Browsing by Author "Morris, GD"

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    8Li+ β-NMR in the cubic insulator MgO
    (IOP Publishing, 2014-12-16) MacFarlane, WA; Parolin, TJ; Cortie, DL; Chow, KH; Hossain, MD; Kiefl, RF; Levy, CDP; McFadden, RML; Morris, GD; Pearson, MR; Saadaoui, H; Salman, Z; Song, Q; Wang, D
    We present extensive high magnetic field β-NMR measurements of 8Li+ implanted in single crystals of MgO. The narrow resonance, consistent with a cubic 8Li+ site, likely the tetrahedral interstitital, is used routinely as a reference for shift measurements. We show the intrinsic linewidth is on the order of 200 Hz, allowing a frequency determination to an accuracy of a few Hz. We find no implantation energy dependence of the resonance within a few ppm, but there is evidence of slow spin dynamics in hole-burning measurements. The spin lattice relaxation is slow. The temperature dependence reveals interesting changes at low temperature whose origin remains uncertain. Open Access CC-BY
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    Local electronic and magnetic properties of the doped topological insulators Bi2Se3:Ca and Bi2Te3 : Mn investigated using ion-implanted 8Li β-NMR
    (American Physical Society (APS), 2020-12-28) McFadden, RML; Chatzichristos, A; Cortie, DL; Fujimoto, D; San Hor, Y; Ji, H; Karner, VL; Kiefl, RF; Levy, CDP; Li, RH; McKenzie, I; Morris, GD; Pearson, MR; Stachura, M; Cava, RJ; MacFarlane, WA
    We report β-NMR measurements in Bi2Se3:Ca and Bi2Te3:Mn single crystals using Li+8 implanted to depths on the order of 100 nm. Above ∼200K, spin-lattice relaxation reveals diffusion of Li+8, with activation energies of ∼0.4eV (∼0.2eV) in Bi2Se3:Ca (Bi2Te3:Mn). At lower temperatures, the NMR properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift in Bi2Se3:Ca. From this, we make a detailed comparison with the isostructural tetradymite Bi2Te2Se [McFadden, Phys. Rev. B 99, 125201 (2019)2469-995010.1103/PhysRevB.99.125201]. In the magnetic Bi2Te3:Mn, the effects of the dilute Mn moments predominate, but remarkably the Li+8 signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the spin-lattice relaxation that is suppressed in a high applied field. This detailed characterization of the Li+8 NMR response is an important step toward using depth-resolved β-NMR to study the low-energy properties of the chiral topological surface state in the Bi2Ch3 tetradymite topological insulator. ©2024 American Physical Society.
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    Observation of a charge-neutral muon-polaron complex in antiferromagnetic Cr2O3
    (American Physical Society (APS), 2019-11-26) Dehn, MH; Shenton, JK; Holenstein, S; Meier, QN; Arseneau, DJ; Cortie, DL; Hitti, B; Fang, ACY; MacFarlane, WA; McFadden, RML; Morris, GD; Salman, Z; Luetkens, H; Spaldin, NA; Fechner, M; Kiefl, RF
    We report a comprehensive muon spin rotation (μSR) study of the prototypical magnetoelectric antiferromagnet Cr2O3. We find the positively charged muon (μ+) occupies several distinct interstitial sites and displays a rich dynamic behavior involving local hopping, thermally activated site transitions, and the formation of a charge-neutral complex composed of a muon and an electron polaron. The discovery of such a complex has implications for the interpretation of μSR spectra in a wide range of magnetic oxides and opens a route to study the dopant characteristics of interstitial hydrogen impurities in such materials. We address implications arising from implanting a μ+ into a linear magnetoelectric and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon. © Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license.
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    The Site and High Field βNMR Properties of 8Li+ Implanted in α-Al2O3
    (IOP Publishing, 2023-03-01) MacFarlane, WA; Shenton, JK; Salman, Z; Chatzichristos, A; Cortie, DL; Dehn, M; Fujimoto, D; Karner, VL; Kiefl, RF; Koumoulis, D; Levy, CDP; McFadden, RML; McKenzie, I; Morris, GD; Pearson, MR; Stachura, M; Ticknor, JO
    We present high (> 2 T) magnetic field βNMR measurements of 8Li+ implanted in single crystals of sapphire, a commonly used backing material for other samples. From the well-resolved quadrupolar splitting, we extract the electric field gradient (EFG) at the implanted 8Li+ site. Comparison with supercell density functional theory calculations of the EFG allows us to identify the octahedral interstitial site as the most likely candidate. In contrast to the zero field βNQR spectra, where multiple singals are detected, only a single site is evident at high field. We discuss possible explanations for this discrepancy. The spin lattice relaxation is extremely slow (1/T1 < 0.02 s−1 ) over a broad temperature range from 4 to 300 K, demonstrating that cross relaxation with the 27Al nuclear spins is quenched in such high magnetic fields.