Superconductor

A superconductor is a substance which conducts an electric current with zero resistance. Superconduction is a phase state (like the liquid and solid states of the water); as such, it depends on the temperature among other variables. The temperature where the transition takes place is the critical temperature (Tc). In 1911, H. Kammerlingh Onnes discovered superconductivity working over mercury.

Superconductors are classified as Type I or Type II depending on their transition behavior. In Type I, resistance falls to zero abruptly when Tc is achieved; Type II superconductors hold a mixed area of superconductor and non superconductor behavior.

Some characteristics of superconductors:

• Metals that support superconductivity have critical temperatures close to the absolute zero (Type I).
• Some ceramics can achieve superconductor state at higher temperatures (Type II).
• The last patented superconductor has a Tc=150 K.
• High Tc superconductors can be sustained with cheaper refrigeration like liquid nitrogen based systems (the Japanese maglev train uses this system).
• All superconductors found so far are solid.

Electric conduction implies losses of energy due to resistance of the conducting material. The energy is released as heat. The main undesirable consequences are the need to keep providing energy to sustain the current and the possible burning of the conducting media. A current in a normal metal ring will decay rapidly; if the ring is superconducting, it will show perpetual movement (the decay constant of over a billion years!). See "What is a Ring-shaped Superconductor used for?" for more details.

Research in the area of superconductors is a hot field. New superconducting materials are discovered in a regular basis and its technological applications are endless. New discoveries force the review of the accepted theories and it is, for now, a phenomenon no completely understood.

Magnetic Properties of a Superconductor

Even when recent studies discarded diamagnetism as a generalized property; it is a very well documented property of most superconductors and it is one of the ways to achieve magnetic levitation.

Meissner effect: In 1933 Walter Meissner and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field. If a magnet moves close to a conductor, electromagnetic currents are induced in the conductor. This is the principle behind electric generators. If a superconductor is used instead, the induced currents exactly mirror the field causing the magnet to be repulsed. A magnet can actually levitate over a superconductive material.

The Meissner effect was discarded as a general property in 1997, when an alloy of gold and indium was found to be both a superconductor and a natural magnet at a temperature very near absolute zero. Since then, other compounds have been found with the same property.

Type I Superconductors

They are characterized by a very sharp transition to a superconducting state and perfect diamagnetism (the ability to repel a magnetic field completely). The conductivity curve vs temperature at constant pressure shows a normal decrease with temperature up to a critical transition temperature (known as Tc) below which the conductivity is zero (within experimental error). The critical temperature is usually very low (0-5 K), being Lead (Pb) the higher one with 7.196 K.

Thirty materials lie in this group. They are metals and metalloids that show some conductivity at room temperature. The best metallic conductors (copper, silver and gold) are not among the Type I superconductors.

The accepted explanation is given by BCS theory.

BCS Theory: The molecular vibrations in the lattice slow down when the temperature goes down, bellow the critical temperature this lack of movement allows the flow of electrons without any obstacle which translates in superconductivity. An interesting factor of this theory is the appearance of Cooper pairs (the electrons move coupled in pairs).

Cooper Pairs: The vibration in the lattice is so small that the presence of the electrons actually affects the position of the surrounding nuclei. One moving electron produces a ripple effect in the lattice that will propel the movement of a second electron coupling both of them via the exchange of a phonon (quanta of lattice vibration energy). Those two electrons form a Cooper pair. The pair will be localized in momentum (same momentum magnitude but moving in opposite direction) and unlocalized in space (they can be spatially apart up to 100 nanometers when the separation between two consecutive nuclei is 0.1-0.4 nm). Electrons are "fermions" (i.e. they are electrically charged and as such they repel each other); but under superconductor state they behave as suffering a transition to the fundamental state which is only available to bosons (particles without electrical charge, neutrons are bosoms). The solution to this "problem" is the creation of Cooper pairs; the coupled pair of electrons behaves as a boson. Experimental corroboration of an interaction with the lattice was provided by the isotope effect on the superconducting transition temperature.

Type II Superconductors

Type II superconductors show a gradual transition from a normal to a superconducting state across a region of "mixed state" behavior. Type II superconductors are also known as hard superconductors and the lattice structure plays a vital role in this case. There is no a complete model to explain Type II superconductors in the way BCS Theory explains Type I. Some Type II superconductors show higher critical temperatures making technological applications viable. Others can maintain the superconductor state in very high applied magnetic field. There are also those that are in the range of Type I Tc and supported magnetic fields.

