Hwangho Lee

Fabrication and electrical characteristics of high-performance ZnO nanorod field-effect transistors

We report on fabrication and electrical characteristics of high-mobility field-effect transistors (FETs) using ZnO nanorods. For FET fabrications, single-crystal ZnO nanorods were prepared using catalyst-free metalorganic vapor phase epitaxy. Although typical ZnO nanorod FETs exhibited good electrical characteristics, with a transconductance of ∼140nS and a mobility of 75cm2∕Vs, the device characteristics were significantly improved by coating a polyimide thin layer on the nanorod surface, exhibiting a large turn-ON/OFF ratio of 104–105, a high transconductance of 1.9μS, and high electron mobility above 1000cm2∕Vs. The role of the polymer coating in the enhancement of the devices is also discussed.

Strain-induced magnetoelectric coupling in BaTiO3/Fe3O4 core/shell nanoparticles

We report a strain-induced magnetoelectric coupling in BaTiO3/Fe3O4 ferroelectric core/ferrimagnetic shell nanoparticles. The temperature-dependent magnetoresistance and magnetization clearly show several jumps near the structural phase transition temperatures of BaTiO3. Below the Verwey transition temperature of Fe3O4, i.e., at 20 K, the dielectric constant of BaTiO3/Fe3O4 decreases continuously upon application of an external magnetic field, and the observed magnetodielectric curve does not follow the square of the magnetization. We discuss the effect of strain on the electric field-dependent magnetic anisotropy near the core/shell interface.

Magnetodielectric coupling in core/shell BaTiO3∕γ-Fe2O3 nanoparticles

We report an intriguing magnetodielectric coupling in BaTiO3∕γ-Fe2O3 dielectric core/ferrimagnetic shell nanoparticles. The dielectric constant steeply increases with magnetic field, and the frequency dependent magnetodielectric curve shows a resonancelike peak at high temperatures, while it decreases smoothly with field and no peak appears in the frequency dependent magnetodielectric curve at low temperatures. We attribute the observed magnetodielectric coupling to the Maxwell-Wagner effect combined with magnetoresistance at high temperatures and to possible spin-lattice coupling and its modification near interfaces at low temperatures.

Electronic reconstruction on dimerized IrTe2

We investigated the electronic reconstruction in IrTe2 across the charge order phase transition using the angle-resolved photoemission spectroscopy and the ab initio calculation. The complicated shadow bands are attributed to the band folding effect of the charge ordered unit cell, while the valence transition causes the Fermi level shift as well as the size change of the Fermi surface. Implementation of a band unfolding scheme enables us to resolve the complicated folded band features, and successfully provides us information on the electronic reconstruction involving the transition. As a result, we could identify a flattened band and a segmented dispersion near the Γ and A point. We also identified the surface state contributions of additional bands crossing the Fermi level and a Dirac-cone–like band. The observed electronic reconstruction implies that the transition is induced by the strong correlation between the itinerant electrons and the spin-orbit coupled charge ordered states.

Electronic investigation on topological surface states of Bi0.9Sb0.1

Topological surface states of Bi0.9Sb0.1 single crystal were reinvestigated by using the angle-resolved photoemission measurements. We identify the surface band topology near the point with two separate electron Fermi surfaces. It provides an indisputable picture of two topological surface bands between the and points, excluding the possible existence of a hypothetic trivial surface band proposed previously. We also examine the detailed nature of topological surface bands and estimate the Fermi velocity . The surface band near the point displays a characteristic warping structure originating from the rhombohedral symmetry, and the turns out to be highly anisotropic in both the hexagon-shaped electron and teardrop-shaped hole Fermi surface. Our investigation clarifies the topological surface states on Bi0.9Sb0.1, which is essential for the topological spintronics applications.

Realization of an atomically flat BaSnO3(001) substrate with SnO2 termination

Atomically flat terraces terminated by mostly single layer SnO2 are realized on the surface of a BaSnO3(001) substrate with a lateral dimension of about 3u2009×u20093u2009mm2 by deionized water leaching and thermal annealing. Surface topography studies reveal that by controlling the annealing time and temperature, the topmost surface evolves from having chemically mixed termination to atomically flat terraces with a step height of one unit cell. The step bunching and kinked steps also depend sensitively on the out-of-plane and in-plane miscut angles. X-ray photoemission spectroscopy near the Ba3d5/2 and Sn3d5/2 states with variation in the electron emission angle confirmed that the topmost atomic layer of the BaSnO3−δ(001) surface mostly consisted of SnO2 rather than BaO. The present findings will facilitate the preparation of atomically flat BaSnO3(001) substrates, which will be useful in the studies of exploring possible two-dimensional electron gases at the interface between BaSnO3(001) and other oxides.