Infinite-layer Structure Research Articles

Atomic scale disorder and reconstruction in bulk infinite-layer nickelates lacking superconductivity

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.

Intercalation Reaction Mechanism of Fluoride-Ions in (Ca, Sr)FeO2 Cathodes with Infinite Layer Structure for All-Solid-State Fluoride-Ion Batteries

Fluoride ion batteries, which use fluoride ions as carriers, are attracting attention as the next generation of storage batteries because of their theoretical high energy density. Conventional research has focused on metal/metal fluorides such as Cu/CuF2 as cathode materials.[1] However, these cathodes have disadvantages of the power density and cyclability due to the rapid decrease in electronic conductivity and large volume change during fluorination and defluorination. To solve these problems, cathode materials that utilize topotactic fluoride ion intercalation reactions, similar to electrode materials applied in lithium-ion secondary batteries, are being developed[2]. However, these materials have the disadvantage of relatively small capacity, compared to metal/metal fluorides.In this study, we focused on SrFeO2and Ca-doped SrFeO2 cathodes with infinite layer structure as new high-capacity intercalation cathodes, and evaluated its electrochemical properties and clarified the charge compensation mechanism.Ca1-xSrxFeO2 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) was synthesized by a low-temperature reduction method using CaH2. Ca1-xSrxFeO2/La0.9Ba0.1F2.9/vapor grown carbon fiber (VGCF) was used as the composite cathode, La0.9Ba0.1F2.9 as the electrolyte and Pb/PbF2/ La0.9Ba0.1F2.9/VGCF as the composite anode to construct the electrochemical cell of the piezoelectric material. Charge-discharge measurements were carried out in the cut-off voltage range of -1.5 to 3.0 V at 140°C. X-ray absorption spectroscopy (XAS) measurements of Fe K-edge, O K-edge, and F K-edge and resonant inelastic X-ray scattering (RIXS) measurements of O K-edge were performed on the samples after charging and discharging.All the Ca1-xSrxFeO2 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) cathode showed voltage plateaus around -0.5 V and 1.5 V during charging, and the maximum discharge capacity (580 mAh g-1) was obtained when x = 0.8, and the cathodes could be repeatedly charged and discharged. XAS of the Fe K-edge revealed that Fe compensated for the charge in the flat potential region around -0.5 V. XAS and RIXS measurements of the O K-edge revealed that the charge compensation was carried out by the formation of O2 molecules (oxidation of oxide ions), as which is observed in lithium-excess metal oxide on charge process[4], in the voltage plateau around 1.5 V. Using inexpensive Ca and Fe, we succeeded in developing an intercalation cathode material that significantly exceeds the capacity of conventional lithium-ion battery cathodes.

Metal–insulator–superconductor transition in nickelate-based heterostructures driven by topotactic reduction

The discovery of superconductivity in doped infinite-layer nickelates has attracted great interest recently. Here, a metal–insulator–superconductor transition is demonstrated by engineering the process of topotactic reduction. By employing topotactic reduction, a superconducting Nd0.8Sr0.2NiO2 layer is obtained from high-quality Nd0.8Sr0.2NiO3, which is characterized by layer-by-layer growth and low room-temperature resistivity, rather than the low-quality Nd0.8Sr0.2NiO3 with mixed phases. Moreover, an insulating intermediate state is uncovered within the transition from metallic Nd0.8Sr0.2NiO3 to superconducting Nd0.8Sr0.2NiO2, corresponding to a non-monotonic modulation of resistivity driven by topotactic reduction. In the incompletely reduced Nd0.8Sr0.2NiO2+δ, residual oxygen atoms in the Nd/Sr plane disrupt the long-range order of the infinite-layer structure, resulting in suppressed superconductivity with a low transition temperature and non-zero residual resistivity. On the other hand, the superconductivity is optimized in the fully reduced sample, where a sign change in the Hall coefficient is always observed at low temperatures. Our study highlights the diverse electronic states achievable by controlling topotactic reduction, providing valuable insight into the understanding and manipulating of superconductivity in infinite-layer nickelates.

