Reply to "Validity of the t-J model"

Symmetry of charge order in cuprates

Charge-ordered ground states permeate the phenomenology of 3d-based transition metal oxides, and more generally represent a distinctive hallmark of strongly correlated states of matter. The recent discovery of charge order in various cuprate families has fuelled new interest into the role played by this incipient broken symmetry within the complex phase diagram of high-T(c) superconductors. Here, we use resonant X-ray scattering to resolve the main characteristics of the charge-modulated state in two cuprate families: Bi2Sr(2-x)La(x)CuO(6+δ) (Bi2201) and YBa2Cu3O(6+y) (YBCO). We detect no signatures of spatial modulations along the nodal direction in Bi2201, thus clarifying the inter-unit-cell momentum structure of charge order. We also resolve the intra-unit-cell symmetry of the charge-ordered state, which is revealed to be best represented by a bond order with modulated charges on the O-2p orbitals and a prominent d-wave character. These results provide insights into the origin and microscopic description of charge order in cuprates, and its interplay with superconductivity.

Quantum dimer model for the pseudogap metal

We propose a quantum dimer model for the metallic state of the hole-doped cuprates at low hole density, p. The Hilbert space is spanned by spinless, neutral, bosonic dimers and spin S = 1/2, charge +e fermionic dimers. The model realizes a "fractionalized Fermi liquid" with no symmetry breaking and small hole pocket Fermi surfaces enclosing a total area determined by p. Exact diagonalization, on lattices of sizes up to 8 × 8, shows anisotropic quasiparticle residue around the pocket Fermi surfaces. We discuss the relationship to experiments.

Detection of Zhang-Rice singlets using spin-polarized photoemission

From a spin-resolved photoemission study on the Bi(2)Sr(2)CaCu(2)O(8+delta) superconductor, we show experimentally that the first ionization state is of nearly pure singlet character. This is true both above and below the superconducting transition and in the presence of doping and band formation. This provides direct support for the existence and stability of Zhang-Rice singlets in high-temperature superconductors, justifying the ansatz of single-band models. Moreover, we establish this technique as an important probe for a wide range of cuprates and strongly correlated materials.

Electronic structure of the high T c superconductors

The high T c superconductors have one structural element in common, namely CuO 2 planes which are lightly doped away from an average valence of Cu 2+ . In the absence of this doping the planes are in a Mott insulating state with local S = 1/2 moments on each Cu-site. There is considerable evidence both experimental and theoretical supporting the assignment of the extra holes, introduced by doping, to the antibonding O 2p-orbitals. The strong hybridization between these orbitals and the central Cu 3d-orbitals makes it favourable to bind the extra hole with a Cu 2+ local moment to form a spin singlet state centred on a CuO 4 square. This singlet, however, is mobile and the combination of mobile charged singlets and local spins is described by the so-called t-J model. This has a number of consequences which can be tested experimentally. For example, one can use this model to estimate the hyperfine coupling constants which are measured in NMR experiments. The prediction that the only spin degrees of freedom are the local Cu 2+ spins even upon doping can also be tested in NMR experiments.