Modulation of manganite nano-film properties ... - ACS Publications

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Modulation of manganite nano-film properties mediated by strong influence of strontium titanate excitons Xinmao Yin, Chi Sin Tang, Muhammad Aziz Majidi, Peng Ren, Le Wang, Ping Yang, Caozheng Diao, Xiaojiang Yu, Mark B.H. Breese, Andrew Thye Shen Wee, Junling Wang, and Andrivo Rusydi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15347 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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ACS Applied Materials & Interfaces

Modulation of manganite nano-film properties mediated by strong influence of strontium titanate excitons Xinmao Yin†,‡,∥, Chi Sin Tang†,∥,#, Muhammad Aziz Majidi†,∥,∓, Peng Ren⊥, Le Wang⊥, Ping Yang†, Caozheng Diao†, Xiaojiang Yu†, Mark B. H. Breese†, ∥, Andrew T. S. Wee∥,#,∇, Junling Wang⊥,*, and Andrivo Rusydi†,§,∥,#,* †

Singapore Synchrotron Light Source (SSLS), National University of Singapore, 5 Research Link, 117603, Singapore ‡

SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China §

NUSNNI-NanoCore, National University of Singapore, 117411, Singapore

∥Department

of Physics, Faculty of Science, National University of Singapore, 117542, Singapore

⊥School

of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore #

NUS Graduate School for Integrative Sciences and Engineering, 117456, Singapore

∇ Centre

for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 117551, Singapore. ∓Departemen

Fisika, FMIPA, Universitas Indonesia, Depok 16424, Indonesia

KEYWORDS: manganite thin film, neutral and charged excitonic effects, electronic and optical structures, spectroscopic ellipsometry, electron-electron and electron-hole interactions.

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ABSTRACT: Hole-doped perovskite manganites have attracted much attention because of their unique optical, electronic and magnetic properties induced by the interplay between spin, charge, orbital and lattice degrees of freedom. Here, a comprehensive investigation of the optical, electronic and magnetic properties of La0.7Sr0.3MnO3 thin-films on SrTiO3 (LSMO/STO) and other substrates is conducted using a combination of temperature-dependent transport, spectroscopic ellipsometry, X-ray absorption spectroscopy and X-ray magnetic circular dichroism. A significant difference in the optical property of LSMO/STO that occurs even in thick (87.2nm) LSMO/STO from that of LSMO on other substrates is discovered. Several excitonic features are observed in thin-film nanostructure LSMO/STO at ~4eV, which could be attributed to the formation of anomalous charged excitonic complexes. Based on spectral-weight transfer analysis, anomalous excitonic effects from STO strengthen the electronic-correlation in LSMO films. This results in the occurrence of optical spectral changes related to the intrinsic Mott-Hubbard properties in manganites. We find that while lattice strain from the substrate influences the optical properties of the LSMO thin-films, the coexistence of strong electron-electron (e-e) and electron-hole (e-h) interactions which leads to the resonant excitonic effects from the substrate play a much more significant role. Our result shows that the onset of anomalous excitonic dynamics in manganite oxides may potentially generate new approaches in manipulating exciton-based optoelectronic applications.

INTRODUCTION


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With their unique optical, magnetic and electronic properties, perovskite manganites have potential applications in devices such as magnetic field sensors, infrared detectors, memory, and spintronic devices1-4. Detailed studies have been conducted on La0.7Sr0.3MnO3(LSMO) for applications in efficient spin injection in organic spin-valves5 and giant tunneling magnetoresistance in magnetic tunnel junctions6. Optical spectroscopy is a powerful technique for investigating the fundamental mechanisms associated with the electronic structure of strongly-correlated systems. It has been used extensively to study various novel physical phenomena of perovskite manganites7-11. Of interest are manganite thin-films in which the lattice constant mismatch between the films and substrate play a significant role in modifying the film’s optical and electronic properties. Unusual phenomena have been observed in the electronic, magnetic and optical properties of manganite thin-films in contact with substrates such as DyScO3(DSO) and LaAlO3 due to large lattice constant mismatches11-17. While the role of lattice constant mismatch has been extensively studied, the influence of electronic correlations from substrates remains unexplored.