Lattice source for charge and spin inhomogeneity in 2D perovskite cuprates

In the work we highlight the structural features of 2D perovskite cuprates (tilted CuO6 octahedra with different orientation with respect to spacer rocksalt layers), where sources of charge and spin inhomogeneity can be hidden. We used the impurity Anderson model with the Jahn–Teller(JT) local cells to show the charge inhomogeneity arises at any low doping concentration x, but disappears when the doping level exceeds threshold concentration xc, and the smaller the magnitudes xc, the larger the area of regions with JT pseudo effect. It is expected that spontaneous chiral symmetry breaking in the dynamic JT state of the stripe CuO2 layer as a whole can lead to the appearance of the goldstone phonon mode. As consequence, the giant thermal Hall effect could be observed in the 2D perovskite cuprates with CuO6 octahedra, rather than with CuO4 squares, e.g. in the Tl-based n layer cuprates or cuprates based on infinite-layer CaCuO2 structure.

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

Charge ordering (CO) phenomena have been widely debated in strongly-correlated electron systems mainly regarding their role in high-temperature superconductivity. Here, the structural and charge distribution in NdNiO2 thin films prepared with and without capping layers, and characterized by the absence and presence of CO are elucidated. The microstructural and spectroscopic analysis is done by scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) and hard X-ray photoemission spectroscopy (HAXPES). Capped samples show Ni1+ , with an out-of-plane (o-o-p) lattice parameter of around 3.30 Å indicating good stabilization of the infinite-layer structure. Bulk-sensitive HAXPES on Ni-2p shows weak satellite features indicating large charge-transfer energy. The uncapped samples evidence an increase of the o-o-p parameter up to 3.65 Å on the thin film top with a valence toward Ni2+ in this region. Here, 4D-STEM demonstrates (303)-oriented stripes which emerge from partially occupied apical oxygen. Those stripes form quasi-2D coherent domains viewed as rods in the reciprocal space with Δqz ≈ 0.24 reciprocal lattice units (r.l.u.) extension located at Q = ( ) and ( ) r.l.u. The stripes associated with oxygen re-intercalation concomitant with hole doping suggest a possible link to the previously reported CO in infinite-layer nickelate thin films.

High nitrogen polymer in silver polynitrides

The Ag–N system is enriched by proposing four polymeric structures in first-principles calculations at high-pressure. The proposed P-1-AgN2 and P21/c-AgN2 not only break the new record in the polymeric configuration of MN2 compound, but also in the energy density. In the MN2 stoichiometric ratio, the infinite armchair-antiarmchair chain and the layered polymer structure are reported for the first time in P-1-AgN2 and P21/c-AgN2, indicating that the highly polymerized structure can also be formed in nitrides with low nitrogen content at high pressure. The mechanism of structure evolution behaviors of AgN2 under pressure is revealed by the analysis of atomic orbital hybridization. Meanwhile, the high energy densities are obtained in P-1-AgN2 (1.76 kJ g−1) and P21/c-AgN2 (2.01 kJ g−1) compared to all the reported MN2 nitrides. In addition, the infinite band-shaped structure and infinite curve layer structure are obtained in P-1-AgN5 and C2/m-AgN6, respectively. The energy densities of P-1-AgN5 (3.82 kJ g−1) and C2/m-AgN6 (4.51 kJ g−1) are comparable to or larger than that of TNT (4.3 kJ g−1). Moreover, P-1-AgN2, P21/c-AgN2, P-1-AgN5, and C2/m-AgN6 exhibit excellent explosive performance in terms of detonation velocity and detonation pressure.

Magnetic and Electronic Properties of Sr Doped Infinite-Layer NdNiO2 Supercell: A Screened Hybrid Density Functional Study.

