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Photon energy dependent photoemission study of La0.7Sr0.3MnO3
Surface Science 575 (2005) 29–34 www.elsevier.com/locate/susc
Photon energy dependent photoemission study of La0.7Sr0.3MnO3 M.C. Falub
a,*
, M. Shi a, J. Krempasky a, K. Hricovini b, Ya.M. Mukovskii c, M. Neumann d, V.R. Galakhov e, L. Patthey a a
e
Swiss Light Source, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland b Universite de Cergy-Pontoise, 95031 Cergy-Pontoise, France c Moscow Institute of Steel and Alloys, Moscow, Russia d Faculty of Physics, University of Osbnabrueck, D-49069 Osnabrueck, Germany Institute of Metal Physics, Ural Division, Russian Academy of Sciences, Yekaterinburg 620219, Russia Received 14 February 2004; accepted for publication 27 October 2004 Available online 28 December 2004
Abstract The electronic structure of the single crystal La0.7Sr0.3MnO3 in ferromagnetic metallic phase has been investigated by photoelectron spectroscopy. Valence band spectra were measured at 40 K for various photon energies, in the range of 45–70 eV. The Mn 3d partial density of states was derived from resonant photoemission near the Mn 3p–3d threshold. Besides the occupied 3d states, resonant photoemission data has provided information about the unoccupied 3d states. The value of the on-site Coulomb interaction strength (correlation energy) was estimated to be 6.7 eV. 2004 Elsevier B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Resonant photoemission; Manganites; Electron correlation
1. Introduction In the last decade there has been a considerable interest in the doped manganese perovskites due to the colossal magnetoresistance (CMR) effect *
Corresponding author. Address: EPFL SB IPN LSE, Institut de Physique des Nanostructures, Ph A1 397 (Baˆtiment PH), Station 3, CH-1015 Lausanne, Switzerland. Tel.: +41 21 693 3399; fax: +41 21 693 3604. E-mail address: mihaelacarmen.falub@epfl.ch (M.C. Falub).
[1]. Among them, the Sr-doped compounds La1xSrxMnO3 exhibit interesting and quite unusual transport and magnetic properties [2–4]. The parent compound LaMnO3 is an A-type antiferromagnetic insulator (TN = 140 K), in which the MnO2 ferromagnetic layers are stacked along c-axis with alternating spin directions. In an ionic picture, the electronic configuration of the Mn3+ ions is 3d4 (t32g e1g ). The crystal field splits the Mn 3d orbitals into the threefold degenerate t2g and twofold degenerate eg orbitals. Whereas the
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.10.053
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M.C. Falub et al. / Surface Science 575 (2005) 29–34
eg orbitals, pointing directly towards oxygen are strongly hybridized with the O 2p states, the t2g orbitals are less hybridized with the ligand states and can be regarded as localized (S = 3/2). Not only the t2g electrons, but also the eg electrons are strongly affected by the correlation effect and tend to localize (Mott insulator). However, when La3+ is substituted by Sr2+ (or another divalent ion) holes are created in the Mn eg orbital states and the eg electrons become itinerant (conduction electrons). The series La1xSrxMnO3 exhibits a very rich phase diagram, including electronic (insulator/metal), magnetic (antiferromagnetic/ferromagnetic/ paramagnetic) and structural (orthorhombic/rhombohedral/tetragonal/monoclinic/hexagonal) transitions [3,4]. For Sr-content from x = 0.2 to x = 0.5, colossal magnetoresistance was reported [2]. At 30% Sr-doping, besides the colossal magnetoresistance effect, half-metallicity was also reported [5], which makes this material particularly interesting, being a possible candidate for applications in magnetic devices. However, while the spin-resolved photoemission study of the thin film La0.7Sr0.3MnO3 has shown half-metallic behaviour [5], other investigations of single crystals of the same composition have found spin-polarization lower than 100%, as one would expect for a half-metal [6]. Although many photoemission studies have been performed for polycrystalline samples, thin films and, more recently, single crystals of La1xSrxMnO3 [7–14], their electronic structure is not yet fully understood. Photoemission spectroscopy near the Mn 3p–3d threshold can provide information about the electronic structure [15–18]. For 3d transition metal compounds, the 3d photoemission is strongly enhanced when the energy of the incoming light equals the energy necessary to excite a 3p electron to an unoccupied 3d state. The intermediate excited state may decay into a final state identical to that obtained after the direct 3d photoemission. The interference of the direct 3d photoemission and the 3p–3d transition followed by a 3p–3d–3d Coster–Kronig decay is denoted as resonant photoemission (RPES) at the 3p–3d threshold. In previous RPES studies of manganites at the Mn 3p–3d threshold, valence band spectra were re-
