Superconductors

Superconductors

Superconductors are materials that can conduct electricity with zero resistance when they are cooled below a certain temperature, known as the critical temperature or T_c. This means that they can carry electric current without any energy loss, which makes them very useful for a wide range of applications in areas such as power generation, medical imaging, and transportation.

The applications of superconductivity are varied, ranging from medical imaging (such as MRI machines) to transportation (such as maglev trains) to power generation and distribution (such as high-field magnets for fusion experiments). However, the challenge with superconductivity is that it requires low temperatures to work, which can be expensive and impractical for some applications. Nevertheless, scientists are continually researching and developing new materials that exhibit superconductivity at higher temperatures, which could lead to more widespread and practical applications in the future.

Types of Superconductors 

Superconductors can be classified into two main types:

1. Type I superconductors: These superconductors have a single critical magnetic field, below which they exhibit perfect conductivity, and above which they lose their superconducting properties abruptly. They are also called “soft” superconductors. Examples of type I superconductors include mercury, lead, and tin.
2. Type II superconductors: These superconductors have two critical magnetic fields, and in between them, they exhibit a mixed state where only some parts of the material are superconducting. They are also called “hard” superconductors. Examples of type II superconductors include niobium-titanium, niobium-tin, and YBCO (yttrium barium copper oxide).

Type II superconductors are more widely used in practical applications because they can operate at higher magnetic fields and temperatures than type I superconductors. They can also maintain their superconducting properties in the presence of strong magnetic fields, which is essential for applications such as MRI machines and particle accelerators.

In addition to these two main types, there are also unconventional superconductors which do not fit into the conventional BCS (Bardeen-Cooper-Schrieffer) theory of superconductivity. These include high-temperature superconductors and heavy fermion superconductors.

Critical Temperature

The critical temperature (Tc) is a key parameter for superconductors, as it is the temperature below which the material exhibits zero electrical resistance and perfect diamagnetism.

For conventional superconductors, such as Nb3Sn and NbTi, the critical temperature is relatively low, ranging from about 9 K (-264 °C) to 18 K (-255 °C) depending on the material and the conditions of the sample.

In contrast, high-temperature superconductors, such as cuprates and iron-based superconductors, have much higher critical temperatures, ranging from about 30 K (-243 °C) to 138 K (-135 °C) for the record-high Tc material HgBa2Ca2Cu3O8+δ.

Application of Superconductors

Superconductors have a wide range of applications in various fields today, including:

1. Magnetic Resonance Imaging (MRI) – Superconducting magnets are used in MRI machines, which generate strong magnetic fields to produce detailed images of internal body structures.
2. Particle accelerators – Superconducting materials are used to create strong magnetic fields for accelerating charged particles in high-energy particle accelerators.
3. Power transmission – Superconductors can carry electricity with zero resistance, leading to much more efficient power transmission over long distances.
4. Magnetic levitation (Maglev) trains – Superconducting materials can be used to create powerful magnetic fields, which allow Maglev trains to float above the tracks and travel at high speeds.
5. Quantum computing – Superconducting qubits are a promising technology for building quantum computers, which could revolutionize computing by solving problems that are currently intractable on classical computers.
6. Magnetic confinement fusion – Superconducting coils are used in experimental fusion reactors to create the magnetic fields needed to confine the plasma.
7. High-speed digital circuits – Superconducting materials can be used to make extremely fast digital circuits with low power consumption.
8. Sensors – Superconducting materials can be used to create extremely sensitive sensors for detecting magnetic fields, temperature, and other physical quantities.

These are just a few examples of the many applications of superconductors in modern technology.

Superconductors – Materials

Here’s a table of 10 superconductors with their key characteristics.

Superconductor	Chemical Formula	Type	Critical Temperature (K)	Critical Magnetic Field (T)
Tin (Sn)	Sn	Type I	3.72	0.005
Lead (Pb)	Pb	Type I	7.19	0.015
Mercury (Hg)	Hg	Type I	4.15	0.091
Niobium-titanium (NbTi)	NbTi	Type II	10.4	12.5
Niobium-tin (Nb3Sn)	Nb3Sn	Type II	18.1	25
Yttrium Barium Copper Oxide (YBCO)	YBa2Cu3O7-x	Type II	92	0.2
Bismuth Strontium Calcium Copper Oxide (BSCCO)	Bi2Sr2Ca2Cu3O10+x	Type II	107	0.2
Lanthanum Barium Copper Oxide (LBCO)	La1.85Ba0.15CuO4	Type II	40	0.2
Magnesium Diboride (MgB2)	MgB2	Type II	39	0.2
Iron-based superconductor (FeSe)	FeSe	Type II	8	0.17

Superconductivity

Superconductivity is a phenomenon much different from conductivity. In fact, the best of the normal conductors, such as silver and copper, cannot become superconducting at any temperature, and the new ceramic superconductors are actually good insulators when they are not at low enough temperatures to be in a superconducting state.

