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A delicate bridge

Electrons can change their behavior in nanowire connections

It is not just enough to produce wires of the lowermost dimensions, with a thickness of 1 millionth of a millimeter, in order to set up the promise of nanotechnology, which is the miniaturizing of electronic gadgets. Also one needs to learn to tie up these wires with connections that are the size of a simple molecule, a task that is beginning to be understood by physicists. A recent article in the magazine Physical Review Letters threw light upon a phenomenon essential to the behavior of electrons in these tie-ups, the nanojunctions. The study, signed by researchers from Brazil, Italy and the United States, showed that the laws that regulate the behavior of electron transportation in electronic circuits, even in those of microelectronics, can not be applied within the new problems involving nanoelectronics. For example, a nanojunction, an extremely narrow metallic system, up to the width of an atom, which connects two conductors, has new properties.

Both in materials of normal dimensions and in nanoscopic wires, one can consider that the behavior of an electron transported in the current suffers the influence of the “average effect” of all of the surrounding electrons. But there are special metals, in which the existence of a narrow gauge or confinement – the typical case of a nanojunction – can affect the passage of current. These metals form the basis of many proposals for molecular electronics. The work took into account a nanojunction made of platinum, a metal with many electrons located in the same atomic sub-shell. With this lack of space, the conducted electrons are obliged to avoid the host electrons in the atom. Instead of the “average effect” the repulsion between each pair of electrons dominates, a phenomenon known as “electronic correlation”.

“Either this effect is taken into consideration, or the calculations are going to be wrong”, says Marilia Caldas, a professor at the Physics Institute of the University of São Paulo, who signed the article in partnership with Andrea Ferretti, Arrigo Calzolari, Rosa Di Felice, Franca Manghi and Elisa Molinari, from the University of Modena, in Italy, and Marco Buongiorno Nardelli, from the University of North Carolina. In the article, the authors proposed a mathematical manner of including electronic correlation in the calculation of electronic transportation and in the application of a platinum nanojunction. The representation of the passage of electrons through the junction (conductance or transmission) is different when the new calculation is made use of. The diagram shows the forms of the wide levels in the case of the average field, that which would indicate good conductance. But when the repulsion of each electron pair is included the levels are deformed, resulting in a drastic loss of conductance.

The work by professor Marilia Caldas is one example among many of the expertise within the Physics Institute in theoretical research concerning the properties of new materials. At the end of 2001, a study led by another of the institution’s professor, Adalberto Fazzio, was the cover story of the same magazine Physical Review Letters. The article described discoveries concerning the behavior of atoms of gold nanowires, a strategic material for the manufacture of components for future generations of computers. The challenge is to transform the findings into practical innovations, a mission for which other countries are more prepared than Brazil. “Our research has theoretical importance and in the training of human resources, but it’s worrying that there’s not an industrial policy capable of guaranteeing the transformation of these riches into products”, says Fazzio.