CuO4 Plaquettes Research Articles

Crossover of high-energy spin fluctuations from collective triplons to localized magnetic excitations in Sr14−xCaxCu24O41 ladders

We studied the magnetic excitations in the quasi-one-dimensional (q-1D) ladder subsystem of Sr14−xCaxCu24O41 (SCCO) using Cu L3-edge resonant inelastic X-ray scattering (RIXS). By comparing momentum-resolved RIXS spectra with high (x = 12.2) and without (x = 0) Ca content, we track the evolution of the magnetic excitations from collective two-triplon (2 T) excitations (x = 0) to weakly-dispersive gapped modes at an energy of 280 meV (x = 12.2). Density matrix renormalization group (DMRG) calculations of the RIXS response in the doped ladders suggest that the flat magnetic dispersion and damped excitation profile observed at x = 12.2 originates from enhanced hole localization. This interpretation is supported by polarization-dependent RIXS measurements, where we disentangle the spin-conserving ΔS = 0 scattering from the predominant ΔS = 1 spin-flip signal in the RIXS spectra. The results show that the low-energy weight in the ΔS = 0 channel is depleted when Sr is replaced by Ca, consistent with a reduced carrier mobility. Our results demonstrate that off-ladder impurities can affect both the low-energy magnetic excitations and superconducting correlations in the CuO4 plaquettes. Finally, our study characterizes the magnetic and charge fluctuations in the phase from which superconductivity emerges in SCCO at elevated pressures.

Enhancement of superexchange due to synergetic breathing and hopping in corner-sharing cuprates

Cuprates with corner-sharing CuO4 plaquettes have received much attention owing to the discoveries of high-temperature superconductivity and exotic states where spin and charge or spin and orbital degrees of freedom are separated. In these systems spins are strongly coupled antiferromagnetically via superexchange mechanisms, with high nearest-neighbour coupling varying among different compounds. The electronic properties of cuprates are also known to be highly sensitive to the presence, distance and displacement of apical oxygens perpendicular to the CuO2 planes. Here we present ab initio quantum chemistry calculations of the nearest-neighbour superexchange antiferromagnetic (AF) coupling J of two cuprates, Sr2CuO3 and La2CuO4. The former lacks apical oxygens, whilst the latter contain two apical oxygens per CuO2 unit completing a distorted octahedral environment around each Cu atom. Good agreement is obtained with experimental estimates for both systems. Analysis of the correlated wavefunctions together with extended superexchange models shows that there is an important synergetic effect of the Coulomb interaction and the O–Cu hopping, namely a correlated breathing-enhanced hopping mechanism. This is a new ingredient in superexchange models. Suppression of this mechanism leads to drastic reduction in the AF coupling, indicating that it is of primary importance in generating the strong interactions. We also find that J increases substantially as the distance between Cu and apical O is increased.

Quasi-two-dimensional magnetism in spin- square lattice compound Cu[C6H2(COO)4][H3N-(CH2)2-NH3] · 3H2O

We report the crystal growth and structural and magnetic properties of quasi-two-dimensional quantum magnet Cu[C6H2(COO)4][H3N-(CH2)2-NH3]3H2O. It is found to crystallize in a monoclinic structure with space group . The CuO4 plaquettes are connected into a two-dimensional framework in the ab-plane through the anions of [C6H2(COO)4]4− (pyromellitic acid). The [H3N-(CH2)2-NH3]3H2O groups are located between the layers and provide a weak interlayer connection via hydrogen (H...O) bonds. The temperature-dependent magnetic susceptibility is well described by frustrated square lattice model with nearest-neighbor interaction and next-nearest-neighbor interaction . Our analysis using frustrated rectangular lattice model also confirms almost isotropic nearest-neighbour interactions ( and ) in the ab-plane and . Further, the isothermal magnetization at is also well described by a non-frustrated square lattice model with . Based on the ratio, the compound can be placed in the Néel antiferromagnetic state of the phase diagram. No signature of magnetic long-range order is detected down to 2 K.

Crystal structure and conductive pathway of a novel Na ion conductor NaCu2VP2O10

A novel Na+ ion conductor, NaCu2VP2O10, was synthesized through a solid-state reaction. NaCu2VP2O10 exhibits an orthorhombic crystal structure with polyhedral layers composed of CuO4 plaquettes, VO6 octahedra, and PO4 tetrahedra. Na+ ions are located between these polyhedral layers. The ion conductivity was measured using an AC impedance method in the temperature range of 523 K to 873 K. The total ion conductivity in the polycrystalline materials shows an Arrhenius-type temperature dependence. Moreover, the three-dimensional bond valence sum (BVS) mapping suggested that Na+ ions conduct one dimensionally along the c-axis as ionic carriers passing between the polyhedral layers.

