Search

The observation of high-Tc superconductivity (HTSC) in concomitant with pressure-induced orthorhombic-tetragonal structural transition in bilayer La3Ni2O7 has sparked hopes of achieving HTSC by stabilizing the tetragonal phase at ambient pressure. Chemical pressure, introduced by replacing La3+ with smaller rare-earth R3+ has been considered as a potential route. However, our experimental and theoretical investigation reveals that such substitutions, despite causing lattice contraction, actually produce stronger orthorhombic distortions, requiring higher pressures for the structural transition. A linear extrapolation of Pc versus the average size of A-site cations (A>), yields a putative critical value of A>c ≈ 1.23 Å for Pc ≈ 1 bar. The negative correlation between Pc and A> indicates that replacing La3+ with smaller R3+ ions is unlikely to reduce Pc to ambient pressure. Instead, substituting La3+ with larger cations like Sr2+ or Ba2+ might be a feasible approach. Our results provide guidance for realizing ambient-pressure HTSC in bilayer nickelates. [ABSTRACT FROM AUTHOR]

The recent discovery of superconductivity in infinite-layer nickelate films has sparked significant interest and expanded the realm of superconductors, in which the infinite-layer structure and proper chemical doping are both of the essence. Nonetheless, the reasons for the absence of superconductivity in bulk infinite-layer nickelates remain puzzling. Herein, we investigate atomic defects and electronic structures in bulk infinite-layer Nd0.8Sr0.2NiO2 using scanning transmission electron microscopy. Our observations reveal the presence of three-dimensional (3D) block-like structural domains resulting from intersecting defect structures, disrupting the continuity within crystal grains, which could be a crucial factor in giving rise to the insulating character and inhibiting the emergence of superconductivity. Moreover, the infinite-layer structure, without complete topotactic reduction, retains interstitial oxygen atoms on the Nd atomic plane in bulk nickelates, possibly further aggravating the local distortions of NiO2 planes and hindering the superconductivity. These findings shed light on the existence of structural and atomic defects in bulk nickelates and provide valuable insights into the influence of proper topotactic reduction and structural orders on superconductivity. The authors study the bulk form of infinite-layer Nd0.8Sr0.2NiO2 using scanning transmission electron microscopy. They argue that these structural defects and local distortions could be responsible for the lack of superconductivity in bulk infinite-layer nickelates. [ABSTRACT FROM AUTHOR]

The mechanism of unconventional superconductivity in correlated materials remains a great challenge in condensed matter physics. The recent discovery of superconductivity in infinite-layer nickelates, as an analog to high-Tc cuprates, has opened a new route to tackle this challenge. By growing 8 nm Pr0.8Sr0.2NiO2 films on the (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate, we successfully raise the superconducting onset transition temperature Tc in the widely studied SrTiO3-substrated nickelates from 9 K into 15 K, which indicates compressive strain is an efficient protocol to further enhance superconductivity in infinite-layer nickelates. Additionally, the x-ray absorption spectroscopy, combined with the first-principles and many-body simulations, suggest a crucial role of the hybridization between Ni and O orbitals in the unconventional pairing. These results also suggest the increase of Tc be driven by the change of charge-transfer nature that would narrow the origin of general unconventional superconductivity in correlated materials to the covalence of transition metals and ligands. The recent discovery of superconductivity in the nickelates provides another angle to investigate this phenomenon in the high-Tc cuprates and hopefully help solve the mechanism of their unconventional superconductivity. Here, the authors report an increase in Tc for Pr0.8Sr0.2NiO2 where strain from the underlying LSAT substrate plays a possible role, supporting simulations also reveal the contributing role Ni and O orbitals hybridisation play in the unconventional pairing. [ABSTRACT FROM AUTHOR]

A key open question in the study of layered superconducting nickelate films is the role that hydrogen incorporation into the lattice plays in the appearance of the superconducting state. Due to the challenges of stabilizing highly crystalline square planar nickelate films, films are prepared by the deposition of a more stable parent compound which is then transformed into the target phase via a topotactic reaction with a strongly reducing agent such as CaH2. Recent studies, both experimental and theoretical, have introduced the possibility that the incorporation of hydrogen from the reducing agent into the nickelate lattice may be critical for the superconductivity. In this work, we use secondary ion mass spectrometry to examine superconducting La1−xXxNiO2 / SrTiO3 (X = Ca and Sr) and Nd6Ni5O12 / NdGaO3 films, along with non-superconducting NdNiO2 / SrTiO3 and (Nd,Sr)NiO2 / SrTiO3. We find no evidence for extensive hydrogen incorporation across a broad range of samples, including both superconducting and non-superconducting films. Theoretical calculations indicate that hydrogen incorporation is broadly energetically unfavorable in these systems, supporting our conclusion that extensive hydrogen incorporation is not generally required to achieve a superconducting state in layered square-planar nickelates.The role of hydrogen in engendering superconductivity in layered nickelates is under intense debate. Here, the authors perform secondary ion mass spectroscopy and see no evidence for extensive hydrogen incorporation into superconducting nickelates. [ABSTRACT FROM AUTHOR]

We explore the optical properties of La3Ni2O7 bilayer nickelates by using density functional theory including a Coulomb repulsion term. Convincing agreement with recent experimental ambient-pressure spectra is achieved for U ~ 3 eV, which permits tracing the microscopic origin of the characteristic features. Simultaneous consistency with angle-resolved photoemission spectroscopy and x-ray diffraction suggests the notion of rather moderate electronic correlations in this novel high-Tc superconductor. Oxygen vacancies form predominantly at the inner apical sites and renormalize the optical spectrum quantitatively, while the released electrons are largely accommodated by a defect state. We show that the structural transition occurring under high pressure coincides with a significant enhancement of the Drude weight and a reduction of the out-of-plane interband contribution that acts as a fingerprint of the emerging hole pocket. We further calculate the optical spectra for various possible magnetic phases including spin-density waves and discuss the results in the context of experiment. Finally, we investigate the role of the 2–2 versus 1–3 layer stacking and compare the bilayer nickelate to La4Ni3O10, La3Ni2O6, and NdNiO2, unveiling general trends in the optical spectrum as a function of the formal Ni valence in Ruddlesden–Popper versus reduced Ruddlesden–Popper nickelates. [ABSTRACT FROM AUTHOR]

