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Observation of superconductivity in LaNiO3/La0.7Sr0.3MnO3 superlattice Guowei Zhou, Fengxian Jiang, Julu Zang, Zhi-Yong Quan, and Xiaohong Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17603 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on January 1, 2018

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ACS Applied Materials & Interfaces

Observation of superconductivity in LaNiO3/La0.7Sr0.3MnO3 superlattice

Guowei Zhou,†,‡ Fengxian Jiang,†,‡ Julu Zang,† Zhiyong Quan,†,‡ and Xiaohong Xu*,†,‡ †

School of Chemistry and Materials Science, Key Laboratory of Magnetic Molecules and

Magnetic Information Materials, Ministry of Education, Shanxi Normal University, Linfen 041004, China ‡

Research Institute of Materials Science of Shanxi Normal University & Collaborative Innovation

Center for Shanxi Advanced Permanent Magnetic Materials and Techonology, Linfen 041004, China

ABSTRACT: In pursuit of high temperature superconductivity like that in cuprates, artificial heterostructures or interfaces have attracted tremendous interests. It has been a long sought goal to find similar unconventional superconductivity in nickelates. However, as far as we know, this has not yet been experimentally realized. To approah this objective, we synthesized a prototypical superlattice, which consists of ultrathin LaNiO3 and La0.7Sr0.3MnO3 layers. Both zero resistance and Meissner effect are observed using resistive and magnetic measurements of the superlattice. These are experimental indicator for superconductivity in new superconductors. X-ray linear dichroism causes the NiO2 planes to develop electron occupied x2-y2 orbital order similar to cuprate-based superconductors. Our findings demonstrate that artificial 1

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interface engineering is suitable to investigate novel physical phenomena, such as superconductivity. KEYWORDS: superconductivity, zero resistance, Meissner effect, orbital occupancy, superlattice INTRODUCTION Atomic scale control of the hetero-interfaces in transition metal oxides has attracted significant attention due to the increased chance to observe new phenomena. These may not happen in the bulk phase of the materials but at the interface due to interaction of both sides of the interface1-4. Among the different electronic states, interface-enhanced superconductivity at the two dimensional boundaries has become one of most compelling research areas in condensed matter physics5. Cuprate- and ferrite-based interfacial superconductors have been investigated extensively, while studies of nickelate-based heterostructures is quite rare6-8. In the perovskite nickelates family, RNiO3, where R is a trivalent cation of the lanthanide series, LaNiO3 (LNO) exhibits a paramagnetic metal state at all temperatures. Furthermore, in contrast with the others, it shows antiferromagnetic and insulating behavior at low temperature9,10. Interestingly, Chaloupka and Khaliullin predicted that antiferromagnetism and high temperature superconductivity (SC) may be stabilized in LNO-based superlattices, where the LNO layers undergo tensile strain and the charge transfer along c axis between neighboring LNO layers is suppressed by insulating layers of other perovskite oxides11. The transition from metal to insulator and the antiferromagnetic state in the lower dimensionality of the LNO ultrathin heterostructures was recently 2


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found in experiment12-15. However, high temperature superconductivity has not yet been reported. For high temperature superconductivity, there are some speculations, for example involving the d-orbital electron, quasi two-dimensionality feature, perhaps spin-1/2, and strong antiferromagnetic spin fluctuation16. The d-orbital feature and spin one-half are expected in the Ni-based superlattice, due to a closed t2g shell and one eg electron. In addition, antiferromagnetic fluctuations have been observed in the reduced dimensionality of LNO-based superlattices12. Therefore, the LNO-based heterostructures hold the basic prerequisites for high-temperature superconductivity17-20. In this work, we investigate the properties of a superlattice (SL) composed of an ultrathin LaNiO3 layer, with a ferromagnetic insulating La0.7Sr0.3MnO3 (LSMO) layer. The

