Jinhee Han

Chiral Orbital-Angular Momentum in the Surface States of Bi2Se3

We performed angle-resolved photoemission (ARPES) experiments with circularly polarized light and first-principles density functional calculation with spin-orbit coupling to study surface states of a topological insulator Bi2Se3. We observed circular dichroism (CD) as large as 30% in the ARPES data with upper and lower Dirac cones showing opposite signs in CD. The observed CD is attributed to the existence of local orbital-angular momentum (OAM). First-principles calculation shows that OAM in the surface states is significant and is locked to the electron momentum in the opposite direction to the spin, forming chiral OAM states. Our finding opens a new possibility for strong light-induced spin-polarized current in surface states. We also provide a proof for local OAM origin of the CD in ARPES.

Low-velocity anisotropic Dirac fermions on the side surface of topological insulators

We report anisotropic Dirac-cone surface bands on a side-surface geometry of the topological insulator Bi

Effect of Diffused B During Annealing on the Electronic Structure of the MgO Barrier in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

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Microscopic and electronic roles of B in CoFeB-based magnetic tunnel junctions

Se

First-principles Electronic Structure Calculations of Topological Insulators

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Identifying Antisite and Vacancy Defects in n-doped Bi

revealed by first-principles density-functional calculations. We find that the electron velocity in the side-surface Dirac cone is anisotropically reduced from that in the (111)-surface Dirac cone, and the velocity is not in parallel with the wave vector {\bf k} except for {\bf k} in high-symmetry directions. The size of the electron spin depends on the direction of {\bf k} due to anisotropic variation of the noncollinearity of the electron state. Low-energy effective Hamiltonian is proposed for side-surface Dirac fermions, and its implications are presented including refractive transport phenomena occurring at the edges of tological insulators where different surfaces meet.

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We investigate the chemical state and electronic structure of boron in the MgO layer in CoFeB/MgO/CoFeB magnetic tunnel junctions by synchrotron radiation X-ray photoelectron spectroscopy and first-principles calculations. In the top CoFeB, we observe three boron 1s peaks corresponding to oxidic, intermediate, and metallic boron. As the annealed films were etched, the B2O3 starts to appear together with the Mg 2p peak, indicating that boron atoms diffuse into the MgO barrier. Our calculations show that B impurities in the MgO barrier do not create any midgap states but reduce the MgO barrier height with the increase of boron concentration.

Se

The giant tunneling magnetoresistance (TMR) in lattice-matched crystalline magnetic tunnel junctions (MTJs) strongly depends on the majority-spin Δ1 bands of ferromagnetic electrodes. Our synchrotron radiation X-ray photoelectron spectroscopy and first-principles calculations show that B atoms suppress the formation of CoFe–O during CoFeB deposition in CoFeB/MgO/CoFeB MTJs, thereby lessening an effect that significantly degrades the majority-spin Δ1 bands, and that CoFe–B has properties superior to CoFe–O in electron conduction in the majority-spin Δ1 bands. During annealing, some of the B in CoFeB diffuses out, enhancing the valence bands of metallic CoFe, which improves the TMR value even further. Our present work elucidates the microscopic and electronic roles of B in MgO MTJs with CoFeB electrodes.

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Recently found topological insulators (TIs) have been attracting much attention of scientists in various disciplines. TIs are characterized by insulating bulk states and conducting surface states which are robust against perturbations preserving time-reversal symmetry. First-principles calculational methods for electronic structures are based on the density functional theory (DFT) and they are very efficient and reliable in finding out candidates for topological insulators and predicting their physical properties. This article briefly reviews contributions and achievements of DFT calculations in the investigation of topological insulators. 들어가는 글