Despite obvious similarities in their electronic and crystallographic structures, it remains unclear whether the interactions that shape the normal and superconducting (SC) state properties of high-Tc cuprates and infinite-layer nickelates (ILNs) have the same origin. This question has been brought into sharper focus with recent studies on ILNs of improved crystallinity that reveal a SC dome of comparable extent and similar transport properties above Tc as the hole-doped cuprates. The evolution of these properties in the magnetic-field-induced normal state, however, has yet to be determined. Here, we examine the magnetotransport properties of new-generation Nd1−xSrxNiO2 films in the T → 0 limit across the phase diagram in fields up to 54 T. This extensive study reveals that the limiting low-T form of the normal-state resistivity in ILNs exhibits non-Fermi-liquid behaviour over an extended doping range inside the SC dome, rather than at a singular quantum critical point. While there are clear differences in the charge dynamics of ILNs and cuprates, most notably in the magnetoresistance, our findings reveal that both systems exhibit anomalous metallicity characteristic of a quantum critical phase.Recent advancements in the sample quality of infinite-layer nickelates have made systematic comparisons with cuprates increasingly relevant. Hsu et al. report anomalous electronic transport in high-crystallinity (Nd,Sr)NiO2 at large magnetic fields, map out the transport phase diagram, and discuss similarities and differences with cuprates. [ABSTRACT FROM AUTHOR]

Knowledge of the explicit mechanisms of charge transport is preeminent for a fundamental understanding of the metal-to-insulator transition in ABO3-type perovskite rare-earth nickelates and for potential applications of these technologically promising materials. Here we suggest that owing to intrinsic Jahn–Teller-driven carrier localization, small-polaron transport is innate in nickelates. We demonstrate experimental evidence for such transport by investigating AC conductivity over a broad range of temperatures and frequencies in epitaxial SmNiO3 films. We reveal the hopping mechanism of conductivity, Holstein-type activation energy for hopping, nonclassical relaxation behavior, and nonclassical consistency between activation and relaxation. By analyzing these observations, we validate small-polaron transport. We anticipate that our findings can lead to precise tailoring of the DC and AC conductivity in nickelates as requested for fruitful employment of these materials. We also believe that further investigations of self-trapped small polarons are essential for a comprehensive understanding of nickelates. [ABSTRACT FROM AUTHOR]

The discovery of superconductivity in hole-doped infinite layer nickelates, RNiO2 (R = Nd, Pr, La) has resulted in sustained interest in the field. A definitive picture of low-energy many-body states has not yet emerged. Here we provide insights into the low-energy physics, based on our embedded dynamical mean-field theory calculations, and propose a correlation (U)-temperature (T) phase diagram. The key features are a low-T Fermi liquid (FL) phase, a high-T Curie-Weiss regime, and an antiferromagnetic phase in a narrow U-T region. We associate the onset of the FL phase with partial screening of Ni-d moments; however, full screening occurs at lower temperatures. This may be related to insufficiency of conduction electrons to effectively screen the Ni-d moments, suggestive of Nozieres Exhaustion Principle. Our results suggest that RNiO2 are in the paramagnetic state, close to an antiferromagnetic dome, making magnetic fluctuations feasible. This may be consequential for superconductivity. Motivated by the recent discovery of superconductivity in infinite layer nickelates, the authors study the correlation and temperature phase diagram of LaNiO2 and NdNiO2, using embedded DMFT combined with DFT. Their study offers insight into the low-energy physics of the paramagnetic and magnetic states, shedding some light on the mechanism of superconductivity and lack of magnetic order in these systems. [ABSTRACT FROM AUTHOR]

We report a determinant quantum Monte Carlo study of a two-band model, inspired by infinite-layer nickelates, focusing on the influence of interlayer hybridization between 3 d x 2 − y 2 orbitals derived from Ni (or Ni and O) in one layer and rare-earth (R) 5d orbitals in the other layer, hereafter the Ni and R layers, respectively. For a filling with one electron shared between the two layers on average, interlayer hybridization leads to "self-doped" holes in the Ni layer and the absence of antiferromagnetic ordering, but rather the appearance of spin-density and charge-density stripe-like states. As the interlayer hybridization increases, both the Ni and R layers develop antiferromagnetic correlations, even though either layer individually remains away from half-filling. For hybridization within an intermediate range, roughly comparable to the intralayer nearest-neighbor hopping tNi, the model develops signatures of Kondo-like physics. [ABSTRACT FROM AUTHOR]

High-temperature unconventional superconductivity quite generically emerges from doping a strongly correlated parent compound, often (close to) an antiferromagnetic insulator. The recently developed dynamical vertex approximation is a state-of-the-art technique that has quantitatively predicted the superconducting dome of nickelates. Here, we apply it to study the effect of pressure in the infinite-layer nickelate SrxPr1−xNiO2. We reproduce the increase of the critical temperature (Tc) under pressure found in experiment up to 12 GPa. According to our results, Tc can be further increased with higher pressures. Even without Sr-doping the parent compound, PrNiO2, will become a high-temperature superconductor thanks to a strongly enhanced self-doping of the Ni d x 2 − y 2 orbital under pressure. With a maximal Tc of 100 K around 100 GPa, nickelate superconductors can reach that of the best cuprates. The authors theoretically study the pressure dependence of the phase diagram of the nickelate PrNiO2 with and without Sr doping. At high pressure, they find that the superconducting dome is significantly enhanced in both Tc and doping-range of superconductivity compared with ambient pressure, with a maximal Tc of 100 K around 100 GPa in absence of external doping. [ABSTRACT FROM AUTHOR]

Motivated by the recent observation of superconductivity with Tc ~ 80 K in pressurized La3Ni2O71, we explore the structural and electronic properties of A3Ni2O7 bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La3Ni2O7 at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations between Tc and the in-plane Ni-O-Ni bond angles for La3Ni2O7. Moreover, two structural phases with significant c+ octahedral rotations and in-plane bond disproportionations are uncovered for A = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Ni eg manifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb3Ni2O7 as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism. [ABSTRACT FROM AUTHOR]