resistivity

dependence

on

temperature

of

the

superlattice

shows a

semiconductor-like behavior, and an abrupt superconductivity transition is observed for a TC around 3.7 K. Moreover, the magnetic susceptibility as a function of temperature under zero-field cooling (ZFC) and field cooling (FC) processes at 10 Oe, has unambiguously confirmed the presence of a superconducting phase. A magnetic hysteresis loop at 2 K has also been measured, which shows a type-II superconductor behavior and further supports the superconducting characteristics of the SL. Furthermore, the x-ray linear dichroism results show that the planar NiO2 x2-y2 orbital dominates the electronic structure, which is consistent with previous theoretical predictions11. Our present work opens a new avenue for searching novel superconductors in Ni-based heterostructures. 3



A high quality film with [LaNiO3 (2 u.c.)/La0.7Sr0.3MnO3 (3 u.c.)]20 superlattice was grown in an epitaxial way using pulsed laser deposition on a (001)-oriented single crystal substrate of SrTiO3 (STO). During the deposition, the KrF excimer laser was operated at a wavelength of 248 nm, a frequency of 2 Hz and a laser energy density of ~1.2 J/cm2. The layer-by-layer growth model was monitored by in-situ high-pressure reflection high-energy electron diffraction (Figure S1). The SL was grown under 100 mTorr oxygen pressure and the temperature of the substrate was held at 725°C during the growth process. After deposition, the sample was in-situ annealed for one hour under an oxygen pressure 300 Torr, followed by cooling at a rate of 15°C/min to room temperature. To study the structural quality and interfacial abruptness of the SL, a typical atomic resolution aberration of a corrected high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) was measured. Magnetic properties were measured in a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 1.8-300 K, with the magnetic field applied in the plane of the sample. The electrical transport measurement was performed using a Van der Pauw four-probe with a Quantum Design physical properties measurement system (PPMS) in the range of 2-300 K after appropriate patterning of the sample. The sample was etched by Ar ion beams with a 0.5 mm mask diameter, followed by growing gold in the regular holes through direct-current magnetron sputtering. Indium was firmly pressed onto the superlattice for the resistive measurements, as shown in Figure S2. The x-ray linear dichroism (XLD) measurements were performed at the Beamline BL08U1A station of Shanghai 4


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Synchrotron Radiation Facility (SSRF). The XLD signals were determined by the difference between the XAS in-plane component E//a (with x-rays impinging at 90 degrees with respect to the sample surface), and out-of-plane E//c (with x-rays impinging at 30 degrees grazing incidence with respect to the sample suface). RESULTS AND DISCUSSION Typical x-ray diffraction scans through (002) symmetric reflections of the LNO/LSMO superlattice are shown in Figure 1a. The main peak of the superlattice and satellite peaks of SL-1 and SL+1 are observed, which suggests sharp interfaces are present in the superlattice. From the distance between adjacent satellite peaks, the thickness of one period is estimated to be 1.92 nm. This is consistent with the nominal thickness of 2 u.c. LNO (a-3.84 Å for the bulk) and 3 u.c. LSMO (a-3.87 Å for the bulk) layers21. Coherent epitaxial growth and the absence of secondary phases or dislocations in LNO/LSMO superlattice are both confirmed using high resolution high-angle

annular

dark

field

scanning

transmission

electron

microscopy

(HAADF-STEM), as shown in Figure 1b. The cross-sectional cuts are prepared by mechanical polishing followed by low-energy, low-angle ion milling and the superlattice is found to be coherent with the substrate, with no obvious defects or dislocations at the interface. The STO (a-3.905 Å) substrate was etched in a buffered hydrogen fluoride solution, and the TiO2-termination was exposed at the surface before growing the superlattice, which is a practical way of obtaining high quality atomic-scale layer-by-layer samples22. In the HAADF-STEM, the image intensity is proportional to the atomic number. The brighter features correspond to the position of 5