corded at two photon energies that correspond to ÔOnÕ- and ÔOff’’-resonance and from their difference the Mn 3d-derived partial density of states was deduced [9,15,16]. For polycrystalline samples of LaMnO3 and SrMnO3 an enhancement of the valence band spectra at 10–15 eV and an apparent decrease at 1–7 eV binding energy was found at resonance, the latter being related by the authors to photoionization cross section effects [9]. Photoemission spectra were measured at room temperature for thin films of La0.65Ba0.35MnO3 (Tc = 330 K) using photons of 47 eV and 52 eV energies and Mn 3d-derived bands were determined at 0– 3.5 eV and 5–7 eV below Fermi level, respectively [15,16]. Similarly, for thin films of La0.65Ca0.35MnO3 resonant enhancement of the valence band over a broad energy region (about 8 eV) was reported above, as well as below the critical temperature Tc = 260 K [16]. However, quite different results have recently been obtained for the single crystal La0.66Ca0.33MnO3 (Tc = 258 K) at 80 K using Mn 3p–3d resonant photoemission [17]. Here, a gradual enhancement of the valence band intensity at 2–4 eV below Fermi level was observed when hm increases, reaching maximum for photons of 55 eV energy. There are only few Mn 3p–3d RPES data reported for single crystals [18–20]. We present a photon energy dependent photoemission study of the La0.7Sr0.3MnO3 single crystal. In order to derive the Mn 3d contribution into the valence band we have measured valence band spectra near the Mn 3p absorption edge. Moreover, from RPES data we have obtained information about the Mn 3d unoccupied states and evaluated the on-site Coulomb interaction strength (correlation energy) of the 3d electrons.
2. Experimental Photon energy dependent photoemission spectra of La0.7Sr0.3MnO3 near the Mn 3p–3d threshold were recorded at Surface and Interface Spectroscopy beamline of the Swiss Light Source. Single crystals of La0.7Sr0.3MnO3 were grown by floating-zone method [21]. In general, single crystal samples yield more detailed and more reliable
M.C. Falub et al. / Surface Science 575 (2005) 29–34
information than polycrystals. In case of polycrystals, quite different results were reported by several groups. This could be related to the possible impure phases between boundaries and/or the migration of internal contaminants. Thin films of oxides need to be annealed or sputtered in order to obtain clean surfaces, as required for photoemission. During such ÔcleaningÕ procedures, the stoichiometry of the surface can be strongly modified, regarding the bulk and often the spectra do not show well defined line shape features. However, even for single crystals is not always easy to prepare clean, high quality surfaces. Cleaving may create local structural distortions on the surface, or oxygen depletion at the surface. This could be the cause for the observed vanishing small spectral weight at the Fermi level for single crystalline samples in the metallic phase. For our study we have fractured the single crystals in ultra-high vacuum conditions at low temperatures. No carbon contamination was detected and the O 1s spectrum showed a single peak, indicating that the surface was not contaminated. Valence band spectra were recorded in ultra-high vacuum (<1 · 1010 mbar) with linearly (horizontal) polarized light using the Gammadata Scienta SES 2002 analyzer with an angular resolution of 0.2. For three-dimensional materials there is no preferential cleaving plane and, therefore, no k-dependent photoemission study of the single crystals was possible. We present k-averaged spectra recorded in normal emission. Measurements were repeated several times to ensure reproducibility of the spectra.
