The recent observation of superconductivity in thin film infinite-layer nickelates$^{1-3}$ offers a different angle to investigate superconductivity in layered oxides$^{4}$. A wide range of candidate models have been proposed$^{5-10}$, emphasizing single- or multi-orbital electronic structure, Kondo or Hund's coupling, and analogies to cuprates. Clearly, further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$. We find that the upper critical field is surprisingly isotropic at low temperatures despite the layered crystal structure. In a magnetic field the superconductivity is strongly Pauli-limited, such that the paramagnetic effect dominates over orbital de-pairing. Underlying this isotropic response is a substantial anisotropy in the superconducting coherence length, which is at least four times longer in-plane than out-of-plane. A prominent low-temperature upturn in the upper critical field indicates the presence of an unconventional ground state.
The observation of superconductivity in infinite layer nickelate (Nd,Sr)NiO$_{2}$ thin films has led to rapid theoretical and experimental investigations of these copper-oxide-analogue systems [1-15]. Superconductivity has also been found in (Pr,Sr)NiO$_{2}$ [16,17], but not previously in (La,Sr)NiO$_{2}$ [2], raising a fundamental question whether superconductivity is associated with the presence of rare-earth moments [18,19]. Here we show that with significant materials optimization, substantial portions of the La$_{1-x}$Sr$_{x}$NiO$_{2}$ phase diagram can enter the regime of coherent low-te…
We report the phase diagram of infinite layer Pr$_{1-x}$Sr$_{x}$NiO$_2$ thin films synthesized via topotactic reduction from the perovskite precursor phase using CaH$_2$. Based on the electrical transport properties, we find a doping-dependent superconducting dome extending between $x$ = 0.12 and 0.28, with a maximum superconducting transition temperature $T_{\rm{c}}$ of 14 K at $x$ = 0.18, bounded by weakly insulating behavior on both sides. In contrast to the narrower dome observed in Nd$_{1-x}$Sr$_{x}$NiO$_2$, a local $T_{\rm{c}}$ suppression near $x$ = 0.2 was not observed for the Pr$_{1-x…
The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ calls for further investigation and optimization of the synthesis of this metastable infinite-layer nickelate structure. Here, we present our current understanding of important aspects of the growth of the parent perovskite compound via pulsed laser deposition on SrTiO$_{3}$ (001) substrates, and the subsequent topotactic reduction. We find that to achieve single-crystalline, single-phase superconducting Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$, it is essential that the precursor perovskite Nd$_{0.8}$Sr$_{0.2}$NiO$_{3}$ thin fil…
The recent observation of superconductivity in infinite-layer nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ has received considerable attention. Despite the many efforts to understand the superconductivity in infinite-layer nickelates, a consensus on the underlying mechanism for the superconductivity has yet to be reached, partly owing to the challenges with the material synthesis. Here, we report the successful growth of superconducting infinite-layer Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ films by pulsed-laser deposition and soft chemical reduction. The details on growth process will be discussed.
A variety of nickel oxide compounds have long been studied for their manifestation of various correlated electron phenomena. Recently, superconductivity was observed in nanoscale infinite layer nickelate thin films of Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, epitaxially stabilized on SrTiO$_3$ substrates via topotactic reduction from the perovskite precursor phase. Here we present the synthesis and properties of PrNiO$_2$ thin films on SrTiO$_3$. Upon doping in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, we observe superconductivity with a transition temperature of 7-12 K, and robust critical current density at 2 K of 3…
Infinite-layer nickelates present a new family of potential unconventional superconductors. A key open question is the superconducting pairing symmetry. We present low-temperature measurements of the London penetration depth in optimally doped La_{0.8}Sr_{0.2}NiO_{2}, Pr_{0.8}Sr_{0.2}NiO_{2}, and Nd_{0.8}Sr_{0.2}NiO_{2}. For La and Pr-nickelates, the superfluid density shows a quadratic temperature dependence, indicating nodal superconductivity in the presence of disorder. Nd-nickelate exhibits complex low-temperature behavior, which we attribute to magnetic impurities. These results are consi…
