Self-assembly of 5,6-dihydroxyindole-2-carboxylic acid: Polymorphism of a eumelanin building block on Au(111)

Investigating two-dimensional (2D) self-assembled structures of biological monomers governed by intermolecular interactions is a prerequisite to understand the self-assembly of more complex biomolecular systems. 5,6-Dihydroxyindole carboxylic acid (DHICA) is one of the building blocks of eumelanin-an irregular heteropolymer and the most common form of melanin which has potential applications in organic electronics and bioelectronics. By means of scanning tunneling microscopy, density functional theory and Monte Carlo calculations, we investigate DHICA molecular configurations and interactions underlying the multiple 2D patterns formed on Au(111). While DHICA self-assembled molecular networks (SAMNs) are dominated by the hydrogen bonding of carboxylic acid dimers, a variety of 2D architectures are formed due to the multiple weak interactions of the catechol group. The hydroxyl group also allows for redox reactions, caused by oxidation via O 2 exposure, resulting in molecular rearrangement. The susceptibility of the molecules to oxidation is affected by their SAMNs architectures, giving insights on the reactivity of indoles as well as highlighting non-covalent assembly as an approach to guide selective oxidation reactions

LOCAL PIEZOELECTRICITY IN SrTiO3-BiTiO3 CERAMICS

Local piezoelectric properties of Bi-doped SrTiO3 ceramics have been investigated by piezoresponse force microscopy. The appearance of both out-of-plane and in-plane polarization components confirmed the piezoelectric nature of the obtained signal. The absence of labyrinth-like structures in observed piezoelectric contrast is not consistent with the expected existence of a relaxor ferroelectric state in this material. The close similarity of local piezoelectric properties in Bi-doped SrTiO3 with pure SrTiO3 suggests that the origin of obtained piezoresponse can be attributed to the flexoelectric phenomenon. Bi-doping leads to occurrence of oxygen vacancies and negative charge on the surface of the sample

Coulomb gap in a model with finite charge transfer energy

The Coulomb gap in a donor-acceptor model with finite charge transfer energy $\Delta$ describing the electronic system on the dielectric side of the metal-insulator transition is investigated by means of computer simulations on two- and three-dimensional finite samples with a random distribution of equal amounts of donor and acceptor sites. Rigorous relations reflecting the symmetry of the model presented with respect to the exchange of donors and acceptors are derived. In the immediate neighborhood of the Fermi energy $\mu$ the the density of one-electron excitations $g(\epsilon)$ is determined solely by finite size effects and $g(\epsilon)$ further away from $\mu$ is described by an asymmetric power law with a non-universal exponent, depending on the parameter $\Delta$.Comment: 10 pages, 6 figures, submitted to Phys. Rev.

Two-photon parametric pumping versus two-photon absorption: A quantum jump approach

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Magnetic Stripes on Hexagonal Lattice with Competing Exchange and Dipole-Dipole Interactions

We obtained the phase diagram for the Ising model with competing exchange and dipolar interactions on 2D hexagonal lattice. By using histogram method we determined the order of the phase transition from isotropic stripe phase to low-temperature anisotropic stripe phases AFh of different stripe widths h. The first order phase transition was found to AF1 and AF2 phases and the second order -to AF3 and AF4 phases. We have also found that in AF1 phase stripe domain grows with time as t 0.5 . In phases AF3 and AF4 stripe domain growth is proportional to log t

Simulation of Reaction-Induced Phase Separation in Surface Alloy

Using kinetic Monte Carlo method we simulate the dynamics of biatomic $Au_{0.3}Ni_{0.7}$ surface alloy separation on Ni(111) due to $Ni(CO)_4$ out-reaction. The experiment of Vestergaard et al. is modeled by counterbalancing dynamical processes and interactions between reactants. The simulations demonstrate step flow rate increase with CO coverage, $c_{CO},$ in qualitative agreement with the experiment only for $c_{CO}$ ≲ 0.45 monolayer. Moreover, we demonstrate both CO influence on reaction process and Au domain formation

Magnetic Stripes on Hexagonal Lattice with Competing Exchange and Dipole-Dipole Interactions

We obtained the phase diagram for the Ising model with competing exchange and dipolar interactions on 2D hexagonal lattice. By using histogram method we determined the order of the phase transition from isotropic stripe phase to low-temperature anisotropic stripe phases AFh of different stripe widths h. The first order phase transition was found to AF1 and AF2 phases and the second order - to AF3 and AF4 phases. We have also found that in AF1 phase stripe domain grows with time as $t^{0.5}$. In phases AF3 and AF4 stripe domain growth is proportional to log t

Magnetic Stripes on Hexagonal Lattice with Competing Exchange and Dipole-Dipole Interactions

We obtained the phase diagram for the Ising model with competing exchange and dipolar interactions on 2D hexagonal lattice. By using histogram method we determined the order of the phase transition from isotropic stripe phase to low-temperature anisotropic stripe phases AFh of different stripe widths h. The first order phase transition was found to AF1 and AF2 phases and the second order - to AF3 and AF4 phases. We have also found that in AF1 phase stripe domain grows with time as $t^{0.5}$. In phases AF3 and AF4 stripe domain growth is proportional to log t