In May of 1999, Drs. Yakov Kopelevich and Sérgio Moehlecke, of the Glen Wataghin Physics Institute of the State University of Campinas (Unicamp), were carrying out magnetic measurements in nanotubes of carbon and graphite when something unexpected happened. An extremely pure sample of graphite that was being used as a control in the experiment, called pyrolytic graphite, highly orientated and known by English the acronym HOPG, began to behave in a surprising way . Within a very wide temperature range, between 2 and 300 degrees Kelvin (K), or that is to say, between -271 until 27 degrees Celsius (°C), it behaved as a superconductor, having this property located in small regions like “grains” or “islands”.
It is known that a material becomes a superconductor when, below a determined critical temperature (Tc), it manages to transmit electric current with zero resistance (loss of energy produced in the form of heat). Each material has a different Tc. Up until now, a compound of copper (II) oxide, HgBa2Ca2Cu3O8+X, has been considered to be the superconductor with the highest Tc – the chilly -109°C (164o K). The pyrolytic graphite studied at Unicamp goes a long way beyond this. It covers an wide range of common room temperatures that peak at 27°C.
Though composed of only carbon atoms, like common graphite, the HOPG has totally different physical characteristics from the material that stuffs pencils. Industrially synthesized under conditions of a temperature of 3,000°C and high pressure, it is an extremely pure polycrystal composed of parallel layers of carbon atoms. It is used as a substrate or in the calibration of some types of microscope such as that of tunneling, and of atomic force. A small sample, with millimeters of thickness and of one square centimeter of area, can cost up to US$ 1,000.00 according to the specifications.
The superconductivity rates in the HOPG, whose samples used in the initial experiment had been brought from Russia by a colleague of Dr. Kopelevich, intrigued and spurred the Unicamp researcher, who immediately put aside the studies with the nanotubes to concentrate on an analysis of the graphite. After all, there has never been a single material that has proven to maintain its properties of superconductivity in conditions even close to room temperature, which would be somethingrevolutionary. “We were lucky in this discovery”, says Dr. Kopelevich, who eight years ago changed his career as a researcher in Russia for academic life in Brazil. A second property detected within the HOPG graphite was that of ferromagnetism, a phenomenon through which materials transform themselves into magnets when exposed to a magnetic field.
Confirmation and caution
Surprised with the result of the experiment, Dr. Kopelevich set about to redo it to confirm the measurements. “We managed to reproduce the experiment”, he says. He began the studies with graphite in the project on nanotubes, and continued with a thematic project about superconductors that was kicked off in 1994 under the coordination of Sérgio Moehlecke and, from 1998 onwards of José Antonio Sanjurjo, who died at the beginning of this year. Today, besides participating in this theme, Kopelevich maintains his own individual project about a new method of measuring non-local resistance in superconductors (outside of the region to which the current is applied).
In order to evaluate this work with rigor, any physicist in this area would ask if the HOPG presented the so-called Meissner effect, one of the strongest evidences that the superconductivity is real. The Meissner effect is noted when, on the application of a magnetic field to the material, an electric current is formed on its outside layer and its interior expels a magnetic flux in the opposite direction. It is this effect that makes a magnet, with a permanent magnetic field, levitate when it is placed above a superconductor. The HOPG showed the Meissner effect, in the same way as a superconductor based on bismuth with a Tc of: -183°C, a critical temperature considered high by the physicists (see graphs).
Even convinced that in his work there is no error of measurement or provoked by impurities in the graphite samples, the physicist opted for discretion and prudence. In a partnership with researchers at Unicamp, at the University of Leipzig in Germany, and with the A. F. Ioffe PhysicoTechnical Institute of Saint Petersburg in Russia, Dr. Kopelevich wrote scientific papers relating the results and sent them off for specialized publication.
In the last two years, the work has been published in three magazines: Journal of Low Temperature Physics, Physics of The Solid State and Solid State Communications. In them, he makes it clear that during his experiments the pyrolythic graphite seemed to behave as a superconductor, but he avoids stating categorically and firmly that it is, without a doubt, a superconductor at room temperature. “We need to be cautious”, reasons Dr. Kopelevich.
To strengthen his bet in the superconductive properties of HOPG, the physicist has invested on various fronts. Besides redoing the experiment on the same sheets of pyrolythic graphite brought from Russia, he measured the superconductivity on plates made by the North American company Union Carbide – evidence that the detected phenomenon does not seem to come from a sample with impurities.
Now, he is attempting to find evidence of superconductivity in monocrystals of graphite. “If we manage to do this, it will become clear that the superconductivity is truly from the graphite material”, comments the Unicamp researcher. For the time being , he has not obtained this type of registration. He has only caught ferromagnetism in the monocrystals.