The Mechanism of Doping and the Features of Phase Diagrams of HTSC Cuprates and Ferropnictides

We propose a generalized model of electronic structure modification in HTSC cuprates and ferropnictides under doping. In this model the role of doping consists in only a local change in the electronic structures of the parent phases of cuprates and ferropnictides due to the formation of trion complexes comprising a doped carrier localized in unit cell and charge transfer (CT) excitons around it. These CT excitons emerge in CuO4 or AsFe4 plaquettes in the CuO2 or FeAs basal planes (CT plaquettes) under the influence of doped carrier, restricting its itinerancy. As the dopant concentration is increased, CT plaquettes combine into clusters of the so called CT phase. It is this CT phase that is related in the model to the HTSC phase. In support of this assumption, we determined the ranges of dopant concentrations conforming to the existence of percolation clusters of the CT phase; these ranges were shown to coincide with the positions of the superconducting domes on the phase diagrams of these compounds. The model also perfectly describes subtle features of the phase diagrams of various cuprates and ferropnictides including the 1/8 anomaly, narrow peaks in the dependences of the London penetration depth on the concentration of the dopant, and other specific features. The mechanism of the generation of free carriers in the CT phase, provided by intrinsic self-doping, was considered. The mechanism is not directly related to external doping, but is due to the interaction of band electrons with so called Heitler-London (HL) centres inherently existing in the percolation cluster of CT phase and representing pairs of adjacent CuO4 or AsFe4 CT plaquettes in the CuO2 or FeAs basal planes. Material in CT phase was shown to represent a medium, in which the mechanism of excitonic superconductivity, specified by the interaction of band electrons with HL centres, can be realized.

Oxygen vacancy-induced magnetic moment in edge-sharing CuO2 chains of Li2CuO2−δ

Li2CuO2 is a typical charge transfer insulator with CuO2 chains that are composed of edge-shared CuO4 plaquettes. The existence of oxygen vacancies for single crystals prepared under various oxygen partial pressures has been confirmed by the chemical and thermogravimetric analyses. The puzzling discovery of extra magnetic moment near the oxygen site by earlier neutron scattering studies has been verified by a thorough Curie–Weiss law analysis of spin susceptibilities, and resolved quantitatively with a molecular orbital model of edge-sharing CuO2 chains containing oxygen vacancies.

Hybridization and spin-orbit coupling effects in the quasi-one-dimensional spin-12magnetBa3Cu3Sc4O12

We study electronic and magnetic properties of the quasi-one-dimensional spin-1/2 magnet Ba3Cu3Sc4O12 with a distinct orthogonal connectivity of CuO4 plaquettes. An effective low-energy model taking into account spin-orbit coupling was constructed by means of first-principles calculations. On this basis a complete microscopic magnetic model of Ba3Cu3Sc4O12, including symmetric and antisymmetric anisotropic exchange interactions, is derived. The anisotropic exchanges are obtained from a distinct first-principles numerical scheme combining, on one hand, the local density approximation taking into account spin-orbit coupling, and, on the other hand, projection procedure along with the microscopic theory by Toru Moriya. The resulting tensors of the symmetric anisotropy favor collinear magnetic order along the structural chains with the leading ferromagnetic coupling J1 = -9.88 meV. The interchain interactions J8 = 0.21 meV and J5 = 0.093 meV are antiferromagnetic. Quantum Monte Carlo simulations demonstrated that the proposed model reproduces the experimental Neel temperature, magnetization and magnetic susceptibility data. The modeling of neutron diffraction data reveals an important role of the covalent Cu-O bonding in Ba3Cu3Sc4O12.

Crystal growth and magnetic ordering of Na2Ni2TeO6 with honeycomb layers and Na2Cu2TeO6 with Cu spin dimers

We report on the crystal growth and magnetic property studies of layered Na2Ni2TeO6 and Na2Cu2TeO6. The former has a honeycomb layer composed of NiO6 octahedra and the latter is composed of paired CuO4 plaquettes connected through TeO6 octahedra.

