graphene, as the material is composed almost entirely of symmetric sp2 bonded carbon. After the invention of SLG significant efforts have been made to investigate phonons, electron-phonon, and electron-electron interactions in graphene using Raman sepctroscopy. Among these categories, we studied E2g phonon corrected by Landau quantizations. The phonon is predicted to have a resonant character when the phonon energy matches with the energy of asymmetric inter-LL transition energy, so-called anticrossing behavior. In graphite, we study the E2g phonon shift and broadening coupled to the inter-LL excitations at H- and K-points. Moreover, we probe a series of Raman peaks due to the symmetric inter-LL excitations over a broad magnetic eld range including including the low energy transitions involving the electron-hole mixed LL-1 and LL0 LLs, enables an accurate determination of the SWM parameters. Also, in the highest magnetic eld range (>35 T) the E2g peak narrows due to suppression of electron-phonon interaction. It allow us to determine the phonon life time through energy time conservation. More deeply, we study the temperature dependence of inter-LL excitations. Surprisingly, Raman peaks shift to the higher energy and broad with increasing the temperature. In SLG, we study magneto-phonon resonance (MPR) of E2g phonon. The interesting aspect is that the electron-phonon coupling strength depends on the lling factor and polarization of incident and scattered photons. We probe the MPR dependence on various doping level and polarizations. Moreover, we observe the surprising Raman scattering intensity in the middle of the MPR anticrossing gap. Such an unusual MPR ne structure is shown to be a result of eective mixing and splitting electron-phonon coupled modes caused by random fluctuations of strain-induced pseudo-magnetic fields. Finally, we present a Raman spectroscopy study of Bi2Se3 and Sb2Te3 crystals in the temperature range between 5 K and 300 K. We uncover a characteristic temperature dependence of the phonon peak position and linewidth, and interpret it in the context of thermal expansion and three-phonon anharmonic decay. We present experimental procedures and describe the magneto-Raman probes we built. in order to conduct these experiments. Summaries of collaborative work on magneto-elastic eects of the molecular-based magnets by using magneto-Raman spectroscopy and Aharov- Bohm oscillations of Type-II Quantum dots by using magneto-photoluminescence are given.
November 2, 2012.
A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Dmitry Smirnov, Professor Directing Dissertation; James Brooks, Professor Co-Directing Dissertation; Albert E. Stiegman, University Representative; Pedro Schlottmann, Committee Member; Volker Crede, Committee Member.
Florida State University
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