Insulating state of ultrathin epitaxial LaNiO$_3$ thin films detected by hard x-ray photoemission. Gray, A X, Janotti, A, Son, J, LeBeau, J M, Ueda, S, Yamashita, Y, Kobayashi, K, Kaiser, A M, Sutarto, R, Wadati, H, Sawatzky, G A, Van de Walle, C G, Stemmer, S, & Fadley, C S Phys. Rev. B: Condens. Matter Mater. Phys., August, 2011.
doi  abstract   bibtex   
In order to understand the influence of strain and film thickness on the electronic structure of thin films of strongly correlated oxides, we have applied hard x-ray photoemission (HXPS) at 6 keV, soft x-ray photoemission (XPS) at 1.5 keV, and transmission electron microscopy to epitaxial LaNiO3 films deposited on two substrates: LaAlO3 (compressive strain) and (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) (tensile strain). Using inelastic attenuation lengths in LaNiO3 determined from the HXPS data, we have decomposed valence-band spectra into layer-specific contributions. This decomposition is validated by comparing with the results of first-principles calculations using a hybrid functional. The resultant thin-film LaNiO3 densities of states exhibit significant differences in spectral weights for the thinnest LaNiO3 films. A gap opening consistent with a metal-to-insulator transition is observed for the thinnest 2.7 nm LaNiO3 film on an (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) substrate, with a similar gap opening also being observed in complementary soft x-ray photoemission at 1.5 keV for a thinner 1.4 nm film on an LaAlO3 substrate. A metal-to-insulator transition in very thin nm-scale films of LaNiO3 is thus suggested as a general phenomenon.
@ARTICLE{Gray2011-xl,
  title    = "Insulating state of ultrathin epitaxial {LaNiO$_3$} thin films
              detected by hard x-ray photoemission",
  author   = "Gray, A X and Janotti, A and Son, J and LeBeau, J M and Ueda, S
              and Yamashita, Y and Kobayashi, K and Kaiser, A M and Sutarto, R
              and Wadati, H and Sawatzky, G A and Van de Walle, C G and
              Stemmer, S and Fadley, C S",
  abstract = "In order to understand the influence of strain and film thickness
              on the electronic structure of thin films of strongly correlated
              oxides, we have applied hard x-ray photoemission (HXPS) at 6 keV,
              soft x-ray photoemission (XPS) at 1.5 keV, and transmission
              electron microscopy to epitaxial LaNiO3 films deposited on two
              substrates: LaAlO3 (compressive strain) and
              (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) (tensile strain). Using inelastic
              attenuation lengths in LaNiO3 determined from the HXPS data, we
              have decomposed valence-band spectra into layer-specific
              contributions. This decomposition is validated by comparing with
              the results of first-principles calculations using a hybrid
              functional. The resultant thin-film LaNiO3 densities of states
              exhibit significant differences in spectral weights for the
              thinnest LaNiO3 films. A gap opening consistent with a
              metal-to-insulator transition is observed for the thinnest 2.7 nm
              LaNiO3 film on an (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) substrate, with a
              similar gap opening also being observed in complementary soft
              x-ray photoemission at 1.5 keV for a thinner 1.4 nm film on an
              LaAlO3 substrate. A metal-to-insulator transition in very thin
              nm-scale films of LaNiO3 is thus suggested as a general
              phenomenon.",
  journal  = "Phys. Rev. B: Condens. Matter Mater. Phys.",
  volume   =  84,
  number   =  7,
  month    =  aug,
  year     =  2011,
  keywords = "electron-spectroscopy;parameters;photoelectron
              angular-distribution;LeBeau",
  language = "English",
  issn     = "1098-0121",
  doi      = "10.1103/PhysRevB.84.075104"
}

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