3. Results and discussion Fig. 1 presents the valence band spectra of the single crystal La0.7Sr0.3MnO3 in the ferromagnetic metallic phase (40 K), recorded for photon energies varying from 45 eV to 70 eV. All spectra exhibit similar shapes with several features: a clear tail A from Fermi level to 1.1 eV, a shoulder/peak B at 2.1 eV, two peaks C and D showing little dispersion with hm and a shoulder E at higher binding energies. There is no peak at 9.5–10 eV, as often found when studying polycrystals, thin films or even single crystals, which is associated with ÔdirtÕ
31
Fig. 1. Valence band spectra of the single crystal La0.7Sr0.3MnO3 measured at 40 K with photon energies (hm) from 45 eV to 70 eV. Inset: XPS valence band spectrum of the same sample recorded with Al Ka radiation (1486.6 eV) at room temperature.
or surface modification [9,16,17]. This indicates the clean surface of our single crystal. Moreover, all spectra exhibit a clear cut-off at the Fermi level, as expected for metallic phase [5]. In inset of Fig. 1 we show the valence band (VB) spectrum of the same single crystal measured at room temperature (RT) using the X-ray photoelectron spectroscopy (XPS) technique with AlKa radiation (hm = 1486.6 eV) [22]. The XPS VB spectrum extends over 9–10 eV below the Fermi level. One can recognize the same five structures, as for the spectra recorded with synchrotron radiation of much lower energy. For manganites, the VB spectra consist of Mn 3d and O 2p states. Whereas the Mn 3d states dominate the spectra at lower binding energies (A–C), the O 2p has a major contribution at higher binding energies (C–E) [9]. It is to be noted that the Mn 3d/O 2p relative cross section strongly increases with hm: Mn 3d dominates the spectral shape in XPS spectrum, while contribution from O 2p becomes important at low
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M.C. Falub et al. / Surface Science 575 (2005) 29–34
photon energies [23]. The similarities in the spectra presented in Fig. 1 (45–70 eV and 1486.6 eV) emphasise the important hybridisation of transition metal and ligand states in the valence band. The strong enhancement of the feature A—associated with Mn eg states—observed in the VB XPS spectrum [22] is consistent with the much larger photocross sections of the 3d states for hm = 1486.6 eV [9]. We will analyse now the VB spectra recorded at photon energies in the vicinity of the Mn 3p–3d threshold. For photon energies below 50 eV there are small changes in the spectra: the spectral weight at the Fermi level remains low and the intensity of the shoulder B starts to grow. A further increase in the photon energy causes significant changes in the shape of the spectra. The corresponding states have largely Mn 3d character; this is why they are significantly enhanced at resonance. Feature A and the spectral weight at the Fermi level are strongly enhanced and reach maxima at 56 eV. The spectral intensity at 2.1 eV below Fermi level (feature B) is very much modified: the shoulder in the spectrum at 45 eV develops into a sharp peak with the highest intensity at 56 eV. There is no dispersion for the states ÔBÕ, which indicates their strongly localized character and therefore, we associate them with the occupied t2g" states. In our spectra we did not observe Auger features. The Auger peaks would appear in the valence band spectra at the same kinetic energy. That means, when increasing hm, they should disperse towards higher binding energy. The explanation could be that the M2,3M4,5M4,5 Auger emission was too weak to be detected [24]. To study in more detail the spectral changes related to the resonance process, we have measured VB spectra increasing hm near the Mn 3p–3d threshold with 0.5 eV step. In Fig. 2 we show three representative spectra recorded at 49.0 eV, 51.4 eV and 56 eV, which correspond to off-resonance, resonance onset and resonance maximum, respectively. Fig. 3 shows the difference spectra obtained by subtracting the spectrum at 49 eV from those recorded at higher photon energies. They reflect regions of the VB having strong Mn 3d character. There is a sharp resonant peak (B) at 2.1 eV that
Fig. 2. Valence band spectra of the single crystal La0.7Sr0.3MnO3 in the vicinity of the Mn 3p–3d threshold (49 eV, 51.4 eV and 56 eV). Spectra were normalized to the total area. Inset: zoom-in to illustrate changes that occur near EF.
Fig. 3. Difference spectra obtained by subtracting the valence band spectrum at 49 eV (off-resonance) from those measured at higher photon energies, from 50 eV to 57 eV. Inset: intensity of peak B versus hm. The resonance maximum occurs at hm = 56 eV.
