History of Superconductors

In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University. When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C), its resistance suddenly disappeared.

The 1980’s were a decade of unrivaled discovery in the field of superconductivity. In 1964 Bill Little of Stanford University had suggested the possibility of organic (carbon-based) superconductors. The first of these theoretical superconductors was successfully synthesized in 1980 by Danish researcher Klaus Bechgaard of the University of Copenhagen and 3 French team members. (TMTSF)2PF6 had to be cooled to an incredibly cold 1.2K transition temperature (known as Tc) and subjected to high pressure to superconduct. But, its mere existence proved the possibility of “designer” molecules – molecules fashioned to perform in a predictable way.

Müller and Bednorz’ discovery triggered a flurry of activity in the field of superconductivity. Researchers around the world began “cooking” up ceramics of every imaginable combination in a quest for higher and higher Tc’s. In January of 1987 a research team at the University of Alabama-Huntsville substituted Yttrium for Lanthanum in the Müller and Bednorz molecule and achieved an incredible 92 K Tc. For the first time a material (today referred to as YBCO) had been found that would superconduct at temperatures warmer than liquid nitrogen – a commonly available coolant. Additional milestones have since been achieved using exotic – and often toxic – elements in the base perovskite ceramic. The current class (or “system”) of ceramic superconductors with the highest transition temperatures are the mercuric-cuprates. The first synthesis of one of these compounds was achieved in 1993 at the University of Colorado and by the team of A. Schilling, M. Cantoni, J. D. Guo, and H. R. Ott of Zurich, Switzerland. Tc of 138 K is now held by a thallium-doped, mercuric-cuprate comprised of the elements Mercury, Thallium, Barium, Calcium, Copper and Oxygen. The Tc of this ceramic superconductor was confirmed by Dr. Ron Goldfarb at the National Institute of Standards and Technology-Colorado in February of 1994. Under extreme pressure its Tc can be coaxed up even higher – approximately 25 to 30 degrees more at 300,000 atmospheres.

The first company to capitalize on high-temperature superconductors was Illinois Superconductor (today known as ISCO International), formed in 1989. This amalgam of government, private-industry and academic interests introduced a depth sensor for medical equipment that was able to operate at liquid nitrogen temperatures (~ 77K).

In 1997 researchers found that at a temperature very near absolute zero an alloy of gold and indium was both a superconductor and a natural magnet. Conventional wisdom held that a material with such properties could not exist!  Since then, over a half-dozen such compounds have been found. Recent years have also seen the discovery of the first high-temperature superconductor that does NOT contain any copper (2000), and the first all-metal perovskite superconductor (2001).

Also in 2001 a material that had been sitting on laboratory shelves for decades was found to be an extraordinary new superconductor. Japanese researchers measured the transition temperature of magnesium diboride at 39 Kelvin – far above the highest Tc of any of the elemental or binary alloy superconductors. While 39 K is still well below the Tc’s of the “warm” ceramic superconductors, subsequent refinements in the way MgB2 is fabricated have paved the way for its use in industrial applications. Laboratory testing has found MgB2 will outperform NbTi and Nb3Sn wires in high magnetic field applications like MRI.

The chronology of discoveries of superconductors with higher critical temperatures

Though a theory to explain high-temperature superconductivity still eludes modern science, clues occasionally appear that contribute to our understanding of the exotic nature of this phenomenon. In 2005, for example, Superconductors.ORG discovered that increasing the weight ratios of alternating planes within the layered perovskites can often increase Tc significantly. This has led to the discovery of more than 60 new high-temperature superconductors, including a candidate for a new world record.

The most recent “family” of superconductors to be discovered is the “pnictides”. These iron-based superconductors were first observed by a group of Japanese researchers in 2006. Like the high-Tc copper-oxides, the exact mechanism that facilitates superconductivity in them is a mystery. However, with Tc’s over 50K, a great deal of excitement has resulted from their discovery.

1911: Discovery of the superconductivity phenomenon by Kamerlingh Onnes (Leiden University, Netherlands). (Mercury at 4.2K (or -269°C))

1957: Establishment of the *BCS Theory by Bardeen, Cooper, and Schrieffer.

1986: Discovery of high-temperature superconductive materials by Bedrnoz and Muller. Then this is verified by Tanaka’s group (University of Tokyo).

1987: Discovery of Y-Ba-Cu-O superconductor by Chu (University of Huston) and others.Critical temperature was 90K, which exceeded the temperature of liquid nitrogen (77K).

1988: Critical temperature 110K was achieved in Bi-Sr-Ca-Cu-O superconductor by Maeda (National Research Institute of Metals) and others.

1988: Critical temperature 120K was achieved in Tl superconductor by Harman (University of Arkansas) and others.

1993: Critical temperature 134K was achieved in Hg superconductor by Putilin (Moscow State University) and others

2001: Discovery of MgB2 superconductor by Akimitsu (Aoyama Gakuin University) and others. Critical temperature 40K was achieved, which was the highest in the intermetallic compounds.