⇦Chinese Physics Letters, 2021, Vol. 38, No. 6, Article code 067401 NMR Evidence of Antiferromagnetic Spin Fluctuations in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ Yi Cui (崔祎)1†, Cong Li (李聪)1†, Qing Li (李庆)2†, Xiyu Zhu (祝熙宇)2, Ze Hu (胡泽)1, Yi-feng Yang (杨义峰)3, Jinshan Zhang (张金珊)4, Rong Yu (俞榕)1*, Hai-Hu Wen (闻海虎)2*, and Weiqiang Yu (于伟强)1* Affiliations 1Department of Physics and Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China 2National Laboratory of Solid State Microstructures and Department of Physics, Center for Superconducting Physics and Materials, Collaborative Innovation Center for Advanced Microstructures, Nanjing University, Nanjing 210093, China 3Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 4Mathematics and Physics Department, North China Electric Power University, Beijing 102206, China Received 21 March 2021; accepted 6 April 2021; published online 25 May 2021 Supported by the National Natural Science Foundation of China (Grant Nos. 51872328, 11674392, 11774401, and A0402/11927809), the National Key R&D Program of China (Grant Nos. 2016YFA0300504 and 2016YFA0300401), China Postdoctoral Science Foundation (Grant No. 2020M680797), the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China (Grant Nos. 18XNLG24, 20XNLG19, and 21XNLG18).
†These authors contributed equally to this work.
*Corresponding authors. Email: rong.yu@ruc.edu.cn; hhwen@nju.edu.cn; wqyu_phy@ruc.edu.cn Citation Text: Cui Y, Li C, Li Q, Zhu X Y, and Hu Z et al. 2021 Chin. Phys. Lett. 38 067401    Abstract Despite the recent discovery of superconductivity in Nd$_{1-x}$Sr$_{x}$NiO$_2$ thin films, the absence of superconductivity and antiferromagnetism in their bulk materials remains a puzzle. Here we report the $^{1}$H NMR measurements on powdered Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ samples by taking advantage of the enriched proton concentration after hydrogen annealing. We find a large full width at half maximum of the spectrum, which keeps increasing with decreasing the temperature $T$ and exhibits an upturn behavior at low temperatures. The spin-lattice relaxation rate $^{1}T_1^{-1}$ is strongly enhanced when lowering the temperature, developing a broad peak at about 40 K, then decreases following a spin-wave-like behavior $^{1}T_1^{-1}\propto T^2$ at lower temperatures. These results evidence a short-range glassy antiferromagnetic ordering of magnetic moments below 40 K and dominant antiferromagnetic fluctuations extending to much higher temperatures. Our findings reveal the strong electron correlations in bulk Nd$_{0.85}$Sr$_{0.15}$NiO$_2$, and shed light on the mechanism of superconductivity observed in films of nickelates. DOI:10.1088/0256-307X/38/6/067401 © 2021 Chinese Physics Society Article Text The recent discovery of superconductivity in films of the infinite layer nickelate Nd$_{1-x}$Sr$_{x}$NiO$_2$[1,2] with a maximal transition temperature $T_{\rm c}$ of 14.9 K has attracted a great deal of research attention. Nd$_{1-x}$Sr$_{x}$NiO$_2$ has the same lattice structure as the infinite-layer cuprate superconductor (Sr$_{1-x}$Ca$_x$)$_{1-y}$CuO$_2$.[3] In the undoped NdNiO$_2$, the Ni$^{1+}$ ion has an electron configuration of 3$d^{9}$ with a single hole in the $d_{x^2-y^2}$ orbital, as Cu$^{2+}$ in cuprate superconductors. It was proposed that upon Sr$^{2+}$ substitution for Nd$^{3+}$ the nickelate behaves as a doped Mott insulator, in analogy to the hole-doped cuprates.[4] However, a number of major differences between these two systems have been noted. The charge transfer gap between the Ni 3$d$ to O 2$p$ orbitals was estimated to be much larger than that in cuprates.[5,6] Thus, it is under debate whether the hole introduced by Sr doping would reside on the O site as in cuprates,[7] or on the Ni site forming a $3d^8$ configuration. The latter could cause an $S=1$ high-spin configuration of Ni that involves multiorbital physics.[5,8–13] Moreover, the larger charge transfer gap leads to a smaller exchange energy,[14,15] which may substantially suppress the antiferromagnetism in the nickelate. The $5d$ and $4f$ electrons in Nd$^{3+}$,[16–18] and/or interstitial $s$ electrons[19] may also participate to the low-energy electronic structure. The above concerns make an open question in relation to the doped Mott insulator scenario of the nickelate and stimulates alternative theoretical proposals involving the Kondo effect,[11,12] anti-Kondo effect,[18] multi-orbital effect,[20–22] Hund's coupling,[23] and disorder effect. Surprisingly, more recent measurements on the bulk materials show an absence of superconductivity[24] and no long-range order antiferromagnetism.