Possible high-Tc superconductivity exceeding 100 K in Ir-substituted perovskite-type manganese oxides

doi:10.21203/rs.3.rs-1856661/v1

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Possible high-T_c superconductivity exceeding 100 K in Ir-substituted perovskite-type manganese oxides

https://doi.org/10.21203/rs.3.rs-1856661/v1

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The search for new high-T_c superconductors has commonly focused on layered perovskite compounds with isoelectronic or isostructural properties similar to those of cuprates.^{1, 2} For example, in 2019, a family of 3d nickel-based superconductors (Nd, Sr)NiO₂ was discovered by Hwang et al.³More recently, Ca₂RuO₄,a 4d transition metal oxide, received attention as a superconductor with T_c = 64 K in coexistence with ferromagnetism in nanofilm single crystals.⁴ Even the 5d transition metal oxide Sr₂IrO₄ was thoroughly studied as a possible high-T_c superconductor.⁵ While no zero-resistance state has yet been observed, surface-electron-doped Sr₂IrO₄ has shown a spectroscopic signature consistent with a superconducting gap.⁶ However, despite such intensive exploration of analogous structures, no materials other than cuprates or single-layer FeSe/SrTiO₃ [Ref. 7] have been reported to exhibit high-T_c superconductivity above 100 K under ambient pressure.

In this study, we report observations of possible new high-T_c superconductivity in a non-layered perovskite, Ir-substituted (La, Sr)MnO₃ (LSMIO) thin films. LSMIO samples with Ir compositions of approximately 10% were confirmed to exhibit the zero-resistance state with onset temperatures exceeding 100 K. Furthermore, their magnetic field response was observed to be characteristic of conventional superconductors. All these results strongly indicated that LSMIO showed the most promise as a new high-T_c superconductor among perovskite oxides. The present discovery will boost interest in new theoretical frameworks for unconventional but possible superconductivity in LSMIO, opening a frontier for research on high-T_c superconductors.

LSMO is one of the most popular manganese oxides with a cubic-type perovskite structure (Fig. 1e)[8] and has been widely investigated as a spintronics material due to its half-metallic nature and so-called colossal magnetoresistance.[9]^,[10],[11] Much interest has been shown in the underlying basic physics of such intriguing properties, and recent studies have focused on further modification of the magnetic properties of LSMO. For example, only 5% Ru substitution at the B sites of LSMO [Refs.[12],[13],[14]]can induce perpendicular magnetic anisotropy with a remarkable enhancement in coercive force. Furthermore, fascinating interfacial magnetic phases have been discovered in LSMO films in heterojunctions and superlattice structures with SrRuO₃ [Refs.[15],[16]] or SrIrO₃[Refs.[17],[18]]. In these systems, there may be possible competition between strong spin-orbit interactions and electron correlations among 4d or 5d electrons. Furthermore, unique coupling of the spins of localized Mn 3d electrons with those of itinerant Ru 4d or Ir 5d electrons is expected. Hence, the present study focused on Ir substitution in LSMO, which has not yet been investigated as intensively as Ru substitution, to explore new properties and functionalities.

Ir-substituted LSMO (La : Sr = 7 : 3) epitaxial films with different Ir contents (Samples A-S: from 0 to 20 at%, all structural parameters shown in Table S1) were synthesized on (LaAlO₃)_0.3-(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT) (001) substrates by pulsed laser deposition (PLD). X-ray photoelectron spectroscopy (XPS) survey scans (Fig. S3a) confirmed that no impurities were included in any of the film samples. From two-dimensional reciprocal space mapping (RSM) (Fig. 1a), the ab-plane was found to be fully locked by the LSAT substrate for all the LSMIO films. As a result, since the LSAT had smaller in-plane lattice constants (a = b = 3.868 Å) than the LSMO, the in-plane compressive strain induced tetragonal distortion or elongation of the c-axis in the LSMIO. Lattice volume expansion by Ir substitution further enhanced such tetragonal distortion with a linear relationship between Ir composition and lattice volume (Fig. 1b and Table S2, see supporting information for details). Scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDX) analysis verified that Ir atoms occupied B sites with no segregation at interstitial sites or on the film surface (Figs. 1c, 1d, and S2).

