"The Coldest Spot on Earth": Low Temperature Physics, Superfluidity, and the Discovery of Superconductivity

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"The Coldest Spot on Earth": Low Temperature Physics, Superfluidity, and the Discovery of Superconductivity

Overview

The Dutch experimental physicist and Nobel Prize laureate Heike Kamerlingh Onnes (1853-1926) worked for more than four decades in low temperature physics, a discipline he helped to establish over the years as a complete and independent field of study. When in 1908 Kamerlingh Onnes succeeded in liquefying helium, he became the very first experimentalist to reach a temperature as low as 4.2 Kelvin (or -451.84°F). His discovery of superconductivity three years later opened whole new vistas of theoretical and experimental researches that are still today of the utmost importance to the progress of science and technology.

Background

Low temperature physics really began in the second half of the nineteenth century with the discovery in 1852 of the Joule-Thomson effect, attributed to two British physicists, James Prescott Joule (1818-1889) and Sir William Thomson, Lord Kelvin (1824-1907). That year Thomson, based on his and Joule's thermodynamical studies, observed that when a gas expands in a vacuum its temperature decreases. Indeed if gases were allowed to expand, then compressed under conditions which did not allow them to regain the lost heat, and expanded once more, and so on over and over in cascade, then very low temperatures could be achieved. This Joule-Thomson effect—which gave rise to a whole new refrigeration industry aimed at the long-term conservation of perishable foodstuffs, dominated by industrialists such as the German Karl Ritter von Linde (1842-1934) and the French Georges Claude (1870-1960)—was utilized to reach temperatures never before obtained.

In 1883, Zygmunt Florenty Wroblewski (1845-1888) and Karol Stanislav Olszewski (1846-1915) were, however, the first to maintain a temperature so cold that it liquefied a substantial quantity of nitrogen and oxygen, said until then to be "permanent" gases. Fifteen years later, the Scottish physicist James Dewar (1842-1923) was able to liquefy hydrogen by first cooling the gas with liquid oxygen—kept at its low temperature with a Dewar flask, the first vacuum, or thermos, bottle ever made—then applying the aforementioned cascade method. At the turn of the twentieth century only the last so-called permanent gas, helium, still eluded liquefaction.

This achievement was to be the work of the Dutch experimental physicist Heike Kamerlingh Onnes. After studying physics in the Netherlands and Germany, he started his academic career as an assistant in a polytechnic school at Delft. It took only a few years before he received a call from Leiden University, which resulted in his appointment to the very first chair of experimental physics in the Netherlands. Kamerlingh Onnes's inaugural address leaves no doubt regarding the impetus he wanted to give to his laboratory: "In my opinion it is necessary that in the experimental study of physics the striving for quantitative research, which means for the tracing of measure relations in the phenomena, must be in the foreground. I should like to write Door meten tot weten (knowledge through measurement) as a motto above each physics laboratory." He always remained loyal to this declaration of principle.

It took more than 20 years for Kamerlingh Onnes to build and establish on a firm ground a cryogenic laboratory of international renown. The laboratory workshops were organized as a school, the Leidsche Instrumentmakers School; they were to have a tremendous importance in the training of qualified instrument makers, glassblowers, and glass polishers in the Netherlands. Even though he confided in his measurement aphorism, Kamerlingh Onnes's research was nevertheless upheld by a solid theoretical background ascribed to a couple of brilliant Dutch contemporaries, Johannes Diderik van der Waals (1837-1923) and Hendrik Antoon Lorentz (1853-1928). Their theories helped him understand the physics involved in the liquefaction of gases.

In 1908 Kamerlingh Onnes's efforts resulted in the liquefaction of helium, obtained at the very low temperature of 4.2 Kelvin or -451.84°F (the Kelvin absolute temperature scale, as you have probably guessed by now, was named after William Thomson, Lord Kelvin, who was the first to propose it in 1848). From then on and until his retirement in 1923, Kamerlingh Onnes would remain the world's absolute monarch of low temperature physics.

Impact

The coldest spot on Earth was now found in Leiden. By attaining this new level of temperature Kamerlingh Onnes set the stage for his next big, and probably most important, discovery. Studying the electrical resistance of metals submitted to low temperature, the Dutch physicist expected that after reaching a minimum value, the resistance would increase to infinity as electrons condensed on the metal atoms, thus impinging their movement. Experimental results, though, contradicted his claim.

