What is London penetration depth?

London penetration depth, also known as London penetration length, is a physical property that describes how far magnetic fields can penetrate into a superconductor. It refers to the distance over which the magnetic field inside the superconductor is reduced to a fraction of its original value. In other words, London penetration depth is the measure of the thickness of the superconducting layer below the surface of the material.

The physics behind London penetration depth

London penetration depth is a consequence of the Meissner effect, which occurs when a superconductor expels all magnetic fields from its interior below a certain temperature known as the critical temperature. This expulsion creates a screening current at the edge of the superconductor, which cancels out any magnetic field. The screening current is proportional to the magnetic field and the inverse of the London penetration depth. The deeper the magnetic field penetrates, the weaker the screening current.

Measuring London penetration depth

London penetration depth can be measured using a variety of techniques such as microwave resonators, tunnel diodes, and magnetic susceptibility measurements. The most commonly used method involves measuring the temperature dependence of the surface resistance of the superconductor. By fitting the data to a model, it is possible to determine the London penetration depth. The accuracy of the measure depends on the purity and homogeneity of the superconductor.

Applications of London penetration depth

London penetration depth plays a crucial role in many applications of superconductors. For example, it determines the maximum current that can flow through a superconductor without loss of energy. It also affects the properties of superconducting magnets used in MRI machines, particle accelerators, and fusion reactors. Moreover, London penetration depth can be used as a probe to study the electronic structure of superconducting materials.

Example: Superconducting materials and technologies

The London penetration depth has been extensively studied in various superconducting materials, including cuprates, iron-based superconductors, and conventional superconductors. Understanding the behavior of London penetration depth is essential for developing new superconducting technologies, such as high-temperature superconducting wires, which have the potential to revolutionize power generation and transmission. London penetration depth is also relevant in the emerging field of topological superconductivity, which could lead to the development of a new generation of quantum computers.