En-Jin Cho

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.

Localized character of 4f electrons in CeRhx (x=2,3) and CeNix (x=2,5).

We have measured Ce 4f spectral weights of extremely alpha-like Ce transition metal intermetallic compounds CeRhx (x=2,3) and CeNix (x=2,5) by using the bulk-sensitive resonant photoemission technique at the Ce M5(3d(5/2)-->4f) edge. High energy resolution and longer escape depth of photoemitted electron at this photon energy enabled us to distinguish the sharp Kondo resonance tails at the Fermi level, which can be well described by the Gunnarsson-Schönhammer calculation based on the Anderson impurity Hamiltonian. On the other hand, the itinerant 4f band description shows big discrepancies, which implies that Ce 4f electrons retain localized characters even in extremely alpha-like compounds.

High-resolution Ce 3 d − edge resonant photoemission study of CeNi 2

Resonant photoemission (RPES) at the Ce 3d -> 4f threshold has been performed for alpha-like compound CeNi_2 with extremely high energy resolution (full width at half maximum 4f RPES threshold. This is interpreted as due to the different surface sensitivity, and the bulk-sensitive Ce 3d -> 4f RPES spectra are found to be consistent with other electron spectroscopy and low energy properties for alpha-like Ce-transition metal compounds, thus resolves controversy on the interpretation of Ce compound photoemission. The 4f spectral weight over the whole valence band can also be fitted fairly well with the Gunnarsson-Schoenhammer calculation of the single impurity Anderson model, although the detailed features show some dependence on the hybridization band shape and (possibly) Ce 5d emissions.

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

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.

Temperature-dependent bulk-sensitive Ce 3d edge resonant photoemission study of CeFe2

Abstract We have performed temperature-dependent Ce 3d→4f resonant photoemission and X-ray absorption spectroscopy of CeFe 2 in ferromagnetic and paramagnetic phases with extremely high resolution (full width at half maximum (FWHM) 2 are quite well described by the Gunnarsson-Schonhammer calculation based on the Anderson single impurity model, but show big disagreement with the Ce 4f density-of-states obtained by band structure calculations. This indicates that the Ce 4f state in CeFe 2 has a fairly localized character.

Electronic structure change of NiS 2− x Se x in the metal-insulator transition probed by X-ray absorption spectroscopy

The electronic structure change of NiS2−xSex as a function of Se concentration x has been studied by Ni L-edge X-ray absorption spectroscopy (XAS). The XAS spectra show distinct features in Ni L3 edge, indicating whether the system is insulating or metallic. These features can be semi-quantitatively explained within the framework of the configurational interaction cluster model (CICM). In the S-rich region, relatively large charge-transfer energy (Δ ~ 5 eV) from ligand p to Ni 3d states and a little small p-d hybridization strength (Vpdσ ~ 1.1 eV) can reproduce the experimental spectra in the CICM calculation, and vice versa in the Se-rich region. Our analysis result is consistent with the Zaanen-Sawatzky-Allen scheme that the systems in S-rich side (x ≤ 0.5) are a charge transfer insulator. However, it also requires that the Δ value must change abruptly in spite of the small change of x near x = 0.5. As a possible microscopic origin, we propose a percolation scenario where a long range connection of Ni[(S,Se)2]6 octahedra with Se-Se dimers plays a key role to gap closure.

Auger effect in high-resolution Ce 3d-edge resonant photoemission

The bulk-sensitive Ce 4f spectral weights of various Ce compounds including CeFe 2 , CeNi 2 , and CeSi 2 were obtained with the resonant photoemission technique at the Ce 3d edge. We found that the line shapes change significantly with the small change of the incident photon energy. Detailed analysis showed that this phenomenon results primarily from the Auger transition between different multiplet states of the Ce 3d 5 / 2 4f 2 (bar denotes a hole) electronic configuration in the intermediate state of the resonant process. This tells us that extra care should be taken for the choice of the resonant photon energy when extracting Ce 4f spectral weights from the Ce 3d-edge resonant photoemission spectra. The absorption energy corresponding to the lowest multiplet structure of the Ce 3d 5 / 2 4f 2 configuration seems to be the logical choice to obtain genuine Ce 4f spectral weight from the Ce 3d resonant photoemission data.