M.C. Falub et al. / Surface Science 575 (2005) 29–34
starts to grow and reaches maximum at 56 eV. When hm increases, the intensity of the tail A firstly decreases (hm < 51.4 eV), is then gradually enhanced and at 56 eV has the highest intensity. In the inset of Fig. 3 the spectral intensity for peak B versus photon energy is plotted: the maximum is reached for 56 eV and the resonance window (onset-maximum) is 4.6 eV. When the energy of the incident photons overcomes the 3p–3d threshold, 3p core electrons can be excited on the partially unoccupied eg" states. Once the photon energy is high enough to promote 3p electrons on the empty t2g# states, the spectral intensities for both features, A and B, are rapidly enhanced. In our experiments the corresponding photon energy is 51.4 eV. The spectral intensity is mostly enhanced when the 3d- occupied and 3dempty states involved in the resonant process have the same orbital symmetry. In our case, the Mn 3p–3d resonance maximum will corresponds to the process in which the occupied t2g" and unoccupied t2g# are involved. From our data (see Figs. 2 and 3), we have determined the maximum of the resonance at 56 eV. The t2g# states are unoccupied and therefore, not directly accessible by photoemission. We have evaluated their energy relative to the Fermi level to be 4.6 eV from the difference between the photon energies that correspond to the resonance-onset (51.4 eV) and maximum (56 eV), respectively. Knowing the binding energy for the Mn 3d t2g" levels, we can now estimate the correlation energy U (t2g" t2g#) to about 6.7 eV. This value shows the importance of the electron–electron interactions for the investigated compound. For a similar perovskite, La2/3Ca1/3MnO3 a value of the on-site Coulomb correlation energy of 6.4 eV was evaluated in a combined analysis of the XPS Mn 2p and Mn L2.3–M2.3M4.5 Auger spectra [25].
4. Conclusions hm-dependent photoemission study of the single crystal La0.7Sr0.3MnO3 is presented. From the analysis of the spectra near Mn 3p–3d threshold, we have determined the electronic states in the valence band having predominant Mn 3d character.
33
Very sharp resonant structures were observed in the valence band spectra in the low binding energy region. The t2g" states are quite localized and appear in the valence band spectra as a distinct structure at 2.1 eV, which is strongly enhanced at resonance. The spectral weight near the Fermi level that originates from the Mn 3d eg states is also enhanced at resonance. Besides information about the Mn 3d occupied states, from the RPES data we have also extracted information about the unoccupied Mn 3d states. The correlation energy was evaluated at 6.7 eV, which underlines the importance of the electron–electron correlations.
Acknowledgment This work was performed at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland. We are grateful to the machine and beamline groups whose outstanding efforts have made these experiments possible. Partial support by the Russian Foundation for Basic Research (Grant No. 04-03-96092-Ural) is acknowledged.
References [1] A.P. Ramirez, J. Phys.: Condens. Matter 9 (1997) 8171 (and the references therein). [2] Y. Tokura, A. Urushibara, Y. Moritomo, T. Arima A. Asamitsu, G. Kido, N. Furukawa, J. Phys. Soc. Jpn. 63 (1994) 3931. [3] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [4] J. Hemberger, A. Krimmel, T. Kurz, H.-A. Krug von Nidda, V.Yu. Ivanov, A.A. Mukhin, A.M. Balbashov, A. Loidl, Phys. Rev. B 66 (2002) 094410. [5] J.-H. Park, E. Vascovo, H.-J. Kim, C. Kwon, R. Ramesh, T. Venkatesan, Nature 392 (1998) 794. , Phys. Rev. Lett. 81 (1998) 1953. [6] Y. Ji, C.L. Chien, Y. Tomioka, Y. Tokura, Phys. Rev. B 66 (2002) 012410. [7] M. Abbate et al., Phys. Rev. B 46 (1992) 4511. [8] A. Chainani, M. Mathew, D.D. Sarma, Phys. Rev. B 47 (1993) 15397. [9] T. Saitoh, A.E. Bouquet, T. Mizokawa, H. Namatame, A. Fujimori, M. Abbate, Y. Takeda, M. Takano, Phys. Rev. B 51 (1995) 13942. [10] T. Saitoh, A. Sekiyama, K. Kobayashi, T. Mizokawa, A. Fujimori, D.D. Sarma, Y. Takeda, M. Takano, Phys. Rev. B 56 (1997) 8836.