Superconducting nickelates in thin film perovskite structures, designed to be isostructural with the Cu(II) cuprates, have been discovered recently by Zhou et al. (Appl. Mater. Interfaces 10, 1463-1467, 1) and Li et al. (Nature 572, 624-627, 2) The strategies of both groups, to make nickelates as similar to the cuprates as possible by using planar NiO2 superlattice structures, proved successful. Ni(I) and Ni(III) are superconducting in superlattices, although TC is lower than in most Cu(II) cuprates. Here we find that a "3-oxidation states rule" applies to nickelate superlattices based on Ni(I) and Ni(III). It is predicted that the oxides of Ni(II), Cu(I), and Cu(III) cannot superconduct, because of violation of the 3-rule. This rule is derived here, along with other rules where the number of interacting oxidation states is decisive for the conduction properties of the metal ion. The Mott model is replaced by a free energy model. [ABSTRACT FROM AUTHOR]

The multiple quantum transitions within d-band correlation oxides such as rare-earth nickelates (RENiO3) triggered by critical temperatures and/or hydrogenation opened up a new paradigm for correlated electronics applications, e.g. ocean electric field sensor, bio-sensor, and neuron synapse logical devices. Nevertheless, these applications are obstructed by the present ineffectiveness in the thin film growth of the metastable RENiO3 with flexibly adjustable rare-earth compositions and electronic structures. Herein, we demonstrate a metal-organic decompositions (MOD) approach that can effectively grow metastable RENiO3 covering a large variety of the rare-earth composition without introducing any vacuum process. Unlike the previous chemical growths for RENiO3 relying on strict interfacial coherency that limit the film thickness, the MOD growth using reactive isooctanoate percussors is tolerant to lattice defects and therefore achieves comparable film thickness to vacuum depositions. Further indicated by positron annihilation spectroscopy, the RENiO3 grown by MOD exhibit large amount of lattice defects that improves their hydrogen incorporation amount and electron transfers, as demonstrated by the resonant nuclear reaction analysis and near edge X-ray absorption fine structure analysis. This effectively enlarges the magnitude in the resistance regulations in particular for RENiO3 with lighter RE, shedding a light on the extrinsic regulation of the hydrogen induced quantum transitions for correlated oxides semiconductors kinetically via defect engineering. [ABSTRACT FROM AUTHOR]

We provide a set of computational experiments based on ab initio calculations to elucidate whether a cuprate-like antiferromagnetic insulating state can be present in the phase diagram of the low-valence layered nickelate family (R n + 1 Ni n O 2 n + 2 , R= rare-earth, n = 1 - ∞ ) in proximity to half-filling. It is well established that at d 9 filling the infinite-layer ( n = ∞ ) nickelate is metallic, in contrast to cuprates wherein an antiferromagnetic insulator is expected. We show that for the Ruddlesden-Popper (RP) reduced phases of the series (finite n) an antiferromagnetic insulating ground state can naturally be obtained instead at d 9 filling, due to the spacer RO 2 fluorite slabs present in their structure that block the c-axis dispersion. In the n = ∞ nickelate, the same type of solution can be derived if the off-plane R-Ni coupling is suppressed. We show how this can be achieved if a structural element that cuts off the c-axis dispersion is introduced (i.e. vacuum in a monolayer of RNiO 2 , or a blocking layer in multilayers formed by (RNiO 2 ) 1 /(RNaO 2 ) 1 ). [ABSTRACT FROM AUTHOR]

A charge order (CO) with a wavevector q ≃ 1 3 , 0 , 0 is observed in infinite-layer nickelates. Here we use first-principles calculations to demonstrate a charge-transfer-driven CO mechanism in infinite-layer nickelates, which leads to a characteristic Ni1+-Ni2+-Ni1+ stripe state. For every three Ni atoms, due to the presence of near-Fermi-level conduction bands, Hubbard interaction on Ni-d orbitals transfers electrons on one Ni atom to conduction bands and leaves electrons on the other two Ni atoms to become more localized. We further derive a low-energy effective model to elucidate that the CO state arises from a delicate competition between Hubbard interaction on Ni-d orbitals and charge transfer energy between Ni-d orbitals and conduction bands. With physically reasonable parameters, q = 1 3 , 0 , 0 CO state is more stable than uniform paramagnetic state and usual checkerboard antiferromagnetic state. Our work highlights the multi-band nature of infinite-layer nickelates, which leads to some distinctive correlated properties that are not found in cuprates. Recent experiments reported charge order with a stripe pattern in parent compounds of infinite-layer nickelate superconductors. Chen et al. use first principles and effective model calculations to propose an electronic, charge-transfer-driven mechanism of the charge order. [ABSTRACT FROM AUTHOR]

Recent discovery of superconductivity in the doped infinite-layer nickelates has renewed interest in understanding the nature of high-temperature superconductivity more generally. The low-energy electronic structure of the parent compound NdNiO2, the role of electronic correlations in driving superconductivity, and the possible relationship between the cuprates and the nickelates are still open questions. Here, by comparing LaNiO2 and NdNiO2 systematically within a parameter-free, all-electron first-principles density-functional theory framework, we reveal the role of Nd 4f electrons in shaping the ground state of pristine NdNiO2. Strong similarities are found between the electronic structures of LaNiO2 and NdNiO2, except for the effects of the 4f electrons. Hybridization between the Nd 4f and Ni 3d orbitals is shown to significantly modify the Fermi surfaces of various magnetic states. In contrast, the competition between the magnetically ordered phases depends mainly on the gaps in the Ni 3 d x 2 − y 2 band. Our estimated value of the on-site Hubbard U in the nickelates is similar to that in the cuprates, but the value of the Hund's coupling JH is found to be sensitive to the Nd magnetic moment. In contrast with the cuprates, NdNiO2 presents 3D magnetism with competing antiferromagnetic and (interlayer) ferromagnetic exchange, which may explain why the Tc is lower in the nickelates. The recent discovery of superconducting nickelates has reignited interest in these materials and whether they can shed light on the mechanism of unconventional superconductivity in the cuprates. Here, the authors use first principles calculations to investigate the f electrons and magnetic ordering effects in the infinite layer nickelates and elaborate on the role of the cuprate-like 3dx2-y2 band. [ABSTRACT FROM AUTHOR]

