Soobin Sinn

Latent instabilities in metallic LaNiO3 films by strain control of Fermi-surface topology

Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by misfit strain to change its fermiology. As tensile strain increases, the hole pocket centered at the A point elongates along the kz-axis and seems to become open, thus changing Fermi-surface (FS) topology from three- to quasi-two-dimensional. Concomitantly, the FS shape becomes flattened to enhance FS nesting. A FS superstructure with Q1 = (1/2,1/2,1/2) appears in all LNO films, while a tensile-strained LNO film has an additional Q2 = (1/4,1/4,1/4) modulation, indicating that some instabilities are present in metallic LNO films. Charge disproportionation and spin-density-wave fluctuations observed in other nickelates might be their most probable origins.

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.

Avoiding fatal damage to the top electrodes when forming unipolar resistance switching in nano-thick material systems

When forming unipolar resistance switching in material systems, fatal damage often occurs to the top electrodes of Pt/SrTiOx/Pt, Pt/TiOy/Pt and Pt/NiOz/Pt cells. To develop a means of overcoming this problem, we systematically investigated the forming process by applying triangular- and pulse-waveform voltage signals to the cells. By investigating the dependence on sweep rate of the triangular-waveform voltage signals and amplitude of the pulse-waveform voltage signals, we discovered that the forming process occurred by two different mechanisms, irrespective of the material: either a thermally assisted dielectric breakdown or a purely electrical dielectric breakdown. During the former process, the top electrodes remained clean, even though oxygen bubbles formed on them. We observed that the top electrodes were blown off only for the latter (electrical) breakdown as a result of the formation of many conducting channels. We were able to overcome the fatal damage to the top electrodes by modifying the forming process into the thermally assisted dielectric breakdown.

Electronic Structure of the Kitaev Material α -RuCl 3 Probed by Photoemission and Inverse Photoemission Spectroscopies

Recently, α-RuCl3 has attracted much attention as a possible material to realize the honeycomb Kitaev model of a quantum-spin-liquid state. Although the magnetic properties of α-RuCl3 have been extensively studied, its electronic structure, which is strongly related to its Kitaev physics, is poorly understood. Here, the electronic structure of α-RuCl3 was investigated by photoemission (PE) and inverse-photoemission (IPE) spectroscopies. The band gap was directly measured from the PE and IPE spectra and was found to be 1.9 eV, much larger than previously estimated values. Local density approximation (LDA) calculations showed that the on-site Coulomb interaction U could open the band gap without spin-orbit coupling (SOC). However, the SOC should also be incorporated to reproduce the proper gap size, indicating that the interplay between U and SOC plays an essential role. Several features of the PE and IPE spectra could not be explained by the results of LDA calculations. To explain such discrepancies, we performed configuration-interaction calculations for a RuCl63− cluster. The experimental data and calculations demonstrated that the 4d compound α-RuCl3 is a Jeff = 1/2 Mott insulator rather than a quasimolecular-orbital insulator. Our study also provides important physical parameters required for verifying the proposed Kitaev physics in α-RuCl3.

Insulating-layer formation of metallic LaNiO3 on Nb-doped SrTiO3 substrate

We investigated the electronic structures of strongly correlated metallic LaNiO3 (LNO) and semiconducting Nb-doped SrTiO3 (Nb:STO) heterostructures by varying the LNO film thickness using in situ photoemission spectroscopy. We found that, contrary to other interfaces with SrTiO3 and LaAlO3, insulating LNO layers are formed between metallic LNO layers and Nb:STO. Such behavior seems to be related with an electron transfer from Nb:STO to LNO due to Schottky-barrier formation at the interface.

Element-Specific Orbital Character in a Nearly-Free-Electron Superconductor Ag 5 Pb 2 O 6 Revealed by Core-Level Photoemission

Ag5Pb2O6 has attracted attentions due to its novel nearly-free-electron superconductivity, but its electronic structure and orbital character of the Cooper-pair electrons remain controversial. Here, we present a method utilizing core-level photoemission to show that Pb 6s electrons dominate near the Fermi level. We observe a strongly asymmetric Pb 4 f7/2 core-level spectrum, while a Ag 3d5/2 spectrum is well explained by two symmetric peaks. The asymmetry in the Pb 4 f7/2 spectrum originates from the local attractive interaction between conducting Pb 6s electrons and a Pb 4 f7/2 core hole, which implies a dominant Pb 6s contribution to the metallic conduction. In addition, the observed Pb 4 f7/2 spectrum is not explained by the well-known Doniach-Šunjić lineshape for a simple metal. The spectrum is successfully generated by employing a Pb 6s partial density of states from local density approximation calculations, thus confirming the Pb 6s dominant character and free-electron-like density of states of Ag5Pb2O6.