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[11] Y. Kuwata et al., J. Electron Spectrosc. Relat. Phenom. 88–91 (1998) 281. [12] D.D. Dessau, Y.D. Chuang, A. Gromko, T. Saitoh, T. Kimura, Y. Tokura, J. Electron Spectrosc. Relat. Phenom. 117–118 (2001) 265. [13] V.R. Mastelaro, D.P.F. de Souza, R. Mesquita, X-ray Spectrom. 31 (2002) 154. [14] Y. Tokura (Ed.), Colossal Magnetoresistive Oxides, vol. 2, Gordon and Breach Science Publishers, 2002. [15] C. Waldfried, D.N. McIlroy, S.-H. Liou, R. Sabiryanov, S.S. Jaswal, P.A. Dowben, J. Phys.: Condens. Matter 9 (1997) 1031. [16] D.N. McIlroy, C. Waldfried, J. Zhang, J.-W. Choi, F. Foong, S.H. Liou, P.A. Dowben, Phys. Rev. B 54 (1996) 17438. [17] R. Liu, W.C. Tonjes, C.G. Olson, J.J. Joyce, A.J. Arko, J.J. Neumeier, J.F. Mitchell, H. Zheng, J. Appl. Phys. 88 (2000) 786.
[18] A. Sekiyama, S. Suga, M. Fujikawa, S. Imada, T. Iwasaki, K. Matsuda, T. Matsushita, Phys. Rev. B 59 (1999) 15528. [19] T. Takeuchi et al., J. Synchrotron Rad. 9 (2002) 237. [20] K.V. Kaznacheyev et al., Phys. Rev. B 58 (1998) 13491. [21] A.M. Balbashov, S.K. Egorov, J. Crystal Growth 52 (1981) 498. [22] M. Demeter, M. Neumann, V.R. Galakhov, N.A. Ovechkina, E.Z. Kurmaev, N.I. Labachevskaya, Acta Phys. Pol. A 98 (2000) 587. [23] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 31 (1985) 1. [24] J. Okabayashi, A. Kimura, T. Mizokawa, A. Fujimori, T. Hayashi, M. Tanaka, Phys. Rev. B 59 (1999) R2486. [25] R. Zalecki, A. Kolodziejczyk, Cz. Kapusta, K. Krop, J. Alloys Compd. 328 (2001) 175.
Photon energy dependent photoemission study of La0.7Sr0.3MnO3 M.C. Falub
a,*
, M. Shi a, J. Krempasky a, K. Hricovini b, Ya.M. Mukovskii c, M. Neumann d, V.R. Galakhov e, L. Patthey a a
e
Swiss Light Source, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland b Universite de Cergy-Pontoise, 95031 Cergy-Pontoise, France c Moscow Institute of Steel and Alloys, Moscow, Russia d Faculty of Physics, University of Osbnabrueck, D-49069 Osnabrueck, Germany Institute of Metal Physics, Ural Division, Russian Academy of Sciences, Yekaterinburg 620219, Russia Received 14 February 2004; accepted for publication 27 October 2004 Available online 28 December 2004
Abstract The electronic structure of the single crystal La0.7Sr0.3MnO3 in ferromagnetic metallic phase has been investigated by photoelectron spectroscopy. Valence band spectra were measured at 40 K for various photon energies, in the range of 45–70 eV. The Mn 3d partial density of states was derived from resonant photoemission near the Mn 3p–3d threshold. Besides the occupied 3d states, resonant photoemission data has provided information about the unoccupied 3d states. The value of the on-site Coulomb interaction strength (correlation energy) was estimated to be 6.7 eV. 2004 Elsevier B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Resonant photoemission; Manganites; Electron correlation
1. Introduction In the last decade there has been a considerable interest in the doped manganese perovskites due to the colossal magnetoresistance (CMR) effect *
Corresponding author. Address: EPFL SB IPN LSE, Institut de Physique des Nanostructures, Ph A1 397 (Baˆtiment PH), Station 3, CH-1015 Lausanne, Switzerland. Tel.: +41 21 693 3399; fax: +41 21 693 3604. E-mail address: mihaelacarmen.falub@epfl.ch (M.C. Falub).