We report the observation of superconductivity in infinite-layer Ca-doped LaNiO$_2$ (La$_{1-x}$Ca$_x$NiO$_2$) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La1-xCaxNiO2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15<x<0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient $R_H$ changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelate…
To understand the superconductivity recently discovered in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, we carried out LDA+DMFT (local density approximation plus dynamical mean-field theory) and magnetic force response calculations. The on-site correlation in Ni-$3d$ orbitals causes notable changes in the electronic structure. The calculated temperature-dependent susceptibility exhibits the Curie-Weiss behavior, indicating the localized character of its moment. From the low-frequency behavior of self-energy, we conclude that the undoped phase of this nickelate is Fermi-liquid-like contrary to cuprates. Intere…
We use first-principles calculations to find that in infinite-layer nickelates $R$NiO$_2$, the widely studied tetragonal $P4/mmm$ structure is only dynamically stable for early lanthanide elements $R$ = La-Sm. For late lanthanide elements $R$ = Eu-Lu, an imaginary phonon frequency appears at $A=(π,π,π)$ point. For those infinite-layer nickelates, condensation of this phonon mode into the $P4/mmm$ structure leads to a more energetically favorable $I4/mcm$ structure that is characterized by an out-of-phase rotation of "NiO$_4$ square". Special attention is given to two borderline cases: PmNiO$_2…
Nickelate films have recently attracted broad attention due to the observation of superconductivity in the infinite layer phase of $Nd_{0.8}Sr_{0.2}NiO_2$ (obtained by reducing Sr doped $NdNiO_3$ films) and their similarity to the cuprates high temperature superconductors. Here we report on the observation of a new type of transport in oxygen poor $Nd_{0.8}Sr_{0.2}NiO_{3-δ}$ films. At high temperatures, variable range hopping is observed while at low temperatures a novel tunneling behavior is found where Josephson-like tunneling junction characteristic with serial resistance is revealed. We at…
The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott-Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally-resolved electron energy loss spectroscopy to directly probe the Mott-Hubbard character of Nd$_{1-x}$Sr$_x$NiO$_2$. Upon doping, we observe emergent hybridization reminiscent of the Zhang-Rice singlet via the oxygen-pr…
Recently, superconductivity at about 9-15K was discovered in Nd$_{1-x}$Sr$_x$NiO$_2$ infinite-layer thin films, which has stimulated enormous interests in related rare-earth nickelates. Usually, the first step to synthesize this 112 phase is to fabricate the RNiO$_3$ phase, however, it was reported that the 113 phase is very difficult to be synthesized successfully due to the formation of unusual Ni$^{3+}$ oxidation state. And the difficulty of preparation is enhanced as the ionic radius of rare-earth element decreases. In this work, we report the synthesis and investigation on multiple physic…
Recently Li et al (2019 Nature 572 624) discovered a new type of oxide superconductor Nd0.8Sr0.2NiO2 with T$_c$ = 14 K. To classify superconductivity in this infinite-layer nickelate experimental upper critical field, B$_{c2}$(T), and the self-field critical current densities, J$_c$(sf,T), reported by Li et al (2019 Nature 572 624), are analysed in assumption of s-, d-, and p-wave pairing symmetries and single- and multiple-band superconductivity. Based on deduced the ground-state superconducting energy gap, $Δ$(0), the London penetration depth, $λ$(0), the relative jump in electronic specific…
The discovery of superconductivity in the infinite-layer nickelates has opened new perspectives in the context of quantum materials. We analyze, via first-principles calculations, the electronic properties of La$_2$NiO$_3$F -- the first single-layer T'-type nickelate -- and compare these properties with those of related nickelates and isostructural cuprates. We find that La$_2$NiO$_3$F is essentially a single-band system with a Fermi surface dominated by the Ni-3$d_{x^2-y^2}$ states with an exceptional 2D character. In addition, the hopping ratio is similar to that of the highest $T_c$ cuprate…
Achieving superconductivity in infinite-layer nickelate thin films by aluminum sputtering deposition
Laboratoire Albert Fert, CNRS, Thales, Université Paris-Saclay（阿尔伯特·费尔特实验室，法国国家科学研究中心，泰雷兹集团，巴黎萨克雷大学）；Laboratoire de Physique des Solides, CNRS, Université Paris-Saclay（固体物理实验室，法国国家科学研究中心，巴黎萨克雷大学）；Synchrotron SOLEIL（索莱尔同步辐射中心）
