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Physics

Carbon Hives

Five centuries after its discovery, graphite is surprising physicists with its uncommon electrical and magnetic properties

Bending over a white formica table, the Peruvian physicist Juan Medina Pantoja glues an adhesive tape onto one of the sides of a square silvered block smaller than a thumb nail. The material between his fingers is a sample of ultra pure graphite, which only under certain conditions behaves like a metal. Produced at extremely high temperatures, close to those found in the deepest regions of the planet, this is the highly oriented pyrolytic graphite, so-called because of its structure: the carbon atoms arrange themselves in regular hexagons like those of the honeycombs of a beehive and form one atom thick layers, like the sheets of graphene, which stack themselves one upon the other.

Finer layers
Medina slowly pulls the tape and separates off a film of graphite with a few layers of graphene. Next he places this sample between two glass slides which will be dried under a very strong light. It was more or less in this manner, at the beginning of this year, that he verified that perhaps it would be possible to reduce even more the width of the graphite samples. “At this temperature, the atoms of carbon begin to detach themselves from the graphite and to combine with the oxygen of the air, forming carbon dioxide gas”, explains the physicist Yakov Kopelevich, the laboratory coordinator at the Physics Institute of the State University of Campinas (Unicamp) in which Medina is carrying out his research. By augmenting the drying stage, researchers Kopelevich and Medina believe they have found a manner of obtaining sheets of graphite that are even finer than those obtained only with the use of an adhesive tape.

Conductor or insulator
The advances by this team are not restricted to this refined gluing. With the results from the research that has counted upon R$ 1 million from FAPESP and the National Scientific and Technological Research Development Council (CNPq), Kopelevich and his two students Medina and Robson Ricardo da Silva, have identified electrical and magnetic properties that it had never been imagined that graphite could present – and have helped to understand why graphite can behave both as a metal and conduct electricity, or as an insulating material.

Quantum Hall Effect
One of its properties is the so called Quantum Hall Effect, which coordinates the movement of electrically charged particles – in the case of graphite, its electrons – along flat surfaces. Discovered by Klaus von Klitzing, a physicist from the Max Planck Institute who received the Nobel Physics prize in 1985 because of this finding, this effect is the version for the microscopic world of a phenomenon identified a century before by the American physicist Edwin Hall. Physicist Hall observed the effect that led to the use of his name on applying a magnetic field to a conductor bar with an electric current passing through it. The magnetic field, perpendicular to the direction of the current, causes a deviation in the electrons’ trajectory, which accumulate in one of the extremities of the bar, generating an electric field in the transversal direction to the current.

Variable resistance
Hall brought together these concepts, clearly abstract for the majority of people, in an equation of only three variables that forecast how the capacity of a material to conduct an electric current would vary depending upon how the intensity of the magnetic field would be altered. Used by physicists in the investigation of the electrical and magnetic properties of metals and semiconductors, this equation shows that the Hall resistance grows in a continuous manner with the magnetic field. But this phenomenon is only valid for macroscopic objects in which the electrically charged particles move themselves in 3-dimensions – in depth, width and height. In the world of particles, ruled by the laws of quantum physics, little understood, even by the specialists, everything is different.

The jumps
When physicists submit a material to whatever low temperature and to a magnetic field, an increase in the intensity of this field makes the Hall resistance increase in proportional jumps, allowing for a constant value between one increase and another. This phenomenon takes the graphics form that reminds one of the rungs of a ladder interspaced by landings. It was this standard of increase of the Hall resistance as a consequence of the variation of the magnetic field that physicist Kopelevich’s team detected in graphite and detailed out in an article published in 2003 in Physical Review Letters. “The resistance to the passage of an electric current between one sheet and another of graphene is 100,000 times superior to the resistance along the plane”, says Kopelevich, who some 13 years ago changed his work at the Physics-Technical Institute of A. F. Ioffe, in the frozen city of St. Petersburg, in Russia, for the stuffy heat of Campinas. Not even the physicists, that two decades ago had imagined that they would discover everything about graphite, had hoped for these results. “It’s surprising that the Quantic Hall Effect has been observed in graphite”, comments Douglas Galvão, from Unicamp, who is studying the properties of another composite material of carbon, nanotubes, formed by layers of rolled up graphene.

Mystery
As yet it is not known for certain why the Quantum Hall Effect, previously observed in silicon and in other semiconductor materials, also occurs in graphite submitted to temperatures close to 200º Celsius negative (- 200ºC) and to reasonably intensive magnetic fields. An explanation comes from the very atomic structure of graphene, whose electrons, under these conditions, can only move themselves in two dimensions. The electrons responsible for the conduction of electrical current locate themselves slightly above and below the plane of the carbon atoms, situated on the vertices of the hexagons and united one to the other by the interaction between the other electrons. In graphite, the sheets of graphene are weakly united one to another – this is why they dislocate easily and leave a gray trace when a matter of fact pencil runs across a piece of paper.

Superconductivity
In another article published in Physical Review Letters, physicists Kopelevich and Igor Luk’yanchuk, from the University of Picardie Jules Verne, in France, discovered yet another property of graphite. By varying the intensity of the magnetic field, they verified that the free electrons of this material exhibit untypical behavior, described by the equations of quantum mechanics created in 1928 by the English physicist Paul Dirac: these electrons move themselves as particles without mass, in a manner similar to that of light, namely photons. In 2005 physicists Andre Geim, in England, and Philip Kim, in the United States, saw this same effect in sheets of graphene. The results obtained by the Unicamp team and published in 2003 also indicate the relationship between the Quantum Hall Effect and superconductivity.

Like a magnet
Actually, Kopelevich, Sergio Moehlecke, José Henrique Spahn Torres and Vladislav Lemanov had described superconductivity in pure graphite some six years previously in the magazine Physics of the Solid State. This effect needs to be confirmed, but it reiterates the possibility that this material can gain other technological applications: physicist Kopelevich also verified that graphite, under special conditions, can function as a magnet.

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