The Discovery of Superconductivity

Superconductivity was a laboratory mystery starting with its discovery by the Dutch low-temperature physicist H.Kamerlingh Onnes in 1911.

Onnes had liquefied helium gas for the first time only a few years earlier and in so doing extended the range of available temperatures down to 1K. This advance made it possible to study material properties over three decades of temperature rather than just two. The electrical resistance of a sample of mercury being studied to see how pure metals behave suddenly became too small to detect when the mercury was cooled below 4.2K(see Fig.2).

Onnes and his collaborators at Leiden University soon found that superconductivity is not confined to pure mercury alone. Corresponding sudden decreases of resistance were found in tin and lead and even mercury amalgams at various low temperatures. Since the drop was sudden, the temperature at which it occurred was recognized as a critical temperature Tc.

The question immediately arose of how small was the remaining resistance. To answer this, the Leiden researchers induced a supercurrent in a lead coil and found no measurable decay of the current by observing the associated magnetic moment over the 24 h it was kept below Tc.

Further experiments and theory have shown that even if one waited the life of the universe there would be no measurable decay of induced current. Hence, the prefix super is justified.

figure 1

Superconductivity is a state of matter that is characterized by two distinct effects: zero resistance and diamagnetism, which means the explusion of magnetic fields. Superconductivity is a macroscopic phenomenon that involves amplitudes and phases asserted with the energy gap parameter. Interference and diffraction effects as well as the Josephson effect have been observed at cryogenic temperatures. These effects are employed in signal processors, rf circuits, EO components, data storage devices, and computer-related technology.
The universally accepted theory of superconductivity was formulated by Bardeen, Cooper, and Schrieffer in 1957, and is commonly known as the BCS theory. The BCS theory is the backbone of the superconductivity phenomenon,which states that if metallic mobile electrons interact efficiently with each other, then they exhibit
Zero de resistivity
The Meissner effect, discovered by Meissner and Ochsenfeld in 1933
A second-order phase transition to normal metallic state at a transition temperature
Perfect diamagnetism in the presence of weak magnetic fields.

Historical Progress

Shortly after the discovery of the superconducting transition in mercury, Onnes and his co-workers found that, at temperatures below Tc, a magnetic field of a few hundred gauss applied to the sample caused the superconductivity to disappear.

Normal electrical resistance was restored at a well-defined value of this quenching or critical magnetic field Hc; it was also discovered that superconductivity could be quenched by a sufficiently high critical current density Jc.

The possibility of using superconductivity for electric power transmission or in electrical equipment such as generators or motors seemed at first to be ruled out by the low quenching fields and currents found in these early experiments; however, these limitations have to a great extent been removed by further advances. Along with Tc and Hc, the critical direct-current density, Jc, forms the third leg of the fundamental triad of superconductor characteristics that are the most significant for engineering purposes. The operating region for a superconductor may be considered as lying below a temperature-current-density-magnetic field surface; see Figure 3.

After World War One, research on superconductors was expanded from Leiden to laboratories in Germany, the Soviet Union, the UK, Canada and the USA as cryogenic technology gradually spread around the world.

By 1928 the known superconductors were all low-melting polyvalent post-transition metals. The search for new superconductors was begun in other laboratories. W. Meissner and his coworkers in Berlin took an important new direction and they discovered many new ones. They investigated the occurrence of superconductivity in the transition elements of the periodic table, which have atoms with incomplete d shells, and in their compounds. These elements are high-melting refractory metals and are difficult to prepare in pure form. Initially they were known as hard superconductors partly because they were brittle due to the presence of dissolved gases such as oxygen and partly because the superconductivity was harder to destroy by magnetic fields than in the nontransition metal elements. This did not prevent discovery that, in fact, new superconductors are often discovered in impure systems. Meissner and his colleagues found most of the elements of groups IV and V, that is, titanium, zirconium, niobium, vanadium and tantalum, to be superconducting. It is noteworthy that niobium in one form or another is the material of choice for all low-temperature superconducting applications at present. Its alloys and compounds are used in making wires and cables for large-scale applications such as magnets for magnetic resonance imaging (MRI) scans, and niobium tunnel junctions are the basis for electronic applications such as high-speed digital circuits, the voltage standard and the most sensitive detectors of electromagnetic signals. Meissner's group took a bold step when they extended their investigations to transition metal compounds and were rewarded by finding even more superconductors. They found a large number of carbides and nitrides with the rock salt structure including NbC with Tc > 10K.

Working with pure elements, Meissner and Ochsenfeld discovered that superconductors exhibit a property almost as unexpected as zero resistance. When cooled below Tc in the presence of a magnetic field, the flux that penetrates the superconductor above Tc is completely expelled, a behavior known as the Meissner effect. On the other hand, the Messner effect, that is, the expulsion of flux, is separate and distinct from zero resistance and indicates reversible thermodynamic behavior. The exclusion of flux when the magnetic field applied to a superconductor is changed is a property predicted for any zero-resistance metal by Maxwell's equations.

After World War II, investigators led initially by B.T. Matthias and J.K. Hulm in the USA and others including N.E. Alexseevski in the Soviet Union made renewed efforts to find new superconductors. They attempted to determine the structural and electronic features favorable for the occurrence of superconductivity. Their timing was good for several reasons including the availability of much purer materials (the use of metallurgical processes developed during the Manhattan Project made pure rare-earth metals and transition metals available for the first time) and better methods of synthesis (the employment of arc furnaces with water-cooled copper hearths and the emergence of high-vacuum processing for removing oxygen made it possible to prepare many new structures and compositions with high melting points and with little contamination).

Particularly fruitful was the study of binary solid solution alloys of the d-band transition metals. This work revealed relatively high Tc binary materials with high critical fields and ultimately led to high-field superconducting magnets, an important new technology which developed rapidly after 1962 based on the alloy niobium-titanium.

Figure 3: For a material to remain superconducting, a particular temperature/H-field current density environment must exist. The material must operate below a surface similar to the one shown in this illustration.

TOP