To understand the influence of doping Sr atoms on the structural, magnetic, and electronic properties of the infinite-layer NdSrNiO2, we carried out the screened hybrid density functional study on the Nd9-nSrnNi9O18 (n = 0-2) unit cells. Geometries, substitution energies, magnetic moments, spin densities, atom- and lm-projected partial density of states (PDOS), spin-polarized band structures, and the average Bader charges were studied. It showed that the total magnetic moments of the Nd9Ni9O18 and Nd8SrNi9O18 unit cells are 37.4 and 24.9 emu g-1, respectively. They are decreased to 12.6 and 4.2 emu g-1 for the Nd7Sr2Ni9O18-Dia and Nd7Sr2Ni9O18-Par unit cells. The spin density distributions demonstrated that magnetic disordering of the Ni atoms results in the magnetism decrease. The spin-polarized band structures indicated that the symmetry of the spin-up and spin-down energy bands around the Fermi levels also influence the total magnetic moments. Atom- and lm-projected PDOS as well as the band structures revealed that Ni(dx2-y2) is the main orbital intersecting the Fermi level. As a whole, electrons of Sr atoms tend to locate locally and hybridize weakly with the O atoms. They primarily help to build the infinite-layer structures, and influence the electronic structure near the Fermi level indirectly.

Critical role of hydrogen for superconductivity in nickelates.

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.

Geometric frustration of Jahn-Teller order in the infinite-layer lattice.

The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, has a key role in many symmetry-lowering crystal deformations1. Lattices of Jahn-Teller ions can induce a cooperative distortion, as exemplified by LaMnO3 (refs. 2,3). Although many examples occur in octahedrally4 or tetrahedrally5 coordinated transition metal oxides due to their high orbital degeneracy, this effect has yet to be manifested for square-planar anion coordination, as found in infinite-layer copper6,7, nickel8,9, iron10,11 and manganese oxides12. Here we synthesize single-crystal CaCoO2 thin films by topotactic reduction of the brownmillerite CaCoO2.5 phase. We observe a markedly distorted infinite-layer structure, with ångström-scale displacements of the cations from their high-symmetry positions. This can be understood to originate from the Jahn-Teller degeneracy of the dxz and dyz orbitals in the d7 electronic configuration along with substantial ligand-transition metal mixing. A complex pattern of distortions arises in a [Formula: see text] tetragonal supercell, reflecting the competition between an ordered Jahn-Teller effect on the CoO2 sublattice and the geometric frustration of the associated displacements of the Ca sublattice, which are strongly coupled in the absence of apical oxygen. As a result of this competition, the CaCoO2 structure forms an extended two-in-two-out type of Co distortion following 'ice rules'13.

Research progress of control of physical properties of topological phase change oxide films by external field

Perovskite transition-metal oxides can undergo significant structural topological phase transition between perovskite structure, brownmillerite structure, and infinite-layer structure under the external field through the gain and loss of the oxygen ions, accompanied with significant changes in physical properties such as transportation, magnetism, and optics. Topotactic phase transformation allows structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for utilizing the redox reaction and oxygen diffusion within transition metal oxides, and establishing great potential applications in solid oxide fuel cells, oxygen sensors, catalysis, intelligent optical windows, and neuromorphic devices. In this work, we review the recent research progress of manipulating the topological phase transition of the perovskite-type oxide films and regulating their physical properties, mainly focusing on tuning the novel physical properties of these typical films through strong interaction between the lattice and electronic degrees of freedom by the action of external fields such as strain, electric field, optical field, and temperature field. For example, a giant photoinduced structure distortion in SrCoO<sub>2.5</sub> thin film excited by photons is observed to be higher than any previously reported results in the other transition metal oxide films. The SrFeO<sub>2</sub> films undergo an insulator-to-metal transition when the strain state changes from compressive state to tensile state. It is directly observed that perovskite SrFeO<sub>3</sub> nanofilament is formed under the action of electric field and extends almost through the brownmillerite SrFeO<sub>2.5</sub> matrix in the ON state and is ruptured in the OFF state, unambiguously revealing a filamentary resistance switching mechanism. Utilizing <i>in situ</i> electrical scanning transmission electron microscopy, the transformation from brownmillerite SrFeO<sub>2.5</sub> to infinite-layer SrFeO<sub>2</sub> under electric field can be directly visualized with atomic resolution. We also clarify the relationship between the microscopic coupling mechanism and the macroscopic quantum properties of charges, lattices, orbits, spin, etc. Relevant research is expected to provide a platform for new materials, new approaches and new ideas for developing high-sensitivity and weak-field response electronic devices based on functional oxides. These findings about the topological phase transition in perovskite oxide films can expand the research scope of material science, and have important significance in exploring new states of matters and studying quantum critical phenomena.

