Yoonyoung Koh

Anisotropic Dirac fermions in a Bi square net of SrMnBi2.

We report the observation of highly anisotropic Dirac fermions in a Bi square net of SrMnBi(2), based on a first-principles calculation, angle-resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the (SrBi)(+) layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi(2) is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of ~8. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provides a new platform that hosts highly anisotropic Dirac fermions.

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.

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.

Microscopic mechanism for asymmetric charge distribution in Rashba-type surface states and the origin of the energy splitting scale

Microscopic mechanism for the Rashba-type band splitting is examined in detail. We show how asymmetric charge distribution is formed when local orbital angular momentum (OAM) and crystal momentum get interlocked due to surface effects. An electrostatic energy term in the Hamiltonian appears when such OAM and crystal momentum dependent asymmetric charge distribution is placed in an electric ?eld produced from an inversion symmetry breaking (ISB). Analysis by using an effective Hamiltonian shows that, as the atomic spin-orbit coupling (SOC) strength increases from weak to strong, originally OAM-quenched states evolve into well-de?ned chiral OAM states and then to total angular momentum J-states. In addition, the energy scale of the band splitting changes from atomic SOC energy to electrostatic energy. To con?rm the validity of the model, we study OAM and spin structures of Au(111) system by using an effective Hamiltonian for the d-orbitals case. As for strong SOC regime, we choose Bi2Te2Se as a prototype system. We performed circular dichroism angle resolved photoemission spectroscopy experiments as well as ?rst-principles calculations. We ?nd that the effective model can explain various aspects of spin and OAM structures of the system.

Intrinsic quasi-particle dynamics of topological metallic states

We investigate the intrinsic quasi-particle (QP) dynamics in the topological metallic (TM) states of Bi2Se3, Bi2−xSnxTe3 (x=0.0067) and Sb. Surprisingly, the intrinsic QP dynamics are mostly determined by the scattering within the TM states and are unaffected by the bulk electronic states. We observe the binding energy-independent imaginary part of the self-energy Im Σ for the QPs in the TM states. We attribute the binding energy-independent Im Σ to momentum mixing due to flakes or warped surfaces of the cleaved samples. This makes the intrinsic scattering rate of the QPs extremely small in the TM states. We discuss a few possible contributions to the smallness of the Im Σ for QPs. Our new observations on the extremely small intrinsic QP scattering rates may open up the possibility of room-temperature quantum devices.

Growth and Electronic Structure Studies of Metal Intercalated Transition Metal Dichalcogenides MxNbSe2 (M: Fe and Cu)

There has been renewed interest in transition metal dichalcogenides (TMDs) as electronic materials. The modification of the electronic structures of TMDs by means of metal intercalation would be of interest in that respect. With this motivation, we have synthesized and performed angle-resolved photoemission studies on Fe- and Cu-intercalated TMD FexNbSe2 and CuxNbSe2 with various x values. FexNbSe2 (x = 0, 0.25) and CuxNbSe2 (x = 0, 0.06) were grown by the iodine vapor transport method. Intercalation and doping were confirmed by changes in c-axis lattice constant and electronic structure, respectively. A significant amount of electron doping from Fe and Cu was observed, as indicated in the ARPES data measured as the rigid downshift of bands. In addition, band folding was observed in FexNbSe2, for which the unit cell is doubled owing to the Fe order. However, the extra band folding expected from the antiferromagnetic order of Fe was not observed.