[25–27] These results not only challenge the similarity between nickelates and cuprates, but also raise an intriguing question: What could the critical ingredients manipulating the superconductivity in film and bulk nickelates be? Both the Nd$_{1-x}$Sr$_{x}$NiO$_2$ films and the powered bulk crystals are made from Nd$_{1-x}$Sr$_{x}$NiO$_3$ by post annealing in hydrogen atmosphere to remove the apical oxygen in the lattice.[2,24–26] However, the resistivity of the powdered bulk crystals show an insulating behavior from room temperature down to 1.5 K. One may consider the question of the role of hydrogen annealing: besides removing the apical oxygens, could it be a key factor in tuning the superconductivity by introducing effective charge doping or inhomogeneity? Experimental studies on the electronic structure and magnetism are much demanded to address the above questions and to get a convergent understanding of the superconductivity in general.[28–31] NMR is an ideal probe for low-energy physics in condensed matter systems, and the hydrogen annealing motivates us to search for $^{1}$H NMR in bulk Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ samples, whose film counterpart is near optimal doping.[2] In this Letter, we show that the $^{1}$H NMR intensity of Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ is much larger than that of Nd$_{0.85}$Sr$_{0.15}$NiO$_3$, which implies a higher concentration of hydrogen after annealing. More strikingly, although the hyperfine shift $^{1}K_{\rm s}$ in the annealed sample is very small, a large increase of full width at half maximum (FWHM) of the spectrum is observed in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$. The spin-lattice relaxation rate is very high, and is largely enhanced with decreasing temperature, developing a broad peak at about 40 K. These results signal dominant antiferromagnetic (AFM) fluctuations in the system. Below 40 K, the FWHM of the spectrum increases dramatically with an upturn behavior, and the $^{1}T_1^{-1}\propto T^2$ showing a spin-wave-like behavior. These suggest a short range or glassy AFM order developing below 40 K. The observed enhanced AFM fluctuations and the short-range AFM ordering are inherent in the strong electron correlations in the bulk materials, and are crucial in the understanding of magnetism and superconductivity in nickelates. The powdered samples of Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ in this study are made by the topotactic hydrogen annealing method as reported in earlier papers.[24,32] The magnetization data are measured by a VSM in a Quantum Design physical property measurement system. In principle, it is hard to perform NMR measurements on Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ without isotope enrichment. However, after annealing, hydrogen can be injected to the system given its small size. Taking this advantage, $^{1}$H (spin-1/2, $\gamma=42.5759$ MHz/T) NMR signal is found by using a standard spin-echo pulse sequence $\pi/2$–$\tau$–$\pi$, where $\pi/2$ and $\pi$ pulse lengths are 1.5 µs and 3.0 µs, respectively, and $\tau \sim 1$ µs is the blank time between two pulses. Such short pulses and $\tau$ are selected to minimize the $^{1}T_2$ effect which is found to be about 20 µs at low temperatures. The full NMR spectrum is obtained by frequency sweep at a fixed field. The spin-lattice relaxation rate $^{1}T_{1}$ is measured by the inversion-recovery method, and the spin recovery is fitted by stretched exponential functions, with a stretching factor $\beta\sim 0.7$ at temperatures above 50 K and is reduced to about 0.3 when cooled down to 5 K, indicating either magnetic anisotropy or sample inhomogeneity. In addition to this short $^{1}T_1$ NMR signal, a long $^{1}T_1$ component with a narrow bandwidth (about 100 kHz) is also found, and does not vary with sample, which should be from the environment. Considering the single-Lorentzian lineshape, we believe that the spectra in Fig. 1 represent the majority phase of the sample, instead of an impurity phase.
cpl-38-6-067401-fig1.png
Fig. 1. The $^{1}$H NMR spectra (open circles) of Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ at a fixed field of $\mu_{0}H=9$ T, obtained by frequency sweep at selected temperatures. The NMR intensity is obtained from the integration of the spin-echo spectrum multiplying by the temperature. For clarity, vertical shifts are applied to data at different temperatures, and vertical scales in panels (a) and (b) are chosen differently. The solid lines are function fits to the spectra with the single-Lorentzian form.
