Superexchange in the cuprates

Origins of multi-sublattice magnetism and superexchange interactions in double-double perovskite CaMnCrSbO6

The multi-sublattice magnetism and electronic structure in double-double perovskite compound CaMnCrSbO6is explored using density functional theory. The bulk magnetization and neutron diffraction suggest a ferrimagnetic order (TC∼49 K) between between Mn2+and Cr3+spins. Due to the non-equivalent Mn atoms (labelled as Mn(1) and Mn(2) which have tetrahedral and planar oxygen coordinations, respectively) and the Cr atom in the centre of distorted oxygen octahedron in the unit cell, the exchange interactions are more complex than that expected from a two sublattice magnetic system. The separations between the on-site energies of thed-orbitals of Mn(1), Mn(2) and Cr obtained from Wannier function analysis are in agreement with their expected crystal field splitting. While the DOS obtained from non spin-polarized calculations show a metallic character, starting from HubbardU = 0 eV the spin-polarized electronic structure calculations yield a ferrimagnetic insulating ground state. The band gap increases withUeff(U - J), thereby showing a Mott-Hubbard nature of the system. The inclusion of anti-site disorder in the calculations show decrease in band-gap and also reduction in the total magnetic moment. Due to the ∼90∘superexchange, nearest neighbour exchange constants obtained from DFT are an order of magnitude smaller than those reported for various magnetic perovskite and double-perovskite compounds. The Mn(1)-O-Mn(2) (out of plane and in-plane), Mn(1)-O-Cr and Mn(2)-O-Cr superexchange interactions are found to be anti-ferromagnetic, while the Cr-O-O-Cr super-superexchange is found to be ferromagnetic. The Mn(2)-O-Cr superexchange is weaker than the Mn(1)-O-Cr super-exchange, thus effectively resulting in ferrimagnetism. From a simple 3-site Hubbard model, we derived expressions for the antiferromagnetic superexchange strengthJAFMand also for the weaker ferromagneticJFM. The relative strengths ofJAFMfor the various superexchange interactions are in agreement with those obtained from DFT. The expression for Cr-O-O-Cr super-superexchange strength (J~SS), which has been derived considering a 4-site Hubbard model, predicts a ferromagnetic exchange in agreement with DFT. Finally, our mean field calculations reveal that assuming a set of four magnetic sub-lattices for Mn2+spins and a single magnetic sublattice for Cr3+spins yields a much improvedTC, while a simple two magnetic sublattice model yields a much higherTC.

High-T c Cuprates: a Story of Two Electronic Subsystems

A review of the phenomenology and microscopy of cuprate superconductors is presented, with particular attention to universal conductance features, which reveal the existence of two electronic subsystems. The overall electronic system consists of 1 + p charges, where p is the doping. At low dopings, exactly one hole is localized per planar copper-oxygen unit, while upon increasing doping and temperature, the hole is gradually delocalized and becomes itinerant. Remarkably, the itinerant holes exhibit identical Fermi liquid character across the cuprate phase diagram. This universality enables a simple count of carrier density and yields comprehensive understanding of the key features in the normal and superconducting state. A possible superconducting mechanism is presented, compatible with the key experimental facts. The base of this mechanism is the interaction of fast Fermi liquid carriers with localized holes. A change in the microscopic nature of chemical bonding in the copper oxide planes, from ionic to covalent, is invoked to explain the phase diagram of these fascinating compounds.

Silver route to cuprate analogs

The parent compound of high-[Formula: see text] superconducting cuprates is a unique Mott insulator consisting of layers of spin-[Formula: see text] ions forming a square lattice and with a record high in-plane antiferromagnetic coupling. Compounds with similar characteristics have long been searched for without success. Here, we use a combination of experimental and theoretical tools to show that commercial [Formula: see text] is an excellent cuprate analog with remarkably similar electronic parameters to [Formula: see text] but larger buckling of planes. Two-magnon Raman scattering and inelastic neutron scattering reveal a superexchange constant reaching 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the [Formula: see text] planes could have an antiferromagnetic coupling that matches or surpasses the cuprates.

