Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in:The European Physical Journal B 56.3 (2007) We employ density-functional theory to calculate realistic parameters for an extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra-and intermolecular screening in the crystal, we find significant longer-range Coulomb interactions along the molecular stacks, as well as inter-stack coupling. We show that the long-range Coulomb term of the extended Hubbard model leads to a broadening of the spectral density, likely resolving the problems with the interpretation of photoemission experiments using a simple Hubbard model only.

We consider quantum critical points (QCP) in which quantum fluctuations associated with charge rather than magnetic order induce unconventional metallic properties. Based on finite-T calculations on a two-dimensional extended Hubbard model we show how the coherence scale T * characteristic of Fermi liquid behavior of the homogeneous metal vanishes at the onset of charge order. A strong effective mass enhancement reminiscent of heavy fermion behavior indicates the possible destruction of quasiparticles at the QCP. Experimental probes on quarter-filled layered organic materials are proposed for unveiling the behavior of electrons across the quantum critical region.

ABSTRACT. The rational assembly of monomers, in principle, enables the design of a specific periodicity of polymeric frameworks, leading to a tailored set of electronic structure properties in

Combining an analytical and numerical approach we investigate the dispersion of the topologically protected spin-filtered edge-states of the Quantum Spin Hall state on honeycomb and ruby nets with zigzag (ZZ) and armchair (AC) edges. We show that the Fermi velocity of the helical edge-states on ZZ edges increases linearly with the strength of the spin-orbit coupling (SOC) whereas for AC edges the Fermi velocity is independent of the SOC. Also the decay length of edge states into the bulk is dramatically different for AC and ZZ edges, displaying an inverse functional dependence on the SOC.PACS numbers: 72.25.Hg,73.61.Wp, Introduction -In their seminal paper [1], Kane and Mele have shown that spin orbit coupling (SOC) in a single plane of graphene leads to a time-reversal invariant Quantum Spin Hall (QSH) state that is characterized by a bulk energy gap and a pair of topologically protected gapless edge-states. However, the SOC energy scale in graphene is so tiny that the predicted gap [2] is merely ∼ 0.01K. It is therefore not possible in practice to establish the existence of the underlying Z 2 topological order in graphene [3][4][5] so that the hunt for the QSH effect was continued in other materials. The theoretical prediction [6] and experimental observation [7] of the QSH effect in HgTe thin films have firmly categorized this material as the sought-after two-dimensional (2D) Z 2 topological insulator (TI). Having a zinc-blende crystal structure this material is of course very different from graphene from both an electronic and a structural point of view.The recently discovered topological insulator Bi 14 Rh 3 I 9 , however, provides an entirely novel platform for the observation of the QSH effect in graphene-like systems with a honeycomb structure [8]. Bi 14 Rh 3 I 9 consists of stacks of bismuth-based layers each of which forms a honeycomb net composed of RhBi 8 cubes.These cubes form what is commonly referred to as a ruby lattice, see Fig. 1, which has the same point group symmetry as the hexagonal graphene honeycomb lattice. Each such a layer of Bi 14 Rh 3 I 9 forms a 2D Z 2 TI, with a large spin-orbit gap of ∼ 2400K due to the strong bismuth-related SOC [8]. The gap being six order of magnitudes larger than graphene opens the avenue for the actual observation of the QSH effect in a hexagonal graphene-like system.For a future use of this QSH effect a fundamental understanding of the topologically-protected spin-polarized edge-states is essential. We have therefore investigated the electronic characteristics of these topological edgestates in both honeycomb and ruby lattices in the presence of SOC. We find in these hexagonal systems a dramatic dependence of the edge-state dispersion, decay

The interplay of Coulomb repulsion and geometrical frustration on charge-driven quantum phase transitions is explored. The ground-state phase diagram of an extended Hubbard model on an anisotropic triangular lattice relevant to quarter-filled layered organic materials contains homogeneous metal, "pinball," and threefold charge ordered metallic phases. The stability of the pinball phase occurring for strong Coulomb repulsions is found to be strongly influenced by geometrical frustration. A comparison with a spinless model reproduces the transition from the homogeneous-metallic phase to a pinball liquid, which indicates that the spin correlations should play a much smaller role than the charge correlations in the metallic phase close to the charge-ordering transition. Spin degeneracy is, however, essential to describe the dependence of the system on geometrical frustration. Based on finite-temperature Lanczos diagonalization we find that the effective Fermi temperature scale T * of the homogeneous metal vanishes at the quantum phase transition to the ordered metallic phase driven by the Coulomb repulsion. Above this temperature scale "bad" metallic behavior is found which is robust against geometrical frustration in general. Quantum critical phenomena are not found whenever nesting of the Fermi surface is strong, possibly indicating a first-order transition instead. "Reentrant" behavior in the phase diagram is encountered whenever the 2k F charge-density wave instability competes with the Coulomb driven threefold charge order transition. The relevance of our results to the family of quarter-filled materials, θ-(BEDT-TTF) 2 X, is discussed.