The $^{1}$H NMR spectra measured by frequency sweep under a constant field 9 T and various temperatures are shown in Fig. 1, where zero frequency corresponds to zero Knight shift. Upon cooling, the NMR spectra follow the Lorentzian form and barely shift with temperature. The hyperfine shift $K_{\rm s}$ [defined as $K_{\rm s}=f_{\rm p}/({\gamma}H)-1$, where $f_{\rm p}$ is the peak frequency of the spectrum] and the FWHM are then calculated from the spectra. Their temperature evolutions at several fields are shown in Figs. 2(a) and 2(b), respectively. $K_{\rm s}$ is very small, with a typical value less than 0.1%. Yet a weak upturn upon cooling is still resolved, which is consistent with the increase of the measured bulk susceptibility data [inset of Fig. 2(a)]. The small $K_{\rm s}$ values and the upturn behavior barely depend on the field. On the other hand, as shown in Fig. 2(b) the FWHM is much larger than the hyperfine shift (relative to the zero frequency), as shown in Fig. 1, and increases with decreasing temperature from 250 K to 40 K, indicating a large magnetic broadening of the spectra. Below 40 K, the FWHM increases rapidly with decreasing temperature and follows an upturn behavior, as also shown by a kinked downturn (marked by the up arrow) in the inverse of the FWHM [Fig. 2(b) inset]. Such a large broadening is a typical character of inhomogeneous NMR linewidth, and is much bigger than the measured $1/T_2$ ($T_2\approx 20$ µs at low temperatures). This large linewidth and the upturn behavior are observed over a broad field range. Though the powdered sample is chemically and structurally inhomogeneous, the strong temperature dependence of the FWHM below 40 K at large fields can only be attributed to a precursor to static AFM ordering, because in an antiferromagnet the projection of local hyperfine fields along the external field is also randomly distributed between negative and positive values when the crystalline orientations are random in powders. This is further discussed in the following.
cpl-38-6-067401-fig2.png
Fig. 2. (a) The NMR hyperfine shift of Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ versus temperatures at different fields. The solid line is a polynomial fit to the 9 T data below 150 K. Inset: the susceptibility as a function of temperature with a fixed field $\mu_0H=1$ T. (b) The FWHM of the spectra of Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ versus temperatures. Inset: the inverse of the FWHM, which exhibits a downturn at 40 K marked by the up arrow and a linear temperature dependence below 40 K as shown by the straight line.
cpl-38-6-067401-fig3.png
Fig. 3. (a) Measured $^{1}T_1^{-1}$ with temperature of Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ under different fields. Inset: the spin recovery data at three typical temperatures. The solid lines are the fits by the stretched exponential functions to obtain the $T_1$. (b) The low-temperature $^{1}T_1^{-1}$ plotted in a log-log scale. The solid line is a function fit to $T_1^{-1}\propto T^2$.
The spin-lattice relaxation rate $T_1^{-1}$ is a sensitive probe of low-energy spin fluctuations,[33] defined as $T_1^{-1} =T \lim _{\omega \to 0} \sum_{q} A_{\rm hf}(q) \, {\rm Im} \chi(q,\omega)/\omega$, where $\chi(q, \omega)$ is the dynamical susceptibility, and $A_{\rm hf}(q)$ represents the hyperfine coupling. Since $^1$H nuclear spin is 1/2, the spin-lattice relaxation rate $^{1}T_1^{-1}$ of $^{1}$H is only sensitive to magnetic fluctuations. The $^{1}T_1^{-1}$ is then measured and plotted against temperatures under various fields in Fig. 3. Upon warming from 2 K, the $^{1}T_1^{-1}$ first increases dramatically, then forms a peaked feature at $T\sim 40$ K, and finally flattens out at temperatures above 100 K. This peaked behavior is demonstrated by the spin-recovery curve for the $T_1$, as shown in the inset of Fig. 3(a), where the spin recovers more rapidly at 40 K than that at 200 K and 10 K. A sharp peak in $^{1}T_1^{-1}$ is