A Tale of Two Entangled Instabilities: Dual Role of delta-O in HgBa2Ca(n-1)Cu(n)O(2(n+1)+delta)

Low-energy instabilities in the hole doped cuprates include, besides short range antiferromagnetic fluctuations and superconductivity, also ubiquitous translational and rotational symmetry breakings. The overwhelming majority of interpretations of these possibly related properties rely on mappings onto three bands spanned by the three atomic orbitals Cu3d(x2-y2)(sigma), O2px(sigma), and O2py(sigma), these three local orbitals spanning the Zhang-Rice band (ZRB), the lower Hubbard bands (LHB) and the upper Hubbard bands (UHB), respectively. Here we demonstrate by means of supercell Density Functional Theory (DFT) (a) how oxygen intercalation affects the structures of the buffer layers, and (b) how the attenuated crystal field pulls two additional oxygen bands in the CuO2 plane to the Fermi level. The self-consistent changes in electronic structure reflected in the corresponding changes in external potential comprise formal properties of the Hohenberg-Kohn theorems. Validation of present days' approximate exchange-correlation potentials to capture these qualitative effects by means of supercell DFT is made by comparing computed doping dependent structural shifts to corresponding experimentally observed correlations. The simplest generalization of Bardeen-Cooper-Schrieffer (BCS) theory is offered to articulate high critical temperature superconductivity (HTS) from a normal state where crystal field causes states related to two non-hybridizing bands to coalesce at EF.Comment: 18 pages, 1 table, 6 figure

Super-Atom Representation of High-TC Superconductivity

A resonating valence bond RVB approach is taken to demonstrate formation of real-space Cooper pairs and High-TC superconductivity HTS. Non-adiabatic coupling between holes aggregates (super-atoms) and undoped anti-ferromagnet cause virtual excitations in either system due to inter-system coupling. HTS is said to reflect cooperative co-existence of two Bose-Einstein condensates in terms of one real-space Cooper pair condensate, and a second magnon condensate, which form at the same critical temperature. TC is formulated in terms of the super-exchange interaction. Connection is made to an equivalent real-space BCS formulation of HTS. Novel perspectives on the HTS in the electron-doped Sr1-xLaxCuO2 and Nd2-xCexCuO4 emerge.Comment: 25 pages, 4 figure

Entertaining the Possibility of RT Superconductivity in LK-99

An intuitive chemical perspective on the LK-99 material is outlined and supported by DFT calculations. A hidden flat Lead band that exhibits instability toward charge density wave formation is exposed. Electron transfer between Lead CDW/conduction bands and Cu$_3$d$^9$/Cu$_3$d$^{10}$ impurity states is suggested to embody the observed phenomenology reminiscent of room temperature superconductivity. The inter-system electronic instability is reflected in a chemical instability involving 2Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O => Pb$_{20-2x}$Cu$_{2x}$(PO$_4$)$_{12}$(O$_2$). Implications for tuning posttreatments as well as handling of the LK-99 material emerge.Comment: 6 pages, 3 figure

A Tale of Two Entangled Instabilities—The Dual Role of δ-O in HgBa2Can-1CunO2(n+1)+δ

Low-energy instabilities in the hole-doped cuprates include, besides short range antiferromagnetic fluctuations and superconductivity, also ubiquitous translational and rotational symmetry breakings. The overwhelming majority of interpretations of these possibly related properties rely on mappings onto three bands spanned by the three atomic orbitals Cu3d(x2−y2)(σ), O2px(σ), and O2py(σ), these three local orbitals spanning the Zhang–Rice band (ZRB), the lower Hubbard bands (LHB) and the upper Hubbard bands (UHB), respectively. Here we demonstrate by means of supercell Density Functional Theory (DFT) (a) how oxygen intercalation affects the structures of the buffer layers, and (b) how the attenuated crystal field pulls two additional oxygen bands in the CuO2 plane to the Fermi level. The self-consistent changes in electronic structure reflected in the corresponding changes in external potential comprise formal properties of the Hohenberg–Kohn theorems. Validation of present days’ approximate exchange-correlation potentials to capture these qualitative effects by means of supercell DFT is made by comparing computed doping dependent structural shifts to corresponding experimentally observed correlations. The simplest generalization of Bardeen–Cooper–Schrieffer (BCS) theory is offered to articulate high-critical temperature superconductivity (HTS) from a normal state where crystal field causes states related to two non-hybridizing bands to coalesce at EF