[1]. Among them, the Sr-doped compounds La1xSrxMnO3 exhibit interesting and quite unusual transport and magnetic properties [2–4]. The parent compound LaMnO3 is an A-type antiferromagnetic insulator (TN = 140 K), in which the MnO2 ferromagnetic layers are stacked along c-axis with alternating spin directions. In an ionic picture, the electronic configuration of the Mn3+ ions is 3d4 (t32g e1g ). The crystal field splits the Mn 3d orbitals into the threefold degenerate t2g and twofold degenerate eg orbitals. Whereas the
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.10.053
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M.C. Falub et al. / Surface Science 575 (2005) 29–34
eg orbitals, pointing directly towards oxygen are strongly hybridized with the O 2p states, the t2g orbitals are less hybridized with the ligand states and can be regarded as localized (S = 3/2). Not only the t2g electrons, but also the eg electrons are strongly affected by the correlation effect and tend to localize (Mott insulator). However, when La3+ is substituted by Sr2+ (or another divalent ion) holes are created in the Mn eg orbital states and the eg electrons become itinerant (conduction electrons). The series La1xSrxMnO3 exhibits a very rich phase diagram, including electronic (insulator/metal), magnetic (antiferromagnetic/ferromagnetic/ paramagnetic) and structural (orthorhombic/rhombohedral/tetragonal/monoclinic/hexagonal) transitions [3,4]. For Sr-content from x = 0.2 to x = 0.5, colossal magnetoresistance was reported [2]. At 30% Sr-doping, besides the colossal magnetoresistance effect, half-metallicity was also reported [5], which makes this material particularly interesting, being a possible candidate for applications in magnetic devices. However, while the spin-resolved photoemission study of the thin film La0.7Sr0.3MnO3 has shown half-metallic behaviour [5], other investigations of single crystals of the same composition have found spin-polarization lower than 100%, as one would expect for a half-metal [6]. Although many photoemission studies have been performed for polycrystalline samples, thin films and, more recently, single crystals of La1xSrxMnO3 [7–14], their electronic structure is not yet fully understood. Photoemission spectroscopy near the Mn 3p–3d threshold can provide information about the electronic structure [15–18]. For 3d transition metal compounds, the 3d photoemission is strongly enhanced when the energy of the incoming light equals the energy necessary to excite a 3p electron to an unoccupied 3d state. The intermediate excited state may decay into a final state identical to that obtained after the direct 3d photoemission. The interference of the direct 3d photoemission and the 3p–3d transition followed by a 3p–3d–3d Coster–Kronig decay is denoted as resonant photoemission (RPES) at the 3p–3d threshold. In previous RPES studies of manganites at the Mn 3p–3d threshold, valence band spectra were re-
corded at two photon energies that correspond to ÔOnÕ- and ÔOff’’-resonance and from their difference the Mn 3d-derived partial density of states was deduced [9,15,16]. For polycrystalline samples of LaMnO3 and SrMnO3 an enhancement of the valence band spectra at 10–15 eV and an apparent decrease at 1–7 eV binding energy was found at resonance, the latter being related by the authors to photoionization cross section effects [9]. Photoemission spectra were measured at room temperature for thin films of La0.65Ba0.35MnO3 (Tc = 330 K) using photons of 47 eV and 52 eV energies and Mn 3d-derived bands were determined at 0– 3.5 eV and 5–7 eV below Fermi level, respectively [15,16]. Similarly, for thin films of La0.65Ca0.35MnO3 resonant enhancement of the valence band over a broad energy region (about 8 eV) was reported above, as well as below the critical temperature Tc = 260 K [16]. However, quite different results have recently been obtained for the single crystal La0.66Ca0.33MnO3 (Tc = 258 K) at 80 K using Mn 3p–3d resonant photoemission [17]. Here, a gradual enhancement of the valence band intensity at 2–4 eV below Fermi level was observed when hm increases, reaching maximum for photons of 55 eV energy. There are only few Mn 3p–3d RPES data reported for single crystals [18–20]. We present a photon energy dependent photoemission study of the La0.7Sr0.3MnO3 single crystal. In order to derive the Mn 3d contribution into the valence band we have measured valence band spectra near the Mn 3p absorption edge. Moreover, from RPES data we have obtained information about the Mn 3d unoccupied states and evaluated the on-site Coulomb interaction strength (correlation energy) of the 3d electrons.
