Changyoung Kim

Fermi surface, surface states, and surface reconstruction in Sr2RuO4

The electronic structure of Sr2RuO4 is investigated by high angular resolution ARPES at several incident photon energies. We address the controversial issues of the Fermi surface (FS) topology and the van Hove singularity at the M point, showing that a surface state and the replica of the primary FS due to sqrt[2]xsqrt[2] surface reconstruction are responsible for previous conflicting interpretations. The FS thus determined by ARPES is consistent with the de Haas-van Alphen results, and it provides additional information on the detailed shape of the alpha, beta, and gamma sheets.

Fermi Surface and Band Dispersion in La2 xSrxCuO4

Using angle-resolved photoemission spectroscopy (ARPES), we observe the band structure, the Fermi surface and their doping dependences in La 2- x Sr x CuO 4 . The results reveal that the Fermi surface undergoes a dramatic change: it is holelike and centered at (π, π) in underdoped ( x =0.1) and optimally doped ( x =0.15) samples as in other cuprates, while it is electronlike and centered at (0, 0) in heavily overdoped ( x =0.3) ones. The peak in the ARPES spectra near (π/2, π/2) is broad and weak unlike that in other cuprates. In the underdoped and optimally doped samples, a superconducting gap (Δ= 10–15 meV) is observed near (π, 0).

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.

Buffer layer effect on the structural and electrical properties of rubrene-based organic thin-film transistors

The structural and electrical properties of organic thin-film transistors with rubrene/pentacene and pentacene/rubrene bilayered structures were investigated using x-ray diffraction, atomic force microscopy, and x-ray emission spectroscopy. High-quality rubrene thin films with orthorhombic structure were obtained in the rubrene/pentacene bilayer while the pentacene/rubrene bilayer only had an amorphous rubrene phase present. The rubrene/pentacene thin-film transistor shows more desirable current-voltage characteristics compared to the pentacene/rubrene transistor. The overall results suggest that the presence of a chemically active organic buffer layer and its associated crystal structure are crucial in enhancing the structural and electrical properties of rubrene-based transistors.

Core level photoelectron microscopy

Abstract Recent instrumental developments have extended the spatial resolution of photoelectron spectroscopy into the micron range. In this paper, one approach for high lateral resolution photoelectron microscopy which employs a superconducting magnet as a magnifying/projection lens is described along with recent results on photoemission (core level and valence band) and photoabsorption (near edge) spectroscopies.

Ambipolar organic thin-film transistors using C60/pentacene structure: Characterization of electronic structure and device property

We fabricated ambipolar organic thin-film transistors (OTFTs) using C60 and pentacene. The electronic structure of the interface was investigated by using ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy. The magnitude of the interface dipole and the band bendings at the interface was determined, and the complete energy level diagram for C60 on pentacene (C60/pentacene) was obtained. The lowered band offsets, due to the enhanced charge redistribution in C60/pentacene relative to pentacene on C60, are favorable for the ambipolar OTFTs. The measured field-effect mobilities were 0.017cm2∕Vs and 0.007cm2∕Vs for the p-channel and the n-channel operations, respectively. The threshold voltages were −2V for the p channel and 15.6V for the n channel, comparable to those of unipolar OTFTs using C60 or pentacene.

Orbital selective Fermi surface shifts and mechanism of high Tc superconductivity in correlated AFeAs (A=Li,Na)

Based on the dynamical mean field theory and angle resolved photoemission spectroscopy, we have investigated the mechanism of high T(c) superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the measured angle resolved photoemission spectroscopy. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect within the dynamical mean field theory method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that a fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.

Warping effects in the band and angular-momentum structures of the topological insulator Bi2Te3

We performed angle resolved photoemission (ARPES) experiments on Bi2Te3 with circularly polarized light. ARPES data show very strong circular dichroism, indicating existence of orbital angular momentum (OAM). Moreover, the alignment of OAM is found to have a strong binding energy dependence. Such energy dependence comes from a relatively strong band warping effect in Bi2Te3 compared to Bi2Se3. OAM close to Dirac point has an ideal chiral structure (sin ?) without out-of-plane component. Warping effect comes in as the binding energy decreases and circular dichroism along a constant energy contour can no longer be explained by a simple sin? function but requires a sin3? term. When the warping effect becomes even stronger near the Fermi energy, circular dichroism gains an additional sin6? term. Such behavior is found to be compatible with the theoretically predicted OAM structure.

Analysis of octadecyltrichlorosilane treatment of organic thin-film transistors using soft x-ray fluorescence spectroscopy

The effect of octadecyltrichlorosilane (OTS) treatment on the electronic properties of organic thin-film transistors is investigated using soft x-ray absorption and emission spectroscopy. Analysis of Carbon Kα x-ray emission spectra reveals that treating the SiO2 layer with OTS prior to pentacene deposition increases the number of π-bonding states in the active pentacene layer, which is strongly correlated with the conduction of charge carriers. The role of the increased π-bonding states is verified by measuring the current-voltage characteristics of pentacene thin-film transistors with and without OTS treatment. Drain current and field-effect mobility of OTS-treated samples are significantly enhanced as anticipated from the spectroscopic analysis.

Spin and orbital angular momentum structure of Cu(111) and Au(111) surface states

We performed angle resolved photoemission (ARPES) studies on Cu(111) and Au(111) surface states with circularly polarized light. Existence of local orbital angular momentum (OAM) is confirmed as has been predicted to be broadly present in a system with an inversion symmetry breaking (ISB). The single band of Cu(111) surface states is found to have chiral OAM in spite of very small spin-orbit coupling (SOC) in Cu, which is consistent with theoretical prediction. As for Au(111), we observe split bands for which OAM for the inner and outer bands are parallel, unlike the Bi2Se3 case. We also performed first principles calculation and the results are found to be consistent with the experimental results. Moreover, majority of OAM is found to be from d-orbitals and a small contribution has p-orbital origin which is anti-aligned to the spins. We derive an effective Hamiltonian that incorporates the role of OAM and used it to extract the OAM and spin structures of surface states with various SOC strength. We discuss the evolution of angular momentum structures from pure OAM case to a strongly spin-orbit entangled state. We predict that the transition occurs through reversal of OAM direction at a k-point in the inner band if the system has a proper SOC strength.