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Published: 20 April 2020

• physics
• history
• electromagnetism

Today marks the birthday of Swiss physicist K. Alexander Müller, who shared the 1987 Nobel Prize in Physics with Georg Bednorz for their discovery of the first high temperature superconductor.

 Born on April 20^th, 1927 in Basel, Switzerland, Alex Müller attended the Eidgenössische Technische Hochschule Zürich, the Federal Institute of Technology at Zurich, where he received his PhD in 1957. He worked at a variety of institutions throughout Switzerland, studying various aspects of what we now term Condensed Matter Physics.

 In the 1980s, Müller and Bednorz were working together searching for high temperature superconductors. “High temperature,” in the context of superconductors, can be misleading to newcomers, as they are still very cold!

 Superconductors are materials whose resistance drops to zero at low temperatures. These materials have many fascinating properties – they can transmit electricity with no loss, and they repel all magnetic flux. Generally, a superconductor has a critical temperature below which it exhibits superconducting properties; above this temperature it does not, behaving as ordinary materials do. For many superconducting materials, and all of those discovered in the first seventy years of them being studied, this temperature is around ten to twenty Kelvin, a temperature very difficult to achieve, maintain, or work with.

 Müller and Bednorz, however, in 1986 discovered a ceramic compound material, lanthanum barium copper oxide, with a critical temperature of 35 Kelvin. Still very cold, but a definite improvement! More crucially, in addition to showing that higher critical temperatures were possible, they showed that superconductivity could be achieved in ceramics, driving other researchers to investigate similar compounds for this effect. Within a year, other such materials had been discovered, including the now popular yttrium barium copper oxide by Paul Chu of the University of Houston. This new material had a critical temperature of 92 Kelvin!

 92 Kelvin is still almost -300 degrees Fahrenheit below zero, obviously much colder than any temperature found naturally on Earth! But it is much warmer than the early metal superconductors. And crucially, it crosses an important line: 77 Kelvin is the temperature of liquid nitrogen, a refrigerant that is much cheaper and easier to manufacture than the liquid helium used in earlier studies, and vastly easier to work with. Since these newest materials can exhibit superconducting behaviour at liquid nitrogen temperatures, it means we can use them in practical technology and experiment with them more easily… including in classroom demonstrations!

We currently have two demonstrations that use high temperature superconductors, both taking advantage of their effect of excluding magnetic flux. Demonstration I7-21: Superconductor – Magnet Levitation uses a yttrium barium copper oxide (YBCO) disc bathed in a liquid nitrogen bath. When a small permanent magnet is placed on top of the disc, the strong magnetic field is repelled from the superconductor, so strongly that the magnet itself levitates above the disc!

Taking the opposite approach, demonstration I7-23: Magnetic Track and Superconductor, built by our own Don Lynch, consists of an array of powerful neodymium magnets. A puck of high temperature superconducting material wrapped in a Teflon sheath is soaked ahead of time in liquid nitrogen, cooling it down such that it will hold its temperature for a few minutes. The puck is cooled while resting above a small block of magnets. When taken out of its bath and placed on the track, it again holds itself at the same height above the magnets of the track.

Resources:

The Nobel Prize in Physics 1987 at nobelprize.org

J. G. Bednorz and K. A. Müller (1986). "Possible highT_c superconductivity in the Ba−La−Cu−O system". Z. Phys. B. 64 (1): 189–193.