In the present study, we explore superconductivity in NdNiO$_2$ and LaNiO$_2$ employing a first-principles derived low-energy model Hamiltonian, consisting of two orbitals: Ni $x^{2}$-$y^{2}$, and an {\it axial} orbital. The {\it axial} orbital is constructed out of Nd/La $d$, Ni 3$z^{2}$-$r^{2}$ and Ni $s$ characters. Calculation of the superconducting pairing symmetry and pairing eigenvalue of the spin-fluctuation mediated pairing interaction underlines the crucial role of inter-orbital Hubbard interaction in superconductivity, which turns out to be orbital-selective. The axial orbital brings in materials dependence in the problem, making NdNiO$_2$ different from LaNiO$_2$, thereby controlling the inter-orbital Hubbard interaction assisted superconductivity.
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…
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 …
We have revisited the electronic structure of infinite-layer RNiO$_2$ (R= La, Nd) in light of the recent discovery of superconductivity in Sr-doped NdNiO$_2$. From a comparison to their cuprate counterpart CaCuO$_2$, we derive essential facts related to their electronic structures, in particular the values for various hopping parameters and energy splittings, and the influence of the spacer cation. From this detailed comparison, we comment on expectations in regards to superconductivity. In particular, both materials exhibit a large ratio of longer-range hopping to near-neighbor hopping which …
The recent discovery of superconductivity in infinite-layer nickelate films has aroused great interest since it provides a new platform to explore the mechanism of high-temperature superconductivity. However, superconductivity only appears in the thin film form and synthesizing superconducting nickelate films is extremely challenging, limiting the in-depth studies on this compound. Here, we explore the critical parameters in the growth of high quality nickelate films using molecular beam epitaxy (MBE). We found that stoichiometry is crucial in optimizing the crystalline structure and realizing…
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.
Fermi liquid and isotropic superconductivity of Hund scenario for bilayer nickelates
Hangzhou Normal University（杭州师范大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Songshan Lake Materials Laboratory（松山湖材料实验室）
The discovery of superconducting nickelates reignited hope for elucidating the high-$T_{\textrm{c}}$ superconductivity mechanism in the isostructural cuprates. While in the cuprates, the superconducting gap opens up on a single-band of the quasi-2D Fermi surface, the nickelates are known to have 3D nature of electronic structure with multi-band. This raises a serious question about the role of 2D nature for the high-$T_{\textrm{c}}$ superconductivity. Here, employing dynamical mean field theory combined with GW method, we found the Kondo effect driven by the strong correlation of Nd-4$f$ and N…
Superconductivity at the metal-insulator phase boundary in a bulk nickelate at ambient pressure
Argonne National Laboratory（阿贡国家实验室）；University of Illinois Chicago（伊利诺伊大学芝加哥分校）；School of Physics, Southeast University（东南大学物理学院）；Norfolk State University（诺福克州立大学）
After the reward of more than two 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 4f orbital in th…
The discovery of superconductivity in thin films ($\sim$10 nm) of infinite-layer hole-doped NdNiO$_2$ has invigorated the field of high-temperature superconductivity research, reviving the debate over contrasting views that nickelates that are isostructural with cuprates are either (1) sisters of the high-temperature superconductors, or (2) that differences between nickel and copper at equal band filling should be the focus of attention. Each viewpoint has its merits, and each has its limitations, suggesting that such a simple picture must be superseded by a more holistic comparison of the two…
The discovery of superconductivity in infinite-layer nickelates has attracted much attention due to their association to the high-$T_c$ cuprates. Cuprate superconductivity was first demonstrated in bulk samples and subsequently in thin films. In the nickelates, however, the situation has been reversed: although surging as a bulk phenomenon, nickelate superconductivity has only been reported in thin films so far. At the same time, the specifics of infinite-layer nickelates yield distinct interface and surface effects that determine their bulk vs thin-film behavior. In this paper, we provide an …
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…
The recent discovery of superconductivity in oxygen-reduced monovalent nickelates has raised a new platform for the study of unconventional superconductivity, with similarities and differences with the cuprate high temperature superconductors. In this paper we investigate the family of infinite-layer nickelates $R$NiO$_2$ with rare-earth $R$ spanning across the lanthanide series, introducing a new and non-trivial "knob" with which to tune nickelate superconductivity. When traversing from La to Lu, the out-of-plane lattice constant decreases dramatically with an accompanying increase of Ni $ d_…
Motivated by the recent observation of superconductivity in strontium doped NdNiO$_2$, we study the superconducting instabilities in this system from various vantage points. Starting with first-principles calculations, we construct two distinct tight-binding models, a simpler single-orbital as well as a three-orbital model, both of which capture the key low energy degrees of freedom to varying degree of accuracy. We study superconductivity in both models using the random phase approximation (RPA). We then analyze the problem at stronger coupling, and study the dominant pairing instability in t…
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…
Evidence for Clean d-wave Superconductivity in Samarium Nickelates
The State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry & Chemical Engineering, Xiamen University（厦门大学化学化工学院固体表面物理化学国家重点实验室）；School of Physics, University of Electronic Science and Technology of China（电子科技大学物理学院）；State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China（电子科技大学电子薄膜与集成器件国家重点实验室）；State Key Laboratory of Cryogenic Science and Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences（中国科学院理化技术研究所低温科学与技术国家重点实验室）
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.
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…
Bulk superconductivity in quintuple-layer square-planar nickelate La6Ni5O12
University of Science and Technology of China（中国科学技术大学）；Collaborative Innovation Center of Advanced Microstructures, Nanjing University（南京大学先进微结构协同创新中心）
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$_{…
Employing first-principles density functional theory calculations and Wannierization of the low energy band structure, we analyze the electronic structure of undoped, infinite-layer nickelate compounds, NdNiO$_2$, PrNiO$_2$ and LaNiO$_2$. Our study reveals important role of non-zero $f$-ness of Nd and Pr atoms, as opposed to $f^{0}$ occupancy of La. The non-zero $f$-ness becomes effective in lowering the energy of the rare-earth 5$d$ hybridized axial orbital, thereby enhancing the electron pockets and influencing the Fermi surface topology. The Fermi surface topology of NdNiO$_2$ and PrNiO$_2$…
Mode-selective phonon effects on magnetism and superconductivity in trilayer nickelates
Universität Würzburg（维尔茨堡大学）；Nagoya University（名古屋大学）；Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所北京凝聚态物理国家研究中心）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Hefei National Laboratory, and New Cornerstone Science Laboratory（合肥国家实验室和新基石科学实验室）；Institute of Theoretical Physics, Chinese Academy of Sciences（中国科学院理论物理研究所）
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…