The relationship between superconductivity and low temperatures is fundamental. For a material to become a superconductor, it must be cooled below a critical temperature called the superconducting transition temperature or critical temperature (Tc). This transition temperature is different for different superconducting materials and can range from a few Kelvin (K) to several hundred K.

At low temperatures, the atomic vibrations in a material decrease, which means that there are fewer impediments to the flow of electrons. This leads to the formation of Cooper pairs, which are pairs of electrons that are bound together by lattice vibrations. In a normal metal, electrons experience resistance when they collide with the atoms in the lattice structure of the material, but in a superconductor, the Cooper pairs move through the lattice structure without any resistance.

History of Superconductors

Superconductivity was first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes, who observed that the resistance of mercury dropped to zero when it was cooled to a temperature of 4.2 Kelvin (-268.8°C). Since then, many other materials have been discovered that exhibit superconductivity at higher temperatures and with different properties.

This phenomenon of superconductivity is of vast potential importance in technology because it means that charge can flow through a superconducting conductor without losing its energy to thermal energy. Currents created in a superconducting ring, for example, have persisted for several years without loss; the electrons making up the current require a force and a source of energy at start-up time but not thereafter. 

BCS theory or Bardeen–Cooper–Schrieffer theory, is the first microscopic theory of superconductivity since Heike Kamerlingh Onnes’s 1911 discovery. The theory describes superconductivity as a microscopic effect caused by a condensation of Cooper pairs. The theory is also used in nuclear physics to describe the pairing interaction between nucleons in an atomic nucleus.

Prior to 1986, the technological development of superconductivity was throttled by the cost of producing the extremely low temperatures required to achieve the effect. 

In 1986, however, new ceramic materials were discovered that become superconducting at considerably higher (and thus cheaper to produce) temperatures. 

High-temperature superconductors

High-temperature superconductors (HTS) are a type of unconventional superconductors that exhibit superconductivity at relatively high temperatures compared to conventional superconductors.

The first high-temperature superconductor was discovered in 1986 by Bednorz and Müller, who found that a compound made of lanthanum, copper, and oxygen had a critical temperature (Tc) of 35 K (-238 °C), much higher than the previous record of 23 K (-250 °C) for Nb3Ge. Since then, many other high-temperature superconductors have been discovered, with critical temperatures as high as 138 K (-135 °C).

The mechanism of superconductivity in high-temperature superconductors is not well understood and is still an active area of research. Unlike conventional superconductors, which can be explained by the BCS theory, high-temperature superconductors are believed to have a more complex mechanism that involves strong electron-electron interactions and possibly a quantum phase transition.

High-temperature superconductors have the potential to revolutionize many areas of technology, including power transmission, magnetic levitation, and high-field magnets for fusion reactors and particle accelerators. However, their widespread use is limited by the difficulty and cost of cooling them to their critical temperature, which requires liquid nitrogen or even colder refrigerants.

LaBaCuO superconductor

LaBaCuO (lanthanum barium copper oxide) is a type of high-temperature superconductor. It has a layered crystal structure consisting of superconducting copper oxide planes and insulating layers. LaBaCuO was one of the first high-temperature superconductors discovered and has a critical temperature of around 30 K (-243 °C), which is higher than that of conventional low-temperature superconductors.

LaBaCuO is a type-II superconductor, which means that it can support strong magnetic fields without losing its superconducting properties. It also exhibits anisotropic behavior, with its electrical and magnetic properties depending on the direction of the applied field.

LaBaCuO is used in various applications, such as in superconducting magnets, power transmission cables, and electronic devices.

Niobium-titanium superconductor

Niobium-titanium (NbTi) is a type II superconductor that is commonly used in applications that require strong magnetic fields, such as MRI machines and particle accelerators. Some of its characteristics include:

1. Critical temperature: The critical temperature of NbTi is around 9 K (-264°C), which is relatively high compared to other superconductors.
2. Critical magnetic field: NbTi has a high critical magnetic field, which allows it to generate very strong magnetic fields when it is superconducting.
3. Ductility: NbTi is a ductile material that can be easily formed into wires or other shapes, which makes it well-suited for use in superconducting magnets.
4. Low resistivity: When NbTi is superconducting, it has zero resistivity, which means that it can carry electrical currents with very low losses.
5. Mechanical properties: NbTi has good mechanical properties, such as high strength and stiffness, which make it suitable for use in high-stress applications.
6. Stability: NbTi is a stable material that does not undergo any significant changes in its superconducting properties over time, which makes it well-suited for long-term use in applications such as MRI machines.
7. Cost: While NbTi is more expensive than some other superconductors, its high critical magnetic field and good mechanical properties make it a cost-effective choice for many applications.

The most commonly used superconductor is niobium-titanium (NbTi), which is widely used in superconducting magnets for MRI machines, particle accelerators, and fusion reactors.