Superconductivity of rocksalt structure LaO epitaxial thin film

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Superconductivity of rocksalt structure LaO epitaxial thin film Kenichi Kaminaga, Daichi Oka, Tetsuya Hasegawa, and Tomoteru Fukumura J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03009 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Journal of the American Chemical Society

Superconductivity of rocksalt structure LaO epitaxial thin film Kenichi Kaminaga,†,‡ Daichi Oka,‡ Tetsuya Hasegawa† and Tomoteru Fukumura‡,§,* † ‡

Department of Chemistry, The University of Tokyo, Tokyo 113-0033, Japan Department of Chemistry, Tohoku University, Sendai 980-8578, Japan

§WPI-Advanced

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Supporting Information Placeholder film during deposition was in-situ observed with reflection high energy electron diffraction. After deposition, the film surface was insitu capped with about 3 nm thick AlOx layer at room temperature to prevent the oxidation. The typical film thickness was approximately 20 nm. More comprehensive growth conditions are summarized in Fig. S1. La metal thin film on CaF2 (111) substrate, La2O3 thin film on c-sapphire substrate were also deposited as references. X-ray diffraction (XRD; D8 DISCOVER, Bruker AXS) measurements with Cu K1 radiation including two-dimensional reciprocal space mapping were used for structural analyses. X-ray photoelectron spectroscopy (XPS; PHI5000 VersaProbe, ULVAC-PHI) using monochromated Al K source equipped with Ar ion sputtering was used for depth profiling measurements of ionic valence and composition, where the peak positions were calibrated by C 1s peak position (284.8 eV). Resistivity and Hall effect were measured to evaluate carrier density (n), mobility (), and upper critical magnetic field under various magnetic fields by physical property measurement system equipped with sample rotator system (PPMS, Quantum Design), using Hall bar-shaped sample with 1 mm wide and 3 mm long. Out-of-plane and in-plane upper critical magnetic fields were evaluated at the onset of the transition in temperature dependence of resistivity at out-of-plane and in-plane magnetic fields, respectively. Magnetization measurements were conducted by superconducting quantum interference magnetometer (MPMS2, Quantum Design). Experimental and physical parameters for all samples are summarized in Table S1. Data shown in Figs. 1 and 2 were obtained from sample D.

ABSTRACT: We report superconducting transition in LaO epitaxial thin film with the onset of superconducting transition temperature (Tc) at around 5 K. The Tc is the highest among lanthanum monochalcogenides opposite to the chemical trend of their Tc: 0.84, 1.02, and 1.48 K for LaX (X = S, Se, Te), respectively. The carrier control resulted in a dome-shaped Tc as a function of electron carrier density. In addition, the Tc was significantly sensitive to epitaxial strain in spite of its highly symmetric crystal structure. This rocksalt superconducting LaO could be a building block to design novel superlattice superconductors.

Rocksalt structure LaO is recognized as an insulating block layer embedded in mother compounds of high-temperature superconducting layered perovskite cuprates (La,M)2CuO4 (M: Ba, Sr).1,2 On the other hand, lanthanum monoxide bulk polycrystal, synthesized several decades ago, has shown ordinary metallic conduction,3,4 and the further investigation has been scarcely seen probably due to their poor chemical stability. Recently, various rare earth monoxide epitaxial thin films have been synthesized owing to state-ofthe-art oxide thin film epitaxy, unveiling unique electronic functionalities in rare earth monoxides. For example, YO with unusual valence of Y2+, which has been known as a gas phase,5 was found to be a rocksalt narrow gap semiconductor with tunable electrical conduction.6 Also, SmO with valence fluctuating state between Sm2+ and Sm3+ was proposed to be a heavy fermion metallic system even in the absence of high pressure,7 in contrast with SmX (X = S, Se, Te).8 In this study, we report on the discovery of superconductivity in LaO epitaxial thin film with the highest Tc of about 5 K among lanthanum monochalcogenides LaX (X = S, Se, Te), opposite to the chemical trend of their Tc: 0.84, 1.02, and 1.48 K, respectively.9 The Tc of LaO is variable with carrier density and lattice strain. LaO (001) epitaxial thin films were deposited on YAlO3 (110) (a = 5.176 Å, b = 5.307 Å, c = 7.355 Å), LaSrAlO4 (001) (a = 3.745 Å), and LaAlO3 (001) (a = 3.79 Å) substrates by pulsed laser deposition method. La metal pellet of three nines purity was used as a target. All oxide substrates were annealed in air at 1200 C for 2 h to obtain step-and-terrace surface prior to film deposition. O2 gas of five nines purity was introduced during deposition, in which oxygen partial pressure during deposition PO2 was controlled from  1  10−8 to 4  10−8 Torr for varying the amount of oxygen vacancy, monitored with quadrupole mass analyzer. LaO thin films (samples A−I) were deposited at 280 C using KrF excimer laser ( = 248 nm) with energy density of 1.0 J/cm2 and repetition ratio of 10 Hz. Sample J was deposited at 300 C. The streak pattern of LaO thin