Motivated by the recent discovery of superconductivity in doped NdNiO$_2$, we study the magnetic exchange interaction $J$ in layered $d^9$ nickelates from first principles. The mother compounds of the high-$T_{\rm c}$ cuprates belong to the charge-transfer regime in the Zaanen-Sawatzky-Allen diagram and have $J$ larger than 100 meV. While this feature makes the cuprates very different from other transition metal oxides, it is of great interest whether layered $d^9$ nickelates can also have such a large $J$. However, one complexity is that NdNiO$_2$ is not a Mott insulator due to carrier doping…
Superconductivity can be destroyed by a magnetic field with an upper bound known as the Pauli-limit in spin-singlet superconductors. Almost all the discovered superconductors are spin-singlet, with the highest transition temperature $T_c$ at ambient pressure achieved in the cuprate family. The closest cuprate analogue is the recently discovered infinite-layer nickelate, which hosts substantial structural and electronic similarity to the cuprate. A previous magnetotransport study on Nd$_{0.775}$Sr$_{0.225}$NiO$_2$ has observed an isotropic Pauli-limited upper critical field. Here, we report a l…
High-temperature superconductivity in Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ membranes under pressure
SLAC National Accelerator Laboratory（SLAC国家加速器实验室）；Stanford Institute for Materials and Energy Sciences（斯坦福材料能源科学研究所）；Stanford University（斯坦福大学）；Fudan University（复旦大学）
Using \emph{ab initio} band structure methods and DFT+dynamical mean-field theory approach we explore the possible formation of spin and charge stripes in the Ni-O plane of hole-doped infinite-layer nickelates, $R$NiO$_2$. Our results reveal a remarkable instability of the $C$-type $(110)$ spin state with undistorted lattice towards the formation of the spin-density, charge- and bond-disproportionation stripe phases accompanied by in-plane``breathing''-like distortions of the crystal structure. Our work gives a comprehensive picture of competing charge and spin stripe states, with possible fru…
Geometric effects in the infinite-layer nickelates $R$NiO$_2$ associated with the relative size of the $R$-site atom are investigated via first-principles calculations. We consider, in particular, the prospective YNiO$_2$ material to illustrate the impact of these effects. Compared to LaNiO$_2$, we find that the La $\to$ Y substitution is equivalent to a pressure of 19 GPa and that the presence of topotactic hydrogen can be precluded. However, the electronic structure of YNiO$_2$ departs from the cuprate-like picture due to an increase in both self-doping effect and $e_g$ hybridization. Furthe…
In this paper we investigate the electronic and superconducting properties in nickelate compounds of NiO2 through Perturbative Density Functional Theory (P-DFT) and multi-band BCS theory. We consider three different cases of study (i) the crystalline structure composed by two layers of NiO2, with a central single atom of Nd, Yb or Pr, which is subjected to mechanical compression and tension forces (pressure), (ii) the crystalline structure composed by three layers of NiO2 with central atom pairs of Nd-Nd, Nd-Sr or Sr-Sr, where the level doping x correspond to change between Nd/Sr and (iii) thr…
We analyze the electronic structure of different surface terminations for infinite-layer nickelates. Surface NiO$_2$ layers are found to be buckled, in contrast to planar bulk layers. While the rare-earth terminated surface fermiology is similar to the bulk limit of the nickelates, the NiO$_2$ terminated surface band structure is significantly altered, originating from the effect of absence of rare-earth atoms on the crystal field splitting. Contrary to the bulk Fermi surfaces, there are two Ni-$3d$ Fermi pockets, giving rise to enhanced spectral weight around the $\bar{\text{M}}$ point in the…
Dimensionality of vortex matter in superconducting infinite-layer nickelates
Laboratoire Albert Fert, CNRS, Thales, Université Paris-Saclay（阿尔贝·费尔实验室，法国国家科学研究中心，泰雷兹集团，巴黎-萨克雷大学）；Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos（实验科学与技术高等学院，胡安卡洛斯国王大学）；GFMC, Dpto. de Física de Materiales, Facultad de Ciencias Físicas, Universidad Complutense de Madrid（马德里康普顿斯大学物理科学学院材料物理系，铁磁性与巨磁阻研究组）
We compute the nominal magnetic penetration depth of $R$NiO$_2$ ($R =$ La, Nd) from first principles calculations and discuss the results in relation to the superconducting $T_c$. We find a marked discrepancy with the well established phenomenology that correlates these two quantities in cuprates (Uemura plot). We also consider the 2D ultrathin limit and estimate the maximum attainable $T_c$ to be $\sim 180$ K according to the Nelson-Kosterliz universal relation between the superfluid density and the transition temperature.
