The essence of superconductivity and superconductors.

Many additional elements can be coaxed into asuperconductive state with the application of highpressure. For example, phosphorus appears to be theType 1 element with the highest Tc. But, it requirescompression pressures of 2.5 Mbar to reach a Tc of22 K (-251ºC).Type I superconductors are very pure metals thattypically have critical fields too low for use insuperconducting magnets.Type 2 superconductorsSuperconductors made from metallic compoundsand alloys are called Type 2 superconductors. Therecently-discovered superconducting "perovskites"(metal-oxide ceramics that normally have a ratio of 2metal atoms to every 3 oxygen atoms) belong to thisType 2 group. Besides being mechanically harderthan Type 1 superconductors, they exhibit muchhigher critical magnetic fields and higher Tc's thanType 1 superconductors by a mechanism that is stillnot completely understood.Type 2 superconductors - also known as the "hard"or high temperature superconductors - differ fromType 1 in that their transition from a normal to asuperconducting state is gradual across a region of"mixed state" behavior. Since a Type 2 will allowsome penetration by an external magnetic field intoits surface, this creates some rather novelmacroscopic phenomena where superconductorusually exist in a mixed state of normal andsuperconducting regions.Fig. 10. Mixed state of type 2 superconductor.This is sometimes called a vortex state, becausevortices of superconducting currents surround coresof normal material. As their critical temperatures areapproached, the normal cores are more closelypacked and eventually overlap as thesuperconducting state is lost. A size of about 300 nmis typical for the normal cores. While the Meissnereffect is modified to allow magnetic fields throughthe normal cores, magnetic fields are still excludedfrom the superconducting regions.The superconducting cuprates (copper-oxides) haveachieved astonishingly high Tc's when consideredthat by 1985 known Tc's had only reached 23 K forType 1. To date, the highest Tc attained at ambientpressure has been 138 K (-135ºC) and is held by athallium-doped, mercuric-cuprate comprised of theelements Mercury, Thallium, Barium, Calcium,Copper and Oxygen.One theory predicts an upper limit of about 200 Kfor the layered cuprates (Vladimir Kresin, Phys.1997). Others assert there is no limit. Either way, itis almost certain that other, more-synergisticcompounds still await discovery among the hightemperaturesuperconductors.Atypical superconductorsIn 1986 a breakthrough discovery was made in thefield of superconductivity. Alex Müller and GeorgBednorz created a brittle ceramic compound thatsuperconducted at the highest temperature thenknown: 30 K (-243ºC). What made this discovery soremarkable was that ceramics are normallyinsulators. The Lanthanum, Barium, Copper andOxygen compound that Müller and Bednorzsynthesized, behaved in a not-as-yet-understoodway.Organic superconductors are part of the organicconductor family which includes: molecular salts,polymers and pure carbon systems. Since these Tc'sare in the range of Type 1 superconductors,engineers have not yet found a practical applicationfor them. However, their rather unusual propertieshave made them the focus of intense research. Theseproperties include giant magnetoresistance, rapidoscillations, quantum hall effect, and more.Discovered in 1993 by Bob Cava, borocarbides areone of the least-understood superconductor systems of all.It has always been assumed that superconductorscannot be formed from ferromagnetic transitionmetals - like iron, cobalt or nickel. It's the equivalentof trying to mix oil and water. However, in someborocarbides there is a "soap" that acts to bring theseadversaries together. Another not understoodphenomenon is that below Tc, where it shouldremain superconductive, there is a discordanttemperature at which the material retreats to a"normal", non-superconductive state.The Heavy Fermions are yet another example ofatypical superconductors. Heavy fermions arecompounds containing rare-earth elements such asCe or Yb, or actinide elements such as U. Theirconduction electrons often have effective massesseveral hundred times as great as that of "normal"electrons, resulting in what's known as low "Fermienergy". This makes them reluctant superconductors.Some of those materials display superconductivitythrough a mechanism that quickly runs afoul of BCStheory. Research suggests cooper-pairing in theheavy fermion systems arises from the magneticinteractions of the electron spins, rather than bylattice vibrations.In 1997 researchers discovered that at a temperaturevery near absolute zero an alloy of gold and indiumwas both a superconductor and a natural magnet.Conventional wisdom held that a material with suchproperties could not exist!In June 1999 New Zealand researcher Dr. Tallon andDr. Bernard discovered a ruthenium-cuprate whose136