Superconductors and Superconductivity


Superconductors and Superconductivity
Superconductors conduct electricity with no resistance, below a certain temperature. They achieve superconductivity, where electric current flows continuously without energy loss.

Superconductors and superconductivity are a fascinating field in modern physics and materials science, with applications ranging from magnetic resonance imaging (MRI) to quantum computing. Here is a look at the concept of superconductors, how they are classified, their properties, how they work, and their applications.

What Are Superconductors and Superconductivity?

Superconductivity is a quantum mechanical phenomenon where a material exhibits zero electrical resistance and expels magnetic fields when cooled below a characteristic critical temperature (Tc). Superconductors are the materials that exhibit this phenomenon.

Properties of Superconductors

A superconductor is a perfect electrical conductor, where direct current flows continuously with no energy loss through heat or sound. It is a perfect diamagnetic material. Superconductors also expel external magnetic fields, leading to the Meissner effect. A permanent magnet levitating above a superconductor demonstrates this effect. While zero resistance and the Meissner effect are properties common to all superconductors, there are also properties that vary according to the material.

Material-Independent Properties

  • Zero Electrical Resistance: This is the most defining feature, allowing superconductors to carry electric current without energy loss.
  • Meissner Effect: The Meissner effect is the expulsion of magnetic fields from the interior of the superconductor.

Material-Dependent Properties

  • Critical Temperature (Tc): The critical temperature is the temperature below which the material becomes superconductive.
  • Critical Magnetic Field (Hc): This is the magnetic field strength that destroys superconductivity.
  • Critical Current Density (Jc): This is the maximum current density the material carries without losing its superconducting properties.

Superconductor Materials and Examples

About half of the elements on the periodic table display superconductivity. But, many other materials also display the effect.

  • Metallic elements: Mercury was the first superconductor discovered.
  • Organics: For example, carbon nanotubes and fullerenes are superconductors.
  • Alloys: Examples include niobium-titanium and germanium-niobium.
  • Ceramics: Examples include cuprates (e.g., Yttrium Barium Copper Oxide or YBCO) and magnesium diboride.
  • Iron-based: Iron selenide (FeSe) and fluorine-doped LaOFeAs.

Classification of Superconductors

There are several methods of classifying superconductors:

1. Type I and Type II

  • Type I Superconductors: These are also known as conventional superconductors. They show a complete loss of resistance and exhibit the Meissner effect completely. They usually have lower critical temperatures and are destroyed by relatively weak magnetic fields.
  • Type II Superconductors: These remain superconductive in higher magnetic fields. They exhibit partial Meissner effect and are divided into mixed states.

2. Conventional and Unconventional

  • Conventional Superconductors: The BCS theory (Bardeen-Cooper-Schrieffer theory) explains these superconductors, which mostly consist of elemental metals and metalloids.
  • Unconventional Superconductors: These materials do not conform to the BCS theory and include high-temperature superconductors.

3. High-Temperature and Low-Temperature

  • High-Temperature Superconductors: These materials become superconductive at temperatures that are significantly higher than absolute zero, though still extremely cold (above 30 K or −243.15 °C).
  • Low-Temperature Superconductors: They become superconductive at temperatures close to absolute zero.

4. By Material Class

For example:

  • Metallic Superconductors: Such as niobium, lead, and mercury.
  • Cuprate Superconductors: High-temperature superconductors that are copper-based.
  • Iron-Based Superconductors: A newer class exhibiting high-temperature superconductivity.

How Superconductors Work

The BCS theory explains how conventional (Type I) superconductors work. The theory gets it name for John Bardeen, Leon Cooper, and Robert Schrieffer, who formulated it in 1957. Electrons form pairs known as Cooper pairs. These pairs move through the lattice of the material without scattering, leading to zero electrical resistance.

Here’s how it works:

1. Cooper Pairs and Electron Pairing

In a normal conductor, electrical resistance is primarily due to electrons scattering off impurities and lattice vibrations (phonons) in the material. In contrast, superconductivity involves electrons forming pairs known as Cooper pairs.