Electronically Inactive Intercalated La$_2$NiO$_4$ Layer in Superconducting La$_5$Ni$_3$O$_{11}$
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（北京凝聚态物理国家实验室和中国科学院物理研究所）；Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences（中国科学院大学卡尔·萨根理论科学研究所）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；New Cornerstone Science Laboratory, Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（新基石科学实验室、北京凝聚态物理国家实验室和中国科学院物理研究所）
Heterostructuring as Gateway to Electron Doping of Nickelate Superconductors
Northwest University（西北大学）；University of Hyogo（兵库大学）；Kyushu Institute of Technology（九州工业大学）；Soochow University（苏州大学）；Universität Würzburg（维尔茨堡大学）；Southeast University（东南大学）；University of Science and Technology of China（中国科学技术大学）；TU Wien（维也纳工业大学）
Based on first-principles calculations including a Coulomb repulsion term, we identify trends in the electronic reconstruction of $A$NiO$_2$/SrTiO$_3$(001) ($A=$ Pr, La) and $A$CuO$_2$/SrTiO$_3$(001) ($A=$ Ca, Sr). Common to all cases is the emergence of a quasi-two-dimensional electron gas (q2DEG) in SrTiO$_3$(001), albeit the higher polarity mismatch at the interface of nickelates vs. cuprates to the nonpolar SrTiO$_3(001)$ substrate (${3+}/0$ vs. ${2+}/0$) results in an enhanced q2DEG carrier density. The simulations reveal a significant dependence of the interfacial Ti $3d_{xy}$ band bendi…
The newly found superconductivity in infinite-layer nickelate superconducting films has attracted much attention, because their crystalline and electronic structures are similar to high-$T_c$ cuprate superconductors. The upper critical field can provide much information on superconductivity, but detailed experimental data are still lacking in these films. Here we present temperature and angle dependence of resistivity measured under different magnetic fields ($H$) in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ thin films. The onset superconducting transition occurs at about 16.2 K at 0 T. Temperature depend…
Motivated by the recent discovery of superconductivity in infinite-layer nickelate thin films, we report on a synthesis and magnetization study on bulk samples of the parent compounds ${R}$NiO$_{2}$ (${R}$=La, Pr, Nd). The frequency-dependent peaks of the AC magnetic susceptibility, along with remarkable memory effects, characterize spin-glass states. Furthermore, various phenomenological parameters via different spin glass models show strong similarity within these three compounds as well as with other rare-earth metal nickelates. The universal spin-glass behaviour distinguishes the nickelate…
{\it Ab initio} prediction of $d_{x^2-y^2}$-wave superconductivity in infinite-layer nickelates
National Taiwan University（台湾大学）；National Center for Theoretical Sciences（国家理论科学中心）；Institute of Atomic and Molecular Sciences, Academia Sinica（中央研究院原子与分子科学研究所）
We derive the effective single-band Hamiltonian in the flat NiO$_2$ planes for nickelate compounds Nd$_{1-x}$Sr$_x$NiO$_2$. We first implement the first-principles calculation to study electronic structures of nickelates using the Heyd-Scuseria-Ernzerhof hybrid density functional and derive a three-band Hubbard model for Ni-O $pdσ$ bands of Ni$^+$ $3d_{x^2-y^2}$ and O$^{2-}$ $2p_{x/y}$ orbitals in the NiO$_2$ planes. To obtain the effective one-band $t$-$t'$-$J$ model Hamiltonian, we perform the exact diagonalization of the three-band Hubbard model for the Ni$_5$O$_{16}$ cluster and map the lo…
We report a muon spin rotation ($μ$SR) study of the cuprate-analog nickelate La$_4$Ni$_3$O$_8$, which undergoes a transition at 105 K to a low-temperature phase with charge-stripe and antiferromagnetic (AFM) order on square planar NiO$_2$ layers. Zero-field $μ$SR shows that this transition is abrupt and that the AFM configuration is commensurate. Comparison of observed muon precession frequencies with Ni dipolar field calculations yields Ni moments $\lesssim 0.5μ_B$. Dynamic muon spin relaxation above 105 K suggests critical slowing of Ni spin fluctuations, but is inconsistent with correspondi…
Neutron and X-ray scattering studies have provided strong evidence for coupled spatial modulations of charge and spin densities in layered nickelates and cuprates. The accumulated results for La(2-x)Sr(x)NiO(4+d) are consistent with the strongly-modulated topological-stripe concept. Clues from Nd-doped La(2-x)Sr(x)CuO(4) suggest similar behavior for the cuprates. The experimental results are summarized, and features that conflict with an interpretation based on a Fermi-surface instability are emphasized. A rationalization for the differences in transport properties between the cuprates and nic…
We propose that the superconductivity recently observed in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ with critical temperature in the range $9$ K to $15$ K results from the same charge carriers and the same mechanism that we have proposed give rise to superconductivity in both hole-doped and electron-doped cuprates: pairing of hole carriers in oxygen $pπ$ orbitals, driven by a correlated hopping term in the effective Hamiltonian that lowers the kinetic energy, as described by the theory of hole superconductivity. We predict a large increase in $T_c$ with compressive epitaxial strain.