Kamerlingh Onnes supposed next—based on Max Planck's (1858-1947) hypothesized vibrators used to theoretically explain the blackbody, giving birth to the quantum concept—that the resistance would decrease to zero. Using purified mercury, he found out what he had anticipated: at very low temperature electrical resistance showed a continuous decrease to zero. Superconductivity was discovered. The year was 1911. When Kamerlingh Onnes received the Nobel Prize two years later, it was for his œuvre complète in low temperature physics, which of course led to the production of liquid helium. But what about superconductivity? Was it ignored? In a sense, yes.

In the early 1910s this phenomenon was considered to be some sort of "peculiar oddity," for it could not yet be theoretically understood, much less used to practical ends. The reason is really quite easy to grasp when you look back at history from our modern point of view: the theoretical foundation of superconductivity is quantum mechanics, still at an embryonic stage of development when Kamerlingh Onnes discovered the empirical properties of superconductors.

From then on, the quest for absolute zero began. Large electromagnets were built in order to reach that temperature where every molecular movement stops. It became a matter of national pride to be able to say that the coldest spot on Earth was on your territory. The successor of Kamerlingh Onnes used such an electromagnet, in 1935, to achieve a temperature of only a few thousandths of a degree Kelvin. Leiden's star shined again. But astonishingly new phenomena did not always require temperatures so extreme. Since the 1920s it was shown that at 2.17K (achieved by applying moderate pressure) liquid helium (He I) changed into an unusual form, named He II.

In 1938 Pjotr Leonidovich Kapitza (1894-1984) demonstrated that He II had such great internal mobility and near vanishing viscosity, that it could better be characterized as a "superfluid." Kapitza's experiments indicated that He II is in a macroscopic quantum state, and that it is therefore a "quantum fluid." It now was indisputable to ascertain that low temperature physics rested on the principles of quantum mechanics. Superconductivity thus had to be tackled with this understanding in mind.

It took, however, no less than 46 years before John Bardeen (1908-1991), Leon N. Cooper (1930-), and J. Robert Schrieffer (1931-) finally found the underlying mechanism to Kamerlingh Onnes's discovery. Nicknamed the BCS theory, it can be theoretically outlined as the coupling of electrons (called Cooper pairs) attuned to the inner vibrations of the superconductor's crystal lattice. As the first electron in the pair flows through the lattice, it attracts toward the positively charged nuclei of the superconductor's atoms. The second electron is then "pulled" forward because it feels the attraction engendered by those same nuclei in front. The Cooper pair of electrons thus stay together as they flow through the superconductor, an unbroken interaction which helps them progress without resistance through the superconductive material.

One of the things that the BCS theory predicted was the superfluidity of the helium-3 isotope. Lev Davidovic Landau (1908-1968) theoretically explained the superfluidity of helium-4 (He II) already in the 1940s. Helium-4 is said to be a boson since each atom has an even number of particles (two protons, two neutrons, and two electrons). Helium-4, as Landau showed, must then follow Bose-Einstein statistics, which, among other things, means that under certain circumstances the bosons condense in the state that possesses the least energy.

Helium-3, however, having one neutron less than helium-4 (and therefore an odd number of particles), is not a boson but a fermion. Since fermions follow Fermi-Dirac statistics they cannot according to this theory be condensed to the lowest energy state. For this reason superfluidity should not be possible in helium-3—which, like helium-4, can be liquefied at a temperature of some degrees above absolute zero. Three Americans discovered at the beginning of the 1970s, in the low temperature laboratory at Cornell University, the superfluidity of helium-3, something that occurs at a temperature of only about two thousandths of a degree above absolute zero.

Where do all these theories and experimental facts lead? Up until 1986 the highest temperature superconductors could operate was 23.2K. Since liquid helium (expensive and inefficient) is the only gas usable for cooling to that range of temperature, superconductors were just not practical. New superconductors were found after 1986 that are operated at 77K. This higher temperature allows the use of liquid nitrogen as a coolant, far less expensive and far more efficient than liquid helium. As electronics, the designs for superconductors went from refrigerators weighing hundreds of pounds, running at several kilowatts, to far smaller units that can weigh as little as a few ounces and run on just a few watts of electricity.

This breakthrough lead to a wider use of superconductors: they are now found in hospitals as magnetic resonance imaging (or MRI) machines, in the fields of high-energy physics and nuclear fusion, and finally in the study of new means of transportation, in the form of levitating trains. Furthermore, fuel cell vehicles, run by liquid hydrogen, could one day replace the petroleum motorized cars of today. Also, by studying the phase transitions to superfluidity in helium-3, scientists may have found a theoretical explanation of how cosmic strings are formed in the universe. In light of all this we may conclude, as the 1996 Nobel Prize laureate Robert C. Richardson (1937- ) did 20 years ago, that the end of physics—viewed from the lenses of low temperature physics—is yet to be at our doors.

JEAN-FRANÇOIS GAUVIN