Belated honour

Vitaly L. Ginzburg with colleagues at the P.N. Lebedev Physical Institute in Moscow. - ALEXANDER ZEMLIANICHENKO/AP

The Nobel Prize in Physics goes to three scientists for their work in the area of superconductivity, done several decades ago.

THREE scientists share this year's Nobel Prize in Physics for their work concerning quantum phenomena at very low temperatures that become spectacularly visible on the macro-scale. Seventy-five-year-old Alexei A. Abrikosov, an outstanding physicist from the former Soviet Union who now holds a dual Russian-United States citizenship and is at the Argonne National Laboratory (ANL) in the U.S., and 87-year-old Vitaly L. Ginzburg, another great Russian physicist and a former head of the P.N. Lebedev Physical Institute in Moscow, have been awarded the prize for their work in the 1950s in the area of superconductivity. Superconductivity refers to a phenomenon when a material loses all its electrical resistance when cooled to extreme low temperatures.

The other third of the prize has gone to the 65-year-old Anthony J. Leggett, a reputed British physicist and a dual British-U.S. citizen, formerly of the University of Sussex and currently of the University of Illinois, U.S., for his work in explaining one type of superfluidity. When a liquid, cooled to extreme low temperatures, loses all viscosity and flows freely, it is called a superfluid. In fact, conceptually, superconductors are also superfluids; only that in the former the fluid is made of the electrons in the material - it is an `electron liquid' - and not the material molecules themselves as it is in the latter.

Collectively, the two form one class of quantum liquids with the quantum behaviour manifesting in the macro-world. This occurs because the participating micro-particles, which normally obey quantum mechanics that rules the micro-world, act collectively to set up an ordered state in the large scale. Both superconductivity and superfluidity display long-range correlation or long-range order among the participating entities.

While all the three have made outstanding contributions to the field, and certainly deserve the prize, to award them this year is somewhat odd. It is as if the Nobel Committees of the many intervening years overlooked them. In the case of the first two, it is clear that the long-due award has come because of the end to the politics of the Cold War during the last decade. Work in superconductivity that came after their work has been awarded the Nobel earlier. American physicists John Bardeen, Robert Schrieffer and Leon Cooper were awarded in 1972 for their `BCS Theory' of superconductivity developed in 1957, which described the quantum mechanism responsible for the phenomenon. Even the discovery of the newer phenomenon of high-temperature superconductivity (HTS), which essentially belongs to the type of superconductivity for which Abrikosov had formulated his theory, brought the prize to Georg Bednorz and Karl Alex Mueller in 1987.

In the case of the latter too, it is as strange that he was not given the award in 1996 when the American experimentalists David Lee, Douglas Osheroff and Robert Richardson were awarded in 1996 for discovering superfluidity in liquid helium-3 (ordinary helium is helium-4, which also becomes a superfluid) - a phenomenon that his theory explained. This was indeed as strange as the British failing to offer him a Chair of Eminence in one of the top British universities, forcing him to move to the U.S. after retirement. The choice of awardees is also perhaps a reflection of the fact that there is no compelling Nobel-worthy work in physics in recent years that is in evidence. Be that as it may, it is better late than never, as the adage goes.

Superconductivity itself was discovered in 1911 by the Dutch physicist Kammerlingh Onnes who found that when mercury was cooled to liquid helium temperatures (about 40 Kelvin; that is 40 above absolute zero which is - 2730 C), all its electrical resistance vanished. The phenomenon was entirely unexpected and, obviously, there could be no theoretical understanding at that time.

Onnes was awarded the Nobel Prize in 1913. As it is understood today, superconductivity arises because electrons in a superconductor form pairs - called Cooper pairs - and these pairs flow along pathways of attraction formed by the positively charged atoms of the regular lattice structure of the material (usually a metal or an alloy). The Cooper pairs are regarded as a `condensate'.

These metallic superconductors that the microscopic BCS theory seeks to explain display a very characteristic behaviour when placed in a magnetic field. They totally expel the magnetic field from their interior - a phenomenon known as Meissner Effect - as long as the strength of the magnetic field is below a certain critical value. Above that value, superconductivity itself is destroyed. Superconductivity is also destroyed by passage of current that exceeds a critical value because of the higher-than-critical magnetic field that the current gives rise to. These are called Type I superconductors. But even as BCS were formulating their theory, researchers of the former Soviet Union had found that there were superconductors that lacked or only partially displayed Meissner Effect. These are Type II superconductors and are generally alloys of various metals or compounds consisting of non-metals and copper.

In these, superconductivity can coexist with a magnetic field between a lower and an upper threshold of magnetic fields. This phase is known as the Shubnikov phase. Superconducting magnets, which are used today for generating very high magnetic fields as in a magnetic resonance imaging (MRI) machine or in superconducting particle accelerators, make use of this property. It was Abrikosov who provided a theoretical understanding of this class of superconductors in his path-breaking paper of 1957. In fact, Abrikosov had worked out his theory in 1952 itself but published it only five years later.

Abrikosov's work actually sprung from an insightful analysis of a phenomenological theory of superconductivity formulated in 1950 by Lev Landau (who was awarded the Nobel Prize in 1962 for his other work relating to superfluidity) and Ginzburg (who gets the Nobel this year).

