Emergence of superconductivity, valence bond order, and Mott insulators in Pd[(dmit)2] based organic salts

Absence of superconductivity and valence bond order in the Hubbard-Heisenberg model for organic charge-transfer solids

A frustrated, effective ½-filled band Hubbard-Heisenberg model has been proposed for describing the strongly dimerized charge-transfer solid families κ-(ET)2X and Z[Pd(dmit)2]2. In addition to showing unconventional superconductivity, these materials also exhibit antiferromagnetism, candidate spin-liquid phases, and, in the case of Z=EtMe3P, a spin-gapped phase that has sometimes been referred to as a valence bond solid. We show that neither superconductivity nor the valence bond order phase occurs within the Hubbard-Heisenberg model. We suggest that a description based on ¼-filling, that is reached when the carrier concentration per molecule instead of per dimer is considered, thus may be appropriate.

Theory of the magnetic and metal-insulator transitions in RNiO3 bulk and layered structures

A slave rotor--Hartree-Fock formalism is presented for studying the properties of the p-d model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within a unified formulation, the significant aspects of the rare-earth nickelate phase diagram, including the paramagnetic metal state observed for the LaNiO3 and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and unstrained thin-film form. For ultrathin films, epitaxial strain and charge transfer to the apical out-of-plane oxygen sites are shown to have significant impact on the phase diagram.

Geometrical frustration in the spin liquid β'-Me3EtSb[Pd(dmit)2]2 and the valence-bond solid Me3EtP[Pd(dmit)2]2

We show that the electronic structures of the title compounds predicted by density functional theory are well described by tight binding models. We determine the frustration ratio, J'/J, of the Heisenberg model on the anisotropic triangular lattice, which describes the spin degrees of freedom in the Mott insulating phase for a range of Pd(dmit)2 salts. All of the antiferromagnetic materials studied have J'/J is < or approximately equal to 0.5 or J'/J > or approximately equal to 0.9, and all salts with 0.5 < or approximately equal to J'/J < or approximately equal to 0.9 are known, experimentally, to be charge ordered valence-bond solids or spin liquids.