A simple scheme for finding magnetic aromatic hydrocarbon molecules.

Polycyclic aromatic hydrocarbon (PAH) molecules such as quasi-unidimensional oligo-acene and fused azulene display interesting properties for increasing chain length. However, these molecules can be hard to explore computationally due to the number of atoms involved and the fast-increasing numerical cost when using many-body methods. The identification of magnetic PAH molecules is most relevant for technological applications and hence it would be of particular interest to develop rapid preliminary checks to identify likely candidates for both theoretical and experimental pursuits. In this article, we show that an analysis based on a second-order perturbation treatment of electronic correlations for the Hubbard model qualitatively predicts the outcome of more extensive and accurate methods. Based on these results we propose a simple computational protocol for screening molecules and identifying those worthy of a more sophisticated analysis on the magnetic nature of their ground states. Using this protocol we were able to identify two new magnetic molecules made from the combination of only two naphthalene monomers and two azulene ones (both isomers with formula C34H20). For further confirmation of this result, these molecules were also studied by means of density matrix renormalization group and density functional theory.

Building two-dimensional metal-organic networks with tin.

We show that Sn atoms combined with organic ligands can be used to build 2D coordination networks on Au(111) surfaces.

Ubiquitous deprotonation of terephthalic acid in the self-assembled phases on Cu(100).

We performed an exhaustive study of terephthalic acid (TPA) self-assembly on a Cu(100) surface, where first-layer molecules display two sequential phase transitions in the 200-400 K temperature range, corresponding to different stages of molecular deprotonation. We followed the chemical and structural changes by means of high-resolution X-ray photoelectron spectroscopy (XPS) and variable-temperature scanning tunneling microscopy (STM), which were interpreted on the basis of density functional theory (DFT) calculations and photoemission simulations. In order to reveal the spectroscopic contributions of the molecules in different states of deprotonation, we modified the substrate reactivity by deposition of a small amount of Sn, which hampers the deprotonation reaction. We found that the characteristic molecular ribbons of the TPA/Cu(100) α-phase at a low temperature contain a significant fraction of partially deprotonated molecules, in contrast to the expectation of a fully protonated phase, where the self-assembly was claimed to be simply driven by the intermolecular double hydrogen bonds [OHO]. On the basis of our simulations, we propose a model where the carboxylate groups of the partially deprotonated molecules form single hydrogen bonds with the carboxylic groups of the fully protonated molecules. Using real time XPS, we also monitored the kinetics of the deprotonation reaction. We show that the network of mixed single and double hydrogen bonds inhibits further deprotonation up to ∼270 K, whereas the isolated molecules display a much lower deprotonation barrier.

Abiotic degradation of glyphosate into aminomethylphosphonic acid in the presence of metals.

Glyphosate [N-phosphono-methylglycine (PMG)] is the most used herbicide worldwide, particularly since the development of transgenic glyphosate-resistant (GR) crops. Aminomethylphosphonic acid (AMPA) is the main glyphosate metabolite, and it may be responsible for GR crop damage upon PMG application. PMG degradation into AMPA has hitherto been reckoned mainly as a biological process, produced by soil microorganisms (bacteria and fungi) and plants. In this work, we use density functional calculations to identify the vibrational bands of PMG and AMPA in surface-enhanced Raman spectroscopy (SERS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra experiments. SERS shows the presence of AMPA after glyphosate is deposited from aqueous solution on different metallic surfaces. AMPA is also detected in ATR-FTIR experiments when PMG interacts with metallic ions in aqueous solution. These results reveal an abiotic degradation process of glyphosate into AMPA, where metals play a crucial role.

Anisotropic magnetoresistance in manganites: experiment and theory.

We present measurements of the anisotropic magnetoresistance (AMR) of La(0.75)Sr(0.25)MnO(3) films deposited on (001) SrTiO(3) substrates, and a model that describes the experimental results. The model, based on the electronic structure of manganites plus the spin-orbit coupling, correctly accounts for the dependence of the AMR on the direction of the current to the crystalline axes. We measure an AMR of the order of 10(-3) for the current I parallel to the [100] axis of the crystal and vanishing AMR for I , in agreement with the model predictions. Further, we calculate the planar Hall effect and show its connection to AMR.

The effect of Coulomb interaction at ferromagnetic-paramagnetic metallic perovskite junctions.

We study the effect of Coulomb interactions in transition metal oxide junctions. In this paper we analyze charge transfer at the interface of a three layer ferromagnetic-paramagnetic-ferromagnetic metallic oxide system. We choose a charge model considering a few atomic planes within each layer and obtain results for the magnetic coupling between the ferromagnetic layers. For large numbers of planes in the paramagnetic spacer we find that the coupling oscillates with the same period as in Ruderman-Kittel-Kasuya-Yoshida (RKKY) theory but the amplitude is sensitive to the Coulomb energy. At small spacer thickness however, large differences may appear as a function of the number of electrons per atom in the ferromagnetic and paramagnetic materials, the dielectric constant at each component, and the charge defects at the interface plane, emphasizing the effects of charge transfer.