It was 100 years ago this April that Dutch physicist Heike Kamerligh Onnes and his collaborators in Leiden, The Netherlands discovered superconductivity of pure metals such as mercury, tin and lead at very low temperatures. This work was an extension of Onnes’ Nobel-prize-winning efforts to reach extremely low temperatures that culminated in the liquefaction of helium in 1908.
While the milestone is being marked with celebrations and observances around the world, it is also being noted by researchers at Ames Laboratory who have their own long history of advancements in the field of superconductivity. Senior physicist Paul Canfield wrote a perspective piece for the April issue of Nature Materials that looks at the unpredictability that both confounds and captivates researchers.
“The reason that Onnes was measuring the temperature-dependent electrical resistivity of mercury was to test the proposed ‘death of conductivity’ or ‘freezing out of carrier motion’ that had been proposed by Lord Kelvin as the fate of electrons when absolute zero was approached,” Canfield writes. “Mercury, given its high vapor pressure, allowed for purification via distillation. The sudden phase transition to an infinitely conducting state was an unimagined result. This first, surprising, appearance of superconductivity was a herald of things to come. Over the ensuing century superconductivity would continue to pop up in what were thought to be extremely unexpected places, repeatedly: in organic compounds, in oxides, in magnetic compounds, in an overlooked boride and most recently in a whole family of FeAs- and FeSe-based compounds.”
A major advance in the field began in the 1950s with niobium-tin (Nb3Sn) and Ames Lab researchers were active participants. Physicist Clayton Swenson was involved in that research.
“When I came to the Department of Physics and the Ames Lab in 1955, my major interests were in high pressure research at low temperature, but also superconductivity involving high purity metals,” Swenson said. “We published a number of papers involving superconductivity in my first 10 years, with Doug Finnemore taking over primary involvement in this area when he joined the Department and Lab in the early 1960’s.”
According to Swenson, one paper in Physical Review looked at the “Effects of Pressure on the Super-conducting Transition Temperatures of Sn, In, Ta, Tl and Hg (1958).”
“We also were able to obtain and then additionally purify high quality samples of niobium metal,” Swenson said. “Measurements of the magnetization vs magnetic field for these samples indicated that pure niobium for the first time showed a complex (Type II) behavior which previously had been seen only for superconducting metallic alloys.”
Ames Lab researchers were key players in the cuprate high-transition-temperature superconductors, the next major advancement in the field.
“Our pioneering theoretical and experimental research done soon after we learned about the discovery of the high-temperature layered copper-oxide superconductors was an important highlight for me,” senior physicist John Clem said. “We also made an enormous worldwide impact on the field of superconductivity by publishing a twice-monthly newsletter, High-Tc Update, during the period 1987-2000.”
When a new class of magnesium-diboride superconductors was discovered, Canfield’s group was instrumental in development of MgB2 wires and later with carbon-doped versions of the wires. Even more recently, Ames Lab researchers have been heavily involved in the development of iron-arsenide superconductors, generating more than 100 papers on various aspects of this latest material to exhibit properties first shown by Onnes.
“Given the discoveries of MgB2 and FeAs-based superconductivity over the past decade, and given renewed experimental efforts as well as funding support, I am very optimistic that we (humanity) will discover further examples of new, and even improved, superconductors,” Canfield writes. “Remarkably, even 100 years after the discovery of superconductivity, these are very exciting times for what promises to remain an active area of basic as well as applied research for decades to come and I’m waiting with anticipation to see just where superconductivity will pop up next and force yet another rewriting of our understanding of where it can and cannot exist.”
John R. Clem on an aluminum disk, underneath which
are several rings of Nd-Fe-B magnets levitating over 200
melt-textured disks of the high-temperature cuprate
superconductor (YBCO) at 77 K. This device was made
at the Superconductivity Research Laboratory in (SRL)
of the International Superconductivity Technology
Center (ISTEC) in Tokyo, Japan.
A cross-section of a carbon-doped magnesium
diboride wire developed at Ames Laboratory.
An iron-arsenide crystal grown at Ames Laboratory.