arXiv subjects

N. Gomes

Publications and source records attributed to N. Gomes.

5 recordsLinked to original sources

arXiv2019

Theory of triangular lattice quasi-one-dimensional charge-transfer solids

Recent investigations of the magnetic properties and the discovery of superconductivity in quasi-one-dimensional triangular lattice organic charge-transfer solids have indicated the severe limitations of the effective 1/2-filled band Hubbard model for these and related systems. Our computational studies of these materials within a 1/4-filled band Hubbard model in which the organic monomer molecules, and not their dimers, constitute the sites of the Hamiltonian are able to reproduce the experimental results. We ascribe the spin gap transition in kappa-(BEDT-TTF)_2B(CN)_4 to the formation of a two-dimensional paired-electron crystal and make the testable prediction that the spin gap will be accompanied by charge-ordering and period doubling in two directions. We find enhancement of the long-range component of superconducting pairing correlations by the Hubbard repulsive interaction for band parameters corresponding to kappa-(BEDT-TTF)_2CF_3SO_3. The overall results strongly support a valence bond theory of superconductivity we have proposed recently.

cond-mat.str-el↗

arXiv2016

Bond patterns and charge order amplitude in 1/4-filled charge-transfer solids

Metal-insulator transition accompanied by charge-ordering has been widely investigated in quasi-one-dimensional conductors, including in particular organic charge-transfer solids. Among such materials the 1/4-filled band charge-transfer solids are of strong interest, because of the commensurate nature of the charge-ordering in these systems. The period-four charge-order pattern ...1100... here is accompanied by two distinct bond distortion patterns, giving rise to bond-charge-density waves (BCDW) of types 1 and 2. Using quantum Monte Carlo methods, we determine the phase diagram within the extended Hubbard Hamiltonian that gives both types 1 and 2 BCDW in the thermodynamic limit. We further investigate the effect of electron-electron and electron-phonon interactions on the amount of charge disproportionation. Our results show that between these two bond patterns, one (BCDW2) in general coexists with a large magnitude charge order, which is highly sensitive to electron-phonon interactions, while the other (BCDW1) is characterized by weak charge order. We discuss the relevance of our work to experiments on several 1/4-filled conductors, focusing in particular on the materials (EDO-TTF)_2X and (DMEDO-TTF)_2X with large amplitude charge-order.

cond-mat.str-el↗

arXiv2016

Coulomb enhancement of superconducting pair-pair correlations in a $\frac{3}{4}$-filled model for $κ$-(BEDT-TTF)$_2$X

We present the results of precise correlated-electron calculations on the monomer lattices of the organic charge-transfer solids $κ$-(BEDT-TTF)$_2$X for 32 and 64 molecular sites. Our calculations are for band parameters corresponding to X = Cu[N(CN)$_2$]Cl and Cu$_2$(CN)$_3$, which are semiconducting antiferromagnetic and quantum spin liquid, respectively, at ambient pressure. We have performed our calculations for variable electron densities $ρ$ per BEDT-TTF molecule, with $ρ$ ranging from 1 to 2. We find that $d$-wave superconducting pair-pair correlations are enhanced by electron-electron interactions only for a narrow carrier concentration about $ρ=1.5$, which is precisely the carrier concentration where superconductivity in the charge-transfer solids occurs. Our results indicate that the enhancement in pair-pair correlations is not related to antiferromagnetic order, but to a proximate hidden spin-singlet state that manifests itself as a charge-ordered state in other charge-transfer solids. Long-range superconducting order does not appear to be present in the purely electronic model, suggesting that electron-phonon interactions also must play a role in a complete theory of superconductivity.

cond-mat.str-el↗

arXiv2013

Organic superconductors: the need to go beyond effective 1/2-filled band models

A frustrated, effective 1/2-filled band Hubbard-Heisenberg model has been proposed to describe the strongly dimerized charge-transfer solid families kappa-(ET)_2X and Z[Pd(dmit)_2]_2. In addition to unconventional superconductivity these materials also exhibit antiferromagnetism, candidate spin-liquid phases, and in the case of Z=EtMe_3P, a so-called valence-bond solid phase. We show that neither superconductivity nor the valence-bond solid phase occurs within the Hubbard-Heisenberg model, indicating that the effective 1/2-filled band model is unsuitable for these materials.

cond-mat.str-el↗

arXiv2010

First results from fringe tracking with the PRIMA fringe sensor unit

The fringe sensor unit (FSU) is the central element of the phase referenced imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility for the Very Large Telescope interferometer (VLTI). It has been installed at the Paranal observatory in August 2008 and is undergoing commissioning and preparation for science operation. Commissioning observations began shortly after installation and first results include the demonstration of spatially encoded fringe sensing and the increase in VLTI limiting magnitude for fringe tracking. However, difficulties have been encountered because the FSU does not incorporate real-time photometric correction and its fringe encoding depends on polarisation. These factors affect the control signals, especially their linearity, and can disturb the tracking control loop. To account for this, additional calibration and characterisation efforts are required. We outline the instrument concept and give an overview of the commissioning results obtained so far. We describe the effects of photometric variations and beam-train polarisation on the instrument operation and propose possible solutions. Finally, we update on the current status in view of the start of astrometric science operation with PRIMA.

astro-ph.IM↗