Due to the mixed area, some penetration by an external magnetic field (B) into its surface will be allowed. As a consequence, new mesoscopic phenomena like superconducting "stripes" and "flux-lattice vortices" can be observed. This partial penetration gives the applied magnetic field power to break the superconductivity state (critical magnetic field Bc). In Type II superconductors, temperature and applied magnetic field will be the main variables of the phase diagram.

The first Type II superconductor, an alloy of lead and bismuth, was created in 1930 by W. de Haas and J. Voogd. Its superconducting properties were not observed until the Meissner effect was discovered. To date, the highest Tc obtained at room pressure is 138 K for a stoichiometric material (formed by formula) and 150K for a patent-pending material which does not form stoichiometrically.

Different compound families have shown to have Type II superconducting characteristics; a brief classification follows:

• The most abundant substnces that display Type II superconductivity are metallic compounds and alloys. Known exceptions are the elements vanadium, technetium and niobium.
• Combinations of Vanadium, Technetium and Niobium are used in the fabrication of superconducting magnets. Niobium-tin and niobium-titanium shaped into wires support high magnetic fields, their Tc forces refrigeration with liquid helium. Usually they are thin filaments (20 .m) embedded in a copper matrix to maximize affectivity (the charges move only over the surface of the wire).
• Ceramic superconductors ("perovskites") are metal-oxide ceramics that normally have a ratio of 2 metal atoms over 3 oxygen atoms. They display higher Tcs.
• Superconducting cuprates (copper-oxides) can achieve the highest critical temperatures among the Type II superconductors.
• Organic superconductors are part of the organic conductor family (molecular salts, polymers and pure carbon systems including carbon nanotubes and C60 compounds). Molecular salts have low Tc at room pressure (0.4-12 K), in the range of Type I superconductors. The advantage they show is a much higher Bc; in (TMTSF)2PF6 the critical magnetic field is around 6T, an order of magnitude higher than the usual Bcs.
• Borocarbides are one of the least-understood superconductor systems. They are formed from ferromagnetic transition metals (it was thought impossible). When combined with peculiar elements like holmium, they retreat from the superconductor state for certain temperature bellow Tc. They were discovered in 1993 by Bob Cava.
• Heavy Fermions are compounds containing rare-earth elements such as Ce or Yb, or actinide elements such as U. At low temperatures, some of these materials display superconductivity. The mechanism is not completely understood, some theories propose the presence of Cooper pairs formed by interaction with the electron spins instead of lattice phonons. The first observation was made by E. Bucher, et al, in 1973 but it was not recognized as superconductivity until 1979. Their transition temperatures are in the range of Type I superconductors.

Superconductors and Technology

Magnetic Levitation: Meissner Effect on ceramic superconductors is used to keep trains levitating. Magnetic Levitating trains can move at speeds of around 400 Km/h. Even when the technology is fully developed, economical and environmental issues have delayed its generalized use. See also:

• What is Magnetic Levitation?
• What are Magnetic Levitation Vehicles?
• What is a MagLev Train?
• How does a MagLev Train work?

Superconducting Transmission Lines: In Brookhaven National Laboratory, prototype superconducting transmission lines transport 1000 MW of power within an enclosure of diameter 40 cm. If scale problems don't arise it would be possible to transport the full output of a power plant with only one line. Superconducting lines would save the 10%-15% of energy, the amount usually dissipated in the transmission lines. The problem to be solved yet is that superconductors that can be shaped as wires so far need to be refrigerated with liquid helium (very expensive). High Tc superconductors are hard and cannot be shaped into wires.

Electronics industry: ISCO International and Superconductor Technologies are currently offering ultra-high-performance filters based on superconducting wire. Having near zero resistance, even at high frequencies, many more filter stages can be applied to obtain the desired frequency. This is useful in the cellular phone industry among others.

Computers:

• Processors based in superconducting materials are among the competing technologies in the race to obtain petaflop computers.
• Recently it was also obsrved that the small magnetic field that penetrates Type II superconductors can be used to store and retrieve digital information.

Military uses:

• Superconducting microwave antenna: superconductive tape is used to reduce the length of low frequency antennas employed on submarines.
• E-bombs: a superconductor magnet creates a strong electromagnetic pulse that disables the enemy's electronic equipment. It was used in the war with Iraq over a radio station.

Some interesting applications appear in the field of superconducting magnets. See "What is a Ring-shaped Superconductor used for?" for more details.