Structural and electronic properties of SrCuO2+δ thin films

The layered structure of superconducting cuprates is considered to be a key ingredient to achieve high superconducting transition temperatures. In this work, we investigate the possibility of doping the SrCuO2 infinite-layer compound by inserting additional oxygen into its structure. We observe that the infinite-layer SrCuO2 structure is epitaxially stabilized in thin films grown by pulsed laser deposition in pure O2. Increasing the oxidizing power by introducing ozone during the growth leads to a different phase with an elongated c axis. Scanning transmission electron microscopy analysis suggests that the films with an elongated c axis are composed of SrCuO2.5 blocks separated by SrCuO2 layers arranged to match the substrate spacing. X-ray absorption spectroscopy measurements show that this SrCuO2+δ phase is associated with a more isotropic Cu orbital configuration and hole doping. This hole doping leads to a dramatic reduction in the resistivity of the films, with a magnitude that depends on the precise oxygen content in the structure.

Metal-organic frameworks derived from chalcone dicarboxylic acid: New topological characters and initial catalytic properties

Two metal–organic frameworks, [Ag2L(bpy)0.5]n (1) and [CoL(bpy)]n (2), were successfully assembled through mixed-ligands strategy with different metal resource. Remarkably, both MOFs manifest multi-nuclear metal units as nodes and diverse topological structures. Compound 1 is an infinite two-dimensional layer structure with unsaturated four-nuclear silver unit nodes. While compound 2 exhibits a novel three-dimensional structure. The heterogeneous catalysis efficiency of the MOFs towards cycloaddition of N-methyl propargylamine with CO2 has been explored, and compound 1 displays higher catalytic activity derived from bare Lewis acid silver active site.

Magnetic correlations in infinite-layer nickelates: An experimental and theoretical multimethod study

We report a comprehensive study of magnetic correlations in ${\mathrm{LaNiO}}_{2}$, a parent compound of the recently discovered family of infinite-layer (IL) nickelate superconductors, using multiple experimental and theoretical methods. Our specific heat, muon-spin rotation ($\ensuremath{\mu}\mathrm{SR}$), and magnetic susceptibility measurements on polycrystalline ${\mathrm{LaNiO}}_{2}$ show that long-range magnetic order remains absent down to 2 K. Nevertheless, we detect residual entropy in the low-temperature specific heat, which is compatible with a model fit that includes paramagnon excitations. The $\ensuremath{\mu}\mathrm{SR}$ and low-field static and dynamic magnetic susceptibility measurements indicate the presence of short-range magnetic correlations and glassy spin dynamics, which we attribute to local oxygen nonstoichiometry in the average infinite-layer crystal structure. This glassy behavior can be suppressed in strong external fields, allowing us to extract the intrinsic paramagnetic susceptibility. Remarkably, we find that the intrinsic susceptibility shows non-Curie-Weiss behavior at high temperatures, in analogy to doped cuprates that possess robust nonlocal spin fluctuations. The distinct temperature dependence of the intrinsic susceptibility of ${\mathrm{LaNiO}}_{2}$ can be theoretically understood by a multimethod study of the single-band Hubbard model in which we apply complementary cutting-edge quantum many-body techniques (dynamical mean-field theory, cellular dynamical mean-field theory, and the dynamical vertex approximation) to investigate the influence of both short- and long-ranged correlations. Our results suggest a profound analogy between the magnetic correlations in parent (undoped) IL nickelates and doped cuprates.