Ruddlesden-Popper and reduced Ruddlesden-Popper nickelates are intriguing candidates for mimicking the properties of high-temperature superconducting cuprates. The degree of similarity between these nickelates and cuprates has been the subject of considerable debate. Resonant inelastic x-ray scattering (RIXS) has played an important role in exploring their electronic and magnetic excitations, but these efforts have been stymied by inconsistencies between different samples and the lack of publicly available data for detailed comparison. To address this issue, we present open RIXS data on La4Ni3O10 and La4Ni3O8. [ABSTRACT FROM AUTHOR]

The newly discovered nickelate superconductors so far only exist in epitaxial thin films synthesized by a topotactic reaction with metal hydrides1. This method changes the nickelates from the perovskite to an infinite-layer structure by deintercalation of apical oxygens1–3. Such a chemical reaction may introduce hydrogen (H), influencing the physical properties of the end materials4–9. Unfortunately, H is insensitive to most characterization techniques and is difficult to detect because of its light weight. Here, in optimally Sr doped Nd0.8Sr0.2NiO2H epitaxial films, secondary-ion mass spectroscopy shows abundant H existing in the form of Nd0.8Sr0.2NiO2Hx (x ≅ 0.2–0.5). Zero resistivity is found within a very narrow H-doping window of 0.22 ≤ x ≤ 0.28, showing unequivocally the critical role of H in superconductivity. Resonant inelastic X-ray scattering demonstrates the existence of itinerant interstitial s (IIS) orbitals originating from apical oxygen deintercalation. Density functional theory calculations show that electronegative H– occupies the apical oxygen sites annihilating IIS orbitals, reducing the IIS–Ni 3d orbital hybridization. This leads the electronic structure of H-doped Nd0.8Sr0.2NiO2Hx to be more two-dimensional-like, which might be relevant for the observed superconductivity. We highlight that H is an important ingredient for superconductivity in epitaxial infinite-layer nickelates.In optimally doped Nd0.8Sr0.2NiO2H epitaxial film, combined state-of-the-art experimental and theoretical approaches show abundant hydrogen with zero resistivity, and its critical role in superconductivity in epitaxial infinite-layer nickelates. [ABSTRACT FROM AUTHOR]

The recent and exciting discovery of superconductivity in the hole-doped infinite-layer nickelate Nd1−δSrδNiO2 draws strong attention to correlated quantum materials. From a theoretical view point, this class of unconventional superconducting materials provides an opportunity to unveil a physics hidden in correlated quantum materials. Here we study the temperature and doping dependence of the local spectrum as well as the charge, spin and orbital susceptibilities from first principles. By using ab initio LQSGW+DMFT methodology, we show that onsite Hund's coupling in Ni-d orbitals gives rise to multiple signatures of Hund's metallic phase in Ni-eg orbitals. The proposed picture of the nickelates as an eg (two orbital) Hund's metal differs from the picture of the Fe-based superconductors as a five orbital Hund's metal as well as the picture of the cuprates as doped charge transfer insulators. Our finding uncover a new class of the Hund's metals and has potential implications for the broad range of correlated two orbital systems away from half-filling. [ABSTRACT FROM AUTHOR]

The discovery of unconventional superconductivity in hole doped NdNiO2, similar to CaCuO2, has received enormous attention. However, different from CaCuO2, RNiO2 (R = Nd, La) has itinerant electrons in the rare-earth spacer layer. Previous studies show that the hybridization between Ni- d x 2 − y 2 and rare-earth-d orbitals is very weak and thus RNiO2 is still a promising analog of CaCuO2. Here, we perform first-principles calculations to show that the hybridization between Ni- d x 2 − y 2 orbital and itinerant electrons in RNiO2 is substantially stronger than previously thought. The dominant hybridization comes from an interstitial-s orbital rather than rare-earth-d orbitals, due to a large inter-cell hopping. Because of the hybridization, Ni local moment is screened by itinerant electrons and the critical UNi for long-range magnetic ordering is increased. Our work shows that the electronic structure of RNiO2 is distinct from CaCuO2, implying that the observed superconductivity in infinite-layer nickelates does not emerge from a doped Mott insulator. The discovery of superconductivity in doped NdNiO2 has generated excitement due to similarities with cuprates. Here, the authors use first-principles calculations to show that different from cuprates, a hybridization between Ni d-orbitals and itinerant electrons in NdNiO2 disfavours magnetism by screening Ni moment, as in Kondo systems. [ABSTRACT FROM AUTHOR]

Waterfalls are anomalies in the angle-resolved photoemission spectrum where the energy-momentum dispersion is almost vertical, and the spectrum strongly smeared out. These anomalies are observed at relatively high energies, among others, in superconducting cuprates and nickelates. The prevalent understanding is that they originate from the coupling to some boson, with spin fluctuations and phonons being the usual suspects. Here, we show that waterfalls occur naturally in the process where a Hubbard band develops and splits off from the quasiparticle band. Our results for the Hubbard model with ab initio determined parameters well agree with waterfalls in cuprates and nickelates, providing a natural explanation for these spectral anomalies observed in correlated materials. In angle-resolved photoemission spectra, nearly vertical energy-momentum dispersions, often termed waterfalls, can occur at high energies in materials including nickelates and cuprates. Using dynamical mean-field theory, the authors show that these features naturally emerge when a Hubbard band develops and splits off from the central quasiparticle band. [ABSTRACT FROM AUTHOR]