Recent studies have shown the importance of electron-electron(e-e) and electron-hole(e-h) interactions yielding to anomalous excitonic effects in STO18. The strong many-body interactions can be manifested in the form of strongly bound neutral and charged excitonic complexes in nanostructures.19-21 Excitons are photo-induced quasiparticles that consist of electrons and holes in a bound state. They are pivotal in exciton-based optoelectronic applications such as light-emitting diodes22-23, optical interconnectors24 and quantum logical devices25-26. Interestingly, when thinfilms such as graphene were grown on STO substrate, the film’s electronic-correlation was modified and it leads to significant renormalization of the film optical properties27. In the case of


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LSMO, it is commonly assumed that STO would have minimal effect on the properties of LSMO thin-films due to comparable lattice spacing. However, this has yet to be carefully investigated.

Here, we observe significant differences in the optical properties involving LSMO grown on STO and other substrates which cannot be explained by epitaxial strain alone as in previous studies. Unique features in the form of split peaks have been observed in the UV-Vis energy regime (~4eV) of the ultra-thin nanostructure LSMO/STO. These unique features are attributed to the onset of anomalous charged excitons. The excitonic effects from the STO substrate are believed to contribute to the anomalous optical properties and electronic-correlation of LSMO thin-films. This in-depth investigation not only provides a deeper understanding towards electronic correlations at heterointerfaces not accounted for by lattice strain, it also unlocks new opportunities for photonic and optoelectronic applications based on excitonic effects in LSMO.

We perform a comprehensive analysis on LSMO grown on STO and other substrates using a combination of experimental techniques including spectroscopic ellipsometry, X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and transport measurements. This comprehensive methodology allows for a complete spectral-weight transfer analysis28. Due to the strong local interactions and hybridization in manganites, spectral-weight transfers that is associated with charge-transfer or Mott-Hubbard physics can take place over a broad energy range29-31. We find that the effects of strain on the film optical conductivity and electronic correlation are significantly different from that of excitonic effects caused by the substrate.


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EXPERIMENTAL High-quality La0.7Sr0.3MnO3 thin-films are grown by pulsed laser deposition on atomically smooth [001]-oriented

SrTiO3,

[110]-oriented

([001]PC)

DyScO3

and

[001]-oriented

(LaAlO3)0.3(Sr2AlTaO6)0.7 single-crystal substrates. Four samples are prepared: 11.2nm LSMO/STO, 87.2nm LSMO/STO, 12.6nm LSMO/DSO and 9.8nm LSMO/LSAT. The crystallographic structure of La0.7Sr0.3MnO3 films are characterized by High-resolution X-ray diffractometry (HR-XRD) in the XDD beamline at Singapore Synchrotron Light Source (SSLS). HR-XRD study reveals fully-strained and coherent interfaces between all LSMO films and their respective substrates. The thickness, lattice constant and strain are obtained. (see Supporting Information Section S4) The strain value of each film are as follows: 𝜖𝑎 =

𝑎𝑠𝑡𝑟𝑎𝑖𝑛𝑒𝑑 −𝑎𝑏𝑢𝑙𝑘 𝑎𝑏𝑢𝑙𝑘

, 0.6%

(11.2 and 87.2nm LSMO/STO); 1.9% (12.6nm LSMO/DSO); -0.3% (9.8nm LSMO/LSAT). Transport measurements are performed using a low-temperature probe station system (Janis Pte. Ltd.). The 11.2nm LSMO/STO sample shows a monotonic metallic behavior below the metalinsulator transition (MIT) temperature of TMIT~325K (see Supporting Information Fig. S9). The spectroscopic ellipsometer (photon energy range: from 0.4 to 5.6eV) is used to measure the ellipsometric parameters Ψ and Δ (the ratio and phase difference between the p- and s-polarized reflected light) at 70o incident angle (Figure 1(c)). Dielectric functions and optical conductivity have been extracted from Ψ and Δ by utilizing an air/LSMO/substrate (STO, LSAT, or DSO) multilayer model (see Supporting Information Section S5). X-ray absorption spectroscopic (XAS) data at the O K-edge and Mn L3,2-edges are obtained using the total-electron-yield detection method at the SINS beamline of the Singapore Synchrotron Light Source. X-ray magnetic circular dichroism(XMCD) utilizes the differences in helicities of circularly polarized light when the