The Landau-Ginzburg (LG) theory sought to describe the relationship between superconductivity and critical magnetic field strengths (above which superconductivity disappeared) for superconductors that were known at that time. They found that an `order' parameter describing the density of the superconductive condensate in the material - which seemed to play the role of a macroscopic quantum mechanical wave function - was necessary if the weak interaction between superconductivity and magnetism was to be explained.

In fact, they had found that the theory predicted two classes of superconductors for two ranges of the parameter value: one type for it was < 1/v2, in which magnetism and superconductivity cannot coexist, and another type for which the parameter was > 1/v2. Since the values of the parameter for superconductors of that time were all << 1 (mercury had 0.16), they rejected the latter as unphysical. This parameter is now called the Landau-Ginzburg parameter.

It was left to Abrikosov to pick it up from where Landau and Ginzburg had left. In fact, Abrikosov was provoked to look at the theory by his roommate, N. V. Zavaritzkii, at Moscow University, who found that some of his samples did not behave according to the predictions of the LG theory. Abrikosov wondered about it and began to investigate theoretically the nature of superconductivity for the LG parameter > 1/v2. The new materials that Zavaritzkii had looked at indeed had values in this range and Abrikosov found that the critical values of the magnetic field corresponded to his measurements.

Abrikosov found that his theory actually predicted vortices - narrow thread-like channels - along which magnetism could penetrate the interior of the superconductor. These channels formed a lattice-like structure in the material. Abrikosov also provided a detailed prediction of how the number of vortices would grow as the magnetic field increased in strength and how superconductivity is lost when the field exceeds the upper threshold, causing the vortices to overlap.

Abrikosov's theory of superconductors in a magnetic field has, in fact, opened up a new field of physics -- the study of Type II superconductors. His theory was a breakthrough and is now increasingly being used in the analysis HTS materials, which are extreme Type II superconductors, and superconducting magnets. His papers from the 1950s have begun to be cited more frequently during the last decade. Indeed, the term `vortex matter' is now being used to describe certain HTS materials. The LG theory, on the other hand, has found new applications in recent times, particularly in String Theory of Elementary Particles. The theory is used to describe spatially varying and time-dependent superconducting order and superconductivity in strong magnetic fields.

Helium, the lightest rare gas, exists as two isotopes in nature: He-4 (the numeral denotes the total number of nucleons in its nucleus - two protons and two neutrons) and He-3 (which has one neutron and two protons in its nucleus). The latter is, therefore, lighter; this form, however, is rarer in nature by a factor of about 10 million. When liquid He-4 - which becomes a liquid at about 4K - is cooled further (down to 2K), it becomes a superfluid. In 1938 Piyotr Kapitsa discovered this phenomenon, for which he received the Nobel 40 years later. Superfluidity, however, sets in at quite different temperatures for the two isotopes, that for He-3 occurring at 1,000 times lower temperature. For this reason, discovery of superfluidity in He-3 had to wait until the 1970s. There are other important differences as well.

Superfluidity was almost immediately explained by Landau, for which he got the Nobel in 1962. The transition of He-4 from the normal liquid state to a superfluid state is a manifestation of what is known as Bose-Einstein (BE) condensation, a phenomenon that has been more conclusively demonstrated in recent years using gaseous atoms (for which the 2001 Nobel went to Eric Cornell, Wolfgang Ketterle and Carl Wieman). In BE condensation, particles that obey Bose-Einstein statistics (like He-4 atoms) condense into a single lowest energy state at very low temperatures. Current-carrying electrons, however, obey Fermi-Dirac statistics, which do not permit this collective condensation into a single state. For this reason, while today we know that superconductivity is also a manifestation of BE condensation, it took nearly five decades for a proper quantum theory, namely the BCS theory, to emerge. The key to the understanding was the formation of electron pairs, with the pairs obeying BE statistics, resulting in the condensate.

If there was no BCS theory, understanding superfluidity would have been problematic for the same reason. Now it could be understood along similar terms: He-3 atoms pair like electrons in a superconductor and form the condensate. There is, however, an important difference. He-3 atoms pair in a manner such that their magnetic properties add up while in the case of electrons they cancel out. Because of this selected direction along which the He-3 magnetic vectors point, superfluid He-3 is anisotropic , that is, unlike superfluid He-4 and Type-I superconductors, it has different properties in different directions.

The property of anisotropy was used to investigate the thermodynamic properties of superfluid He-3 in detail and it was found that it displayed very complex properties. It was found to exhibit a mixture of three different phases. The three phases have different properties and their proportions in the mixture depended upon temperature, pressure and external magnetic fields.

In has recently been demonstrated that if a vortex is created in a rotating vessel containing superfluid He-3, the result depends critically on the temperature. Above a critical temperature, the vortex is aligned along the axis of rotation. Below the critical temperature the vortices form a disorderly state.

In 1972, Leggett formulated the theory of anisotropic superfluids, which was based on the concept of `spontaneously broken symmetry' - that is the lowest energy or ground-state of the system is not one which is isotropic or having total symmetry but one where there is a preferred direction and the symmetry is broken.

In fact, he showed that several simultaneously broken symmetries can occur simultaneously in condensed matter. This could explain the various superfluid phases whose properties cannot be understood otherwise. Leggett's theory has also proved useful in other fields like liquid crystal physics, particle physics and cosmology.