Recent observations of superconductivity in infinite-layer nickelates offer insights into high-temperature superconductivity mechanisms. However, defects and dislocations in doped films complicate the realization of superconductivity, limiting current research to superconducting nickelate heterostructures. The lack of research on superconductivity in superlattices composed of ultrathin nickelates severely impedes not only the exploration of the interface effect on superconductivity, but also the utilization of heterostructure engineering for exploring higher superconducting temperature Tc. Here, we demonstrated superconducting infinite-layer nickelate superlattices [(Nd0.8Sr0.2NiO2)8/(SrTiO3)2]10 via topotactic reduction. Our study uncovered that only above a critical thickness can high-quality superlattices be achieved, with structural formation dependent on nickelate layer thickness. The superconducting superlattice showed a Tc of 12.5 K and a 2D superconducting feature, indirectly indicate the intrinsic superconductivity of infinite-layer nickelates. Our study offers promising avenues for delving into the superconducting mechanism and for exploring multilevel interface engineering of infinite-layer nickelates, thus opening new horizons for the study of infinite-layer nickelates. So far infinite-layer nickelates have only been observed to superconduct in single-thin-film form. Here, the authors demonstrate superconductivity in [(Nd0.8Sr0.2NiO2)8/(SrTiO3)2]10 superlattices. [ABSTRACT FROM AUTHOR]

The Ni1−xGdxFe2O4 (x = 0.02–0.08) (NGF) nanoparticles were synthesized via hydrothermal method. The X-ray diffraction study confirmed the single-phase cubic spinel structure. The Da value decreases from 25 to 19 nm (strain increases from 0.0438 to 0.0613) (for x = 0.2–0.4). Further, it is increased to 43 nm (strain decreases from 0.0283 to 0.0138) from x = 0.4–0.8. The morphology indicated the stones (approximately nanocubes) like structures in the FESEM images. The mass loss and thermal transition temperatures were evaluated as a function of Gd-content using TG-DTG and TG-DSC measurements. The high mass reduction was observed for x = 0.02 (7.81%) and 0.04 (8.45%). The M-H loops evidenced the high magnetization values ranging between 49.1 and 62.1 emu/g for NGF as compared to pure nickel ferrite. Cation distribution evidenced the concentration of each cation at A- and B-site. The dielectric, conductivity, and impedance parameters were elucidated with frequency and ‘x’. The Maxwell–Wagner polarization is noticed at low frequency. The short- and long-range polarization mechanisms were found in the dielectric modulus plots. The impedance analysis shows the partial relaxation behavior of samples. [ABSTRACT FROM AUTHOR]

Copyright of Rare Metals is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Results obtained in synthesis and certification of new materials based on Ba-doped Ln2NiO4+δ as possible electrode candidates for electrochemical devices with solid-oxide proton-conducting electrolytes are presented. Single-phase materials of composition Ln1.95Ba0.05NiO4+δ (Ln = La, Nd, and Pr) were obtained by using the citrate-nitrate synthesis method, and their thermomechanical, electrical, and electrochemical functionality was examined with the use of the dilatometric method, four-probe dc measurements, and electrochemical impedance spectroscopy, respectively. An integrated analysis of the data obtained shows that the highest electrical transport properties, which compare well with modern results, were reached for the Pr-containing nickelite. At the same time, this nickelite has an increased thermal expansion coefficient, which exceeds by ~20% that for the electrolyte material. These characteristics can be optimized by creating composite materials based on Pr1.95Ba0.05NiO4+δ with a small fraction of the electrolyte component. [ABSTRACT FROM AUTHOR]

The mechanism of oxygen transfer in layered nickelates having a Ruddlesden-Popper structure and their nanocomposites with CeGdO (GDC) and Y(TiZr)MnO (YTZM) solid electrolytes having fluorite and pyrochlore structures were studied by the oxygen isotope heteroexchange method in a flow and static reactor, thermoprogrammed desorption, and semiempirical interacting bonds method. The experimental heteroexchange data were adequately described by assuming that all atoms were equivalent in exchange in the bulk of layered nickelates, which was consistent with the cooperative oxygen migration model with fast exchange between the interstitial and regular positions. Strong interaction between the domains of the nickelate phases and solid electrolytes in nanocomposites, accompanied by a redistribution of cations between the phases, hindered the cooperative oxygen migration and led to a decrease in the diffusion coefficient as the exchange rate increased. [ABSTRACT FROM AUTHOR]

Strongly correlated materials respond sensitively to external perturbations such as strain, pressure, and doping. In the recently discovered superconducting infinite-layer nickelates, the superconducting transition temperature can be enhanced via only ~ 1% compressive strain-tuning with the root of such enhancement still being elusive. Using resonant inelastic x-ray scattering (RIXS), we investigate the magnetic excitations in infinite-layer PrNiO2 thin films grown on two different substrates, namely SrTiO3 (STO) and (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) enforcing different strain on the nickelates films. The magnon bandwidth of PrNiO2 shows only marginal response to strain-tuning, in sharp contrast to the enhancement of the superconducting transition temperature Tc in the doped superconducting samples. These results suggest the bandwidth of spin excitations of the parent compounds is similar under strain while Tc in the doped ones is not, and thus provide important empirics for the understanding of superconductivity in infinite-layer nickelates. Nickelates have been shown to host unconventional superconductivity, and recently it has been found that the choice of substrate can significantly change the superconducting critical temperature. This suggests, that like some Cuprates, strain could be important. Here Gao, Fan, Wang, and coauthors find that magnetic excitations in a parent Nickelate are insensitive to substrate choice, and therefore strain, which differs markedly from the case of Cuprates. [ABSTRACT FROM AUTHOR]

We report the crystal structures and physical properties of trilayer nickelates Nd4Ni3O10 and Nd4Ni3O8. Measurements of magnetization and electrical resistivity display contrasting behaviors in the two compounds. Nd4Ni3O10 shows a paramagnetic metallic behavior with a metal-to-metal phase transition (T*) at about 162 K, as revealed by both magnetic susceptibility and resistivity. Further magnetoresistance and Hall coefficient results show a negative magnetoresistance at low temperatures and the carrier type of Nd4Ni3O10 is dominated by hole-type charge carriers. The significant enhancement of Hall coefficient and resistivity below T* suggests that effective charge carrier density decreases when cooling through the transition temperature. In contrast, Nd4Ni3O8 shows an insulating behavior. In addition, this compound shows a paramagnetic behavior with the similar magnetic moment as that of Nd4Ni3O10 derived from the Curie-Weiss fitting. This may suggest that the magnetic moments in both systems are contributed by Nd3+ ions. By applying pressures up to about 49 GPa, the insulating behavior is still present and becomes even stronger under a high pressure. Our results suggest that the different Ni configurations (Ni1+/2+ or Ni2+/3+) and the changes of coordination environment of Ni sites may account for the contrasting behaviors in trilayer nickelates Nd4Ni3O10 and Nd4Ni3O8. [ABSTRACT FROM AUTHOR]

