Some solids lose all resistance to electrical current in certain conditions. This is the superconductivity phenomenon and today’s subject.
How to obtain superconductivity?
To a material become superconductor, must be cooled to extremely low temperatures, until reaches critical temperature T_{C}, where electric resistance drops to zero. Not all materials can become superconductors, even though resistance drops with temperature.
A material under a magnetic field H_{C} strong enough can be kept in normal state, even below critical temperature. There is a critical magnetic field which must be below a value to obtain superconductivity. The H_{C} value depends on material and room temperature.
Superconductor material
When material reaches critical temperature, interesting properties appear. A superconductor shows the Meissner effect, do not allow magnetic field inside and becomes a perfect diamagnetic.
Thanks to Meissner effect, a magnet can float over a superconductor’s surface, the opposite also can happen.
This effect happens due to currents in surface (green), whose magnetic fields (red) are strong enough to cancel external field (blue) inside material for being in opposite direction.
In superconductivity state, an electric current can be kept without a potential difference or voltage.
BCS theory
BCS theory, whose name came from three authors (Bardeen, Cooper and Schrieffer), was created to explain superconductivity. According to this theory, electrons with opposite spin inside material form Cooper pairs and electrically interact with crystalline molecular structure. Front electron distorts lattice, and attract positive charges. Positive charges closer to each other attract the back electron, following the front one.
The energy package which unite electrons, created by lattice distortion is called phonon.
Isotopic effect
Isotopes are atoms of same element with different masses. Was verified that different mercury isotopes have different critical temperatures. This is the isotopic effect predicted by BCS theory.
Types of superconductors
Exist two types of superconducting materials: Type 1, made of a single element, and type 2, formed by alloys and composites.
- Type 1 examples: Mercury (Hg), aluminum (Al), lead (Pb).
- Type 2 examples: SmFeAs(O,F), YBCO (YBa_{2}Cu_{3}O_{7}), BSSCO (Bi_{2}Sr_{2}CuO_{6}).
Other differences are:
- Type 1 has only one critical field intensity. While type 2 has two field values: upper H_{C2} and lower H_{C1}. These mark transition state between normal and superconducting, where there are superconducting current vortices.
- Type 2 isn’t a perfect diamagnetic in transition state. Therefore, has an incomplete Meissner effect.
- Type 2 has much higher critical temperature than type 1. BCS theory claims that maximum critical temperature is 30 Kelvin (-243ºC) and can’t explain the type 2.
Search for hot superconductors
Cool materials to obtain superconductivity is very expensive and complex. Because of that, many laboratories are seeking out superconductors with higher temperatures. Until now, material with highest critical temperature is hydrogen sulphide, whose value is 203 K (-70ºC). But to have superconductivity, must be subjected to very high pressure to be in solid state.
Superconductivity is a very extensive area, other related topics will be for future posts.
