Superconductivity with Tc≈15 K was recently found in doped NdNiO2. The Ni1+O2 layers are expected to be Mott insulators, so hole doping should produce Ni2+ with S=1, incompatible with robust ...superconductivity. We show that the NiO2 layers fall inside a critical region where the large pd hybridization favors a singlet A11 hole-doped state like in CuO2. However, we find that the superexchange is about one order smaller than in cuprates, thus a magnon "glue" is very unlikely and another mechanism needs to be found.
Magnetism alone was thought to be responsible for superconductivity in copper oxides. The finding of superconductivity in a non-magnetic compound that is structurally similar to these copper oxides ...challenges this view.
We study a model for the metal-insulator (M-I) transition in the rare-earth-element nickelates RNiO3, based upon a negative charge transfer energy and coupling to a rocksaltlike lattice distortion of ...the NiO6 octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with a ground state of predominantly d8L character, where L_ denotes a ligand hole. For sufficiently large distortions (δdNi-O∼0.05-0.10 Å), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with (d8L_2)S=0(d8)S=1 character, where S is the total spin. Thus the M-I transition may be viewed as being driven by an internal volume "collapse" where the NiO6 octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the (1/2, 1/2, 1/2) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of charge ordering achieved without any actual movement of the charge.
The recent observation of superconductivity in Formula: see text has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system ...falls in the Mott-Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally resolved electron energy-loss spectroscopy to directly probe the Mott-Hubbard character of Formula: see text Upon doping, we observe emergent hybridization reminiscent of the Zhang-Rice singlet via the oxygen-projected states, modification of the Nd 5d states, and the systematic evolution of Ni 3d hybridization and filling. These experimental data provide direct evidence for the multiband electronic structure of the superconducting infinite-layer nickelates, particularly via the effects of hole doping on not only the oxygen but also nickel and rare-earth bands.
The recent observation of replica bands in single-layer FeSe/SrTiO3 by angle-resolved photoemission spectroscopy (ARPES) has triggered intense discussions concerning the potential influence of the ...FeSe electrons coupling with substrate phonons on the superconducting transition temperature. Here we provide strong evidence that the replica bands observed in the single-layer FeSe/SrTiO3 system and several other cases are largely due to the energy loss processes of the escaping photoelectron, resulted from the well-known strong coupling of external propagating electrons to Fuchs-Kliewer surface phonons in ionic materials in general. The photoelectron energy loss in ARPES on single-layer FeSe/SrTiO3 is calculated using the demonstrated successful semiclassical dielectric theory in describing low energy electron energy loss spectroscopy of ionic insulators. Our result shows that the observed replica bands are mostly a result of extrinsic photoelectron energy loss and not a result of the electron phonon interaction of the Fe d electrons with the substrate phonons. The strong enhancement of the superconducting transition temperature in these monolayers remains an open question.
In cuprate high-temperature superconductors, an antiferromagnetic Mott insulating state can be destabilized toward unconventional superconductivity by either hole or electron doping. In hole-doped ...(p-type) cuprates, a charge ordering (CO) instability competes with superconductivity inside the pseudogap state. We report resonant x-ray scattering measurements that demonstrate the presence of charge ordering in the n-type cuprate Nd2–xCexCuO4 near optimal doping. We find that the CO in Nd2–xCexCuO4 occurs with similar periodicity, and along the same direction, as in p-type cuprates. However, in contrast to the latter, the CO onset in Nd2–xCexCuO4 is higher than the pseudogap temperature, and is in the temperature range where antiferromagnetic fluctuations are first detected. Our discovery opens a parallel path to the study of CO and its relationship to antiferromagnetism and superconductivity.
We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites X Bio sub(3) (X = Ba, Sr) that recognizes the bismuth-oxygen hybridization as a dominant ...energy scale. It is demonstrated by using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecularlike orbitals of the A sub(1g) symmetry composed of O-2p sub(sigma) and Bi-6s atomic orbitals of collapsed BiO sub(6) octahedra. The primary importance of oxygen p states is thus revealed, in contrast to a popular picture of a purely ionic Bi super(3+)/Bi super(5+) charge disproportionation. Octahedra tilting is shown to enhance the breathing instability by means of a nonuniform band narrowing. We argue that the formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice, and we expect similar hybridization effects in other perovskites involving formally high oxidation state cations.
Understanding what controls the dynamics of the quasiparticle that results when a hole is doped into an antiferromagnetically ordered CuO2 layer is the rst necessary step in the quest for a theory of ...the high-temperature superconductivity in cuprates. Here we show that the long-held belief that the quantum spin uctuations of the antiferromagnetic background play a key role in determining this dynamics is wrong. Indeed, we demonstrate that the correct, experimentally observed quasiparticle dispersion is generically obtained for a three-band model describing the hole moving on the oxygen sublattice and coupled to a Nel lattice of spins without spin uctuations. We argue that results from one-band model studies actually support this conclusion, and that this signicant conceptual change in our understanding of this phenomenology opens the way to studying few-hole dynamics, to accurately gauge the strength of the magnetic glue and its contribution to superconductivity. Twenty-seven years after the discovery of high-temperature superconductivity, consensus on its theoretical explanation is still absent. To a good extent, this is due to the difficulty of studying strongly correlated systems near half-filling, needed to understand the behaviour of one or a few holes doped into a CuO2 layer. To simplify this task it is customary to replace three-band models describing the doping holes as entering the O 2p orbitals of these charge-transfer insulators with much simpler one-band Hubbard or tJ models.
There is major interest, in condensed matter physics, in understanding the role of topology: remarkable progress has been made in classifying topological properties of non-interacting electrons, and ...on understanding the interplay between topology and electron-electron interactions. We extend such studies to interactions with the lattice, and predict non-trivial topological effects in infinitely long-lived polaron bands. Specifically, for a two-dimensional many-band model with realistic electron-phonon coupling, we verify that sharp level crossings are possible for polaron eigenstates, and prove that they are responsible for a novel type of sharp transition in the ground state of the polaron that can occur at a fixed momentum. Furthermore, they result in the appearance of Dirac cones stabilized by electron-phonon coupling. Thus, electron-phonon coupling opens an avenue to create and control Dirac and Weyl semimetals.
We derive and investigate numerically a minimal yet detailed spin-polaron model that describes lightly doped CuO2 layers. The low-energy physics of a hole is studied by total-spin-resolved exact ...diagonalization on clusters of up to 32 CuO2 unit cells, revealing features missed by previous studies. In particular, spin-polaron states with total spin 3/2 are the lowest eigenstates in some regions of the Brillouin zone. In these regions, and also at other points, the quasiparticle weight is identically zero indicating orthogonal states to those represented in the one electron Green's function. This highlights the importance of the proper treatment of spin fluctuations in the many-body background.
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