High-temperature superconductivity

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Unsolved problems in physics: Why do certain materials exhibit superconductivity at temperatures much higher than 50 K? Is room-temperature superconductivity possible?

High-temperature superconductors are generally considered to be those that demonstrate superconductivity at or above the temperature of liquid nitrogen, or −196 °C (77 K), since this is the most easily attainable cryogenic temperature.

Conventional superconductors, by contrast, require temperatures no higher than a few degrees above absolute zero (−273.15 °C or −459.67 °F). Though it is extremely cold by everyday standards, in the field of superconductivity, 77 K is considered high temperature.

1 History and Progress
2 Types of High-Temperature Superconductors
3 Ongoing Research
4 Notes
5 See also
6 External links

[edit] History and Progress

A "high-temperature" superconductor levitates a magnet (with boiling liquid nitrogen underneath) demonstrating the Meissner effect, one of the hallmark properties of superconductivity.

The term 'high-temperature superconductor' was first used to designate the new family of cuprate-perovskite ceramic materials discovered by Johannes Georg Bednorz and K.A. Müller in 1986<ref>J. G. Bednorz and K. A. Müller, Z. Physik, B 64, 189 (1986)</ref>, for which they won the Nobel prize the following year. Their discovery of the first high-temperature superconductor L B CO, with a transition temperature of 35 K, generated much excitement because it was previously widely assumed to be impossible for superconductivity to occur at such "high" temperatures.

Recently, other unconventional superconductors have been discovered. Some of them also have unusually high values of the critical temperature T_c, and hence they are sometimes also called high-temperature superconductors, although the record is still held by a cuprate perovskite material (T_c=138 K, that is −135 °C) (although slightly higher transition temperatures have been achieved under pressure<ref>L. Gao et al. Phys. Rev. B 50, 4260-4263 (1994).</ref>). Nevertheless it is believed by some researchers that if room temperature superconductivity is ever achieved it will be in a different family of materials.

[edit] Types of High-Temperature Superconductors

Most prominent materials in the high-T_c range are the so-called cuprates, such as La_1.85Ba_0.15CuO₄, YBCO (Yttrium-Barium-Copper-Oxide) and related substances.

All known high-T_c superconductors are so-called Type-II superconductors. A Type-II superconductor allows magnetic field to penerate its interior in the units of flux quanta, creating 'holes' (or tubes) of normal metallic regions in the superconducting bulk. This property makes high-T_c superconductors capable of sustaining much higher magnetic fields.

[edit] Ongoing Research

One of the top unsolved problems in modern physics is the question of how superconductivity arises in these materials, that is, what mechanism causes the electrons in these crystals to form pairs (see Cooper pairs).

Despite much intensive research and many promising leads, an answer to this question has so far eluded scientists. One reason for this is that the materials in question are generally very complex, multi-layered crystals (for example, BSCCO), making theoretical modeling difficult. But with the rapid rate of new, important discoveries in the field, many researchers are optimistic that a complete understanding of the process is possible within the next decade or so.