Photon Factory Activity Report 2004 #22 Part B (2005)
Electronic Structure of Condensed Matter

2C/2003G149

Bulk electronic structure of Ca1-xSrxRuO3 studied using epitaxial thin films
Masaru TAKIZAWA*1, Daisuke TOYOTA2, Hiroki WADATI1, Akira CHIKAMATSU2,Ryuji HASHIMOTO2, Hiroshi KUMIGASHIRA2, Atsushi FUJIMORI1, Masaharu OSHIMA2,
345Mikk LIPPMAA , Masashi KAWASAKI , and Hideomi KOINUMA
1
Department of Physics and Department of Complexity Science and Engineering,
University of Tokyo, Kashiwa 277-8561, Japan2
Department of Applied Chemistry, University of Tokyo, Tokyo 113-8656, Japan
3Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan
4
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
5Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Introduction
Metal-insulator transition has been extensively studied
because of its fundamental importance as well as its close
relationship to interesting phenomena such as hightemperature superconductivity in cuprates and colossal
magnetoresistance in manganites [1]. Ca1-xSrxVO3 (CSVO)
is a typical bandwidth control system but remains metallic
for the entire x range. With Ca doping, ultra-violet
photoemission spectra of CSVO have shown the spectral
weight transfer from the coherent part to the incoherent
part [2], while using high photon energies there were nospectral weight transfer [3]. Very recently, a more bulksensitive measurement of the same system using a UV
laser as a light source indicated that spectral weighttransfer indeed occurs [4]. The problem therefore remains
highly controversial and further studies are stronglyrequired. In the present work we have observed clear
spectral weight transfer in another bandwidth-control
system Ca1-xSrxRuO3 (CSRO) after decomposition of thespectra into surface and bulk components [5].
Experiment
CSRO thin films were fabricated in a laser MBEchamber connected to a synchrotron radiation photoemission system at BL-2C of Photon Factory [6]. Thefilms were deposited on Nb-doped TiO2-terminatedoSrTiO3 (001) substrates [7] at 900 C at an oxygenpressure of 0.1 Torr. The fabricated CSRO films weretransferred into the photoemission chamber under an-10ultrahigh vacuum of 10 Torr. The PES spectra weretaken at room temperature with the total energy resolutionof about 200 ~ 500 meV depending on photon energy.
Results and Discussion
We have measured PES spectra at 400 eV and 900 eV
as shown in Fig. 1. As we are interested in the Ru 4d band,the tail of the O 2p band has been subtracted. In order toobtain the bulk and surface components of the Ru 4d band,
we have followed the procedure of Ref. [8]. The
measured photoemission spectra are assumed to be
expressed as I(E) = exp(-s/λ)Ibulk(E) + [1 - exp(s/λ)]Isurface(E), where s is the thickness of the surface layer,
λ is the photoelectron mean-free path, and Ibulk and Isurface

denote the spectra of the bulk and surface regions,respectively. We have used the mean-free paths of λ900 =18 Å and λ400 = 6 Å at hν = 900 and 400 eV, respectively
and the surface layer thickness of 4 Å, the dimension ofthe unit cell. As shown in Fig. 1, the incoherent part of thebulk component thus obtained is weaker than that of theraw spectra, and manifests itself as a shoulder rather thana separate feature from the main structure near EF.
Nevertheless, there is still spectral weight transfer fromthe coherent part to the incoherent part with Ca doping.

Fig. 1: Ru 4d band of Ca1-xSrxRuO3. Top: Data taken at hν
= 400 and 900 eV. Bottom: The bulk and surface
components deduced from different energies.
References[1] M. Imada, A. Fujimori, and Y. Tokura, Rev. Mod. Phys.
70 1039 (1998), and references therein.
[2] I. H. Inoue et al., Phys. Rev. Lett. 74, 2539 (1995).
[3] A. Sekiyama et al., Phys. Rev. Lett. 93, 156402 (2004).
[4] R. Eguchi et al., cond-mat/0504576 (2005).
[5] M. Takizawa et al., cond-mat/0412718 (2004).
[6] K. Horiba et al., Rev. Sci. Instr. 74, 3406 (2003).
[7] M. Kawasaki et al., Science 266, 1540 (1994).
[8] K. Maiti, P. Mahadevan, and D. D. Sarma, Phys. Rev.
Lett. 80, 2885 (1998).
* takizawa@wyvern.phys.s.u-tokyo.ac.jp

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