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TEAM II: Oxides
Oxide materials are abundant in nature because oxygen is a main ingredient of our atmosphere and has strong chemical reactivity. Among them, transition-metal and rare-earth oxides have attracted much attention because of their novel physical properties that can be controlled by external parameters such as electromagnetic fields, temperature, pressure. Most of them are electrically insulating but some show fascinating electrical properties that are believed to originate from strong electron-electron correlation in an unfilled d- or f-shell. Our goal is to understand their electronic structures, which are not tractable by the density functional theory, using various electron spectroscopy tools such as ARPES, XPS, XAS, and IPES.
Strain control of the electronic structure of ultrathin oxide films
Utilizing the lattice mismatch between an ultrathin film and a substrate, we can vary the lattice parameter of a film, thus changing the hybridization between metal-ion and oxygen 2p orbitals. Especially in transition-metal oxides, triply degenerate t2g and doubly degenerate eg orbitals under a cubic crystal field can be split further due to compressive or tensile strain. Figure 1 illustrates such an example in LaNiO3, a paramagnetic metal close to the phase boundary toward an antiferromagnetic insulator. Under a strong tensile strain, the 3z2-r2 band crosses the Fermi level, thus the Fermi-surface topology changes from three- to two-dimensional one, concomitantly increasing Fermi-surface nesting to enhance spin-density-wave instability. (Figure 1)
Measurement of electronic energy gaps
In an insulator, its energy gap is an important parameter in determining the electronic properties. There are many methods to measure an energy gap such as optical absorption and transport measurement. However, most of them do not directly correspond to the energy gap obtained from the density functional theory in which the energy gap is defined as the energy difference between one-hole and one-electron excitations from the ground state. An way to measure the exact gap is to combine photoemission (PES) and inverse photoemission (IPES) spectroscopies, which measure one-hole and one-electron excitation spectra, respectively. Figure 2 depicts such measurment for RuCl3. While optical absorption shows an optical gap of 1.2 eV, PES-IPES measurements show an electronic gap of 1.9 eV, thus providing valuable information for accurate theoretical calculation. (Figure 2)
Figure 1. Strain control of the electronic band dispersions in ultrathin LaNiO3 films [Sci. Rep. 5, 8746 (2015)].
Figure 2. Photoemission and inverse photoemission spectra of RuCl3 for the measurement of the electronic energy gap [in preparation].