Superposition of ferromagnetic and antiferromagnetic spin chains in the quantum magnet BaAg2Cu[VO4]2

Based on density functional theory band structure calculations, quantum Monte-Carlo simulations, and high-field magnetization measurements, we address the microscopic magnetic model of BaAg2Cu[VO4]2 that was recently proposed as a spin-1/2 anisotropic triangular lattice system. We show that the actual physics of this compound is determined by a peculiar superposition of ferromagnetic and antiferromagnetic uniform spin chains with nearest-neighbor exchange couplings of Ja(1) ~ -19 K and Ja(2) ~ 9.5 K, respectively. The two chains featuring different types of the magnetic exchange perfectly mimic the specific heat of a triangular spin lattice, while leaving a clear imprint on the magnetization curve that is incompatible with the triangular-lattice model. Both ferromagnetic and antiferromagnetic spin chains run along the crystallographic 'a' direction, and slightly differ in the mutual arrangement of the magnetic CuO4 plaquettes and non-magnetic VO4 tetrahedra. These subtle structural details are, therefore, crucial for the ferromagnetic or antiferromagnetic nature of the exchange couplings, and put forward the importance of comprehensive microscopic modeling for a proper understanding of quantum spin systems in transition-metal compounds.

Structural ordering principles in quasi one-dimensional Wigner crystals of the series Na1+x CuO2 (0 ≤ x ≤ 1)

Abstract The general building principle of the commensurate subset of mixed valent sodium cuprates Na1+x CuO2 (0 ≤ x ≤ 1) has been revealed. Starting from parent ­NaCuO2, spin doping of the chain of edge sharing CuO4 plaquettes requires to incorporate further sodium ions for charge compensation. The accessible space needed for this purpose is generated by elongating the NaO6 octahedra present in NaCuO2. In this way, from one octahedron two square pyramidal and two tetrahedral sites result, out of which at best one each tetrahedral and square pyramidal site per original octahedron can be occupied, excluding combinations of electrostatically blocked sites. This general building principle has been employed to predict further structure candidates of this series (x = 1/4, 1/3, 2/3, 5/7, 3/4, 1). Density functional calculations (x = 0, 5/7, 3/4, 1) confirm the predicted charge ordering as well as the related structural features. The structure candidates are identified as local minima on the energy landscape.

Lattice coupling and Franck–Condon effects in K-edge resonant inelastic x-ray scattering

In this paper, we discuss our recent Cu K-edge resonant inelastic x-ray scattering (RIXS) results for CuB2O4, a model system consisting of electronically isolated CuO4 plaquettes, which exhibits anomalies in the incident photon energy dependence that are not captured by electron-only models. We show that these anomalous features can be qualitatively described by extending the electron-only considerations to include the lattice degrees of freedom. Our findings have important general implications for the interpretation of K-edge RIXS results for electronically higher-dimensional transition metal oxides.

Resonant inelastic x-ray scattering in electronically quasi-zero-dimensionalCuB2O4

We explore the general phenomenology of resonant inelastic scattering (RIXS) using CuB2O4, a network of CuO4 plaquettes electronically isolated by B+3 ions. Spectra show a small number of well-separated features, and we exploit the simple electronic structure to explore RIXS phenomenology by developing a calculation which allows for intermediate-state effects ignored in standard approaches. These effects are found to be non-negligible and good correspondence between our model and experiment leads to a simple picture of such phenomenology as the genesis of d-d excitations at the K edge and intermediate-state interference effects.

Incommensurate Spin-Density Modulation in a Copper Oxide Chain Compound with Commensurate Charge Order

Neutron diffraction has been used to determine the magnetic structure of Na8Cu5O10, a stoichiometric compound containing chains based on edge-sharing CuO4 plaquettes. The chains are doped with 2/5 hole per Cu site and exhibit long-range commensurate charge order with an onset well above room temperature. Below TN=23 K, the neutron data indicate long-range collinear magnetic order with a spin-density modulation whose propagation vector is commensurate along, and incommensurate perpendicular to, the chains. Competing interchain exchange interactions are discussed as a possible origin of the incommensurate magnetic order.

Evidence for two types of low-energy charge transfer excitations in Sr2CuO3.

A comparative analysis of electron energy-loss spectroscopy (EELS) spectra for the 1D insulating cuprate Sr2CuO3 with transferred momentum q--> axially and radially to the chain axis allows one to elucidate the structure of the charge transfer gap in in-chain response. It is determined by the superposition of two types of excitations with different magnitudes of dispersion. The low-energy response with q--> radially to the chain direction, but yet within the plane of CuO4 plaquettes, exhibits also a dispersionless peak near 2 eV. The theoretical simulation of the EELS data using exact diagonalizations of an appropriate extended Hubbard Hamiltonian for relevant clusters requires the explicit consideration of low-lying oxygen 2p pi states within the CuO4 plaquette plane beyond the standard pd sigma extended Hubbard model widely used for cuprates with corner-shared CuO4 plaquettes.