usually a signature of magnetic phase transition. Although the magnetization data reveal a spurious ferromagnetic phase in both Nd$_{0.85}$Sr$_{0.15}$NiO$_2$[24] and Nd$_{4}$Ni$_3$O$_8$,[32,34] their high onset temperatures (above 300 K) are incompatible with our observation. Since no additional magnetic hysteresis was emergent at 40 K by any other probes,[24] the magnetic transition must be an AFM type. The broad peak at about 40 K in bulk Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ suggests the onset of glassy antimagnetic order, where the transition temperature varies across the sample due to inhomogeneity. This glassy behavior is similar to underdoped cuprate superconductor.[35] Nevertheless, it indicates the onset of low-energy spin fluctuations at 40 K. Below 40 K, $^{1}T_1^{-1}\propto T^\alpha$, following a power-law behavior with a low power-law exponent $\alpha=2$, much smaller than that caused by AFM spin wave excitations ($\alpha=5$),[36] again it evidences remaining spin excitations. These results, together with the broad peak in $^{1}T_1^{-1}$ at 40 K and the large FWHM below 40 K, are fully consistent with a glassy AFM order caused by inhomogeneity at low temperatures. In order to understand the effects of hydrogen annealing and the origin of the $^{1}$H signal, we performed comparative studies on four bulk nickelate compounds. Nd$_{0.85}$Sr$_{0.15}$NiO$_3$[37] and Nd$_{4}$Ni$_3$O$_{10}$[32] are made without hydrogen annealing, and Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ and Nd$_{4}$Ni$_3$O$_8$ are synthesized after hydrogen annealing from the former two compounds,[32,38] respectively. Although NdNiO$_3$ is insulating and antiferromagnetically ordered below 200 K,[39–42] Nd$_{0.85}$Sr$_{0.15}$NiO$_3$ shows a metallic behavior at low temperatures.[37] Nd$_{4}$Ni$_3$O$_{10}$ is also metallic at low temperatures,[32,34,43] while Nd$_{4}$Ni$_3$O$_{8}$[32] shows an insulating behavior at low temperatures and its magnetic properties remain unknown. The spectra of the four compounds at a representative field 9 T are shown in Fig. 4(a). Interestingly, clear $^1$H signals are resolved for the two samples without hydrogen annealing. This is not too surprising because hydrogen is hard to avoid in materials, given its small atomic size. However, the total integrated spectral weight of $^{1}$H in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ is enhanced by a factor of $\sim $45 compared with Nd$_{0.85}$Sr$_{0.15}$NiO$_3$, which suggests much higher hydrogen concentration in the annealed sample. However, by comparing to some organic materials, the concentration of hydrogen in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ is estimated to be less than 1%. Though largely enhanced after annealing, the level of hydrogen concentration remains small and can not provide an effective carrier doping. We also tried proton injection using the ionic liquid method,[44–46] and no additional effect was observed, implying a similar hydrogen concentration. On the other hand, the measured NMR spin-lattice relaxation rate takes a typical value about 1000 s$^{-1}$ for temperatures between 10 K and 200 K, which is much larger than that of the environmental protons, indicating that the hydrogen ions strongly couple to the magnetic moment in the sample. Therefore, instead of providing effective doping, hydrogen entered in the system serves as a sensitive local probe for the magnetic properties of the system.
cpl-38-6-067401-fig4.png
Fig. 4. (a) The $^1$H spectra under a field of 9 T measured on four compounds of nickelates. The spectra are measured under identical condition, such as the same sample mass, coil, NMR pulse sequences, etc. Solid lines are Lorentzian-form fit to the data. (b) Corresponding FWHM versus temperatures, obtained by the Lorentzian-form fit to the spectra.