Stability of superconducting Nd0.8Sr0.2NiO2 thin films

The discovery of superconducting states in the nickelate thin film with infinite-layer structure has paved a new way for studying unconventional superconductivity. So far, research in this field is still very limited due to difficulties in sample preparation. Here we report on the successful preparation of superconducting Nd0.8Sr0.2NiO2 thin film (Tc = 8.0 - 11.1 K) and study the stability of such films in ambient environment, water and under electrochemical conditions. Our work demonstrates that the superconducting state of Nd0.8Sr0.2NiO2 is remarkably stable, which can last for at least 47 days continuous exposure to air at 20 degree Celsius and 35% relative humidity. Further we show the superconductivity disappears after being immersed in de-ionized water at room temperature for 5 hours. Surprisingly, it can also survive under ionic liquid gating conditions with applied voltage up to 4 V, which is even more stable than conventional perovskite complex oxides.

Topotactic Oxygen Release and Incorporation in AFeO3 with Fe4+, AFeO2.5 with Fe3+, and AFeO2 with Fe2+ (A = Ca and Sr): Dedicated to the Occasion of the 100th Birthday of Prof. John B. Goodenough

Oxygen contents in perovskite-structure Fe oxides can change in accordance with the valence states of Fe, i.e., AFeO3 with Fe4+, AFeO2.5 with Fe3+, and AFeO2 with Fe2+ (A = Ca and Sr). AFeO3 has a fully oxygenated simple-perovskite structure, and the unusual high valence Fe4+ in AFeO3 is easily reduced to relatively stable Fe3+ by releasing oxygen. On the other hand, AFeO2 has an infinite-layer structure, and the unusual square-planar coordination of Fe2+ in AFeO2 changes to tetrahedral and octahedral Fe3+ by incorporating oxygen. Sample weight measurements by thermogravimetry and corresponding phase analysis with synchrotron X-ray diffraction data revealed that the difference in the A-site cation strongly influenced the oxygen release and incorporation behaviors. In ambient air, topotactic changes of AFe4+O3 → AFe3+O2.5 ← AFe2+O2 for both A = Ca and Sr can occur by releasing and incorporating oxygen in the perovskite structure frameworks. Nonstoichiometric phases with oxygen vacancies are present between SrFeO3 and SrFeO2.5.

Unintended consequence of topochemical reduction of SrFeO3 to SrFeO2 : Design of infinite layered oxides

The infinite layer structure type has been known to host high-temperature superconductivity since the discovery of ${\mathrm{Ca}}_{0.86}{\mathrm{Sr}}_{0.14}{\mathrm{CuO}}_{2}$, yet little progress has been made to synthesize many analogs. Here, using ${\mathrm{SrFeO}}_{x}$ as a prototype system, we explore the thermodynamic obstacles behind the scarcity of $3d$ elements adopting the infinite layer structure type. In this context, synthetic considerations to achieve the $AB{\mathrm{O}}_{3}$ to $AB{\mathrm{O}}_{2}$ transformation are discussed. Specifically, we demonstrate that the conventionally reported topochemical reduction can result in hydride incorporation into $\mathrm{SrFe}{\mathrm{O}}_{2}$, causing a decrease in the magnetic ordering temperature of the infinite layered oxide. First-principles simulations further confirm that the incorporation of H is necessary for stabilizing the $\mathrm{SrFe}{\mathrm{O}}_{2}$ phase by decreasing the thermodynamic cost of individual steps required to transform $\mathrm{SrFe}{\mathrm{O}}_{3}$ into $\mathrm{SrFe}{\mathrm{O}}_{2}$, and is the driving factor behind the changes in magnetic exchange interactions that ultimately change the N\'eel temperature (${T}_{\mathrm{N}}$). Additionally, inspired by recent reports of superconductivity in another low-dimensional oxide ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_{2}$, ${\mathrm{Sr}}_{0.95}{\mathrm{Nd}}_{0.05}{\mathrm{FeO}}_{2}$ was synthesized via a more traditional topochemical reduction procedure. Both physical characterization and accompanying density-functional theory calculations show that this $A$-site doping can have similar effects on $A{\mathrm{FeO}}_{2}$ stability and magnetic ordering temperatures as with the incorporation of hydrogen. Ultimately, these results suggest that charge doping either through the incorporation of H or $A$-site substitution may be fruitful routes in tuning stability and magnetic properties, with direct consequences on superconducting behavior.