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samples are applied with an external magnetic field of 1T and -1T at θ=60o. The degree of circular polarization (Pc) was 88%. RESULTS Differences in optical spectra of LSMO on different substrates Figure 1a shows the optical conductivity, σ1(ω), of the LSMO films on different substrates in comparison with that of bulk LSMO32 at low temperature. At the energy region below 2.1eV (Region I in Figure 1a) where the spectral-weight is attributed mainly to the Jahn-Teller and charge-transfer excitations32-33, we observe distinct increment in spectral-weights from bulk LSMO to LSMO films on different substrates. This corresponds to the respective increase in strain values caused by the different substrates (0 strain for bulk LSMO). This shows that both JahnTeller and charge-transfer effects are amplified as strain increases. In comparison with the optical conductivity spectra of bulk LSMO, the increased spectral-weight of LSMO thin-films in Region 𝐼 is due to a transfer from a higher energy range (Region 𝐼𝐼). Interestingly, the optical conductivity above 3.7eV (Region 𝐼𝐼𝐼) for thin LSMO/STO is much lower than that of the LSMO/LSAT and LSMO/DSO samples. The optical conductivity in Region 𝐼𝐼𝐼 for the thick LSMO/STO lies in between that of the thin LSMO/STO, LSMO/LSAT, LSMO/DSO as well as bulk LSMO32. According to f-sum rule, the steep drop in optical conductivity for LSMO/STO in Region 𝐼𝐼𝐼 is due to a spectral-weight shift to a significantly higher energy regime above the instrument measurement range. This indicates that the electroniccorrelations of the respective LSMO film on the STO substrate have been dramatically modified. Despite having small interfacial strains in the thin and thick LSMO/STO samples and the LSMO/LSAT sample, their corresponding spectral-weight in Region III differ significantly. The


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spectral-weight of LSMO/DSO lies close to that of bulk LSMO despite the large strain in the LSMO/DSO sample (1.9%). These indicate that strain is not the main factor affecting the optical spectra in Region III.

Excitonic effects from STO substrate Strong temperature-dependent optical features in STO in Region 𝐼𝐼𝐼 are observed (Figure 2). Optical structures at ~3.8 and ~4.7eV are strongly enhanced as temperature decreases in the STO substrate spectra (Figure 2c, d). These features are absent in the measured energy range of DSO and LSAT substrates because of their wider bandgap (Figure 1b). They are identified as delocalized Wannier-like and resonant excitons respectively34. Therefore, we argue that the difference in optical conductivity in Region 𝐼𝐼𝐼 of Figure 1a is attributed to the excitonic effects from the STO substrate as discussed below. With the propagation of STO excitons into the LSMO films, it affects the electronic-correlation in the LSMO film (Figure 1c, d). Figure 1b displays σ1(ω) of different substrates and all three of them show a near-constant behavior in Regions I and II. However, the optical spectrum for the STO substrate differs significantly from the other substrates in Region III. It is important to note that STO exhibits strong excitonic effects in this region34. Comparing these data with those in Figure 1a, the differences in the optical features of the LSMO films in Regions I and II are due to the differing lattice strains while the strong excitonic effect in STO dominates the optical conductivity of LSMO thin-film in Region III.

Temperature-dependent optical spectra


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To provide more details on how different substrates may influence the optical properties of LSMO thin-films, we make a comparison between the temperature-dependent dielectric functions (ε(ω)=ε1 (ω)+iε2 (ω)) of LSMO/DSO and thin LSMO/STO (Figure 3a, b). We notice that there are differences at the low (3.7eV). At the low-energy range, its optical behavior shows that as temperature decreases, LSMO/DSO does not transform into the metallic state while LSMO/STO does. For LSMO/STO below 260K, the plasma frequency, ωp, at ε1 (ωp)=0 is elucidated35; this supports the metallicity of LSMO/STO at lowtemperature36. Dielectric functions at the low-energy region are consistent with the transport property (see Supporting Information Section S6). In the intermediate region, LSMO/STO has a slightly higher (lower) ε1 (ε2)—both show stronger temperature-dependence than LSMO/DSO. In the high-energy region, ε1 (ε2) of thin LSMO/STO is much higher (lower) than that of LSMO/DSO. The optical differences in the intermediate and high-energy regions are attributed to the differing lattice strains and excitonic effects respectively. To provide a more detailed analysis using f-sum rule31, we present σ1(ω) of thin LSMO/STO in Figure 3c along with their corresponding differences, Δσ1(ω,T)=σ1(ω,T)-σ1(ω,350K), in Figure 3d. For the LSMO/STO sample at 350K (above the metal-insulator transition temperature, TMIT), comparing ε2 (Figure 3b), σ1, and Δσ1 spectra, three optical features appear at the low-energy region (

Modulation of manganite nano-film properties ... - ACS Publications

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