Out-of-plane −2 XRD pattern of LaO (001) epitaxial thin film on YAlO3 (110) substrate showed the c-axis orientation of rocksalt LaO (Fig. 1(a)). The full width at half maximum of rocking curve of LaO 002 peak was 0.166 (Fig. S2), comparable to that of highly crystalline EuO epitaxial thin film on YAlO3 (110) substrate.10,11 Two-dimensional reciprocal space mapping of the LaO thin film (Fig. 1(b)) represented the epitaxial relationship of LaO [001] || YAlO3 [110] and LaO [110] || YAlO3 [001], similar to EuO thin film,12 as schematically shown in Fig. 1(c). The compressive strain along in-plane of LaO due to the smaller in-plane lattice constants of YAlO3 substrate caused the expansion of the c-axis, resulting in the tetragonally distorted rocksalt structure (a  b = 5.198 Å, c  5.295 Å). The ionic radius of 6-coordinated La ion in LaO evaluated from cube root of the unit cell volume was 1.215 Å, being comparable with that of 6-coordinated La2+ (1.25 Å) deduced from an empirical formula.13 XPS showed that the peak position of La 3d was located between those of La metal (La0) and La2O3 (La3+), uggesting divalent state of La2+ in LaO (Fig. 1(d)). Nearly oxygen stoichiometric composition as well as the absence of impurity element was also confirmed by XPS (Fig. S3).

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Figure 1. Structural and spectroscopic properties of LaO (001) epitaxial thin film. (a) −2 XRD pattern for LaO thin film on YAlO3 (110) substrate. (b) Reciprocal space mapping around 224 diffraction peak of LaO thin film and 334 diffraction peak of YAlO3 substrate, representing the epitaxial relationships of LaO [001] || YAlO3 [110] and LaO [110] || YAlO3 [001]. (c) Schematic crystal structure of LaO thin film on YAlO3 substrate. LaO unit cell is rotated 45 degrees with respect to the YAlO3 pseudocubic cell. The structure was drawn by the VESTA program.23 (d) La 3d XPS spectrum (open symbols) with fitting curves for La metal polycrystalline thin film on CaF2 (111) substrate (top panel), LaO thin film on YAlO3 (110) substrate (middle panel), and La2O3 polycrystalline thin film on c-sapphire substrate (bottom panel).

Figure 2. Superconductivity of LaO (001) epitaxial thin film on YAlO3 substrate. (a) Temperature dependence of resistivity (solid curves) for LaO thin film at 0 T and 9 T. A dashed curve is fitting result of the Bloch–Grüneisen term (see text). (b) Temperature dependence of normalized resistivity /10K at out-of-plane (upper panel) and in-plane (bottom panel) magnetic fields. (c) In-plane (Hc2//) and out-of-plane (Hc2) upper critical magnetic fields as a function of temperature for LaO thin films obtained from ρ−T curves (Fig. 2(b)). Solid and dashed curves denote the fitting results with the Werthamer-Helfand-Hohenberg model for Hc2// and Hc2, respectively.