the La (or La0.7Sr0.3) atoms, while the weaker spots in between show the columns of Mn and Ni atoms. Because the A site is similar to the superlattice, and the atomic number of B site is contrasted in this image, the LNO layers appear brighter than the LSMO layers (Figure S3). In the schematic inset shown in Figure 1b, the NiO2 plane is surrounded by the neighboring MnO2 plane, where the ultrathin LSMO layer is in the insulating ferromagnetic state. Due to the STO substrate induced c-axis compression of the NiO6 octahedra, the low-energy electronic states are confined to the further stabilized x2-y2 orbital in NiO2 planes and the LNO layer has quasi-two dimensional nature11. In addition, the surface resistance dependence of temperature in the superlattice is measured for the range of 2 K to 300 K without etching, which suggests semiconducting properties (Figure S4). In the LNO material, the nominal ଺ ଵ configuration of Ni is in a low-spin ‫ݐ‬ଶ௚ ݁௚ analogue to the electronic configuration of

the Cu, that is d9 (one unoccupied electronic state in the d shell)23. The net effect is a strong enhancement of antiferromagnetic correlation among spin one-half electrons residing predominantly in a single dx2-y2 orbital, as observed in previous experiment12. In addition, the orbital nondegenerate spin one-half electronic structure is expected to occur in LaNiO3-based superlattices11. Therefore, the LNO-based heterostructure exhibits the properties required for cuprate-like superconductivity.

6


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Figure 1. (a) X-ray diffraction radial scans through the (002) reflections of LNO/LSMO superlattice on STO substrate. Note the satellite peaks around the (002) Bragg reflections. (b) High-angle annular dark field scanning transmission electron microscope images corresponding to LNO/LSMO superlattice. The inset shows the schematic view of the partial atomic arrangement.

We have grown a high quality epitaxial [(LNO)2/(LSMO)3]20 superlattice on single crystal substrates of SrTiO3 cut along the (001) crystallographic direction, where the ultrathin LSMO film shows ferromagnetic insulating behavior, as previously reported24. The prepared sample was measured with a current of 5×10-3 mA in a Van der Pauw geometry. In Figure 2a, the electric resistivity increases with the 7



temperature decrease from 300 to 10 K, indicating a semiconducting characteristic of the SL. In order to observe the superconducting property at low temperatures directly, the temperature dependence of the resistivity was measured, as shown in the inset of Figure 2a. The superlattice displays metallic behavior at a temperature below 10 K and the resistivity drops abruptly around 3.7 K, clearly indicating superconductivity. We also measured the detection current dependence of the onset of TC (Figure S5a), and found that the superconductivity vanishes in SL for currents larger than 0.1 mA, which indicates that the superconductivity really originates from the SL. To verify the superlattice nature of the observed superconductivity, we measured the magnetic susceptibility (χ) as a function of temperature at magnetic field (H) strength of 10 Oe. As shown in Figure 2b, the zero field cooled (ZFC) and field cooled (FC) susceptibility are essentially temperature independent at low temperature. A sharp drop in magnetic susceptibility is observed for both ZFC and FC processes, which indicates that the magnetic onset of superconductivity appears around 3.5 K. This is the same as the zero-resistivity temperature. The χ at 1.8 K in the field cooled process is -0.5 emu·cm-3·Oe-1, and three times as high diamagnetism (-1.7 emu·cm-3·Oe-1) is observed in the zero-filed cooling process. We conclude that the present phase undergoes a superconducting transition at about 3.5 K, since only superconductivity can account for such a large diamagnetic signal. With H increasing from 10 Oe to 500 Oe, the onset temperature of superconductivity is suppressed noticeably and disappears as the field increases above 150 Oe (Figure S5b). Further confirmation of superconductivity in the superlattice is shown in the inset of Figure 8


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2b. It displays the typical magnetic hysteresis curve for a superconductor at 2 K after the zero-field cooling process. The characteristic M-H loop indicates that the present superlattice is a superconductor of the second kind with a lower critical field of 50 Oe.

