Thursday, 1 November 2012

Superconductivity, a brief history

Major advances in low-temperature refrigeration were made during the late 19th century. Superconductivity was first discovered in 1911 by the Dutch physicist,Heike Kammerlingh Onnes. Onnes dedicated his scientific career to exploring extremely cold refrigeration. On July 10, 1908, he successfully liquified helium by cooling it to 452 degrees below zero Fahrenheit (4 Kelvin or 4 K). Onnes produced only a few milliliters of liquid helium that day, but this was to be the new beginnings of his explorations in temperature regions previously unreachable. Liquid helium enabled him to cool other materials closer to absolute zero (0 Kelvin), the coldest temperature imaginable. Absolute zero is the temperature at which the energy of material becomes as small as possible.

In 1911, Onnes began to investigate the electrical properties of metals in extremely cold temperatures. It had been known for many years that the resistance of metals fell when cooled below room temperature, but it was not known what limiting value the resistance would approach, if the temperature were reduced to very close to 0 K. Some scientists, such as William Kelvin, believed that electrons flowing through a conductor would come to a complete halt as the temperature approached absolute zero. Other scientists, including Onnes, felt that a cold wire's resistance would dissipate. This suggested that there would be a steady decrease in electrical resistance, allowing for better conduction of electricity. At some very low temperature point, scientists felt that there would be a leveling off as the resistance reached some ill-defined minimum value allowing the current to flow with little or no resistance.
Onnes passed a current through a very pure mercury wire and measured its resistance as he steadily lowered the temperature. Much to his surprise there was no leveling off of resistance, let alone the stopping of electrons as suggested by Kelvin. At 4.2 K the resistance suddenly vanished. Current was flowing through the mercury wire and nothing was stopping it, the resistance was zero.
According to Onnes, "Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state". The experiment left no doubt about the disappearance of the resistance of a mercury wire. Kamerlingh Onnes called this newly discovered state, Superconductivity.

Onnes recognized the importance of his discovery to the scientific community as well as its commercial potential. An electrical conductor with no resistance could carry current any distance with no losses. In one of Onnes experiments he started a current flowing through a loop of lead wire cooled to 4 K. A year later the current was still flowing without significant current loss. Onnes found that the superconductor exhibited what he called persistent currents, electric currents that continued to flow without an electric potential driving them. Onnes had discovered superconductivity, and was awarded the Nobel Prize in 1913.

By 1933 Walther Meissner and R. Ochsenfeld discovered that superconductors are more than a perfect conductor of electricity, they also have an interesting magnetic property of excluding a magnetic field. A superconductor will not allow a magnetic field to penetrate its interior. It causes currents to flow that generate a magnetic field inside the superconductor that just balances the field that would have otherwise penetrated the material.
In 1957 scientists began to unlock the mysteries of superconductors. Three American physicists at the University of Illinois, John Bardeen, Leon Cooper, and Robert Schrieffer, developed a model that has since stood as a good mental picture of why superconductors behave as they do. The model is expressed in terms of advanced ideas of the science of quantum mechanics, but the main idea of the model suggests that electrons in a superconductor condense into a quantum ground state and travel together collectively and coherently. In 1972, Bardeen, Cooper, and Schrieffer received the Nobel Prize in Physics for their theory of superconductivity,which is now known as the BCS theory, after the initials of their last names.

In 1986, Georg Bednorz and Alex Müller, working at IBM in Zurich Switzerland, were experimenting with a particular class of metal oxide ceramics called perovskites. Bednorz and Müller surveyed hundreds of different oxide compounds. Working with ceramics of lanthanum, barium, copper, and oxygen they found indications of superconductivity at 35 K, a startling 12 K above the old record for a superconductor. Soon researchers from around the world would be working with the new types of superconductors. In February of 1987, a perovskite ceramic material was found to superconduct at 90 K. This discovery was very significant because now it became possible to use liquid nitrogen as a coolant. Because these materials superconduct at significantly higher temperatures they are referred to as High Temperature Superconductors. Since then scientists have experimented with many different forms of perovskites producing compounds that superconduct at temperatures over 130 K.

A Bose-Einstein condensate is a state of matter composed of very large number of particles, bosons, that have the same energy level, that of the lowest energy. In a superconductor, the Cooper pairs are the bosons and they form the condensate, but this is an unusual case where pairs are in a very strong interaction. The “actual” condensate that Einstein theoretically predicted in 1925 after the works of the Indian physicist Bose was only directly observed in 1995 in an atom gas. The gas had to be cooled [Refroidissement d’atomes] to a few billionths of a degree to absolute zero, thanks to very recent methods using lasers. Just like superconductors, these gas condensates have spectacular properties such as the coherence of a unique wave, or superfluidity. These recent experimental developments have been at the origin of two Nobel Prizes, in 1997 for cooling atoms and in 2001 for the observation of gas condensates. The field of research concerning gas condensates has developed a lot since the first discoveries in 1995, and is very promising today. For instance, a large number of research teams take advantage of the possibility of playing with various experimental parameters of the condensate in a controlled maner to study complex and still mysterious phenomena. This approach is called quantum simulation and it could provide a new means of understanding superconductivity.
An international team led by University of Toronto physicists has developed a simple new technique using Scotch poster tape that has enabled them to induce high-temperature superconductivity in a semiconductor for the first time. The method paves the way for novel new devices that could be used in quantum computing and to improve energy efficiency.
Cuprates were believed to be impossible to incorporate with semi-conductors, and so their real-world use has been severely limited as has the exploration of new effects they may generate. For example, observing the phenomenon of the proximity effect wherein the superconductivity in one material generates superconductivity in an otherwise normal semi-conductor has been difficult because the fundamental quantum mechanics require the materials to be in nearly perfect contact.
The team used Scotch poster tape and glass slides to place high-temperature superconductors in proximity with a special type of semi-conductor known as a topological insulator. Topological insulators have captured world-wide attention from scientists because they behave like semi-conductors in the bulk, but are very metallic at the surface. The result was induced superconductivity in these novel semi-conductors: a physics first.
Brookhaven National Laboratory, and the Central Research Institute of Electric Power Industry in Japan has used Scanning Tunneling Microscopy to investigate for the first time how this happens on the nanoscale. As they warmed a superconducting sample, they saw that superconductivity died out at different temperatures in regions just a few nanometers apart. Superconductivity didn't just depend on the characteristics of the local region, but on what was going on nearby. Regions of stronger superconductivity seemed to help regions of weaker superconductivity survive at higher temperatures.
Researchers might exploit this interplay by micromanaging a superconductor's structure, so that regions of strong superconductivity have the maximum benefit to weak regions, potentially resulting in a new material that's superconducting at a higher overall temperature than is possible with randomly arranged ceramic superconductors. Controlling structure on the nanoscale could lead to better Superconductors, considering at this point in time we are beginning to introduce superhydrophobic materials. Altering on the surface structures in the nanoscale to suit a need of self cleaning surfaces. So to will our understanding of structure within a material to allow superconductivity at high temperatures. There are several ideas currently looking into this matter and yet room temperature superconductivity remains elusive. Only when we understand more on the atomic level, will the goal of efficient power cables or levitating trains and portable MRI machines will be in our grasp...

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