Photon Factory Activity Report 2016 #34 (2017)

BL-2A/2016G621

Electron correlations, oxygen vacancies, or both ?
Revisiting a foundational paradigm of strongly-correlated electron systems
T. C. Rödel1, 2, F. Fortuna1, E. Frantzeskakis1, P. Le Fèvre2, F. Bertran2, M. Kobayashi3, R. Yukawa3,
T. Mitsuhashi3, M. Kitamura3, K. Horiba3, H. Kumigashira3, and A. F. Santander-Syro1*
12

CSNSM, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, 91405 Orsay Cedex, France
Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin-BP48, 91192 Gif-sur-Yvette, France3
Photon Factory, 1-1 Oho, Tsukuba, 305-0801, Japan

1 Introduction
SrTiO3 and SrVO3 are model examples of, respectively,a band insulator and a correlated electron metal. However,
our previous work unveiled that a 2D electron gas (2DEG)
can be created at the surface of SrTiO3 [1]. Moreover thespectrum of such 2DEG looks identical to that of SrVO3,
namely composed of dispersing quasi-particles near theFermi level (EF) and an incoherent peak around -1.5 eV,assigned to the lower Hubbard band in the case of SrVO3 [2],but known to arise merely from oxygen vacancies in the caseof SrTiO3 [1]. The purpose of this work is to determine whatpart of the measured spectrum of SrVO3 is due tocorrelations, and what part is imputable to oxygen vacancies
created by the preparation methods and/or by the UV lightused for photoemission measurements. The issue at stakeis to understand to what extent the drosophila of correlatedelectron materials can be described in the framework ofmodern theories of interacting electrons in solids.
2 Experiment
The bulk-like, crystalline SrVO3(001) thin films weregrown by pulsed laser deposition (PLD) in a chamber
directly connected to the ARPES setup at beamline 2A ofKEK-PF. The PLD growth was performed in a pressure
below 10-7 Torr, to obtain clean surfaces, using a Sr2V2O7
target, which has excess oxygen with respect to SrVO3,
thus minimizing the formation of vacancies during thegrowth. For the ARPES measurements we used linearlypolarized photons in the energy range 30–110 eV and ahemispherical electron analyzer with horizontal slits. The
angular and energy resolutions were 0.25◦ and 15 meV.
The mean diameter of the incident photon beam wassmaller than 100 μm. The UV light brilliance was about
5x107 photons/(s μm2). Such low brilliance, about 100
times smaller than the one found in standard 3d generationsynchrotrons, was crucial for our measurements, to avoid
the rapid generation of oxygen vacancies under UV dose.
The samples were measured at T = 20 K.
3 Results and Discussion
The experiments successfully provided the information
we were looking for. As shown in Fig. 1(a), when thepreparation procedure carefully avoids the introduction ofvacancies, and the UV dose is kept sufficiently small, a weak
but clearly dispersive Hubbard peak is observed. On theother hand, as seen in Fig. 1(b), once the UV dose is large
again, hence creating a significant amount of oxygen
vacancies, the incoherent peak becomes very intense andnon-dispersive. Such dramatic enhancement of theincoherent peak at -1.5 eV demonstrates that states arisingfrom oxygen vacancies can completely mask any intrinsic
Hubbard band. All these observations prove that themeasured line-shape of the Mott-Hubbard band and theratio of intensities between the quasi-particle and theMott-Hubbard peaks, which are crucial to quantify thestrength of electron correlations, can be dramaticallyaffected by the conditions used in the photoemission
experiments, and must be seriously considered to validate
theories and first-principles calculation schemes of thephysics of strongly-interacting electron materials [3].

Fig. 1: (a) ARPES energy-momentum map at the Γ point
in an SrVO3 sample prepared and measured at BL-2A ofKEK-PF under conditions that minimize the introductionof oxygen vacancies. (b) Same as (a) after irradiation witha large UV dose during complementary measurements atsynchrotron SOLEIL (France). Adapted from Ref. [3]References[1] A. F. Santander-Syro et al., Nature 469, 189 (2011).
[2] I. H. Inoue et al., Phys. Rev. Lett. 74, 2539 (1995).
[3] S. Backes et al., Phys. Rev. B 94, 241110(R) (2016).
Research Achievements
1. Our results were published in Phys. Rev. B Rapid
Communications [3]. The paper was an “Editors’Suggestion” for the issue of Dec. 2016.
* andres.santander@csnsm.in2p3.fr