Superconductors and Superconducting Materials

- Overview

Superconductivity is a set of physical properties observed in certain materials where electrical resistance disappears and magnetic flux fields are expelled from the material. Any material that exhibits these properties is a superconductor. 

Unlike ordinary metal conductors, the resistance of superconductors decreases gradually with decreasing temperature, even to near absolute zero. Superconductors have a characteristic critical temperature below which the resistance suddenly drops to zero. Electric current through a superconducting wire loop can continue indefinitely without a power source.

Superconductivity was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes. Like ferromagnetism and atomic lines, superconductivity is a phenomenon that can only be explained by quantum mechanics. It is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of a superconductor during its transition into a superconducting state. The emergence of the Meissner effect shows that superconductivity cannot simply be understood as the idealization of perfect conductivity in classical physics.

- Superconductors and Materials

Superconductors are materials which transport electric charge without resistance and with the display of associated macroscopic quantum phenomena such as persistent electrical currents and magnetic flux quantization.

Superconductor material classes include chemical elements (e.g. mercury or lead), alloys (such as niobium–titanium, germanium–niobium, and niobium nitride), ceramics (YBCO and magnesium diboride), superconducting pnictides (like fluorine-doped LaOFeAs) or organic superconductors (fullerenes and carbon nanotubes; though perhaps these examples should be included among the chemical elements, as they are composed entirely of carbon).

In 1987, researchers discovered the first "high-temperature" superconductor, a material that only needed to be cooled to 77 Kelvin (–321 degrees Fahrenheit), a temperature easily reached with cheap and plentiful liquid nitrogen. These materials are literally and figuratively exciting, fueling enthusiasm among scientists and the public for the possibility of warmer superconductivity. But as progress slowed, most of the enthusiasm faded, and "high-temperature" superconductors remained at frigid temperatures and were still too fragile to be practical. 

Over the past decade, researchers have been pursuing an intriguing alternative: They found that hydrogen-based compounds are superconductors at relatively high temperatures, but only at pressures greater than a million atmospheres. Maintaining such high pressures is even more impractical than maintaining ultra-low temperatures.  

[Haifa, Israel]

- Superconductivity

Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. 

Unlike an ordinary metallic conductor, whose resistance decreases gradually as its temperature is lowered, even down to near absolute zero, a superconductor has a characteristic critical temperature below which the resistance drops abruptly to zero. 

An electric current through a loop of superconducting wire can persist indefinitely with no power source.

When electrons flow through standard conductive materials such as aluminum wire, they act like bumper cars, bouncing off atoms. All that bouncing creates resistance, which reduces current flow. But if the aluminum wire is cooled to about a Kelvin (–459 degrees Fahrenheit) above absolute zero, something strange happens: The rules of the road change, and electrons combine to pair up and slide frictionlessly between the aluminum atoms with zero resistance. 

- Properties of Superconductors

Some physical properties of superconductors vary from material to material, such as critical temperature, superconducting gap value, critical magnetic field, and critical current density at which superconductivity is destroyed. On the other hand, there is a class of properties that are independent of the base material. 

The Meissner effect, the quantization of magnetic flux or permanent current, i.e. the state of zero resistance are the most important examples. The existence of these "universal" properties is rooted in the appearance of symmetry breaking and off-diagonal long-range order in superconductors. 

Superconductivity is a thermodynamic phase and as such has some remarkable properties that are largely independent of microscopic details.

 [More to come ...]