2. Experimental Photon energy dependent photoemission spectra of La0.7Sr0.3MnO3 near the Mn 3p–3d threshold were recorded at Surface and Interface Spectroscopy beamline of the Swiss Light Source. Single crystals of La0.7Sr0.3MnO3 were grown by floating-zone method [21]. In general, single crystal samples yield more detailed and more reliable
M.C. Falub et al. / Surface Science 575 (2005) 29–34
information than polycrystals. In case of polycrystals, quite different results were reported by several groups. This could be related to the possible impure phases between boundaries and/or the migration of internal contaminants. Thin films of oxides need to be annealed or sputtered in order to obtain clean surfaces, as required for photoemission. During such ÔcleaningÕ procedures, the stoichiometry of the surface can be strongly modified, regarding the bulk and often the spectra do not show well defined line shape features. However, even for single crystals is not always easy to prepare clean, high quality surfaces. Cleaving may create local structural distortions on the surface, or oxygen depletion at the surface. This could be the cause for the observed vanishing small spectral weight at the Fermi level for single crystalline samples in the metallic phase. For our study we have fractured the single crystals in ultra-high vacuum conditions at low temperatures. No carbon contamination was detected and the O 1s spectrum showed a single peak, indicating that the surface was not contaminated. Valence band spectra were recorded in ultra-high vacuum (<1 · 1010 mbar) with linearly (horizontal) polarized light using the Gammadata Scienta SES 2002 analyzer with an angular resolution of 0.2. For three-dimensional materials there is no preferential cleaving plane and, therefore, no k-dependent photoemission study of the single crystals was possible. We present k-averaged spectra recorded in normal emission. Measurements were repeated several times to ensure reproducibility of the spectra.
3. Results and discussion Fig. 1 presents the valence band spectra of the single crystal La0.7Sr0.3MnO3 in the ferromagnetic metallic phase (40 K), recorded for photon energies varying from 45 eV to 70 eV. All spectra exhibit similar shapes with several features: a clear tail A from Fermi level to 1.1 eV, a shoulder/peak B at 2.1 eV, two peaks C and D showing little dispersion with hm and a shoulder E at higher binding energies. There is no peak at 9.5–10 eV, as often found when studying polycrystals, thin films or even single crystals, which is associated with ÔdirtÕ
31
Fig. 1. Valence band spectra of the single crystal La0.7Sr0.3MnO3 measured at 40 K with photon energies (hm) from 45 eV to 70 eV. Inset: XPS valence band spectrum of the same sample recorded with Al Ka radiation (1486.6 eV) at room temperature.
or surface modification [9,16,17]. This indicates the clean surface of our single crystal. Moreover, all spectra exhibit a clear cut-off at the Fermi level, as expected for metallic phase [5]. In inset of Fig. 1 we show the valence band (VB) spectrum of the same single crystal measured at room temperature (RT) using the X-ray photoelectron spectroscopy (XPS) technique with AlKa radiation (hm = 1486.6 eV) [22]. The XPS VB spectrum extends over 9–10 eV below the Fermi level. One can recognize the same five structures, as for the spectra recorded with synchrotron radiation of much lower energy. For manganites, the VB spectra consist of Mn 3d and O 2p states. Whereas the Mn 3d states dominate the spectra at lower binding energies (A–C), the O 2p has a major contribution at higher binding energies (C–E) [9]. It is to be noted that the Mn 3d/O 2p relative cross section strongly increases with hm: Mn 3d dominates the spectral shape in XPS spectrum, while contribution from O 2p becomes important at low
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photon energies [23]. The similarities in the spectra presented in Fig. 1 (45–70 eV and 1486.6 eV) emphasise the important hybridisation of transition metal and ligand states in the valence band. The strong enhancement of the feature A—associated with Mn eg states—observed in the VB XPS spectrum [22] is consistent with the much larger photocross sections of the 3d states for hm = 1486.6 eV [9]. We will analyse now the VB spectra recorded at photon energies in the vicinity of the Mn 3p–3d threshold. For photon energies below 50 eV there are small changes in the spectra: the spectral weight at the Fermi level remains low and the intensity of the shoulder B starts to grow. A further increase in the photon energy causes significant changes in the shape of the spectra. The corresponding states have largely Mn 3d character; this is why they are significantly enhanced at resonance. Feature A and the spectral weight at the Fermi level are strongly enhanced and reach maxima at 56 eV. The spectral intensity at 2.1 eV below Fermi level (feature B) is very much modified: the shoulder in the spectrum at 45 eV develops into a sharp peak with the highest intensity at 56 eV. There is no dispersion for the states ÔBÕ, which indicates their strongly localized character and therefore, we associate them with the occupied t2g" states. In our spectra we did not observe Auger features. The Auger peaks would appear in the valence band spectra at the same kinetic energy. That means, when increasing hm, they should disperse towards higher binding energy. The explanation could be that the M2,3M4,5M4,5 Auger emission was too weak to be detected [24]. To study in more detail the spectral changes related to the resonance process, we have measured VB spectra increasing hm near the Mn 3p–3d threshold with 0.5 eV step. In Fig. 2 we show three representative spectra recorded at 49.0 eV, 51.4 eV and 56 eV, which correspond to off-resonance, resonance onset and resonance maximum, respectively. Fig. 3 shows the difference spectra obtained by subtracting the spectrum at 49 eV from those recorded at higher photon energies. They reflect regions of the VB having strong Mn 3d character. There is a sharp resonant peak (B) at 2.1 eV that
Fig. 2. Valence band spectra of the single crystal La0.7Sr0.3MnO3 in the vicinity of the Mn 3p–3d threshold (49 eV, 51.4 eV and 56 eV). Spectra were normalized to the total area. Inset: zoom-in to illustrate changes that occur near EF.