Analysis of the singlet-triplet splitting computed by the density functional theory-broken-symmetry method: is it an exchange coupling constant?

We derive an analytical expression of the density functional theory (DFT)-broken-symmetry (BS) estimation J(BS) of the singlet-triplet gap at the "3 sites-4 electrons" level, that is, two S = (1)/(2) metallic sites + one diamagnetic bridge orbital. As originally designed by Noodleman and Davidson (Chem. Phys.1986, 109, 131), J(BS) contains the residual ferromagnetic contribution, single ligand-to-metal and metal-to-metal charge-transfer terms, but no double ligand-to-metal charge-transfer terms or intra/interligand spin-polarization terms. As revealed by the present analysis, the triplet and BS states computed by DFT differ, not only perturbatively (as expected) because of the various physical mechanisms involved (i.e., differential charge-transfer terms) but mainly because of a spurious and unphysical symmetry breaking of the bridge orbitals in the BS state. We examine the consequences of such a difference by deriving two analytical expressions of the exchange coupling constant, one from the BS orbitals designed to match J(BS) and another one from triplet orbitals only. Following and extending on the first paper in the series (J. Phys. Chem. A 2010, 114, 6149), we propose a simple procedure to extract appropriate parameters filling in our analytical expressions. Moreover, we derive the equivalent "3 sites-4 electrons" exchange coupling constant in the configuration-interaction approach, J(CI), for the purpose of comparison. These analytical expressions have been applied to various copper dimers and compared to experimental values.

Unusual superexchange pathways in an NiS2 triangular lattice with negative charge-transfer energy

We have studied the electronic structure of the Ni triangular lattice in NiGa(2)S(4) using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2p core-level spectrum shows that the S 3p to Ni 3d charge-transfer energy is approximately -1 eV and the ground state is dominated by the d(9)L configuration (L is a S 3p hole). Cell perturbation analysis for the NiS(2) triangular lattice indicates that the strong S 3p hole character of the ground state provides the enhanced superexchange interaction between the third-nearest-neighbor sites.

One- and two-triplon spectra of a cuprate ladder

We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was the separation of one-triplon and two-triplon scattering. Two-triplon scattering is observed quantitatively for the first time and so access is realized to the important strong magnetic quantum fluctuations. The spin gap is found to be 26.4+/-0.3 meV. The data are successfully modeled using the continuous unitary transformation method, and the exchange constants are determined by fitting to be Jleg=186 meV and Jrung=124 meV along the leg and rung, respectively; a substantial cyclic exchange of Jcyc=31 meV is confirmed.

Magnetic ordering in CuO from first principles: a cuprate antiferromagnet with fully three-dimensional exchange interactions

We investigate the interplay of bonding and magnetism in CuO by a first-principles self-interaction-free density-functional approach. Our analysis reveals that, at variance with typical low-dimensional cuprates, a fully three-dimensional view of the exchange interactions is needed to describe accurately the magnetic ground state and low-energy excitations in CuO. The apparent one-dimensional behavior of antiferromagnetic order is due to the presence of a single spin-polarized hole of d(z)2 character. This induces a strongly anisotropic magnetic ordering built up by ferromagnetic (x,y) layers, and antiferromagnetic chains along z, with exchange interactions of similar magnitude.

Pairing correlations on t-U-J ladders

We find that the pairing correlations on the usual t-U Hubbard ladder are significantly enhanced by the addition of a nearest-neighbor exchange interaction J. Likewise, these correlations are also enhanced for the t-J model when the on-site Coulomb interaction is reduced from infinity. Moreover, the pairing correlations are larger on a t-U-J ladder than on a t-J(eff) ladder in which J(eff) has been adjusted so that the two models have the same spin gap at half filling. This enhancement of the pairing correlations is associated with an increase in the pair-binding energy and the pair mobility in the t-U-J model and points to the importance of the charge-transfer nature of the cuprate systems.