The respective FWHM of the $^1$H NMR spectra of four compounds are shown in Fig. 4(b). For the annealed compounds, the FWHMs are very large. For example, in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ the FWHM above 50 K corresponds to 0.5% of the resonance frequency, which is over twenty times of the hyperfine shift ($\sim $0.02%). The FWHM of Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ and Nd$_{4}$Ni$_3$O$_8$ are much larger than those of unannealed samples. In particular, the FWHM in Nd$_{4}$Ni$_3$O$_8$ shows a large upturn below 40 K, similar to Nd$_{0.85}$Sr$_{0.15}$NiO$_2$. The larger FWHM reflects stronger inhomogeneity of the annealed compounds. The origin of the inhomogeneity is two-fold. On the one hand, during annealing, the removal of apical oxygen may be incomplete, and the hydrogen and Ni ions may even form Ni–H bond.[47] These can certainly increase the level of inhomogeneity in the system. Inhomogeneity promotes the localization of electrons, which explains the observed insulating behavior up to room temperature in the annealed samples. On the other hand, the prominent upturn of FWHM below 40 K in the annealed samples should be associated with magnetic fluctuations, because at low temperatures the charge and structural inhomogeneities are quenched and cannot account for such a strong temperature dependence. In the following, we discuss the nature of the observed AFM fluctuations in bulk Nd$_{0.85}$Sr$_{0.15}$NiO$_2$. At present, we cannot differentiate whether the magnetic fluctuations are caused by Nd or Ni moments. Nonetheless, the Nd magnetic ordering can be induced by the Ni magnetic ordering through their couplings.[42,48] In either case, our result indicates the existence of intrinsic order of Ni moments. Consider the dipolar hyperfine coupling between one Ni/Nd moment and the neighboring $^1$H nuclei (assuming in the apical positions), the FWHM of 4 MHz at 5.24 T gives an estimation of magnetic moment about 0.2$\mu _{\rm B}$, which is near the limit of resolution of neutron diffraction and may not be observed.[25–27] The broad peak of $^{1}T_1^{-1}$ at around 40 K indicates that the strength of magnetic couplings is in the order of 10 meV. Indeed, the super-exchange coupling among nearest Ni moments is estimated theoretically to be 10–30 meV,[5,6,14,15] consistent with our observations. Our results support the existence of antiferromagnetically interacting Ni moments and are consistent with the doped Mott insulator scenario of nickelates. The onset of short-range ordering at 40 K coincides with the upturn of the low-temperature resistivity,[24] which suggests an enhanced insulating behavior through magnetic scattering, as seen in quasi-1D Bechgaard salts.[49] A recent RIXS measurement suggests the doped hole results in a low-spin state of the Ni $3d^8$ configuration,[50] which is also consistent with this picture. Nonetheless, the alternative picture, in which the doped hole leads to a high-spin $3d^8$ Ni configuration,[23,51] cannot be differentiated from our current data. Measurements on how the AFM fluctuations and short-range order evolve with different rare earth and/or hole doping would provide more information on whether a low-spin or a high-spin configuration is favored. As an interesting proposal, the Ni$^{+1}$ moments could hybridize with Nd $5d$ itinerant electrons to form Kondo singlets.[11] In Nd$_{0.85}$Sr$_{0.15}$NiO$_2$, we do not observe an effect of Kondo-singlet formation which would cause a constant $(T_1T)^{-1}$ at low temperatures.