UCLA scientists develop a method to visualize electronic and magnetic properties of topological insulators at the nanoscale

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Bouchard Small 2015

Breakthrough in technique development enables non-invasive depth profiling of emerging materials for spintronics.

Update – the news of this research was recently reported in UCLA Newsroom.

The seach for novel materials that drive innovation in the semiconductor industry has led to the emergence of new research directions. Topological insulators (TIs) form a class of low dimensional electronic systems characterized by an insulating bulk and conductive surfaces.  Their constituent atoms are typically heavy elements that feature large spin-orbit coupling interactions.  These could lead to the development of novel nanoelectronic devices featuring enhanced electronic transport properties and operating at temperatures higher than superconductors.

However, these materials contain a large number of native defects, making their electronic properties less than ideal for applications.  These defects translate into a material bulk that is conductive rather than insulating.  Methods to compensate for unwanted charges do not work well due to the inhomogeneous nature of the defects.  For example, it is known that the surface states of TIs and magnetically doped TIs exhibit considerable inhomogeneities at the nanoscale. 

To complicate matters, conventional techniques to characterize the physical properties of these materials mix contributions from surface and bulk regions, require the creation of high quality materials, the growth of thin films or the use of exceedingly low temperatures.  Method to characterize these TIs are needed that can differentiate bulk from surface states and cope with imperfect materials up to room temperature.

The research, led by Louis Bouchard, an assistant professor of chemistry & biochemistry, and Dimitrios Koumoulis, a postdoctoral scholar, was published in the Proceedings of the National Academy of Sciences (PNAS).

Bouchard and Koumoulis
Research led by UCLA’s Prof. Louis Bouchard and Dr. Dimitrios Koumoulis
presents a method to visualize topological insulators at the nanoscale.

The article reports the first use of β‑detected nuclear magnetic resonance (β‑NMR) to study the depth dependent properties of TIs.  β‑NMR is a novel experiment in which 8Li+ ions of various energies are implanted in the material of interest, generating signals from layers of interest.   The nucleus of 8Li is unstable and decays via β‑decay.  Because the β‑emissions correlate with the orientation of the nucleus, researchers can measure the precession of nuclear spins in a magnetic field.  The high sensitivity of the β-NMR technique and its ability to probe materials as a function of implantation depth give unique insights into these state-of-the-art materials.

Komoulis and co-workers demonstrated depth resolution of the material properties, enabling the unambiguous identification of surface vs. bulk states of a TI and a magnetically doped TI by monitoring the shift of the nuclear resonance.  In epitaxial thin films they demonstrate an increase in the density of states, a weakening of the ferromagnetic order when approaching the TI edges, as detected by measurements of the electron–nuclear hyperfine interaction, the effective sd exchange integral, and local moment density.  Depth profiling is expected to help uncover exotic physics of pure and ferromagnetic TIs and TI heterostructures.  It could also aid in the development of new materials and devices.

Co-authors of the PNAS research are Gerald Morris, a senior scientist at TRIUMF; Danny King of the UCLA department of chemistry & biochemistry; Masrur Hossain of TRIUMF; Dong Wang of the University of British Columbia’s physics & astronomy department; Liang He, Xufeng Kou and Kang L. Wang of the UCLA department of electrical engineering; Gregory Fiete of the physics department at the University of Texas in Austin, and Mercouri Kanatzidis of the chemistry department at Northwestern University.

The research was funded by the DARPA Mesodynamic Architectures program.

The abstract and paper are available online.

Photos by Penny Jennings, UCLA Department of Chemistry and Biochemistry