Temperature (T) dependence of resistivity () for LaO thin film at 0 T showed metallic conduction accompanied by the superconducting transition with the onset Tc, Tconset = 4.56 K, and the zero resistance Tc, Tczero = 3.70 K, while the superconductivity was completely suppressed at 9 T (Fig. 2(a)). The normal state resistivity was well fitted to the following formula,   (0) +BG(T) E(T), similar to 4d and 5d metallic oxides RuO2 and IrO2,14 where (0), BG(T), and E(T) are the residual resistivity, the usual Bloch–Grüneisen term due to the coupling of electrons with acoustic-mode phonons, and an additional contribution due to the coupling of electrons with optical-mode phonons, respectively (see Supporting Information). The Debye temperature D of 262 K (sample D in Table S1) evaluated by −T fitting (dashed curve in Fig. 2(a)) was comparable with that of LaS (D  276 K),15 while the electron-phonon coupling parameter λe-p of 0.65 evaluated by the McMillan equation16 was higher than those of LaX series (λe-p  0.52, 0.42, 0.34 for X = S, Se, Te, respectively).15 The magnetization measurements showed the type-II superconductivity with approximately full superconducting volume fraction, 81 % at 1.8 K (Fig. S4). Tc was gradually decreased with increasing magnetic field (Fig. 2(b)). The superconductivity was fully suppressed at out-of-plane magnetic field of 3 T (upper panel of Fig. 2(b)) and at in-plane magnetic field of 5 T (bottom panel of Fig. 2(b)). The upper critical field (0Hc2) at 2 K was evaluated as 0Hc2 = 2.0 T and 0Hc2// = 4.1 T, respectively (see Fig. S5). Dependence of 0Hc2 on normalized temperature by Tconset for each magnetic field direction is

shown in Fig. 2(c). According to 0Hc2 vs. T fitting with the Werthamer-Helfand-Hohenberg model (see Supporting Information),17 0Hc2 at absolute zero was calculated as 0Hc2(0) = 2.97 T and 0Hc2//(0) = 4.30 T, then the anisotropy ratio  =Hc2(0)/Hc2(0) was 1.45. The in-plane coherent length  = (0 / 20Hc2⊥(0))1/2 = 10.5 nm (0: the flux quantum) was shorter than the superconducting layer thickness d = (√30) / (0Hc2//(0)) = 25.2 nm,18 suggesting 3D superconductivity of the LaO (001) epitaxial thin film The electron carrier density was controlled by changing the amount of oxygen vacancy in LaO. The c-axis length monotonically increased with decreasing oxygen pressure during deposition (PO2) while the a- and b- axes lengths unchanged (Fig. S6 and Table S1), resulting in systematic change in carrier density approximately proportional to the unit cell volume (Fig. 3(a)). The range of carrier density (n) was 4.8  1021–2.0  1022 cm3 at 300 K without significant temperature dependence, and the range of mobility () was 3.9–6.3 cm2/V∙s at 300 K (Fig. S7 and Table S1), similar to those of LaX series (except LaS with high mobility).19 Figure 3(b) shows −T curves for LaO (001) epitaxial thin films with different carrier density. Figure 3(c) shows the Tconset as a function of n300K. Above n300K = 5  1021 cm3, LaO was superconducting and Tconset showed a maximum at around n300K = 1.6  1022 cm3, exhibiting a domeshaped Tconset vs. n300K. The density of states at the Fermi level N(EF) evaluated from the initial slope of Hc2(T) and the residual resistivity20 (see Fig. S8 and Table S1) increased with increasing carrier density, being much larger than those of LaX series15 (Fig.

