Physics Commons^™

Fluorescence Polarization Of Atomic, Dissociated Atomic, And Molecular Transitions Induced By Spin-Polarized Electron Impact, Jack W. Maseberg

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Excitation of atoms by spin-polarized electron impact yields fluorescence that can generally exhibit both linear and circular polarization. For experiments where the scattered electrons are not detected, symmetry requires that the electron beam be spin polarized in order for non-zero circular polarization to be observed. Extensive theoretical and experimental investigations have been performed regarding fluorescence polarizations (Stokes parameters) resulting from spin-polarized electron impact excitation of atoms. Measurement of fluorescence polarization provides insight into the angular momentum coupling that exists in the atomic state of interest. It also enables the measurement of electron spin polarization and experimental benchmarking of theoretical atomic …

Optical Properties And Magnetochromism In Multiferroic Bifeo3, X. S. Xu, T. V. Brinzari, S. Lee, Y. H. Chu, L. W. Martin, A. Kumar, S. Mcgill, R. C. Rai, R. Ramesh, V. Gopalan, S. W. Cheong, J. L. Musfeldt

Xiaoshan Xu Papers

In order to investigate spin-charge coupling in multiferroic oxides, we measured the optical properties of BiFeO₃. Although the direct 300 K charge gap is observed at 2.67 eV, absorption onset actually occurs at much lower energy with Fe³⁺ excitations at 1.41 and 1.90 eV. Temperature and magnetic-field-induced spectral changes reveal complex interactions between on-site crystal-field and magnetic excitations in the form of magnon sidebands. We employ the sensitivity of these magnon sidebands to map out the magnetic-fieldtemperature phase diagram which demonstrates optical evidence for spin spiral quenching above 20 T and suggests a spin domain reorientation near …

Absence Of Spin Liquid Behavior In Nd3ga5sio14 Using Magneto-Optical Spectroscopy, X. S. Xu, T. V. Brinzari, S. Mcgill, H. D. Zhou, C. R. Wiebe, J. L. Musfeldt

Xiaoshan Xu Papers

We measured the low-lying crystal field levels of Nd³⁺ in Nd₃Ga₅SiO₁₄ via magneto-optical spectroscopy and employed the extracted energies, magnetic moments, and symmetries to analyze the magnetic properties and test the spin liquid candidacy of this material. The exchange interaction is surprisingly small, a discovery that places severe constraints on models used to describe the ground state of this system. Further, it demonstrates the value of local-probe photophysical techniques for rare-earthcontaining materials where bulk property measurements can be skewed by low-lying electronic structure.

Magnon Sidebands And Spin-Charge Coupling In Bismuth Ferrite Probed By Nonlinear Optical Spectroscopy, M. O. Ramirez, A. Kumar, S. A. Denev, N. J. Podraza, X. S. Xu, R. C. Rai, Y. H. Chu, J. Seidel, L. W. Martin, S. -Y. Yang, E. Saiz, J. F. Ihlefeld, S. Lee, J. Klug, S. W. Cheong, M. J. Bedzyk, O. Auciello, D. G. Schlom, R. Ramesh, J. Orenstein, J. L. Musfeldt, V. Gopalan

Xiaoshan Xu Papers

The interplay between spin waves (magnons) and electronic structure in materials leads to the creation of additional bands associated with electronic energy levels which are called magnon sidebands. The large difference in the energy scales between magnons (meV) and electronic levels (eV) makes this direct interaction weak and hence makes magnon sidebands difficult to probe. Linear light absorption and scattering techniques at low temperatures are traditionally used to probe these sidebands. Here we show that optical secondharmonic generation, as the lowest-order nonlinear process, can successfully probe the magnon sidebands at room temperature and up to 723 K in bismuth ferrite, …

Anisotropic Magnetoresistance And Planar Hall Effect In Epitaxial Films Of La0.7ca0.3mno3, N. Naftalis, Y. Bason, J. Hoffman, X. Hong, C. H. Ahn, L. Klein

Xia Hong Publications

We measured the anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) in a [001] oriented epitaxial thin film of La_0.7Ca_0.3MnO₃ (LCMO) as a function of magnetic field, temperature, and current direction relative to the crystal axes. We find that both AMR and PHE in LCMO depend strongly on the current orientation relative to the crystal axes, and we demonstrate the applicability of AMR and PHE equations based on a fourth order magnetoresistance tensor consistent with the film symmetry.

High-Mobility Few-Layer Graphene Field Effect Transistors Fabricated On Epitaxial Ferroelectric Gate Oxides, X. Hong, A. Posadas, K. Zou, C. H. Ahn, J. Zhu

Xia Hong Publications

The carrier mobility μ of few-layer graphene (FLG) field-effect transistors increases tenfold when the SiO₂ substrate is replaced by single-crystal epitaxial Pb(Zr_0.2Ti_0:8)O₃ (PZT). In the electron-only regime of the FLG, μ reaches 7 X 10⁴ cm² / Vs at 300 K for n = 2.4 X 10¹²=cm², 70% of the intrinsic limit set by longitudinal acoustic (LA) phonons; it increases to 1.4 X 10⁵ cm² / Vs at low temperature. The temperature-dependent resistivity p(T) reveals a clear signature of LA phonon scattering, yielding …

Quantum Scattering Time And Its Implications On Scattering Sources In Graphene, X. Hong, K. Zou, J. Zhu

Xia Hong Publications

We determine the quantum scattering time T_q in six graphene samples with mobility of 4 400<μ <17 000 cm² /V s over a wide range of carrier density (1.2<n<6X10¹²/cm2). T_q derived from Shubnikov–de Haas oscillation ranges ~25–74 fs, corresponding to a single-particle level broadening of 4.5–13 meV. The ratio of the transport to quantum scattering time T_t /T_q spans 1.5–5.1 in these samples, which can be quantitatively understood combining scattering from short-ranged centers and charged impurities located within 2 nm of the graphene sheet. Our results suggest that charges residing on the SiO …

Electron Attachment To Trans-Azobenzene, Alberto Modelli, Paul Burrow

Paul Burrow Publications

The temporary anion states of gas-phase trans-azobenzene are characterized by means of electron transmission spectroscopy (ETS) in the 0–6 eV range. The measured energies of vertical electron attachment are compared with the energies of the π* virtual orbitals of the neutral molecule supplied by HF (at MP2 optimized geometries) and B3LYP calculations. The calculated energies, scaled with empirical equations, reproduce quantitatively the energies of the corresponding spectral features and predict a positive vertical electron affinity of 0.83 eV.

The total anion current at the walls of the collision chamber and the mass-selected molecular anion current are also reported as …