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PHYSICS

Many forms of electricity

Special materials find new opportunities to conduct electric current when in contact with semiconductors

Possible directions of electric current spins The arrows indicate the various spin directions at the surface of contact between gallium arsenide and bismuth selenide as a result of the interaction between the materials. Each circle represents different electron energy levels. The electric current spins may vary clockwise (blue circles) or counterclockwise (pink circles)

Source Seixas et al., Nature CommunicationsPossible directions of electric current spins: The arrows indicate the various spin directions at the surface of contact between gallium arsenide and bismuth selenide as a result of the interaction between the materials. Each circle represents different electron energy levels. The electric current spins may vary clockwise (blue circles) or counterclockwise (pink circles)Source Seixas et al., Nature Communications

A new type of special material, able to conduct electricity on its surface, not internally, could become more versatile—and conduct electricity in several directions with different energy levels—when placed in contact with a semiconductor material that has been used for decades in computers, according to simulations carried out by physicists at the University of São Paulo (USP) and Rensselaer Polytechnic Institute in the United States.

This conclusion, from a study using computer modeling, was surprising because it indicated the possibility of reorganizing the electrons responsible for conducting electricity. Another unexpected finding was that electric current could be created and controlled by laser beams incident on the area of contact between the materials.

The researchers arrived at these results by analyzing what could happen when gallium arsenide—the semiconductor material used to manufacture computers, LEDs and lasers—is placed against a material with completely different electronic properties, bismuth selenide, which can conduct special electric currents. In bismuth selenide, a property similar to electron spin is always pointing in the same direction, parallel to the surface of the material. The computer simulations showed that contact between the two different materials would increase the possibility of organizing the spins of the electric currents in the layer between the materials.

“When in contact, gallium arsenide and bismuth selenide change their electronic properties,” noted Brazilian physicist Leandro Seixas, one of the authors of the study and now a researcher at the National University of Singapore. He began to investigate the interactions between the materials during an internship at Rensselaer Polytechnic Institute, done as part of his doctoral research, which he concluded in 2014 under the guidance of Adalberto Fazzio, at the USP Physics Institute (see Pesquisa FAPESP Issue nº 192).

The calculations indicate that, when bismuth selenide is placed in contact with gallium arsenide, the electrons retain the ability to move in an orderly manner along the area of contact between the two materials. In addition, changes in energy level and electron speed appear to allow changing of the direction of rotation (spin) without disordering them.

If confirmed by experimental measurements, this property would allow encoding and manipulation of information in the spins, creating the basis for a new computer science technology called spintronics. In today’s computers, information processing is carried out by transistors made of silicon, a semiconductor material. Silicon transistors control the passage of current, consisting of moving electrons, regardless of their spins, which point in random directions in these semiconductor materials. For physicists, materials such as bismuth selenide—called topological insulators because they conduct electric current only on their surfaces—allow the creation of a new type of transistor that could process information using electric current with ordered spins, which could, in principle, be faster and result in less energy loss.

Project
Electronic, magnetic and transport properties of nanostructures (nº 2010/16202-3);  Grant Mechanism: Thematic Project; Principal Investigator: Adalberto Fazzio (IF-USP); Investment: R$ 1,327,201.88 (FAPESP).

Scientific article
SEIXAS, L. et al. Vertical twinning of the Dirac cone at the interface between topological insulators and semiconductors. Nature Communications. V. 6, No. 7630. July 3, 2015.

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