We report a detailed analysis of the electrical resistivity exponent of thin films of NdNiO3 as a function of epitaxial strain. Thin films under low strain conditions show a linear dependence of the resistivity versus temperature, consistent with a classical Fermi gas ruled by electron-phonon interactions. In addition, the apparent temperature exponent, n, can be tuned with the epitaxial strain between n = 1 and n = 3. We discuss the critical role played by quenched random disorder in the value of n. Our work shows that the assignment of Fermi/Non-Fermi liquid behaviour based on experimentally obtained resistivity exponents requires an in-depth analysis of the degree of disorder in the material. Strong electronic correlations in rare-earth nickelates make them prone to unconventional behaviour but extrinsic effects hamper the interpretation of data. Guo et al. show that apparent non-Fermi liquid transport in NdNiO3 is tuned by strain and disorder, suggesting a more conventional origin. [ABSTRACT FROM AUTHOR]

A unique combination of methods (TPD of O, thermogravimetry, isotopic heteroexchange of oxygen in different modes) was used to carry out detailed studies of oxygen mobility and reactivity in mixed praseodymium nickelates-cobaltites (PrNiCoO) and their composites with doped cerium dioxide (CeYO) as promising cathodic materials stable towards the effect of CO in the intermediate-temperature region. It is shown that in the case of composites of PrNiCoO-CeYO synthesized using the Pechini method and ultrasonic treatment, stabilization of the disordered cubic perovskite phase due to redistribution of cations between the phases provides high oxygen mobility. Preliminary results on tests of cathodic materials of this type supported on planar NiO/YSZ anodes (H.C. Starck) with a thin layer of YSZ electrolyte and a buffer CeYO layer showed that power density of up to 0.4 W/cm was reached in the region of medium (600-700°C) temperatures, which was close to typical values for fuel cells of this type with cathodes based on strontium-doped perovskites and their composites with electrolytes. [ABSTRACT FROM AUTHOR]

A nickel-based compound has shown evidence of a superconducting state at a temperature of 80 kelvin. The material bridges a gap between other nickelates and a notable class of superconductor containing copper. High-temperature superconductivity in a nickel-based compound. [ABSTRACT FROM AUTHOR]

For most metals, increasing temperature (T) or disorder hastens electron scattering. The electronic conductivity (σ) decreases as T rises because electrons are more rapidly scattered by lattice vibrations. The value of σ decreases as disorder increases because electrons are more rapidly scattered by imperfections in the material. This is the scattering rate hypothesis, which has guided our understanding of metal conductivity for over a century. However, for so-called bad metals with very low σ this hypothesis predicts scattering rates so high as to conflict with Heisenberg's uncertainty principle. Bad-metal conductivity has remained a puzzle since its initial discovery in the 1980s in high-temperature superconductors. Here we introduce the rare-earth nickelates (RNiO3, R = rare-earth) as a class of bad metals. We study SmNiO3 thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni-O covalence explains bad-metal conductivity and the insulator-metal transition. This interaction shifts spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low σ without violating the uncertainty principle. [ABSTRACT FROM AUTHOR]

For the manganites RMnO3 (R = La, Pr, Nd), a mechanism is proposed to explain the anomalous temperature dependences of the kinetic coefficients, resistivity, and thermoelectric power during the transition from the pseudocubic O to the orthorhombic O’ crystal structure. The contributions of the bending and stretching modes of the octahedron to the formation of thermal-conductivity maxima and to the deviation from the linear dependence of ln( ρ/T) on 1/ T at low temperatures have been estimated. In nickelates, the metal-insulator transition is caused by lattice-polaron pinning by the stretching mode of the octahedron, and the low-temperature anomaly of the thermal conductivity is related to electron scattering by the bending mode of the octahedron. [ABSTRACT FROM AUTHOR]

Rare-earth nickelates form an intriguing series of correlated perovskite oxides. Apart from LaNiO3, they exhibit on cooling a sharp metal-insulator electronic phase transition, a concurrent structural phase transition, and a magnetic phase transition toward an unusual antiferromagnetic spin order. Appealing for various applications, full exploitation of these compounds is still hampered by the lack of global understanding of the interplay between their electronic, structural, and magnetic properties. Here we show from first-principles calculations that the metal-insulator transition of nickelates arises from the softening of an oxygen-breathing distortion, structurally triggered by oxygen-octahedra rotation motions. The origin of such a rare triggered mechanism is traced back in their electronic and magnetic properties, providing a united picture. We further develop a Landau model accounting for the metal-insulator transition evolution in terms of the rare-earth cations and rationalizing how to tune this transition by acting on oxygen rotation motions. [ABSTRACT FROM AUTHOR]

Hyperfine interactions of Fe impurity nuclei are studied by probe Mössbauer spectroscopy in TmNiO and YbNiO perovskite-like nickelates in the ranges of temperature transitions of an insulator ( T < T) ↔ metal ( T > T) and antiferromagnetic ( T < T) ↔ paramagnetic ( T > T). The changing behavior of hyperfine interaction parameters of Fe nuclei in the ranges of phase transition temperatures ( T and T) is analyzed. The results are interpreted in the context of the charge disproportionation of Ni cations associated with the electronic localization in monoclinic-distorted nickelates at T < T. [ABSTRACT FROM AUTHOR]

