Preparation of superconducting thin films of infinite-layer nickelate Nd_{0.8}Sr_{0.2}NiO_{2}
PPreparation of superconducting thin film of infinite-layernickelate Nd . Sr . NiO Qiang Gao , Yuchen Zhao , , Xingjiang Zhou , , , , ∗ , and Zhihai Zhu , , ∗ National Lab for Superconductivity,Beijing National laboratory for Condensed Matter Physics,Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China School of Physical Sciences, University of ChineseAcademy of Sciences, Beijing 100049, China Beijing Academy of Quantum Information Sciences, Beijing 100193, China Songshan Lake Materials Laboratory, Dongguan 523808, China ∗ Corresponding author: [email protected], [email protected] (Dated: February 24, 2021) a r X i v : . [ c ond - m a t . s up r- c on ] F e b he recent observation of superconductivity in infinite-layer nickelateNd . Sr . NiO has received considerable attention. Despite the many efforts tounderstand the superconductivity in infinite-layer nickelates, a consensus on theunderlying mechanism for the superconductivity has yet to be reached, partlyowing to the challenges with the material synthesis. Here, we report the success-ful growth of superconducting infinite-layer Nd . Sr . NiO films by pulsed-laserdeposition and soft chemical reduction. The details on growth process conditionswill be discussed. Despite decades of research on cuprate superconductors, an understanding of what ex-act features of a material essentially support superconductivity has remained elusive. Onan empirical basis, it seems natural to expect that superconductivity can be achievedthrough synthesis of cuprate analogs that share common features including spin one-half,two-dimensionality, and strong antiferromagnetic correlations etc[1] . Along this direction,Chaloupka et al. proposed that LaNiO /LaMO superlattices would create superconduc-tivity, which however has not been realized to date although the nickel-oxide superlatticeshave been successfully synthesized[2]. The recent report of superconductivity in an infinitelayer nickelate Nd . Sr . NiO has opened an alternative route to achieve superconductivityin transition metal oxide materials in addition to cuprates [3]. Compared with cuprates,the superconducting nicketales comprise a similar crystal structure, isoelectronic 3 d va-lence state, and a superconducting dome in phase diagram [4–9]. But in sharp contrast tocuprates, the parent compound NdNiO of superconducting nickelates do not exhibit anymagnetic order down to 1.7 K [10], indicating the underlying superconducting mechanismdiffers from that of cuprate superconductors where magnetism are generally believed to becrucial for mediating Cooper pairs. Although various theoretical models have been proposedto explain the superconductivity in the superconducting nickelates[11–50], a consensus modelhas not yet reached partly owing to the challenges with the synthesis, and only very fewreports exist on achieving superconducting films of infinite-layer nickelates [3, 5, 51–56]. Atpresent, superconductivity has not been observed in bulk material [57–60].In this paper, we report the successful growth of superconducting infinite-layerNd . Sr . NiO films. The perovskite precursor phase nickelate films were prepared by usingpulsed-laser deposition. The infinite-layer phase was acquired by soft-chemistry reductionmethod. The thickness and out-plane x-ray diffracrtion (XRD) pattern of the prepared2lms were examined using a SmartLab x-ray diffractometer. The superconductivity wasconfirmed by transport measurements using a Quantum Design Physical Property Measure-ment System (PPMS).The laser target contained a stoichiometric mixture of SrCO (Alfa Aesar, 99.99%), Nd O (Alfa Aesar, 99.99%), and NiO (Sigma Aldrich-Chemie GmbH, 99.995%) prepared by asolid-state reaction in the air at 1100 ◦ C for 24 hours. The products of this reaction wereground and reheated, and this process was repeated five times. The resulting polycrystallinematerials was pressed into a pellet and sintered for 24 hours at 1200 ◦ C in the air. Theheating and cooling rate of sintering were kept at 3 ◦ C/min.The nickelate films were grown by pulsed-laser deposition using 248-nm KrF excimerlaser (COMPex 201, Coherent). The SrTiO (001) substrates (5*5 mm, without chemicaletching to achieve a TiO terminated surface) were pre-annealed at 900 ◦ C with an oxygenpartial pressure of 1 × − Torr. During growth, the substrate temperature was kept at 600 ◦ C under an oxygen partial pressure of 150 mTorr. After deposition, the films were cooledto room temperature at a rate of 5 ◦ C per minute in the same oxygen partial pressure. Thelaser beam size was about 0 . × . realized by using an aperture. The pulse energyof the laser was set to 430 mJ and 730 mJ for the growth of NdNiO and Nd . Sr . NiO ,respectively. The laser frequency was set to 4 Hz.The infinite-layer nickelate phase was acquired by soft-chemistry reduction method. Asshown in