The recent discovery of the superconductivity in the doped infinite layer nickelates $R$NiO$_2$ ($R$=La, Pr, Nd) is of great interest since the nickelates are isostructural to doped (Ca,Sr)CuO$_2$ having superconducting transition temperature ($T_{\rm c}$) of about 110 K. Verifying the commonalities and differences between these oxides will certainly give a new insight into the mechanism of high $T_{\rm c}$ superconductivity in correlated electron systems. In this paper, we review experimental and theoretical works on this new superconductor and discuss the future perspectives for the "nickel …
Topotactic transformations between related crystal structures are a powerful emerging route for the synthesis of novel quantum materials. Whereas most such "soft chemistry" experiments have been carried out on polycrystalline powders or thin films, the topotactic modification of single crystals -- the gold standard for physical property measurements on quantum materials -- has proven remarkably difficult. Here, we report the topotactic reduction of La$_{1-x}$Ca$_{x}$NiO$_3$ single-crystals to La$_{1-x}$Ca$_{x}$NiO$_{2+δ}$ using CaH$_2$ as the reducing agent. The transformation from the three-d…
The correlated electronic structure of the infinite-layer compounds NdNiO$_2$ and SrCuO$_2$ at stoichiometry and with finite hole doping is compared. Key differences are elucidated from an advanced first-principles many-body perspective. Contrary to the charge-transfer insulating cuprate, the self-doped nickelate remains non-insulating even for large interaction strength, though the Ni-$d_{x^2-y^2}$ spectral weight is also gapped in that limit. Hybridization between Ni$(3d)$ and Nd$(5d)$ is crucial for the appearance of the self-doping band. Upon realistic hole doping, Sr$_{1-y}$CuO$_2$ shows …
Inspired by a recently proposed superconducting mechanism for a new cuprate superconductor Ba$_2$CuO$_{3+δ}$, we theoretically design an unconventional nickelate superconductor with $d^8$ electron configuration. Our strategy is to enlarge the on-site energy difference between $3d_{x^2-y^2}$ and other $3d$ orbitals by adopting halogens or hydrogen as out-of-plane anions, so that the $3d$ bands other than $d_{x^-y^2}$ lie just below the Fermi level for the $d^8$ configuration, acting as incipient bands that enhance superconductivity. We also discuss a possible relevance of the present proposal t…
Infinite-layer (IL) nickelates are an emerging class of superconductors, where the Ni$^{1+}$ valence state in a square planar NiO$_2$ coordination can only be reached via topotactic reduction of the perovskite phase. However, this topotactic soft chemistry with hydrogenous reagents is still at a stage of rapid development, and there is a number of open issues, especially considering the possibility of hydrogen incorporation. Here we study the time dependence of the topotactic transformation of LaNiO$_3$ to LaNiO$_2$ for powder samples with x-ray diffraction and gas extraction techniques. While…
New dielectric transitions are observed at common temperatures 32K and 245K, in isostructural $La_{2}CuO_{4+x}$ and $La_{5/3}Sr_{1/3}NiO_{4},$ that are signatures of local lattice (octahedral) instabilities. The present dielectric transitions reveal new aspects of the phase diagram of the perovskite cuprates and nickelates. They suggest that competition and coexistence of superconductivity with dielectricity occurs that is analogous to that between superconductivity and anti-ferromagnetism. These results also indicate that inhomogeneous electronic states, such as charge stripes and oxygen orde…
Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials. One prime materials platform has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd$_{0.8}$Sr$_{0.2}$NiO$_2$. Undoped NdNiO$_2$ belongs to a series of layered square-planar nickelates with chemical formula Nd$_{n+1}$Ni$_n$O$_{2n+2}$ and is known as the 'infinite-layer' ($n = \infty$) nickelate. Here, we report the s…