Fig. 2a displays the temperature (T) dependence of resistivity (r) at 0 T for representative samples (see the r-T data for all the samples as summarized in Fig. S4 for more details). For specific LSMIO samples with C (2%), H (7%), K (9.6%), L (10.4%), M (11%) and Q (19%), a metal-insulator (M-I) transition was observed followed by an unexpected sharp drop in resistance with decreasing temperature. In fact, the resistivity of some of these samples showed the zero-resistance state (defined as |r| < 1 μΩ.cm) at, for example, T_c^zero = 86 K for Sample M. In the zero-resistance state, the resistivity fluctuated across the zero-line (Fig. 2b), as is often observed in superconducting (SC) states. The first (onset) and second resistivity drops were observed at T_c^on = 123 K and T_c^sec = 114 K, respectively, before reaching the zero-resistance state (see Fig. S4) due to possible inhomogeneities in the film.

To confirm the presence of superconductivity in the LSMIO, the applied magnetic field response of the transition behaviour was further investigated, as shown in Figs. 3a-b and S5. As expected in the SC state, the transition temperature steadily decreased when the in-plane or out-of-plane applied magnetic field was increased. This behaviour was completely opposite to the M-I transition temperature (T_MI) in the colossal magnetoresistance effect of the LSMO.[19] Except for Sample C (Ir 2 %), the SC state was robust in that the sharp drop in resistance could be observed even under an applied magnetic field µ₀H = 9 T, the upper limit of the PPMS. Emphasis should be placed on the magnetic response which is rather independent of the direction of the applied magnetic field. Such an isotropic response of the transition behaviour to an applied magnetic field can rule out the possibility of two-dimensional superconductivity at the heterointerface, as reported in Refs. [20] and [21].

The data in Fig. 3a and Fig. 3b were then analysed in an attempt to evaluate m₀H_c₂. For example, the m₀H_c₂ values for Sample M (Ir 11%) were plotted against a normalized temperature for in-plane (along the ab-plane of the films, H || ab) and out-of-plane (along the c-axis of the films, H || c) magnetic field directions, respectively, as shown in Fig. 3c. The solid and dotted lines in Fig. 3c represent fits to the obtained m₀H_c2^||ab and m₀H_c2^||c values with the Ginzburg–Landau (GL) formula where H_c2(0) is the upper critical field at absolute zero temperature.^{^[22]} The critical temperature is defined by the drop in the resistivity to 10% of its value in a normal state for various applied magnetic fields, e.g., ρ/ρ_{124 K} = 0.1 in Sample M. Additionally, the zero-temperature GL coherence length along the ab-plane (ξ_ab(0)) and c-axis (ξ_c(0)) were estimated from the relationships where Φ₀ is the quantum flux (h/2e).²²As a result, we found that both ξ_ab(0) and ξ_c(0) corresponded to the lengths on the order of approximately 2–3 unit cells (8–12 Å), for Samples K-M (Table S1 and S3). When compared to previously reported superconductors with T_c values above 100 K, the values of the GL coherence length were between those of cuprates (ξ_ab(0) = 20–30 Å, ξ_c(0)= 3 Å) and hydrides (ξ(0) = 15–30 Å).[23] In stark contrast with layered superconductors of cuprates and iron pnictides/chalcogenides,[24] the anisotropy ratios of approximately unity for H_c2^||ab(0)/H_c2^||c(0) indicated the nearly isotropic SC nature of the LSMIO similar to that of bismuth-based cubic perovskite superconductors such as Ba_xK_1-xBiO₃.^{^[25]}

On the solid-state physics side, one immediate question is the origin of the high SC transition temperatures of the LSMIO. It would be difficult for current theories for high-T_c superconductors based on antiferromagnetic spin fluctuations[26] or charge-density wave fluctuations[27], much less the Bardeen–Cooper–Schrieffer (BCS) theory, to answer this question. Hence, even at this time, it would be helpful to grasp the general trends of superconductivity and magnetism in the LSMIO. Fig. 4a shows the out-of-plane saturation magnetization at 100 K (M_100K) of the films plotted against Ir % (see Fig. S6 for details). An unexpected oscillatory behaviour of M_100K with Ir composition was observed in the LSMIO films. Then, the relatively weak ferromagnetic (FM) LSMIO samples exhibited SC transitions. Nevertheless, they tended to be in the vicinity of the samples with the local maximal values of M_100K, as shown in Fig. 4a. Furthermore, T_c^on and T_MI in the SC samples increased in parallel as Ir % increased, and both seemed to reach maxima at approximately 10% (Fig. 4b). These results were indicative of the correlation of the SC state of the LSMIO with its FM fluctuations as a result of magnetic quantum instability. In fact, unconventional superconductivity has often been discovered in the vicinity of magnetic quantum instabilities surrounded by a host of competing electronic phases.[28] There has been a similar discussion about the origin of superconductivity in Ca₂RuO₄, a possible superconductor that is also a weak FM.⁴ Therefore, further theoretical and experimental investigations will be indispensable to reveal the overall characteristics of this possible new superconductor, LSMIO.

