Nuclear Magnetic Resonance as a Probe of the Topological Insulator Bi 2 Se 3
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Nuclear Magnetic Resonance as a Probe of the Topological Insulator Bi2Se3 David M. Nisson1, Adam P. Dioguardi1, Peter Klavins1, Ching H. Lin1, Kent Shirer1, Abigail C. Shockley1, John Crocker1, and Nicholas J. Curro1 1 Department of Physics, University of California, Davis, Davis, CA 95616, U.S.A. ABSTRACT Topological insulators are a new class of materials with the ability to carry spin-polarized currents on their surfaces. Nuclear magnetic resonance (NMR) measurements can probe the magnetic interactions between specific isotopes and the electronic system of a material. We present 209Bi NMR spectra and relaxation rate data on single crystals of the topological insulator material Bi2Se3 grown under various conditions. Our NMR data on single crystals reveal a significant strength of coupling between the nuclear spins and the bulk carrier spins, suggesting that nuclear spins may have a sizeable effect on spin-polarized surface currents. INTRODUCTION Topological insulators are a new class of materials that display metallic surface states, but are insulating or semiconducting in their bulk.[2] These conducting surfaces have a special property where currents carried on them are spin-polarized, making them potentially useful candidates for building novel spintronic devices. The electron spins in two-dimensional surface currents in topological insulators, unlike conventional bulk currents, should be robustly coherent for a given current direction because of the chirality. However, one phenomenon, of which little is known experimentally in topological materials, could affect the surface states. In any material whose nuclei have spins, those spins interact with the spins of the electrons. This hyperfine interaction is what we aim to probe in this study. The material we use is Bi2Se3, a very promising material for applications because of its large band gap.[3] Nuclear magnetic moments in any given material are randomly oriented. However, when a material is magnetized or subjected to a magnetic field, the nuclear moments preferentially align with the field. Memory devices have been proposed based on the Hall effect, where the surface currents are deflected perpendicularly by a magnetic field to an applied electrical potential.[4] If the hyperfine coupling is significant in a material, the nuclear spins can provide a source of scattering and decoherence, with implications for building such devices. The effect of the coupling in a nuclear magnetic resonance study is to alter the magnetic field “felt” by the nuclei, causing them to resonate at a slightly different frequency than a bare nucleus. However, this frequency depends on the intrinsic concentration of free electrons in the material as well as the strength of the hyperfine coupling. The measurements are thus complicated by the fact that the density of carriers in Bi2Se3 is strongly dependent on growth conditions.[5] When Bi2Se3 is grown, the Se atoms have an intrinsic tendency to be missing from some sites in the lattice, making the material n-type.[6] Because of a eutecti
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