Figure 2. (a) Temperature dependence of electrical resistivity for the LSMO/LNO superlattice between 300 and 2 K with a detection current of 5×10-3 mA. The inset shows the enlarged view of the superconducting transition. (b) Magnetic susceptibility (χ) of SL is measured during zero field cooling (black circles) and the field cooling (red circles) processes with a magnetic field of 10 Oe.

The inset shows the magnetic

hysteresis loop at 2 K after zero-field cooling, which further confirms the superconducting behavior in the superlattice.

In the CuO2-based high TC cuprates, the crystal line electric field lifts the 9



degenerated of the d-derived orbitals, and only the dx2-y2 orbital remain relevant, which dominates the electronic structure and causes a high transition temperature25,26. In a recent letter, Chaloupka and Khaliullin predicted the artificial engineering of nickelate heterostructures with the aim to obtain a cuprate-like electronic structure11. An axially compressive strain, or control of the apical cation, yields a single Fermi surface sheet with a geometry and volume similar to that of cuprates. In order to research the electron orbital occupancy in the LNO layer, we measured the x-ray absorption spectroscopy at the Beamline BL08U1A station of the Shanghai Synchrotron Radiation Facility. The photo polarizations (E) nearly parallel to the sample plane (E//c) and vertical to it (E//a) were performed in total electron yield mode. The XLD is calculated using the intensity difference between the spectra measured with in-plane and out-of-plane polarizations27. The dichroic signal around the high-energy L2 absorption peak is the clearest indicator of the symmetry (x2-y2/3z2-r2) of the occupied orbitals, with a negative XLD indicating x2-y2 electron occupancy and a positive XLD indicating 3z2-r2 electron occupancy28. In Figure 3a, a negative XLD signal is observed for the Ni L2 edge, which indicates preferential x2-y2 electron occupancy in LNO layer. This is consistent with previous theoretical predictions. In the Mn L2 edge, the negative XLD signal also appears and suggests x2-y2 electron occupancy (see Figure 3b). These phenomena coincide with stress effects on electron occupancy in LNO/LSMO superlattice, where the lattice parameter of the STO substrate is slightly larger than the LNO and LSMO layer.

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Figure 3. X-ray linear dichroism spectra of the LNO/LSMO superlattice measured at (a) Ni and (b) Mn L3,2 edges. The area under XLD around the L2 peak represents the difference between the relative occupancies of the 3z2-r2 and x2-y2 orbits. CONCLUSIONS In conclusion, we have unambiguously demonstrated superconductivity in a LNO/LSMO superlattice on a STO substrate using direct electrical transport and Meissner effect measurements at low temperatures. Furthermore, we obtained a perfect NiO2 plane and x2-y2 electron occupancy in the superlattice, which was predicted by Chaloupka et al. and leads to the superconductivity similar to that in cuprates. We believe that superlattice superconductivity should be further investigated using this rather unconventional approach. This discovery opens the possibility to use artificial interface engineering as a platform for exotic phenomena. 11



■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. RHEED patterns for the LNO/LSMO superlattice before and after the deposition, partial oscillating curve of the LNO/LSMO SL, the schematic diagrams of growth process for LNO/LSMO SL, integral HADDF-STEM of LNO/LSMO SL, the surface electrical resistance for LNO/LSMO SL, and R-T、 M-T curves of LNO/LSMO superlattice.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID Xiaohong Xu: 0000-0001-7588-4793 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We thank fruitful discussion with Hai-Hu Wen at Nanjing University and Hong Ding at Chinese Academy of Sciences. We also thank Wen-Sheng Yan for the XAS measurement at Beamline BL12-a in National Synchrotron Radiation Laboratory (NSRL) and XLD measurement at Beamline BL08U1A in Shanghai Synchrotron Radiation Facility (SSRF). This work is financially supported by National Key R&D 12


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Program of China (No. 2017YFB0405703), National Natural Science Foundation of China (Nos. 61434002, 11274214, and 51571136), and the Special Funds of Sanjin Scholars Program.

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