Learn Physics Online: July 2010

“The experiment left no doubt that, as far as accuracy of measurement went, the resistance disappeared. At the same time, however, something unexpected occurred. The disappearance did not take place gradually but abruptly. From 1/500 the resistance at 4.2K, it could be established that the resistance had become less than a thousand-millionth part of that at normal temperature. Thus the mercury at 4.2K has entered a new state, which, owing to its particular electrical properties, can be called th state of superconductivity. “— Heike Kamerlingh Onnes

A GLIMPSE

SUPERCONDUCTIVITY is a fascinating and challenging field of physics. Scientists and engineers throughout the world have been striving to develop an understanding of this remarkable phenomenon for many years. For nearly 75 years superconductivity has been a relatively obscure subject. Today however, superconductivity is being applied to many diverse areas such as: medicine, theoretical and experimental science, the military, transportation, power production, electronics, as well as many other areas. With the discovery of high-temperature superconductors, which can operate at liquid nitrogen temperatures (77 K), superconductivity is now well within the reach of high school students. Unique and exciting opportunities exist today for our students to explore and experiment with this new and important technological field of physics.

HISTORICAL BACKGROUND

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.FIG 1 is a graph of resistance versus temperature in mercury wire as measured by Onnes . 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 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.

UNLOCKING THE MYSTERIES OF SUPERCONDUCTIVITY

Whenever a new scientific discovery is made, researchers must strive to explain their theories. 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.

This effect, called the Meissner Effect, causes a phenomenon that is a very popular demonstration of superconductivity. Figure (2) is an image of magnetic field lines from a magnet levitating above a superconductor. The Meissner Effect will occur only if the magnetic field is relatively small. If the magnetic field becomes too great, it penetrates the interior of the metal and the metal loses its superconductivity. 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.

Fundementals of superconductors

¯ Superconductors have the ability to conduct electricity without the loss of energy. When current flows in an ordinary conductor, for example copper wire, some energy is lost. In a light bulb or electric heater, the electrical resistance creates light.

¯ Inside a superconductor the behavior of electrons is vastly different. As the superconducting electrons travel through the conductor they pass unobstructed through the complex lattice .As these electrons do not bump with anything( unlike the electrons of metal conductors Cu and which collide with tiny impurities and imperfections in lattice), these create no friction and they can transmit electricity with no appreciable loss in the current and no loss of energy.

¯ BCS THEORY by three American physicists-John Bardeen, Leon Cooper, and John Schrieffer explains superconductivity at temperatures close to absolute zero. According to the theory, as one negatively charged electron passes by positively charged ions in the lattice of the superconductor, the lattice distorts. This in turn causes phonons to be emitted which forms a trough of positive charges around the electron illustrates a wave of lattice distortion due to attraction to a moving electron. Before the electron passes by and before the lattice springs back to its normal position, a second electron is drawn into the trough. It is through this process that two electrons, which should repel one another, link up.

¯ The BCS theory successfully shows that electrons can be attracted to one another through interactions with the crystalline lattice. This occurs despite the fact that electrons have the same charge. The electron pairing is favorable because it has the effect of putting the material into a lower energy state. When electrons are linked together in pairs, they move through the superconductor in an orderly fashion.

¯ As the superconductor gains heat energy the vibrations in the lattice become more violent and break the pairs. As they break, superconductivity diminishes. Superconducting metals and alloys have characteristic transition temperatures from normal conductors to superconductors called Critical Temperature (T _c)

¯ Below the superconducting transition temperature, the resistivity of a material is exactly zero. Superconductors made from different materials have different( T _c) values.

¯ There is a certain maximum current that these materials can be made to carry, above

which they stop being superconductors. If too much current is pushed through a superconductor, it will revert to the normal state even though it may be below its transition temperature.

¯ There are two types of superconductors, Type I and Type II. Very pure samples of lead, mercury, and tin are examples of Type I superconductors. High temperature ceramic superconductors such as YBa_2CU_3O_7 (YBCO) and Bi_ CaSr _ 2Cu_2O_9are examples of Type II superconductors.

¯ The superconducting state is defined by three very important factors: critical temperature (Tc), critical field (Hc), and critical current density (Jc). Each of these parameters is very dependant on the other two properties present. Maintaining the superconducting state requires that both the magnetic field and the current density, as well as the temperature, remain below the critical values, all of which depend on the material

¯ Tunneling is a process arising from the wave nature of the electron .It is the passage of electrons through a potential barrier which they would not be able to cross according to classical mechanics, such as a thin insulating barrier between two superconductors.

