Meissner Effect

The Meissner effect is a fundamental phenomenon observed in superconductors when transitioning into their superconducting state below the critical temperature (T_C). It describes the expulsion of a magnetic field from the interior of a superconductor.

When a material becomes superconducting, it transitions into a state of perfect diamagnetism, meaning it completely excludes any external magnetic field from penetrating its interior. On the contrary, when a magnetic field is applied to a superconductor above its critical temperature, it penetrates the material, as depicted in the image below.

German physicists Walther Meissner and Robert Ochsenfeld discovered this phenomenon in 1933, and it is considered one of the defining characteristics of superconductivity.

Theory

The theoretical explanation for the Meissner effect is based on the London equations developed by brothers Fritz and Heinz London in 1935. These equations describe the electrodynamics of superconductors and provide a framework for understanding the perfect diamagnetism observed in the Meissner effect.

According to the London equations, the magnetic field inside a superconductor decays exponentially from the surface, with a characteristic length scale known as the London penetration depth. This decay results from the formation of persistent supercurrent loops within the superconductor, which generate a magnetic field that opposes the applied field, effectively expelling it from the interior.

The Meissner effect is a direct consequence of the thermodynamics of superconductors. When a material transitions into the superconducting state, it undergoes a phase transition accompanied by a decrease in the system’s free energy. The expulsion of the magnetic field allows the superconductor to minimize its free energy and achieve a more stable thermodynamic state.

Significance

Meissner effect can explain two important magnetic phenomena – magnetic levitation and flux pinning.

Magnetic Levitation

Magnetic levitation, often called Maglev, exploits the Meissner effect to achieve levitation and frictionless motion.

In a Maglev system, the superconducting material is often used to construct the vehicle, while the magnetized track is embedded with powerful magnets. As the superconducting vehicle passes over the magnetized track, the Meissner effect generates a repulsive force between the superconductor and the magnetic field, causing the vehicle to levitate above the track. By controlling the magnetic fields along the track and the orientation of the superconductor, the levitating vehicle can be propelled forward, allowing for efficient and frictionless transportation.

Overall, magnetic levitation through the Meissner effect enables the development of high-speed, energy-efficient transportation systems with minimal contact between the vehicle and the track, offering numerous advantages over traditional wheeled transportation methods.

Image Courtesy: Researchgate.net

Flux Pinning

Flux pinning is a phenomenon observed in superconductors where magnetic flux lines become trapped or pinned within the material, even when it transitions into its superconducting state below the critical temperature. This phenomenon occurs due to imperfections or defects in the superconducting material, such as impurities, dislocations, or grain boundaries.

Meissner effect is typically dominant in pristine, defect-free superconductors. However, if imperfections are present, the Meissner effect can be partially or completely suppressed, allowing magnetic flux lines to penetrate the material.

When magnetic flux lines enter a superconductor, they induce currents called screening currents that circulate the flux lines to shield the interior of the superconductor from the magnetic field. In the presence of defects, these screening currents can become trapped or pinned, immobilizing the magnetic flux lines within the material.