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Figure 4. Lattice strain dependences of properties of LaO (001) epitaxial thin films. (a) Reciprocal space mapping around 224 diffraction peak of LaO thin films for sample G on YAlO3, sample H on LaAlO3, sample I on LaSrAlO4, and sample J on LaAlO3 substrates. The epitaxial relationship is LaO [001] || LaAlO3 (LaSrAlO4) [001] and LaO [110] || LaAlO3 (LaSrAlO4) [100]. (b) Temperature dependences of normalized / 300K for samples G–J. (c) Cube root of the unit cell volume dependence of Tconset for samples G–J. The dash line is a guide for eyes.

Figure 3. Carrier density dependences of properties of LaO (001) epitaxial thin film. (a) Relationship between carrier density at 300 K and unit cell volume (V) calculated from reciprocal space mapping. (b) Temperature dependence of resistivity for LaO (001) epitaxial thin films deposited under different PO2 (< 1  10−8–4  10−8 Torr). Inset shows the temperature dependence of normalized resistivity at low temperature. Carrier density dependences of (c) superconducting critical temperature and (d) density of states at Fermi level N(EF) including data of LaS, LaSe, and LaTe (Ref.15).

through electron-phonon interaction varied by the unit cell volume. The simple rocksalt binary oxide superconductor LaO would pave the way to create new superconductors and exotic properties e.g. by designing heteroepitaxial structure.

3(d)). The increased N(EF) could be a possible origin of the highest Tc superconductivity of LaO among LaX series. Effect of epitaxial strain on Tc was significantly large in the LaO (001) epitaxial thin film. In order to fine-tune the epitaxial strain, LaO (001) epitaxial thin films were deposited on YAlO3 (110), LaAlO3 (001), and LaSrAlO4 (001) substrates. The samples G−I were synthesized simultaneously and the sample J had approximately same carrier density with those of the samples G and H to reduce additional factors to vary Tc. From results of reciprocal space mapping (Fig. 4(a)), the LaO thin films deposited simultaneously at 280 C received compressive strain on YAlO3 (110) (sample G: c/a  1.018) and tensile strain on LaSrAlO4 (001) (sample I: c/a  0.986), while the film was almost relaxed on LaAlO3 (001) (sample H: c/a  0.998). LaO thin film on LaAlO3 (001) deposited at 300 C received higher tensile strain (sample J: c/a  0.984). From temperature dependence of resistivity (Fig. 4(b)), Tconset was found to change significantly from 4.25 K to 5.24 K respectively for samples G to J, i.e. Tc increases monotonically from compressive to tensile strain among the samples with similar carrier density and N(EF) (see Fig. S9 and Table S1). The Tc showed approximately linear increase with the cube root of the LaO unit cell V1/3 (Fig. 4(c)), being similar to recent study of pressure-weakened superconductivity in rocksalt TiO thin film under hydrostatic pressure.21 Monotonic increase in λe-p from tensile to compressive strain (Table S1) suggests that epitaxial strain influences the Tc through the strength of electron-phonon interaction similar to TiO (Ref.21) and transition metal nitrides.22 In summary, LaO epitaxial thin film was the first rare earth binary oxide superconductor. Tc about 5 K was the highest among La monochalcogenides and was tunable by means of carrier control, exhibiting a domed-shaped dependence of Tc on electron carrier density. In addition, epitaxial strain also influenced Tc possibly

ASSOCIATED CONTENT Supporting Information Additional experimental details; rocking curve measurement, O1s XPS spectra and typical XPS spectra, in-plane magnetization, isothermal magnetoresistance, carrier density and mobility, table of experimental parameters for samples A-J. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT XPS measurement was conducted at Research Hub for Advanced Nano Characterization, The University of Tokyo, under the support of Nanotechnology Platform by MEXT, Japan (No. 12024046). This work is supported by JST-CREST, JSPS-KAKENHI (Nos. 26105002, JP17J05331, 18H03872), the Mitsubishi Foundation,

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and Center for Spintronics Research Network (CSRN), Tohoku University.

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Superconductivity of rocksalt structure LaO epitaxial thin film

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