Han-Jin Noh

X-ray absorption spectroscopy of graphite oxide

We have investigated the electronic structure of graphite oxide using X-ray absorption spectroscopy at the carbon and oxygen K-edges. The unoccupied π* and σ* states associated with sp2 hybridization in graphite, are also apparent in the graphite oxide, which indicates that it has a graphitic structure even though it experiences oxidation and annealing. Additional electronic states of the graphite oxide which are not present in its precursor, graphite, are ascribed to the functional groups such as epoxide, carboxyl, and hydroxyl groups.

Spin-orbit interaction effect in the electronic structure of Bi2Te3 observed by angle-resolved photoemission spectroscopy

The electronic structure of p-type doped Bi2Te3 is studied by angle-resolved photoemission spectroscopy (ARPES) to experimentally confirm the mechanism responsible for the high thermoelectric figure of merit. Our ARPES study shows that the band edges are located off the Γ-Z line in the Brillouin zone, which provides direct observation that the spin-orbit interaction is a key factor to understand the electronic structure and the corresponding thermoelectric properties of Bi2Te3. A successive time-dependent ARPES measurement also reveals that the electron-like bands crossing EF near the -point are formed in an hour after cleaving the crystals. We interpret these as surface states induced by surface band bending, possibly due to quintuple inter-layer distance change of Bi2Te3.

Core-Level X-Ray Photoemission Satellites in Ruthenates: A New Mechanism Revealing The Mott Transition

Core-level x-ray photoemission spectra for the Mott-Hubbard systems are calculated by the dynamical mean-field theory based on the exact diagonalization method. The spectra show a two-peak structure, screened and unscreened peaks. The screened peak is absent in a Mott insulator, but develops into the main peak when the correlation strength becomes weak and the system turns metallic. The calculated spectral behavior is consistent with the experimental Ru 3d core-level spectra of various ruthenates. This new mechanism of the core-level photoemission satellite can be utilized to reveal the Mott transition phenomenon in various strongly correlated electron systems, especially in nanoscale devices and phase-separated materials.

Quantum oscillations of the metallic triangular-lattice antiferromagnet PdCrO2.

We report the electronic and transport properties of the triangular antiferromagnet PdCrO(2) at high magnetic fields up to 33 T, using measurements of the de Haas-van Alphen oscillations and the Hall resistivity. The de Haas-van Alphen oscillations below the magnetic ordering temperature T(N) reveal several two-dimensional Fermi surfaces of smaller size than those found in nonmagnetic PdCoO(2), consistent with the band structure calculations. This evidences Fermi surface reconstruction due to the 120° helical ordering of the localized Cr spins, suggesting significant coupling of the itinerant electrons to the underlying spin texture. This induces the nonlinear Hall resistivity at low temperatures via the magnetic breakdown in the reconstructed Fermi surface. Furthermore, such a coupling leads to the unconventional anomalous Hall effects near T(N) due to the field-induced spin chirality at high magnetic fields.

Direct Observation of Localized Spin Antiferromagnetic Transition in PdCrO2 by Angle-Resolved Photoemission Spectroscopy

We report the first case of the successful measurements of a localized spin antiferromagnetic transition in delafossite-type PdCrO2 by angle-resolved photoemission spectroscopy (ARPES). This demonstrates how to circumvent the shortcomings of ARPES for investigation of magnetism involved with localized spins in limited size of two-dimensional crystals or multi-layer thin films that neutron scattering can hardly study due to lack of bulk compared to surface. Also, our observations give direct evidence for the spin ordering pattern of Cr3+ ions in PdCrO2 suggested by neutron diffraction and quantum oscillation measurements, and provide a strong constraint that has to be satisfied by a microscopic mechanism for the unconventional anomalous Hall effect recently reported in this system.

Jahn-Teller effect in spinel manganites probed by soft x-ray absorption spectroscopy

We present soft x-ray absorption spectroscopy (XAS) on spinel manganites and show it can be used as a powerful tool to directly probe the Jahn-Teller (JT) splitting energy in transition metal oxides. The MnL2,3-edge XAS spectra of the spinel ZnMnxGa2−xO4(x=0.5,1.0,1.4,and2.0) and AMn2O4(A=MgandCd) change very systematically with the structural distortion of JT active octahedral sublattices. The XAS spectra are well reproduced by the configuration interaction cluster model including full ionic multiplet structure, and the spectral evolution is explained by JT energy splitting due to the elongation of the MnO6 octahedra in the model. In the OK-edge XAS spectra, the JT distortion produces a weak effect, less distinctive than in L2,3-edge spectra.

Controlling the evolution of two-dimensional electron gas states at ametal/Bi2Se3interface

We have demonstrated that the evolution of the two-dimensional electron gas (2DEG) system at an interface of metal and the model topological insulator (TI) Bi2Se3 can be controlled by choosing an appropriate kind of metal elements and by applying a low temperature evaporation procedure. In particular, we have found that only topological surface states (TSSs) can exist at a Mn/Bi2Se3 interface, which would be useful for implementing an electric contact with surface current channels only. The existence of the TSSs alone at the interface was confirmed by angle-resolved photoemission spectroscopy (ARPES). Based on the ARPES and core-level x-ray photoemission spectroscopy measurements, we propose a cation intercalation model to explain our findings. *E-mail: ffnhj@chonnam.ac.kr

X-ray absorption spectroscopy of a poly sodium 4-styrensulfonate intercalated graphite oxide electrode

We investigated the electronic structures of a poly sodium 4-styrensulfonate intercalated graphite oxide (PSSGO) electrode and a precursor graphite oxide (GO) electrode using X-ray absorption spectroscopy (XAS). Both electrodes were obtained from electrochemical cells. We found that in the C K-edge XAS spectra the π* state intensity originating from the sp2 hybridization of graphite decreases predominantly in the graphite oxide and PSSGO electrodes. This indicates that the negatively charged electrolyte ion (BF4−) is absorbed onto the electrodes and is transferred to the π* state of the both electrodes. The analysis of their F K-edge spectra reveals that more BF4− ions were found in the PSSGO electrode than in the graphite oxide electrode. This indicates that more electrolyte ions are absorbed in the PSSGO than in the graphite oxide electrode. We argue that this is the main reason why PSSGO cells have higher capacitance, higher energy density, and higher power density when compared to the graphite oxide cells. We also found that BF4− is the primary working ion that can be inserted into the interlayers of the PSSGO electrode.

Persistence of surface states despite impurities in the surface of topological insulators

We have experimentally demonstrated the robustness and immunity of the surface states to surface impurities and disorder in topological insulator (TI) Bi0.9Sb0.1 in comparison with those of semi-metallic Sb(111) surface by measuring the scattering rates of the quasiparticles (QP) via angle-resolved photoemission spectroscopy. The surface impurities on TIs increase the scattering rate of the QPs but their effects are extrinsic and limited regardless of their magnetic properties, while in the semimetallic Sb case they open new scattering channels making the surface states fragile.

Synthesis of monodisperse chromium nanoparticles from the thermolysis of a Fischer carbene complex

We successfully synthesized monodisperse chromium nanoparticles from the thermolysis of a Fischer carbene complex.