  • Cooper Pairs: These are pairs of electrons that, counterintuitively, attract each other under certain conditions despite their identical negative charges. Lattice vibrations mediate this attraction.
  • Phonon Mediation: When an electron moves through a lattice, it attracts nearby positive ions. This creates a region of positive charge density that attracts electrons, leading to an effective electron-electron attraction. This interaction is mediated by phonons, which are quantum units of vibrational energy arising from oscillating atoms.

2. Energy Gap and Superconducting State

  • Energy Gap: The formation of Cooper pairs leads to an energy gap between the superconducting state and the normal state. This gap means that it takes a certain amount of energy to break the pairs and destroy superconductivity.
  • Resistance-Free Flow: Within the superconducting state, these Cooper pairs move through the lattice without scattering. This is the main reason behind the zero electrical resistance in superconductors.

3. Critical Temperature (Tc)

  • Each superconductor has a specific critical temperature below which it exhibits superconductivity. Above this temperature, thermal energy breaks the Cooper pairs, and the material reverts to a normal conductive state.

4. Meissner Effect

  • In addition to zero resistance, superconductors also exhibit the Meissner effect. When a material transitions into the superconducting state, it expels the magnetic field from its interior. This is due to the superconductor’s diamagnetic nature in the superconducting state, which perfectly opposes any external magnetic field.

High-Temperature Superconductivity and Unconventional Superconductors

The BCS theory does not fully explain high-temperature (Type II) or unconventional superconductors. These materials, such as the cuprates and iron-based superconductors, have higher critical temperatures and more complex structures. Their exact mechanism of superconductivity is still a subject of research. Theories suggest that the electron pairing mechanism might involve interactions more complex than phonon mediation.

History and Nobel Prizes in Superconductivity

Superconductivity was first observed in mercury by Heike Kamerlingh Onnes in 1911.

To date, there have been five Nobel Prize in Physics awards for research into superconductivity:

  • 1913: Heike Kamerlingh Onnes receive the Nobel Prize for the discovery of superconductivity.
  • 1972: John Bardeen, Leon N. Cooper, and J. Robert Schrieffer receive the Nobel Prize for the BCS theory.
  • 1973: Discoveries regarding tunneling phenomena in semiconductors and superconductors earn Leo Esaki, Ivar Giaever, and Brian D. Josephson the Nobel Prize in Physics.
  • 1987: J. Georg Bednorz and K. Alex Müller for the discovery of high-temperature superconductivity in cuprates.
  • 2003: Alexei A. Abrikosov, Vitaly L. Ginzburg, and Anthony J. Leggett receive the Prize for their pioneering contributions to the theory of superconductors and superfluids.

Applications of Superconductors

Superconductors have many uses:

  • Magnetic Resonance Imaging (MRI): Superconducting magnets produce strong and stable magnetic fields.
  • Particle Accelerators: Superconductors play an important role in the construction of large electromagnets.
  • Magnetic Levitation (Maglev) Trains: Trains utilize superconductors for frictionless, high-speed transportation.
  • Electric Power Transmission: Superconducting cables transmit electricity with no loss.
  • Quantum Computing: Superconductors are key in the development of qubits for quantum computers.

References

  • Bardeen, John; Cooper, Leon; Schrieffer, J. R. (1957). “Theory of Superconductivity”. Phys. Rev. 108: 1175. ISBN 978-0-677-00080-0. doi:10.1103/physrev.108.1175
  • Combescot, Roland (2022). Superconductivity. Cambridge University Press. ISBN 9781108428415.
  • Durrant, Alan (2000). Quantum Physics of Matter. CRC Press. ISBN 978-0-7503-0721-5.
  • Hirsch, J. E.; Maple, M. B.; Marsiglio, F. (2015). “Superconducting materials classes: Introduction and overview”. Physica C: Superconductivity and Its Applications. Superconducting Materials: Conventional, Unconventional and Undetermined. 514: 1–8. doi:10.1016/j.physc.2015.03.002
  • Schilling, A.; et al. (1993). “Superconductivity above 130 K in the Hg–Ba–Ca–Cu–O system”. Nature. 363 (6424): 56–58. doi:10.1038/363056a0