Fig. 1 (b), the as-grown nickelate films were wrapped in clean aluminum foil andthen sealed with 0.1 g CaH powder (Alfa Aesar, 98%) in quartz tubes which were pumpedto a vacuum better than 1 × − Torr. The reduction was carried out at a temperature of290 ◦ C for 5 hours, with the heating and cooling rates of 10 ◦ C/min.Figure 2 shows the structural characterization of the nickelate films. The NdNiO filmpeaks at 23.5 ◦ and 48.2 ◦ correspond to the (001) and (002) reflections, respectively (seeFig.2 (a)). After chemical reduction, as shown in Fig. 2 (b) the film peaks at 26.6 ◦ and55.7 ◦ identify the infinite-layer NdNiO and correspond to (001) and (002) reflections, re-spectively. For a typical film of Nd . Sr . NiO shown in Fig. 2(c), the peaks at 23.8 ◦ and48.4 ◦ correspond to (001) and (002) reflections, respectively, and both two peaks are slightlybroader than those of undoped phase in Fig. 2 (a). After chemical reduction, the peaks at26.5 ◦ and 54.8 ◦ again correspond to (001) and (002) reflections, respectively, as expected fora film of an infinite-layer Nd . Sr . NiO (Fig.2 (d)). It was reported that the Nd . Sr . NiO θ less than 48 ◦ [52].The intensity is comparable to the precursor.In Fig.3 we show the measured Nd . Sr . NiO film resistance as a function of tempera-ture revealing that a superconducting transition occurs below 9 K and the zero resistivity isachieved at about 3.5 K. The superconducting transition temperature of our film is a littlelower than that reported in literature, which probably stems from the variation of the holeconcentration in the films prepared under differing growth and reduction conditions. Thetransport measurements were carried out on a quantum design physical property measure-ment system with a standard four-probe configuration. The wire connection was made bymelting indium.In summary, we have successfully synthesized superconducting infinite-layerNd . Sr . NiO thin films by using pulsed-laser deposition and soft-chemistry reduc-tion method. The details of the film growth and subsequent chemical reduction process arediscussed. Our results provide important information for the growth of superconductingnickelate films that is a crucial step in this field. Acknowledgement
We wish to thank Er-jia Guo for helpful discussions.This work was sup-ported in part by the National Natural Science Foundation of China (Grant No. 12074411)and (Grant No. 11888101), the National Key Research and Development Program of China(Grant No. 2016YFA0300300 and 2017YFA0302900), the Strategic Priority Research Pro-gram (B) of the Chinese Academy of Sciences (Grant No. XDB25000000) and the ResearchProgram of Beijing Academy of Quantum Information Sciences (Grant No. Y18G06).
Author Contributions
Z.H.Z., X.J.Z., and Q.G. proposed and designed the research. Q.G., Y.C.Z. and Z.H.Z. grewthe films. Q.G. carried out the XRD and transport measurements with the help from Y.C.Z.Z.H.Z. and Q.G. wrote the paper. All authors participated in discussions and comments onthe paper. [1] Jiˇr´ı Chaloupka and Giniyat Khaliullin. Orbital order and possible superconductivity inLaNiO /LaMO superlattices. Physical Review Letters , 100(1):016404, 2008.
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ONiNd
FIG. 1:
Crystal structure and the reduction process of nickelates. (a) illustrates thecrystal structures of NdNiO (left) and NdNiO (right). (b) The soft-chemistry reduction process.A typical nickelate film wrapped in Aluminum foil is vacuum-sealed with CaH powder in a quartztube. ! " ! $ %" & ’ ( ) * + * , -.///.0/0.1/1.2/2. ! " ! $ %" & ’ ( ) * + * , -.///.0/0.1/1.2/2. ! " ! $ %" & ’ ( ) * + * , -.///.0/0.1/1.2/2. ! " ! $ %" & ’ ( ) * + * , -.///.0/0.1/1.2/2. ’(..8, (9,(:, (;, 456%7 ’(..8,(), 456%7 ’(..2, 456%7 ’(..2,456%7 ’(..8, 456%7 ’(..2, 456%7 ’(..2,456%7 ’(..8,<;<%7 ’(..8, <;<%7 ’(..2, <;<%7 ’(..8, <;<%7 ’(..2,<; .*= .*2 <%7 ’(..8, <; .*= .*2 <%7 ’(..2, <; .*= .*2 <%7 ’(..8, <; .*= .*2 <%7 ’(..2, FIG. 2:
Structural characterization of nickelate films on STO substrates. (a) The XRD θ − θ scans of a typical NdNiO film with a thickness of 14 nm. (b) The XRD θ − θ scans ofthe infinite-layer NdNiO film acquired by performing chemical reduction on the sample (a) at 290 ◦ C for 5 hours. (c) The XRD θ − θ scans of a typical Nd . Sr . NiO film with a thickness of 9nm. (d) The XRD θ − θ scans of the infinite-layer Nd . Sr . NiO film acquired by reducing thesample (c) at 290 ◦ C for 5 hours. !" % & ’ ( ’ )( * () + , - . / . % & ’ ( ’ )( * () + , - . / .
3" $ " ! , ,>,?,9 FIG. 3:
Transport properties of Nd . Sr . NiO thin film. (a) Temperature-dependent resis-tivity of Nd . Sr . NiO thin film from 300 K to 2 K. (b) A zoom-in view of (a) where temperatureis lower than 30 K. The onset of the superconducting transition is about 9 K.thin film from 300 K to 2 K. (b) A zoom-in view of (a) where temperatureis lower than 30 K. The onset of the superconducting transition is about 9 K.