Superconductivity

In the field of condensed matter physics, superconductivity is one of the most fascinating phenomena. It is a state of matter characterized by the complete absence of electrical resistance and the expulsion of magnetic fields in certain materials when they are cooled below a specific critical temperature. The phenomenon was first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes, who observed it in mercury at temperatures below 4.2 K (Kelvin).

Discovery and historical development

The journey to superconductivity began more than a century ago. Onnes conducted experiments on very cold metals and found that the electrical resistance of mercury suddenly dropped to zero at 4.2 K. This surprising result led to the discovery of superconductivity, which aroused great interest in scientific circles. Subsequently, superconductivity was also found in other materials such as lead and niobium, each with its own unique critical temperature.

In the years following Onnes' discovery, researchers tried to understand the underlying mechanism. The phenomenon puzzled scientists for decades, until three physicists, John Bardeen, Leon Cooper, and Robert Schrieffer, introduced the BCS theory in 1957, which provided a satisfactory theoretical explanation for superconductivity in conventional low-temperature superconductors.

Fundamental properties of superconductors

Zero electrical resistance

One of the most remarkable features of a superconductor is the absence of electrical resistance. In normal conductors, electrons scatter off impurities and lattice vibrations, causing resistance. In a superconductor, below its critical temperature, these scattering processes stop, and electrons can move through the material unhindered. This property allows the transmission of electric current without any energy loss.

Meissner effect

The second fundamental feature of superconductivity is the Meissner effect, discovered in 1933 by Walther Meissner and Robert Ochsenfeld. They found that when a superconductor reaches its superconducting state, it excludes magnetic fields from within itself. This exclusion of magnetic fields is what distinguishes an ideal conductor from a superconducting material.

Example of the Meissner effect:

The above figure shows a superconductor (in blue) with magnetic field lines (in green) bent around it, demonstrating the expulsion of magnetic fields, known as the Meissner effect.

Theoretical explanation of superconductivity

BCS principle

The BCS theory, named after its developers Bardeen, Cooper and Schrieffer, answers the question of why electrons, which normally repel each other because of their like charges, form Cooper pairs in a superconductor.

The Cooper Pairs

Cooper pairs are pairs of electrons bound together at low temperatures in a lattice structure, which behave like bosons (particles that obey Bose-Einstein statistics) rather than fermions. As bosons, they can condense into the same quantum state, allowing them to move through the superconductor without scattering.

Cooper pair formation:    
Electron 1 ----- Phonon cloud ----- Electron 2
    

In the above view, an electron moves through the lattice, creating a pattern of positive charge, which attracts another electron, forming a pair via lattice vibrations or phonons.

Band gap

BCS theory predicts an energy gap in the states of electronic density at the Fermi level. This energy gap prevents electron scattering states, which are usually dominant in normal metals, leading to superconductivity. The size of the energy gap depends on temperature and decreases as the temperature approaches the critical temperature.

Types of superconductors

Type I superconductor

Type I superconductors are characterized by a single critical magnetic field, below which they act as perfect diamagnets and exhibit a full Meissner effect. Above this critical field, superconductivity is abruptly lost.

Type II superconductor

Type II superconductors have two critical magnetic fields, known as the low critical field and the high critical field. Below the low critical field, they exhibit a full Meissner effect, but between the low and high fields, they allow partial penetration of magnetic field lines through vortex states, in which small regions of the material become normal. Above the high critical field, superconductivity is lost completely.

Illustration of magnetic field penetration in type II superconductors:

This visualization sheds light on the unique behavior of Type II superconductors and how they allow magnetic fields to enter as vortices between critical fields.

Applications of superconductivity

Magnetically levitated trains (maglev)

Superconductors are used in magnetically levitated trains because they have the ability to cancel out magnetic fields (the Meissner effect), allowing trains to float above the tracks with virtually no friction.

Magnetic resonance imaging (MRI)

MRI machines use powerful superconducting magnets to generate the magnetic fields needed for high-resolution images of the human body, which aid in medical diagnosis.

Power cables

The zero resistance property of superconductors can be exploited in power cables, allowing electricity to be transmitted over long distances with minimal or no energy loss, significantly improving efficiency.

Particle accelerators

In particle accelerators, superconducting magnets are used to direct and focus particle beams. These magnets are essential components in facilities such as CERN's Large Hadron Collider (LHC).

Challenges and future prospects

Despite its promise, widespread adoption of superconductivity has faced challenges, primarily due to the requirement for extremely low operating temperatures. However, the discovery of high-temperature superconductors in the late 1980s, such as those based on copper-oxide ceramics, has highlighted the potential for more practical applications. Research continues to find materials that become superconductors at even higher temperatures, ideally closer to room temperature.

In short, superconductivity is one of the most interesting phenomena in physics, with significant implications across a variety of technological fields. As research progresses, it is hoped that its potential will be further enhanced, making superconductor-based technologies feasible for everyday use.

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