[52] A likely picture could be such that the Nd $5d$ bands are already emptied by the hole doping of Sr$^{2+}$ at $x=0.15$, and the Kondo effect would be more relevant in the underdoped regime. Interestingly, the FWHM with temperature of Nd$_{0.85}$Sr$_{0.15}$NiO$_3$ shows a peak at about 40 K and drops rapidly at low temperatures [Fig. 4(b)]. Whether the Kondo physics is relevant there deserves further study. Finally, we discuss the implication to superconductivity of the nickelate. The observation of AFM fluctuations and short-range magnetic ordering in the bulk materials suggests that the superconductivity in the nickelate is mediated via spin fluctuations, analogous to cuprate and iron-based superconductors. In the bulk materials, the absence of superconductivity and long-range antiferromagnetism would imply strong competition between several ordered phases, while the strong inhomogeneity in the system prevents formation of any long-range order. As for the films, similar AFM fluctuations are expected, from which superconductivity emerges. However, some factors could be unique to the films. First, with typical thickness of $\sim $10 nm,[1,2] it should be easier to achieve oxygen homogeneity upon annealing, and a more homogeneous environment helps in acquiring the phase coherence for superconductivity. Indeed, in superconducting Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ films the resistivity remains metallic up to room temperature,[2] indicating a much higher level of electron homogeneity. Furthermore, the SrTiO$_3$ substrate may provide additional doping or other interfacial effects to suppress the static AFM order and induce superconductivity in thin films. An example of the interface enhanced superconductivity has been realized in the single-layer FeSe film grown on the SrTiO$_3$ substrates.[53,54] In summary, we have performed $^1$H NMR measurements on the hydrogen-annealed powdered Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ samples, and observed enhanced AFM fluctuations below 100 K and emergence of a short-range AFM order below 40 K. These properties are inherent to the strong correlations of Ni magnetic moments in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$. Although superconductivity and long-range AFM order are not observed, possibly due to the high inhomogeneity after annealing, the finding of strong AFM fluctuations pave the way for understanding of superconductivity in nickelates. References Superconductivity in an infinite-layer nickelateSuperconducting Dome in Nd 1 − x Sr x NiO 2 Infinite Layer FilmsSuperconductivity at 110 K in the infinite-layer compound (Sr1-xCax)1-yCuO2Electronic structure of possible nickelate analogs to the cupratesSimilarities and Differences between LaNiO 2 and CaCuO 2 and Implications for SuperconductivityElectronic structure, magnetism, and superconductivity in infinite-layer nickelatesWhere do the hole carriers reside in the new superconducting nickelates?Entering the Nickel Age of SuperconductivityInduced magnetic two-dimensionality by hole doping in the superconducting infinite-layer nickelate Nd 1 − x Sr x NiO 2 Robust d x 2 − y 2 -wave superconductivity of infinite-layer nickelatesSelf-doped Mott insulator for parent compounds of nickelate superconductorsComparative many-body study of Pr 4 Ni 3 O 8 and NdNiO 2 Critical Nature of the Ni Spin State in Doped NdNiO 2 Electronic structure of early 3 d -transition-metal oxides by analysis of the 2 p core-level photoemission spectraElectronic structure of the parent compound of superconducting infinite-layer nickelatesInfinite-layer La Ni O 2 : Ni 1 + is not Cu 2 + Role of 4 f states in infinite-layer NdNiO 2 A substantial hybridization between correlated Ni-d orbital and itinerant electrons in infinite-layer nickelatesOrbital Order and Possible Superconductivity in LaNiO 3 / LaMO 3 SuperlatticesLifshitz transition and frustration of magnetic moments in infinite-layer NdNiO 2 upon hole dopingGiant orbital polarization of Ni 2 + in a square planar environmentHund's metal physics: From SrNiO 2 to LaNiO 2 Absence of superconductivity in bulk Nd1−xSrxNiO2Sodium Hydride as a Powerful Reducing Agent for Topotactic Oxide Deintercalation: Synthesis and Characterization of the Nickel(I) Oxide LaNiO 2Synthesis of the infinite layer Ni(I) phase NdNiO2+x by low temperature reduction of NdNiO3 with sodium hydrideSynthesis and characterization of bulk Nd 1 − x Sr x Ni O 2 and Nd 1 − x Sr x Ni O 3 19 F NMR Study of the Bilayer Iron-Based Superconductor KCa 2 Fe 4 As 4 F 2 *Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in (Li 0.84 Fe 0.16 )OHFe 0.98 SeMott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe 2Pressure-Induced Metallization and Structural Phase Transition in the Quasi-One-Dimensional TlFeSe 2Contrasting physical properties of the trilayer nickelates Nd4Ni3O10 and Nd4Ni3O8The Effect of Electron-Electron Interaction on the Nuclear Spin Relaxation in MetalsMetal-to-metal transition and heavy-electron state in Nd 4 Ni 3 O 10 − δ Muon-spin-rotation study of magnetism in La 1.85 Sr 0.15 CuO 4 and YBa 2 Cu 3 O x below 90 mKHole and Electron Doping of RNiO3 (R = La, Nd)Neutron Diffraction and TEM Studies of the Crystal Structure and Defects of Nd4Ni3O8Photoinduced melting of magnetic order in the correlated electron insulator NdNiO 3 Spin-canted magnetism and decoupling of charge and spin ordering in NdNiO 3 Electronic properties and the nature of metal–insulator transition in NdNiO3 prepared at ambient oxygen pressureInduced noncollinear magnetic order of Nd 3 + in Nd Ni O 3 observed by resonant soft x-ray diffractionCrystal Structure and Properties of Nd4Co3O10+δ and Nd4Ni3O10−δProtonation induced high- T c phases in iron-based superconductors evidenced by NMR and magnetization measurementsIonic-Liquid-Gating Induced Protonation and Superconductivity in FeSe, FeSe 0.93 S 0.07 , ZrNCl, 1 T -TaS 2 and Bi 2 Se 3 *A selective control of volatile and non-volatile superconductivity in an insulating copper oxide via ionic liquid gatingTopotactic Hydrogen in Nickelate Superconductors and Akin Infinite-Layer Oxides A B O 2 Electronic and magnetic structure of infinite-layer NdNiO2: trace of antiferromagnetic metalOrbital and Spin Character of Doped Carriers in Infinite-Layer NickelatesType-II t − J model in superconducting nickelate Nd 1 − x Sr x NiO 2 Interface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO 3What makes the Tc of monolayer FeSe on SrTiO3 so high: a sign-problem-free quantum Monte Carlo study
[1] Li D, Lee K, Wang B, Osada M, Crossley S, Lee H R, Cui Y, Hikita Y, and Hwang H Y 2019 Nature 572 624
[2] Li D, Wang B, Lee K, Harvey S P, Osada M, Goodge B H, Kourkoutis L F, and Hwang H Y 2020 Phys. Rev. Lett. 125 027001
[3] Azuma M, Hiroi Z, Takano M, Bando Y, and Takeda Y 1992 Nature 356 775
[4] Anisimov V I, Bukhvalov D, and Rice T M 1999 Phys. Rev. B 59 7901
[5] Botana A S and Norman M R 2020 Phys. Rev. X 10 011024
[6] Fujimori A 2019 J. Club For Condens. Matter Phys. DOI:10.36471/JCCM_December_2019_02
[7] Liang Z J, Jiang R, and Ku W 2020 arXiv:2005.00022 [cond-mat.supr-con]
[8] Norman M R 2020 Physics 13 85
[9] Ryee S, Yoon H, Kim T J, Jeong M Y, and Han M J 2020 Phys. Rev. B 101 064513
[10] Wu X, Sante D D, Schwemmer T, Hanke W, Hwang H Y, Raghu S, and Thomale R 2020 Phys. Rev. B 101 060504(R)
[11] Zhang G M, Yang Y F, and Zhang F C 2020 Phys. Rev. B 101 020501(R)
[12]Goodge B H, Li D, Lee K, Osada M, Wang B Y, Sawatzky G A, Hwang H Y, and Kourkoutis L F 2020 Proc. Natl. Acad. Sci. USA 12 118
[13] Karp J, Hampe A, Zing M, Botana A S, Park H, Norman M R, and Millis A J 2020 Phys. Rev. B 102 245130
[14] Jiang M, Berciu M, and Sawatzky G A 2020 Phys. Rev. Lett. 124 207004
[15] Bocquet A E, Mizokawa T, Morikawa K, Fujimori A, Barman S R, Maiti K, Sarma D D, Tokura Y, and Onoda M 1996 Phys. Rev. B 53 1161
[16] Hepting M, Li D, Jia C J, Lu H, Paris E, Tseng Y, Feng X, Osada M, Been E, Hikita Y, Chuang Y D, Hussain Z, Zhou K J, Nag A, Garcia-Fernandez M, Rossi M, Huang H Y, Huang D J, Shen Z X, Schmitt T, Hwang H Y, Moritz B, Zaanen J, Devereaux T P, and Lee W S 2020 Nat. Mater. 19 381
[17] Lee K W and Pickett W E 2004 Phys. Rev. B 70 165109
[18] Choi M Y, Lee K W, and Pickett W E 2020 Phys. Rev. B 101 020503(R)
[19] Gu Y, Zhu S, Wang X, Hu J, and Chen H 2020 Commun. Phys. 3 84
[20] Chaloupka J and Khaliullin G 2008 Phys. Rev. Lett. 100 016404
[21] Leonov I, Skornyakov S L, and Savrasov S Y 2020 Phys. Rev. B 101 241108(R)
[22] Mandal P, Patel R K, Rout D, Banerjee R, Bag R, Karmakar K, Narayan A, Freeland J W, Singh S, and Middey S 2021 Phys. Rev. B 103 L060504