A long journey
Dr. Kopelevich believes that the HOPG could be a type of superconductor with the potential to transmit at least weak currents. Contrary to the majority of superconductors, it apparently doesn’t lose its capacity to transmit current with zero resistance even when it shows ferromagnetism, a property which needs to be better analyzed.
What he doesn’t want to happen is to provoke in his research with the pyrolytic graphite the same type of skepticism aroused by the experiments of a group of Croatian scientists. He knows that the path can be long and torturous and that, in the end, “the HOPG might not be as promising a superconductor as we had at first thought.” In the worst of the hypothesis, a new line of research was opened.
The physicist Dr. Carlos Rettori, his colleague at Unicamp, who is also participating in the project, reminds the work with pyrolythic graphite is actually being resumed. “15 years ago, we carried out some measurements with this same material”, recalls Dr. Rettori. “Now Dr. Kopelevich is bringing back to life this theme and we are also going back to study the material. Our recent studies demonstrate, without ambiguities, the presence of itinerant ferromagnetism in the graphite and also the possibility of superconductivity at high temperatures.” Dr. Kopelevich is convinced that superconductors at room temperature exits.
Advances and promises during 90 years
The search for superconductors which work at higher and higher temperatures continues to be a major objective. The potential use of a hypothetical material capable of transmitting electric current with zero resistance under the environmental conditions close to those in which man lives is enormous. The present wires and electric cables, for example, could be substituted for similar ones coated with this room temperature superconductor, and the energy transmission with greater saving and efficiency. However, since they were discovered, some 90 years ago, the superconductors have had restricted applications, exactly because of the low temperatures which they demand.
Those that have discovered in the 15 last years, have accumulated more promises than accomplishments. They still haven’t resulted in any applications because it is difficult to make wires with them. To be able to be used in some practical way, those available on the market have to be cooled to very low temperatures, in very expensive processes. This is the case, for example, with the magnets used in the apparatus of resonance magnetism, done with superconducting metallic alloys. So they can work as they were conceived, these magnets have to be cooled below the Tc of the alloy, through the immersion in liquid helium at -269°C. It is not surprising, therefore, that we frequently see trucks bringing liquid helium to hospitals that use this type of apparatus.
After more than a lukewarm decade, without producing any discovery of great impact, this year research in the area began to heat up throughout the world. In January, Japanese scientists at the Aoyama Gakuin University revealed that a known semi-metallic compound of magnesium and boron, the MgB2, is a cheap and efficient superconductor when cooled to -234°C.
In March, a team from the Bell Laboratories in the United States, showed to the world the first plastic compound, the polymer polythiophene, that behaves as a superconductor, as long as it is integrated to a type of transistor and submitted to -270°C. In spite of the extremely low temperatures necessary to transform these compounds into superconductors, the two discoveries were, for different motives, very celebrated, and would be much more if they worked at room temperature.
The MgB2 is the stable metallic compound with the highest Tc, which makes it a potential candidate to generate new superconductor alloys with higher critical temperatures. Kopelevich comments: “Due to the fact that the MgB2 is an isoelectonic material to graphite, this discovery also motivates a serious reconsideration of the physical properties of graphite. For example, Dr. G. Baskaran, a famous theoretical physicist, recently argued that there are strong superconductor correlations in graphite. Our results provide the experimental evidence that this really occurs. Still on the MgB2, the group led by the physicist Dr. Oscar de Lima, who is also part of the thematic project, made an unprecedented contribution, determining that the superconductivity properties of MgB2 depend on the direction in which the external magnetic field is applied”. The polythiophene is important for it to open up a series of superconducting plastics, something unexpected and unprecedented, since under normal conditions polymers are not good conductors of electricity.
If these two observations were celebrated by the scientific community and are provoking a new outbreak of research into superconductors, a third, even though promising, is seen with reservations. Led by the scientist Dr. Danijel Djurek, a team of researchers of three Croatian institutions, the University of Zagreb, the Ruder Boskovic Institute and the local company Avac claim that they have gone beyond what their Japanese and North American colleagues have obtained. They guarantee that they have evidence of a compound containing silver, lead, carbon and oxygen, that transforms itself into a superconductor at room temperature. Its Tc is, according to the Croatians, an almost unbelievable 70°C. If proven, this Tc would permit the material to transmit current without resistance in any natural environment of the planet, even in the burning sands of a desert. Since nobody has managed to redo the experiment of Dr. Djurek and the Croatians yet, and the Croats have already been mistaken once on this question, the possible discovery has not yet received acceptance.
1. Study of Superconductor Materials; Modality Thematic project; Coordinator Dr. José Antonio Sanjurjo -Physics Institute of Unicamp; Investment R$ 61,890.00
2. Study of the State of Vortices in Superconductors of High Temperature through Non Local Measurements (nº 99/00779-9); Modality Assistance to a research project; Coordinator Dr. Yakov Kopelevich – Physics Institute Unicamp; Investment R$ 18,989.00 and US$ 54,133.40