Seung Ryong Park

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.

Hallmarks of the Mott-metal crossover in the hole-doped pseudospin-1/2 Mott insulator Sr2IrO4

The physics of doped Mott insulators remains controversial after decades of active research, hindered by the interplay among competing orders and fluctuations. It is thus highly desired to distinguish the intrinsic characters of the Mott-metal crossover from those of other origins. Here we investigate the evolution of electronic structure and dynamics of the hole-doped pseudospin-1/2 Mott insulator Sr2IrO4. The effective hole doping is achieved by replacing Ir with Rh atoms, with the chemical potential immediately jumping to or near the top of the lower Hubbard band. The doped iridates exhibit multiple iconic low-energy features previously observed in doped cuprates—pseudogaps, Fermi arcs and marginal-Fermi-liquid-like electronic scattering rates. We suggest these signatures are most likely an integral part of the materials proximity to the Mott state, rather than from many of the most claimed mechanisms, including preformed electron pairing, quantum criticality or density-wave formation.

Dimensionality-controlled Mott transition and correlation effects in single-layer and bilayer perovskite iridates

We studied Sr2IrO4 and Sr3Ir2O7 using angle-resolved photoemission spectroscopy (ARPES), making direct experimental determinations of intra- and inter-cell coupling parameters as well as Mott correlations and gap sizes. The results are generally consistent with LDA+U+Spin-orbit coupling (SOC) calculations, though the calculations missed the momentum positions of the dominant electronic states and neglected the importance of inter-cell coupling on the size of the Mott gap. The calculations also ignore the correlation-induced spectral peak widths, which are critical for making a connection to activation energies determined from transport experiments. The data indicate a dimensionality-controlled Mott transition in these 5d transition-metal oxides (TMOs).

Angle-resolved photoemission spectroscopy of electron-doped cuprate superconductors: isotropic electron-phonon coupling.

We have performed high resolution angle-resolved photoemission (ARPES) studies on electron-doped cuprate superconductors Sm2-xCexCuO4 (x=0.10, 0.15, 0.18), Nd2-xCexCuO4 (x=0.15), and Eu2-xCexCuO4 (x=0.15). Imaginary parts of the electron removal self energy show steplike features due to an electron-bosonic mode coupling. The steplike feature is seen along both nodal and antinodal directions but at energies of 50 and 70 meV, respectively, independent of the doping and rare earth element. Such energy scales can be understood as being due to preferential coupling to half- and full-breathing mode phonons, revealing the phononic origin of the kink structures. Estimated electron-phonon coupling constant lambda from the self energy is roughly independent of the doping and momentum. The isotropic nature of lambda is discussed in comparison with the hole-doped case where a strong anisotropy exists.

Electron Number-Based Phase Diagram of Pr1−xLaCexCuO4−δ and Possible Absence of Disparity between Electron- and Hole-Doped Cuprate Phase Diagrams

We performed annealing and angle resolved photoemission spectroscopy studies on electron-doped cuprate Pr_{1-x}LaCe_{x}CuO_{4-δ} (PLCCO). It is found that the optimal annealing condition is dependent on the Ce content x. The electron number (n) is estimated from the experimentally obtained Fermi surface volume for x=0.10, 0.15 and 0.18 samples. It clearly shows a significant and annealing dependent deviation from the nominal x. In addition, we observe that the pseudo-gap at hot spots is also closely correlated with n; the pseudogap gradually closes as n increases. We established a new phase diagram of PLCCO as a function of n. Different from the x-based one, the new phase diagram shows similar antiferromagnetic and superconducting phases to those of hole doped ones. Our results raise a possibility for absence of disparity between the phase diagrams of electron- and hole-doped cuprates.

Evidence for a charge collective mode associated with superconductivity in copper oxides from neutron and x-ray scattering measurements of La

In superconducting copper oxides, some Cu-O bond-stretching phonons around 70 meV show anomalous giant softening and broadening of electronic origin, and electronic dispersions have large renormalization kinks near the same energy. These observations suggest that phonon broadening originates from quasiparticle excitations across the Fermi surface and the electronic dispersion kinks originate from coupling to anomalous phonons. We measured the phonon anomaly in underdoped (

_{2-x}

x=0.05

Sr

) and overdoped (

_x

x=0.20

CuO

and 0.25) La