Infinite-Layer Nickelate Superconductors: A Current Experimental Perspective of the Crystal and Electronic Structures

Abstract

After the reward of more than 2 decades of pursuit on the high-Tc cuprate analog with the hope to obtain a better understanding of the mechanism of high-Tc superconductivity, the discovery of superconductivity in the infinite-layer nickelate brings more mystery to the picture than expected. Tops in the list of questions are perhaps 1) absence of superconductivity in the bulk nickelate and limited thickness of the infinite-layer phase in thin film, 2) absence of superconductivity in the La-nickelate despite it being the earliest studied rare-earth nickelate, and the role of 4 f orbital in the recipe of superconductivity, 3) absence of Meissner effect and suspect of the origin of superconductivity from the interface, 4) whether nickelate hosts similar pairing symmetry to the single-band high-Tc cuprates or multiband iron-based superconductor. In this perspective article, we will discuss the following aspects: 1) stabilization of the infinite-layer phase on the SrTiO3(001) substrate and the thickness dependency of observables; 2) rare-earth dependence of the superconducting dome and phase diagram of the (La/Pr/Nd)- infinite-layer nickelate thin film; 3) experimental aspects of the measurement of Meissner effect; 4) theoretical framework and experimental study of the pairing symmetry of infinite-layer nickelate superconductor.

Optical Properties of Superconducting Nd0.8Sr0.2NiO2 Nickelate

“…A similar fitting occurred at 20 K (see Table S1 in SI), highlighting a transfer of spectral weight from the Drude term to the MIR band, as further expected for correlated systems , (see Figure a,b). Moreover, the increase of the Drude scattering time from 300 K (τ ∼ 1.3 × 10 –14 s) to 20 K (τ 20K ∼ 6.7 × 10 –14 s) is in good agreement with the DC resistivity temperature behavior of similarly doped samples . Through the scattering time value at 20 K, it is possible to estimate whether the Nd 0.8 Sr 0.2 NiO 2 nickelate is in the clean (ℏ/τ < 2Δ) or in the dirty (ℏ/τ > 2Δ) limit for superconductivity.…”

“…Moreover, the increase of the Drude scattering time from 300 K (τ ∼ 1.3 × 10 −14 s) to 20 K (τ 20K ∼ 6.7 × 10 −14 s) is in good agreement with the DC resistivity temperature behavior of similarly doped samples. 47 Through the scattering time value at 20 K, it is possible to estimate whether the Nd 0.8 Sr 0.2 NiO 2 nickelate is in the clean (ℏ/τ < 2Δ) or in the dirty (ℏ/τ > 2Δ) limit for superconductivity. As the superconductive gap energy 2Δ is measured at 3.9 meV by tunneling spectroscopy, 25 we can calculate the ratio ℏ/2Δ = 1.7 × 10 −13 s, and by comparing it with our scattering time τ, we obtain τ < ℏ/2Δ, suggesting that the material is in the dirty limit.…”

Charge Distribution across Capped and Uncapped Infinite‐Layer Neodymium Nickelate Thin Films

“…The particular way in which nickelates are prepared, that is presence/absence of a capping layer, combined with the subtle control of the crystal structure modification during the reduction process, is pivotal to attaining the zero-resistance state. [2][3][4] Apart from superconductivity, recent studies have also revealed the existence of charge ordering phenomena and magnetic excitations in IL-nickelate thin films, [5][6][7][8] the amplitudes of which mostly depend upon the particular sample preparation, that is presence/absence of a SrTiO 3 (STO) capping-layer and doping level. In particular, charge order (CO) has been observed in uncapped-IL NdNiO 2 samples and it is not much pronounced in the capped ones, which on the contrary host dispersing magnetic excitation with a bandwidth of circa 200 meV.…”

High pO2 Flux Growth and Characterization of NdNiO3 Crystals

“…The discovery of superconductivity in the thin film of Nd 0.8 Sr 0.2 NiO 2 by Li et al [1] has attracted intense attention in the fields of condensed matter physics and materials science and launched the Nickel Age of Superconductivity [2][3][4][5][6][7][8][9][10][11][12][13][14]. Until today, superconductivity with a critical temperature (T c ) in the range of 9-15 K has been observed in the infinite-layer R 1-x M x NiO 2 (R = La, M = Ca, Sr; R = Pr, Nd, M = Sr) [15][16][17][18][19] and the quintuple-layer Nd 6 Ni 5 O 12 [20].…”

“…Under high static pressure, T c has been increased to 30 K in Pr 0.8 Sr 0.2 NiO 2 films [21]. After 3 years of intense research, several fundamental issues still exist [2][3][4][5][6][7][8][9][10][11][12][13][14], including the mechanism of superconductivity, the similarity and difference between nickelate and cuprate superconductivity, whether the ground state is magnetic, and whether competing phases such as pseudogaps and strange metals exist. Single crystals are the ideal platform to solve the abovementioned open questions.…”