Byeong-Gyu Park

Novel Jeff=1/2 Mott state induced by relativistic spin-orbit coupling in Sr2IrO4.

We investigated the electronic structure of 5d transition-metal oxide Sr2IrO4 using angle-resolved photoemission, optical conductivity, x-ray absorption measurements, and first-principles band calculations. The system was found to be well described by novel effective total angular momentum Jeff states, in which the relativistic spin-orbit coupling is fully taken into account under a large crystal field. Despite delocalized Ir 5d states, the Jeff states form such narrow bands that even a small correlation energy leads to the Jeff=1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of Jeff quantum spin driven correlated-electron phenomena.

Magnetism in Mn-doped ZnO bulk samples prepared by solid state reaction

Contrasting magnetic properties were obtained from bulk Mn-doped ZnO synthesized under different processing conditions. While a ferrimagnetic phase transition was observable in a Zn0.95Mn0.05O sample processed at 1170 K, no such transition was found for a sample with the same composition processed at 1370 K. The detailed magnetic, structural, and spectroscopic studies of these two samples have revealed that the ferrimagnetic transition in the former sample is attributable to the secondary phase, (Mn,Zn)Mn2O4, in the system. For the latter sample processed at higher temperature, no secondary phase was detected and the major feature of the system remained paramagnetic down to 4 K. The implication of the present results for Mn-doped ZnO thin films is discussed.

Spectroscopic evidence for limited carrier hopping interaction in amorphous ZnO thin film

The electronic structure of amorphous ZnO film (a-ZnO) was examined by O K- and Zn L3-edge x-ray absorption spectroscopy and valence band photoemission spectroscopy. Comparative studies of a-ZnO and a wurtzite ZnO (w-ZnO) revealed a decrease in Zn 4s-O 2p hybridization strength and the localization of Zn 4s band as a consequence of local structural disorder, indicating limited electron hopping interactions in a-ZnO. The 0.1 eV higher Fermi-level of a-ZnO compared to w-ZnO suggests that the electrical properties of a-ZnO are different from those in w-ZnO due to structural disorder, even in the absence of impurities or grain boundaries.

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.

Photoemission, soft x-ray absorption, and magnetic circular dichroism spectroscopy study of Fe1−xCuxCr2S4 (0.1≤x≤0.5) spinel sulfides

The electronic and magnetic structures of Fe1−xCuxCr2S4 (0.1≤x≤0.5) spinel sulfides have been investigated systematically by performing photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and soft x-ray magnetic circular dichroism (XMCD) measurements using synchrotron radiation. Cr and Cu ions are found to be nearly trivalent (Cr3+) and monovalent (Cu+), respectively, and their valence states do not change with x. The Fe 2p XAS spectra of Fe1−xCuxCr2S4 are very similar to that of Fe metal, indicating that the Fe 3d electrons are strongly hybridized to other valence electrons. The Fe and Cr 2p XMCD spectra show that the magnetic moments of Cr ions and Fe ions are aligned antiparallel to each other and that both the Cr and Fe magnetic moments increase with increasing x. The valence-band PES study reveals that the Cr3+ () 3d states are located at ~1.5 eV below EF. The occupied Fe 3d states consist of the broad states, the states at ~4 eV below EF, and the states very close to EF. The filled Cu 3d10 states lie at ~2.5 eV below EF. This study suggests that the hybridized Fe and S 3p states near EF play an important role in determining the transport properties of Fe1−xCuxCr2S4 for x≤0.5.

Evidence for Anionic Excess Electrons in a Quasi-Two-Dimensional Ca2N Electride by Angle-Resolved Photoemission Spectroscopy.

Angle-resolved photoemission spectroscopy (ARPES) study of a layered electride Ca2N was carried out to reveal its quasi-two-dimensional electronic structure. The band dispersions and the Fermi-surface map are consistent with the density functional theory results except for a chemical potential shift that may originate from the high reactivity of surface excess electrons. Thus, the existence of anionic excess electrons in the interlayer region of Ca2N is strongly supported by ARPES.

Temperature dependent phase transition of EuO on MgO(100)

We investigated the changes of magnetic properties and phase transition in EuO films grown on a MgO(100) substrate as a function of temperature. As the temperature was varied, we observed a clear phase transition and polarization change at 69K using spin resolved photoemission spectroscopy and low energy electron diffraction. We will elucidate the temperature driven changes in the electronic and magnetic properties of EuO films on a MgO(100) substrate.

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.

Phase separation in thermoelectric delafossite CuFe1−xNixO2 observed by soft x-ray magnetic circular dichroism

Electronic structures of Ni-doped CuFe1−xNixO2 delafossite oxides (0 ≤ x ≤ 0.03) have been investigated by employing soft x-ray magnetic circular dichroism (XMCD). Finite XMCD signals are observed for Fe, Ni, and Cu 2p states, and valence states of Cu, Fe, and Ni ions are nearly monovalent (Cu+), trivalent (Fe3+), and divalent (Ni2+), respectively, for all x ≤ 0.03. Tiny magnetic impurities could be detected by employing XMCD. Fe and Ni 2p XMCD signals are identified due to ferrimagnetic spinel impurities of CuFe2O4 and NiFe2O4. XMCD signals for Cu 2p states arise from divalent Cu2+ ions. Thermoelectrical properties are found to be very sensitive to the very little impurity phase present in delafossite oxides.