100 years of superconductivity

April 8, 2011 marked the 100th anniversary of the discovery of superconductivity by Heike Kammerlingh Onnes. Working in his lab at Leiden University on April 8, 1911, he was experimenting with the electrical resistance of mercury at low temperatures. In his notebooks, he noted that at 3 K (-270°C), mercury’s resistance dropped to ‘practically zero’. The discovery was only made possible by Onnes previous achievement of helium liquefaction at 4.22 K, which provided the means to cool samples down to even lower temperatures. At the time, some scientists believed that at low temperatures, electrical resistance would rise exponentially, while others believed it would gradually decrease, which proved to be the case for many materials. However, superconductivity remained a puzzling phenomenon and it was not until 1957, that thanks to Bardeen, Cooper and Schrieffer, that the quantum theory could explain it in certain materials such as mercury. Later, in 1987, “high-temperature” superconductivity was discovered in cuprates which only required to be cooled at the temperatures of liquid nitrogen, but is not, to this day, fully understood.
Superconductors are used to make powerful electromagnets, for example for MRI (Magnetic Resonance Imaging) and NMR (Nuclear Magnetic Resonance) machines in medical diagnostics. Other promising applications include power transmission cables with low losses and highly sensitive devices to measure magnetic fields.