[23] Wang Y, Kang C J, Miao H, and Kotliar G 2020 Phys. Rev. B 102 161118(R)
[24] Li Q, He C, Si J, Zhu X, Zhang Y, and Wen H H 2020 Commun. Mater. 1 16
[25] Hayward M A, Green M A, Rosseinksky M J, and Sloan J 1999 J. Am. Chem. Soc. 121 8843
[26] Hayward M A and Rosseinsky M J 2003 Solid State Sci. 5 839
[27] Wang B X, Zheng H, Krivyakina E, Chmaissem O, Lopes P P, Lynn J W, Gallington L C, Ren Y, Rosenkranz S, Mitchell J F, and Phelan D 2020 Phys. Rev. Mater. 4 084409
[28] Shao Y T, Hong W S, Li S L, Li Z, and Luo J L 2019 Chin. Phys. Lett. 36 127401
[29] Wu Q, Zhou H, Wu Y, Hu L, Ni S, Tian Y, Sun F, Zhou F, Dong X, Zhao Z, and Zhao J 2020 Chin. Phys. Lett. 37 097802
[30] Jia Y T, Gong C S, Liu Y X, Zhao J F, Dong C, Dai G Y, Li X D, Lei H C, Yu R Z, Zhang G M, and Jin C Q 2020 Chin. Phys. Lett. 37 097404
[31] Liu Z Y, Dong Q X, Shan P F, Wang Y Y, Dai J H, Jana R, Chen K Y, Sun J P, Wang B S, Yu X H, Liu G T, Uwatoko Y, Sui Y, Yang H X, Chen G F, and Cheng J G 2020 Chin. Phys. Lett. 37 047102
[32] Li Q, He C, Zhu X, Si J, Fan X, and Wen H H 2021 Sci. Chin. Phys. Mech. & Astron. 64 227411
[33] Moriya T 1963 J. Phys. Soc. Jpn. 18 516
[34] Li B Z, Wang C, Yang P T, Sun J P, Liu Y B, Wu J, Ren Z, Cheng J G, Zhang G M, and Cao G H 2020 Phys. Rev. B 101 195142
[35] Kiefl R F, Brewer J H, Carolan J, Dosanjh P, Hardy W N, Kadono R, Kempton J R, Krahn R, Schleger P, Yang B X, Zhou H, Luke G M, Sternlieb B, Uemura Y J, Kossler W J, Yu X H, Ansaldo E J, Takagi H, Uchida S, and Seaman C L 1989 Phys. Rev. Lett. 63 2136
[36]Abragam A 1961 Principles of Nuclear Magnetism (Oxford: Oxford University Press)
[37] Alonso J A, Martínez-Lope M J, and Hidalgo M A 1995 J. Solid State Chem. 116 146
[38] Retoux R, Rodriguez-Carvaja J, and Lacorre P 1998 J. Solid State Chem. 140 307
[39] Caviglia A D, Först M, Scherwitzl R, Khanna V, Bromberger H, Mankowsky R, Singla R, Chuang Y D, Lee W S, Krupin O, Schlotter W F, Turner J J, Dakovski G L, Minitti M P, Robinson J, Scagnoli V, Wilkins S B, Cavill S A, Gibert M, Gariglio S, Zubko P, Triscone J M, Hill J P, Dhesi S S, and Cavalleri A 2013 Phys. Rev. B 88 220401(R)
[40] Kumar D, Rajeev K P, Alonso J A, and Martínez-Lope M J 2013 Phys. Rev. B 88 014410
[41] Hooda M K and Yadav C S 2016 Physica B 491 31
[42] Scagnoli V, Staub U, Bodenthin Y, García-Fernández M, Mulders A M, Meijer G I, and Hammerl G 2008 Phys. Rev. B 77 115138
[43] Olafsen A, Fjellvåg H, and Hauback B C 2000 J. Solid State Chem. 151 46
[44] Cui Y, Zhang G, Li H, Lin H, Zhu X, Wen H H, Wang G, Sun J, Ma M, Li Y, Gong D, Xie T, Gu Y, Li S, Luo H, Yu P, and Yu W 2018 Sci. Bull. 63 11
[45] Cui Y, Hu Z, Zhang J, Ma W, Ma M, Ma Z, Wang C, Yan J, Sun J, Cheng J, Jia S, Li Y, Wen J, Lei H, Yu P, Ji W, and Yu W 2019 Chin. Phys. Lett. 36 077401
[46] Wei X, Li H B, Zhang Q, Li D, Qin M, Xu L, Hu W, Huan Q, Yu L, Miao J, Yuan J, Zhu B, Kusmartseva A, Kusmartsev F V, Silhanek A V, Xiang T, Yu W, Lin Y, Gu L, Yu P, Chen Q, and Jin K 2020 Sci. Bull. 65 1607
[47] Si L, Xiao W, Kaufmann J, Tomczak J M, Lu Y, Zhong Z, and Held K 2020 Phys. Rev. Lett. 124 166402
[48] Liu Z, Ren Z, Zhu W, Wang Z, and Yang J 2020 npj Quantum Mater. 5 31
[49]Lebed A G 2008 The Physics of Organic Superconductors and Conductors, Springer Series in Materials Science (Berlin: Springer) vol 110
[50] Rossi M, Lu H, Nag A, Li D, Osada M, Lee K, Wang B Y, Agrestini S, Garcia-Fernandez M, Chuang Y D, Shen Z X, Hwang H Y, Moritz B, Zhou K J, Devereaux T P, and Lee W S 2020 arXiv:2011.00595 [cond-mat.str-el]
[51] Zhang Y H and Vishwanath A 2020 Phys. Rev. Res. 2 023112
[52]Hewson A C 2009 The Kondo Problem to Heavy Fermions (Cambridge: Cambridge University Press)
[53] Wang Q Y, Li Z, Zhang W H, Zhang Z C, Zhang J S, Li W, Ding H, Ou Y B, Deng P, Chang K, Wen J, Song C L, He K, Jia J F, Ji S H, Wang Y Y, Wang L L, Chen X, Ma X C, and Xue Q K 2012 Chin. Phys. Lett. 29 037402
[54] Li Z X, Wang F, Yao H, and Lee D H 2016 Sci. Bull. 61 925