Fig. 3. Difference spectra obtained by subtracting the valence band spectrum at 49 eV (off-resonance) from those measured at higher photon energies, from 50 eV to 57 eV. Inset: intensity of peak B versus hm. The resonance maximum occurs at hm = 56 eV.
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starts to grow and reaches maximum at 56 eV. When hm increases, the intensity of the tail A firstly decreases (hm < 51.4 eV), is then gradually enhanced and at 56 eV has the highest intensity. In the inset of Fig. 3 the spectral intensity for peak B versus photon energy is plotted: the maximum is reached for 56 eV and the resonance window (onset-maximum) is 4.6 eV. When the energy of the incident photons overcomes the 3p–3d threshold, 3p core electrons can be excited on the partially unoccupied eg" states. Once the photon energy is high enough to promote 3p electrons on the empty t2g# states, the spectral intensities for both features, A and B, are rapidly enhanced. In our experiments the corresponding photon energy is 51.4 eV. The spectral intensity is mostly enhanced when the 3d- occupied and 3dempty states involved in the resonant process have the same orbital symmetry. In our case, the Mn 3p–3d resonance maximum will corresponds to the process in which the occupied t2g" and unoccupied t2g# are involved. From our data (see Figs. 2 and 3), we have determined the maximum of the resonance at 56 eV. The t2g# states are unoccupied and therefore, not directly accessible by photoemission. We have evaluated their energy relative to the Fermi level to be 4.6 eV from the difference between the photon energies that correspond to the resonance-onset (51.4 eV) and maximum (56 eV), respectively. Knowing the binding energy for the Mn 3d t2g" levels, we can now estimate the correlation energy U (t2g" t2g#) to about 6.7 eV. This value shows the importance of the electron–electron interactions for the investigated compound. For a similar perovskite, La2/3Ca1/3MnO3 a value of the on-site Coulomb correlation energy of 6.4 eV was evaluated in a combined analysis of the XPS Mn 2p and Mn L2.3–M2.3M4.5 Auger spectra [25].
4. Conclusions hm-dependent photoemission study of the single crystal La0.7Sr0.3MnO3 is presented. From the analysis of the spectra near Mn 3p–3d threshold, we have determined the electronic states in the valence band having predominant Mn 3d character.
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Very sharp resonant structures were observed in the valence band spectra in the low binding energy region. The t2g" states are quite localized and appear in the valence band spectra as a distinct structure at 2.1 eV, which is strongly enhanced at resonance. The spectral weight near the Fermi level that originates from the Mn 3d eg states is also enhanced at resonance. Besides information about the Mn 3d occupied states, from the RPES data we have also extracted information about the unoccupied Mn 3d states. The correlation energy was evaluated at 6.7 eV, which underlines the importance of the electron–electron correlations.
Acknowledgment This work was performed at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland. We are grateful to the machine and beamline groups whose outstanding efforts have made these experiments possible. Partial support by the Russian Foundation for Basic Research (Grant No. 04-03-96092-Ural) is acknowledged.
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