Oxygen migration is increasingly acknowledged as playing an important role in the ionic transport in mixed conductors and influencing the electrode electrochemical performance. The aim of this work was to establish correlations between the structural and electrical properties of undoped (Ln2NiO4 + δ, Ln = La, Pr) and doped (La1.7M0.3NiO4 + δ, M = Ca, Sr, Ba, La0.85Pr0.85Ca0.3NiO4 + δ, Pr1.7Ca0.3NiO4 + δ) layered nickelates and the oxygen diffusion in these materials to determine what influences their electrochemical response. A new technique for temperature programmed isotope exchange of oxides with C18O2 in a flow reactor was applied to investigate oxygen mobility and surface reactivity in the polycrystalline powder samples which provided the means to experimentally demonstrate the appearance of two channels of oxygen migration in the doped materials via cooperative mechanism and via near-dopant position. The electrochemical performance of the electrodes based on the developed materials was found to exhibit a strong dependence on their oxygen transport characteristics. [ABSTRACT FROM AUTHOR]

We theoretically investigate the unconventional superconductivity in the newly discovered infinite-layer nickelates Nd1−xSrxNiO2 based on a two-band model. By analyzing the transport experiments, we propose that the doped holes dominantly enter the Ni dxy or/and d3z2−r2 orbitals as charged carriers, and form a conducting band. Via the onsite Hund coupling, the doped holes are coupled to the Ni localized holes in the dx2−y2 orbital band. We demonstrate that this two-band model could be further reduced to a Hund-Heisenberg model. Using the reduced model, we show the non-Fermi liquid state above the critical Tc could stem from the carriers coupled to the spin fluctuations of the localized holes. In the superconducting phase, the short-range spin fluctuations mediate the carriers into Cooper pairs and establish dx2−y2-wave superconductivity. We further predict that the doped holes ferromagnetically coupled with the local magnetic moments remain itinerant even at very low temperature, and thus the pseudogap hardly emerges in nickelates. Our work provides a new superconductivity mechanism for strongly correlated multi-orbital systems and paves a distinct way to exploring new superconductors in transition or rare-earth metal oxides. [ABSTRACT FROM AUTHOR]

Many-body effects produce deviations from the predictions of conventional band theory in quantum materials, leading to strongly correlated phases with insulating or bad metallic behavior. One example is the rare-earth nickelates RNiO3, which undergo metal-to-insulator transitions (MITs) whose origin is debated. Here, we combine total neutron scattering and broadband dielectric spectroscopy experiments to study and compare carrier dynamics and local crystal structure in LaNiO3 and NdNiO3. We find that the local crystal structure of both materials is distorted in the metallic phase, with slow, thermally activated carrier dynamics at high temperature. We further observe a sharp change in conductivity across the MIT in NdNiO3, accompanied by slight differences in the carrier hopping time. These results suggest that changes in carrier concentration drive the MIT through a polaronic mechanism, where the (bi)polaron liquid freezes into the insulating phase across the MIT temperature. [ABSTRACT FROM AUTHOR]

The recent discovery of a superconductivity signature in La3Ni2O7-δ under a pressure of 14 GPa, with a superconducting transition temperature of around 80 K, has attracted considerable attention. An important aspect of investigating electronic structures is discerning the extent to which the electronic ground state of La3Ni2O7-δ resembles the parent state of the cuprate superconductor, a charge transfer insulator with long-range antiferromagnetism. Through X-ray absorption spectroscopy, we reveal the influence of oxygen ligands on the electronic ground states of the Ni ions, displaying a charge transfer nature akin to cuprate but with distinct orbital configurations. Additionally, in La3Ni2O7-δ films, we detect a superlattice reflection (1/4, 1/4, L) at the Ni L absorption edge using resonant X-ray scattering measurements. Further examination of the resonance profile indicates that the reflection originates from the Ni d orbitals. By evaluating the reflection's azimuthal angle dependence, we confirm the presence of collinear antiferromagnetic spin ordering and charge-like anisotropy ordered with the same periodicity. Our findings reveal a microscopic relationship between these two components in the temperature dependence of the scattering intensity of the reflection. This investigation enriches our understanding of high-temperature superconductivity in La3Ni2O7-δ under high pressure. The recent discovery of superconductivity in bilayer nickelates under high pressure offers a new material platform to explore high-temperature superconductivity. In this study, the authors identify the electronic ground states of the La3Ni2O7 film at ambient pressure revealing the critical role of ligand oxygen and the existence of long-range spin ordering along with charge-like anisotropy. [ABSTRACT FROM AUTHOR]

The pursuit of discovering new high-temperature superconductors that diverge from the copper-based model1–3 has profound implications for explaining mechanisms behind superconductivity and may also enable new applications4–8. Here our investigation shows that the application of pressure effectively suppresses the spin–charge order in trilayer nickelate La4Ni3O10−δ single crystals, leading to the emergence of superconductivity with a maximum critical temperature (Tc) of around 30 K at 69.0 GPa. The d.c. susceptibility measurements confirm a substantial diamagnetic response below Tc, indicating the presence of bulk superconductivity with a volume fraction exceeding 80%. In the normal state, we observe a strange metal behaviour, characterized by a linear temperature-dependent resistance extending up to 300 K. Furthermore, the layer-dependent superconductivity observed hints at a unique interlayer coupling mechanism specific to nickelates, setting them apart from cuprates in this regard. Our findings provide crucial insights into the fundamental mechanisms underpinning superconductivity, while also introducing a new material platform to explore the intricate interplay between the spin–charge order, flat band structures, interlayer coupling, strange metal behaviour and high-temperature superconductivity.The application of pressure effectively suppresses the spin–charge order in trilayer nickelate La4Ni3O10−δ single crystals, leading to the emergence of superconductivity. [ABSTRACT FROM AUTHOR]

