Thickness-dependent electronic structure of ultrathin SrRuO3 films studied by in situ photoemission spectroscopy

Abstract

In situ thickness-dependent photoemission spectroscopy (PES) has been performed on SrRuO3 (SRO) layers deposited on SrTiO3 substrates to study the structure-induced evolution of the electronic structure. The PES spectra showing the existence of two critical film thicknesses reveal that a metal-insulator transition occurs at a film thickness of 4–5 monolayers (ML) and the evolution of Ru 4d-derived states around the Fermi level (EF) saturates at about 15 ML. The observed spectral behavior well matches the electric and magnetic properties and thickness-dependent evolution of surface morphology of the ultrathin SRO films. These experimental results suggest the importance of the disorder associated with the unique growth-mode transition in SRO films.

Electronic properties of bulk and thin filmSrRuO3: Search for the metal-insulator transition

“…22,28,32 Here we first revisit the LSDA with our own calculations, with an emphasis on understanding discrepancies between the experimental measured photoemission results 18,19 and the calculated LSDA electronic structure of the orthorhombic material. We then extend our study to the two "beyond-LSDA" approaches to examine correlation effects in bulk Pbnm and Pm3m SrRuO 3 .…”

“…In particular, it has been demonstrated that metallic SrRuO 3 can be transformed into an insulating state by growing films thinner than five monolayers on SrTiO 3 substrates. 18 It was also found that the Curie temperature decreases with reduced film thickness, along with the disappearance of strong ferromagnetic order. Photoemission experiments show a shift in the spectral weight to the incoherent peak features in the spectra, suggesting that these effects are a result of changes in electron-electron correlation effects.…”

“…Many experimental studies have already addressed the degree of electron-electron correlation in SrRuO 3 including x-ray and ultraviolet photoemission spectroscopy, [18][19][20][21] specific-heat measurements, 22 infrared and optical conductivity measurements, 23 and transport experiments. 24 For example, Kim and co-workers 19 use x-ray photoemission spectroscopy ͑XPS͒ to identify how such correlations change within the ruthenate family, and Toyota et al 18 use photoemission spectroscopy ͑PES͒ to detail the metal-insulator transition in SrRuO 3 as a function of film thickness concomitant with the onset of magnetism. In all of these studies, the general consensus is that electron correlation effects do play a role in determining the electronic structure of this itinerant ferromagnet, but to what degree remains unclear.…”

Ab initioand photoemission study of correlation effects in SrRuO3thin films

“…19 and 20, or soft x-ray PES is used. [21][22][23][24] Since it is well known that PES is highly surface sensitive, the cleaning of ex situ prepared specimens by applying heating or scraping techniques may somehow affect the surface and thus further complicate the investigation of intrinsic electronic structure in SrRuO 3 . Moreover, Siemons et al 19 have concluded that a tiny change in ruthenium stoichiometry may have a fairly considerable effect on the PES spectrum, indicating a highly stoichiometry-dependent degree of correlation for this material.…”

“…In addition, one should also take into account the fact that physical properties of this perovskite are sensitive to the strain 26 and thin film thickness. 24 Having in mind that all these factors may influence the experiment, one should be very careful while making rigorous comments about electronic structure in SrRuO 3 . It is evident that interpretation of experimental results is highly dependent on accurate theoretical calculations.…”