Thin film fabrication:

Nonsubstituted and Ir-substituted LSMO (001) epitaxial thin films were deposited on (LaAlO₃)_0.3-(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT, a = 3.868 Å, Crystal GmbH) (001) substrates by a pulsed laser deposition (PLD) method (with rapid beam deflection by a galvanometer mirror custom-made by Pascal Co., Ltd. [Ref.[30]]). Except for a polycrystalline nonsubstituted LSMO pellet purchased from Toshima Manufacturing Co., Ltd., we synthesized polycrystalline Ir-substituted LSMO pellets through the conventional solid-state reaction route for use as targets. Each target was prepared using an arbitrary mixture of the following powders: La₂O₃(purity 99.99%, FUJIFILM Wako Pure Chemical Co.), SrCO₃(purity 99.99%, FUJIFILM Wako Pure Chemical Co.), MnO₂(Wako 1^st Grade, FUJIFILM Wako Pure Chemical Co.) and IrO₂(Practical Grade, FUJIFILM Wako Pure Chemical Co.); the mixture was ground, reacted at 1100 °C for 12 h in open air, reground, pressed into pellets, and sintered again at 1200 °C for 24 h. The atomic compositions (except for that of oxygen) of the targets were determined by inductively coupled plasma‒mass spectrometry (ICP‒MS; Agilent 8800, Agilent), and the results are shown in Table S2. The ICP‒MS measurement was carried out three times, and the relative standard deviation was confirmed to be less than 5%. During deposition, five-nines purity O₂ gas was introduced at a constant partial pressure P_O2 of 50 mTorr, and the growth temperature was maintained at 530 °C for all the films to minimize possible re-evaporation of the Ir species at high temperatures. PLD was carried out using a KrF excimer UV laser (l = 248 nm), for which the energy density and repetition rate were set to 0.55 J/cm² and 2 Hz, respectively, and a substrate-target distance of 5 cm. After deposition, the samples were cooled to room temperature at a rate of 10 °C/min, maintaining the same P_O2= 50 mTorr as during deposition. For some samples (which are listed in Table S1), the desired Ir % was achieved by rapidly alternately ablating two targets with different Ir concentrations, where the ratio of the laser pulse numbers was adjusted during film growth according to the deposition rate for each target material.

Characterizations:

Structural analyses and thickness estimation were carried out using X-ray diffraction (XRD; D8 DISCOVER, Bruker AXS) with a Cu Ka₁ radiation source, including two-dimensional reciprocal space mapping. The film thicknesses of all the samples are shown in Table S2. The microstructure of an LSMIO thin film was analysed using cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF/STEM) and energy dispersive X-ray spectroscopy (EDX) mapping on an FEI Talos F200X G2 TEM (Thermo Fisher Scientific Inc.). Samples with different Ir compositions were subjected to Ar⁺ sputtering followed by X-ray photoelectron spectroscopy (XPS; Quantum 2000, ULVAC-PHI) with a monochromated Al Ka source. The samples were simultaneously deposited on 0.5 wt% Nb:SrTiO₃ (001) substrates and LSAT (001) substrates. The temperature-dependent resistivity of the samples was determined under various magnetic fields by a Physical Property Measurement System (PPMS, Quantum Design) equipped with a sample rotator system based on four-probe measurements. Magnetization measurements were carried out using a superconducting quantum interference magnetometer (MPMS2, Quantum Design).

Competing interests

The authors declare no competing interests.

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Possible high-T_c superconductivity exceeding 100 K in Ir-substituted perovskite-type manganese oxides

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