¯ The tunneling of a pair of electrons between superconductors separated by an insulating barrier was first discovered by Brian Josephson in 1962. Josephson discovered that if two superconducting metals were separated by a thin insulating barrier such as an oxide layer 10 to 20 angstroms thick, it is possible for electron pairs to pass through the barrier without resistance. This is known as the dc Josephson Effect.

¯ A Josephson junction consists of two superconductors separated by a thin insulating barrier. Pairs of superconducting electrons will tunnel through the barrier. The Josephson junction is a superfast switching devise. the junction devices' exceptional switching speed make them ideal for use in super fast and much smaller computers.

APPLICATION OF SUPERCONDUCTORS

Ø Magnetic-levitation is an application where superconductors perform extremely well. A landmark for the commercial use of MAGLEV technology occurred in 1990 when it gained the status of a nationally-funded project in Japan. The Minister of Transport authorized construction of the Yamanashi Maglev Test Line. the wider use of MAGLEV vehicles has been constrained by political and environmental concerns (strong magnetic fields can create a bio-hazard).

Ø superconductors can perform a life-saving function in the field of biomagnetism. By impinging a strong superconductor-derived magnetic field into the body, hydrogen atoms that exist in the body's water and fat molecules are forced to accept energy from the magnetic field. They then release this energy at a frequency that can be detected and displayed graphically by a computer- Magnetic Resonance Imaging (MRI)

Ø superconducting wire are made use in electric generators and their efficiency is above 99% and is half the size of conventional generators.

Ø Recently, power utilities have also begun to use superconductor-based transformers and "fault limiters". The Swiss-Swedish company ABB was the first to connect a superconducting transformer to a utility power network in March of 1997. ABB also recently announced the development of a 6.4MVA (mega-volt-ampere) fault current limiter - the most powerful in the world.

Ø Superconductors can be employed in transmission of power to cities.But due to the high cost and difficulty of cooling miles of superconducting wire this has only happened with short "test runs".

Ø In the electronics industry, ultra-high-performance filters are now being built.

Ø Superconductors have also found widespread applications in the military. HTSC SQUIDS are being used by the U.S. NAVY to detect mines and submarines. And, significantly smaller motors are being built for NAVY ships using superconducting wire and "tape". In mid-July, 2001, American Superconductor unveiled a 5000-horsepower motor made with superconducting wire (below). An even larger 36.5MW HTS ship propulsion motor was delivered to the U.S. Navy in late 2006.

Ø The newest application for HTS(high temperature superconductor) wire is in the degaussing of naval vessels. In addition to reduced power requirements, HTS degaussing cable offers reduced size and weight.

Ø Superconducting x-ray detectors and ultra-fast, superconducting light detectors are being developed due to their inherent ability to detect extremely weak amounts of energy.

Ø Soon superconductors may even play a role in Internet communications also. , superconductor technology may be called upon to meet this need.

Superconductivity milestones

ª 1911 - Dutch physicist Heike Kamerlingh Onnes discovers superconductivity in mercury at temperature of 4 K.

ª 1913-Kamerlingh Onnes is awarded the Nobel Prize in Physics for his research on the properties of matter at low temperature

ª 1933 – W. Meissner and R. Ochsenfeld discover the Meissner Effect.

ª 1941- scientists report superconductivity in niobium nitride at 16 K.

ª 1953- Vanadium-3 silicon found to superconduct at 17.5 K.

ª 1962- Westinghouse scientists develop the first commercial niobium- titanium superconducting wire.

ª 1972- John Bardeen, Leon Cooper, and John Schrieffer win the Nobel Prize in Physics for the first successful theory of how superconductivity works.

ª 1986- IBM researchers Alex Müller and Georg Bednorz make a ceramic compound of lanthanum, barium, copper, and oxygen that superconducts at 35 K.

ª 1987- Scientific groups at the University of Houston and the University of Alabama at Huntsville substitute yttrium for lanthanum and make a ceramic that superconducts at 92 K, bringing superconductivity into the liquid nitrogen range.

ª 1988- Allen Hermann of the University of Arkansas makes a superconducting ceramic containing calcium and thallium that superconducts at 120 K. Soon after, IBM and AT&T Bell Labs scientists produce a ceramic that superconducts at 125 K

ª 1993- A. Schilling, M. Cantoni, J. D. Guo, and H. R. Ott from Zurich, Switzerland, produces a superconductor from mercury, barium and copper, (HgBa _Ca _C u_ O) with maximum transition temperature of 133K.