Particle-hole symmetry breaking in the pseudogap state of Pb0.55Bi1.5Sr1.6La0.4CuO6+d: A quantum-chemical perspective

Two Bi2201 model systems are employed to demonstrate how, beside the Cu-O \sigma-band, a second band of purely O2p\pi character can be made to cross the Fermi level owing to its sensitivity to the local crystal field. This result is employed to explain the particle-hole symmetry breaking across the pseudo-gap recently reported by Shen and co-workers, see M. Hashimoto et al., Nature Physics 6, (2010) 414. Support for a two-bands-on-a-checkerboard candidate mechanism for High-Tc superconductivity is claimed.Comment: 25 pages, 8 figure

Communication: Towards catalytic nitric oxide reduction via oligomerization on boron doped graphene

We use density functional theory to describe a novel way for metal free catalytic reduction of nitric oxide NO utilizing boron doped graphene. The present study is based on the observation that boron doped graphene and O-N=N-O- act as Lewis acid-base pair allowing the graphene surface to act as a catalyst. The process implies electron assisted N=N bond formation prior to N-O dissociation. Two N-2 + O-2 product channels, one of which favoring N2O formation, are envisaged as outcome of the catalytic process. Besides, we show also that the N-2 + O-2 formation pathways are contrasted by a side reaction that brings to N3O3- formation and decomposition into N2O + NO2-

Towards In Silico Mining for Superconductors -- Cutting the Gordian Knot

A random forest regression based supervised machine learning method to predict experimental critical temperature of superconductivity from the electronic band structure, as obtained from Density Functional Theory, is demonstrated. This complementarity between experiment and theory draws inspiration from the merging of Kohn-Sham and Bogoliubov-De Gennes equations [W. Kohn, W, EKU Gross, and LN Oliveira, Int. J. of Quant. Chem., 36(23), 611-615 (1989)]. Features in the Kohn-Sham Density Functional Theory band structure away from EF becoming decisive for the superconducting gap demonstrates this divide-and-conquer physical understanding. Not committing to any microscopic mechanism for the SC at this stage, it implies that in different classes of materials, different electronic features are responsible for the superconductivity. However, training on known members of a class, the performance of new members may be predicted. The method is validated for the A15 materials, including both binary A3X and ternary A6XY intermetallics, A=V, Nb, demonstrating that the two do indeed belong to the same class of superconductors.Comment: 12 pages, 4 figures and Supplementary Information containing 8 additional figure

On the fate of hydrogen during zirconium oxidation by water: Effect of oxygen dissolution in α-Zr

Zirconium oxidation by water is accompanied by hydrogen conversion, either H2 is released or hydrogen is picked up by the alloy. Strategies are sought to mitigate the detrimental hydrogen uptake into the metal. The corrosion phenomenon is subdivided into anode and cathode processes caused by electron release upon O2- oxidation at the metal/oxide interface in case of the former and electron-proton recombination resulting in hydrogen pick-up or H2 evolution in case of the latter. In a previous study, the additive dependence of the cathodic hydrogen evolution reaction was analysed. The present study contributes the oxygen concentration dependence of the anode potential, presents the impact of oxygen concentration on the co-absorption of hydrogen and merges the anode and cathode processes. The computational model is validated by semi-quantitatively reproducing the experimental solubility limit for oxygen in α-Zr. The impact of the emerging conceptual understanding for material development is discussed

Enhanced Manifold of States Achieved in Heterostructures of Iron Selenide and Boron-Doped Graphene

Enhanced superconductivity is sought by employing heterostructures composed of boron-doped graphene and iron selenide. Build-up of a composite manifold of near-degenerate noninteracting states formed by coupling top-of-valence-band states of FeSe to bottom-of-conduction-band states of boron-doped graphene is demonstrated. Intra- and intersubsystem excitons are explored by means of density functional theory in order to articulate a normal state from which superconductivity may emerge. The results are discussed in the context of electron correlation in general and multi-band superconductivity in particular