Direct observation of nodeless superconductivity and phonon modes in electron-doped copper oxide Sr1-xNdxCuO2.

The microscopic understanding of high-temperature superconductivity in cuprates has been hindered by the apparent complexity of crystal structures in these materials. We used scanning tunneling microscopy and spectroscopy to study the electron-doped copper oxide compound Sr1−xNdxCuO2, which has only bare cations separating the CuO2 planes and thus the simplest infinite-layer structure of all cuprate superconductors. Tunneling conductance spectra of the major CuO2 planes in the superconducting state revealed direct evidence for a nodeless pairing gap, regardless of variation of its magnitude with the local doping of trivalent neodymium. Furthermore, three distinct bosonic modes are observed as multiple peak-dip-hump features outside the superconducting gaps and their respective energies depend little on the spatially varying gaps. As well as the bosonic modes, with energies identical to those of the external, bending and stretching phonons of copper oxides, our findings reveal the origin of the bosonic modes in lattice vibrations rather than spin excitations.

Dehydration of Electrochemically Protonated Oxide: SrCoO2 with Square Spin Tubes.

Controlling oxygen deficiencies is essential for the development of novel chemical and physical properties such as high-Tc superconductivity and low-dimensional magnetic phenomena. Among reduction methods, topochemical reactions using metal hydrides (e.g., CaH2) are known as the most powerful method to obtain highly reduced oxides including Nd0.8Sr0.2NiO2 superconductor, though there are some limitations such as competition with oxyhydrides. Here we demonstrate that electrochemical protonation combined with thermal dehydration can yield highly reduced oxides: SrCoO2.5 thin films are converted to SrCoO2 by dehydration of HSrCoO2.5 at 350 °C. SrCoO2 forms square (or four-legged) spin tubes composed of tetrahedra, in contrast to the conventional infinite-layer structure. Detailed analyses suggest the importance of the destabilization of the SrCoO2.5 precursor by electrochemical protonation that can greatly alter reaction energy landscape and its gradual dehydration (H1-xSrCoO2.5-x/2) for the SrCoO2 formation. Given the applicability of electrochemical protonation to a variety of transition metal oxides, this simple process widens possibilities to explore novel functional oxides.

First-principles study of electronic structure and magnetic properties of Sr3Fe2O5 oxide* *Project supported by the National Natural Science Foundation of China (Grant No. 11964039) and Xinjiang “Hundred Young Doctors Introduction Program” Project, China (Grant No. 3010010111).

We investigate the electronic structure and magnetic properties of layered compound Sr3Fe2O5 based on first-principles calculations in the framework of density functional theory with GGA+U method. Under high pressure, the ladder-type layered structure of Sr3Fe2O5 is transformed into the infinite layered structure accompanied by a transition from G-type anti-ferromagnetic (AFM) insulator to ferromagnetic (FM) metal and a spin transition from S = 2 to S = 1. We reproduce these transformations in our calculations and give a clear physical interpretation.

The discovery and research progress of the nickelate superconductors

<p indent=0mm>The discovery of the nickelate superconductors has motivated the search for new unconventional superconductors, analogous to cuprates and spurred the revolving interest on the mechanism of high-temperature superconductivity, with the focus on the relationship between the crystal structure and the detailed electronic structure. This article briefly reviews the discovery of the nickelate superconductors with the infinite-layer structure, the on-going experimental efforts, and the evolving understanding of the physics on this new superconducting system. It also discusses the open questions regarding (but not limited to) the exploration of a whole family of nickelate superconductors, materials growth issues, physics problems, and immediate research topics/directions in this new field.