In 1911, Heike Kamerling Onnes discovered superconductivity (the ability of a material to carry electricity with no resistance) in mercury, cooled by expensive and rare liquid helium to below the critical temperature (T_c) of 4.2 K (Kelvin). During the next 75 years, applications were developed, such as powerful magnets built of superconducting materials for medical magnetic resonance imaging (MRI), high energy accelators like the proposed Superconducting Supercollider (SSC), and very senstive magnetic field detectors called Superconducting Quantum Interference Devices (SQUIDs). Because of the expense and inconvenience of liquid helium refrigeration, however, other applications of the phenomenon were not considered economically feasible.

In April 1986, two researchers at IBM in Switzerland, K. Alex Muller and George Bednorz, detected superconductivity in (La-Ba)₂CuO₄ with a T_c up to 35 K, in contrast to the previous record of 23 K for which they were subsequently awwarded the Nobel Prize. By the end of 1986, superconductivity research achieved revolutionary advances with the effort of Paul C. W. Chu and colleagues at the University of Houston. Signs of superconductivity above 77 K were repeatedly observed in poorly-characterized samples during the period, strongly affirming the belief in the existence of superconductivity in the liquid-nitrogen temperature range. The scientific world knew that the textbooks had to be rewritten after January 1987, when the Houston group in collaboration with M. K. Wu, Chu's former student, achieved stable and reproducible superconductivity above 90 K in Y₁Ba₂Cu₃O_7-d (Y123), with T_c close to 100 K. Superconductivity at such high temperatures defies our common understanding of solids.

In addition to the savings in cost resulting from the displacement of liquid helium by liquid nitrogen for cooling, it is now apparent that superconductivity applications with more inexpensive refrigerants -- or eventually no refrigerant at all -- are possible. The race for new superconductors with higher Tc continues. Bismuth and thallium superconducting systems were discovered in 1988 which superconduct at 110 K and 125 K, respectively. The mercury-based compounds were discovered in 1993, with temperatures up to 164 K under pressure, another world record set at Houston. Many laboratories throughout the world have reported glimpses of superconductivity at much higher temperatures but these have not yet been confirmed.

The observation of superconductivity above 77 K in such unusual classes of materials defied the predictions of earlier theories; but these materials are also intriguing because they behave unusually above their T_c's as well; e.g. the meissner effect. The causes for and consequences of these observations pose great challenges to physicists, chemists, and materials scientists. Even though "the liquid nitrogen barrier" has been broken, many of the great promises of superconductivity technology have yet to be realized. The difficulties with the materials can be attributed to many of the material and engineering problems of HTS's, e.g. making long HTS wire than can carry large current without energy loss and can retain excellent superconducting properties over long periods of time without chemical and physical degradation.

However, commercial applications of HTS technology in fields such as electric power, transportation, electronics and medicine are now appearing. Current applications of HTS include thin films, magnetic resonance imaging (MRI), wireless communication filters, and ultra-fast computer chips. By the year 2010, it is estimated that the global superconductivity market will be in excess of $50 million.

After the discovery of the transistor in 1947, it took almost 40 years to introduce the one megabyte memory chip which is vital to today's powerful computers. Modern discoveries in superconductivity go far beyond piece-meal improvements in electric devices. They have opened the door on a totally new technology and stretch the imagination to the discovery of new applications. Future generations will witness significant changes in electricity generation, transmission and storage; impacts in microelectronics, communication, and computers; and advances in solid state science. If history serves as a guide, the wonderland of HTS applications is destined to be achievable in the forseeable future with determination, persistence and patience all guided by vision and imaginative experimentation.

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