The recently discovered infinite-layer nickelates show great promise in helping to disentangle the various cooperative mechanisms responsible for high-temperature superconductivity. However, lack of antiferromagnetic order in the pristine nickelates presents a challenge for connecting the physics of the cuprates and nickelates. Here, by using a quantum many-body Green's function-based approach to treat the electronic and magnetic structures, we unveil the presence of many two- and three-dimensional magnetic stripe instabilities that are shown to persist across the phase diagram of LaNiO2. Our analysis indicates that the magnetic properties of the infinite-layer nickelates are closer to those of the doped cuprates, which host a stripe ground state, rather than the undoped cuprates. The computed longitudinal-spin, transverse-spin, and charge spectra of LaNiO2 are found to contain an admixture of contributions from localized and itinerant carriers. Theoretically obtained dispersion of magnetic excitations (spin-flip) is found to be in good accord with the results of recent resonant inelastic X-ray scattering experiments. Our study gives insight into the origin of strong magnetic competition in the infinite-layer nickelates and their relationship with the cuprates. The discovery of superconductivity in doped infinite-layer nickelates has recently garnered significant attention. Here, the authors employed a quantum many-body Green's function-based approach to investigate the electronic and magnetic fluctuations in LaNiO2 providing microscopic insight into the origin of suppressed long-range order and magnetic excitation spectrum in the nickelates, along with their potential correlation with the cuprates. [ABSTRACT FROM AUTHOR]

The layered square-planar nickelates, Ndn+1NinO2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd6Ni5O12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd4Ni3O10, and subsequent reduction to the square-planar phase, Nd4Ni3O8. We synthesize our highest quality Nd4Ni3O10 films under compressive strain on LaAlO3 (001), while Nd4Ni3O10 on NdGaO3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd4Ni3O10 on SrTiO3 (001). Films reduced on LaAlO3 become insulating and form compressive strain-induced c-axis canting defects, while Nd4Ni3O8 films on NdGaO3 are metallic. This work provides a pathway to the synthesis of Ndn+1NinO2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy. The discovery of superconductivity in the infinite-layer nickelates reignites an interest in the nickelates as cuprate analogues. Here, the authors investigate the role of epitaxial strain in the synthesis of the n=3 layered nickelate, Nd4Ni3O8. [ABSTRACT FROM AUTHOR]

The discovery of 80 K superconductivity in bilayer La3Ni2O7 at pressures greater than 14 GPa presents a unique opportunity to study a novel class of high-temperature superconductors. Therefore, other bilayer nickelates following the classical (T4+) Ruddlesden-Popper (RP) series of Sr3Ni2O7 would present an interesting new candidate. In this work, we study the stabilization of RP n = 2 phase in Sr3Ni2−xAlxO7−δ, via floating zone growth of crystals. With powder and single-crystal XRD, we study the stability range of the RP-type phase. Our Thermogravimetric Analysis (TGA), X-ray photoelectron spectroscopy (XPS) and gas extraction studies reveal a remarkably high oxidation state of Ni4+ stabilized by chemical strain from Al. The obtained black crystals are insulating in transport and show a magnetic transition around 12 K. [ABSTRACT FROM AUTHOR]

Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Rud-dlesden-Popper nickelate La3Ni2O7, the atomic structure and electronic band structure of the trilayer nickelate La4Ni3O10 under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P21/a space group to the tetragonal I4/mmm around 12.6–13.4 GPa is identified, accompanied by a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni 3 d z 2 orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La4Ni3O10. The trilayer nickelate La4Ni3O10 shows some similarities with the bilayer La3Ni2O7 and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates. [ABSTRACT FROM AUTHOR]

The infinite-layer nickelates, isostructural to the high-Tc cuprate superconductors, have emerged as a promising platform to host unconventional superconductivity and stimulated growing interest in the condensed matter community. Despite considerable attention, the superconducting pairing symmetry of the nickelate superconductors, the fundamental characteristic of a superconducting state, is still under debate. Moreover, the strong electronic correlation in the nickelates may give rise to a rich phase diagram, where the underlying interplay between the superconductivity and other emerging quantum states with broken symmetry is awaiting exploration. Here, we study the angular dependence of the transport properties of the infinite-layer nickelate Nd0.8Sr0.2NiO2 superconducting films with Corbino-disk configuration. The azimuthal angular dependence of the magnetoresistance (R(φ)) manifests the rotational symmetry breaking from isotropy to four-fold (C4) anisotropy with increasing magnetic field, revealing a symmetry-breaking phase transition. Approaching the low-temperature and large-magnetic-field regime, an additional two-fold (C2) symmetric component in the R(φ) curves and an anomalous upturn of the temperature-dependent critical field are observed simultaneously, suggesting the emergence of an exotic electronic phase. Our work uncovers the evolution of the quantum states with different rotational symmetries in nickelate superconductors and provides deep insight into their global phase diagram. The authors study transport in the superconducting state of infinite-layer nickelate Nd0.8Sr0.2NiO2 films using a Corbino-disk configuration, finding that the magnetoresistance changes from isotropic to four-fold anisotropic with increasing magnetic field. At even higher field, an additional two-fold component emerges, which coincides with an anomalous upturn of the critical field. [ABSTRACT FROM AUTHOR]

The occurrence of superconductivity in proximity to various strongly correlated phases of matter has drawn extensive focus on their normal state properties, to develop an understanding of the state from which superconductivity emerges1–4. The recent finding of superconductivity in layered nickelates raises similar interests5–8. However, transport measurements of doped infinite-layer nickelate thin films have been hampered by materials limitations of these metastable compounds: in particular, a high density of extended defects9–11. Here, by moving to a substrate (LaAlO3)0.3(Sr2TaAlO6)0.7 that better stabilizes the growth and reduction conditions, we can synthesize the doping series of Nd1–xSrxNiO2 essentially free from extended defects. In their absence, the normal state resistivity shows a low-temperature upturn in the underdoped regime, linear behaviour near optimal doping and quadratic temperature dependence for overdoping. This is phenomenologically similar to the copper oxides2,12 despite key distinctions—namely, the absence of an insulating parent compound5,6,9,10, multiband electronic structure13,14 and a Mott–Hubbard orbital alignment rather than the charge-transfer insulator of the copper oxides15,16. We further observe an enhancement of superconductivity, both in terms of transition temperature and range of doping. These results indicate a convergence in the electronic properties of both superconducting families as the scale of disorder in the nickelates is reduced.By moving to a substrate that better stabilizes conditions, the doping series of Nd1–xSrxNiO2 is synthesized free from extended defects, resulting in enhancement of superconductivity in terms of transition temperature and range of doping. [ABSTRACT FROM AUTHOR]