Meissner effect

Meissner effect
Name	Meissner effect
Phenomena	Expulsion of magnetic field from a superconductor
Related	Superconductivity, London equations, BCS theory

Meissner effect. The Meissner effect is the complete expulsion of a magnetic field from the interior of a superconductor as it transitions into the superconducting state. This fundamental property, distinct from perfect conductivity, demonstrates that superconductivity is a true thermodynamic phase with a definite order parameter. Its discovery by Walther Meissner and Robert Ochsenfeld in 1933 was pivotal in distinguishing superconductors from mere perfect conductors and underpins critical applications like MRI machines and maglev train systems.

Physical explanation

The effect arises because an applied magnetic field induces supercurrents on the surface of the material, which generate a counter-field that precisely cancels the external flux within the bulk. This screening is described by the London penetration depth, a characteristic length over which external fields decay exponentially inside the superconductor. The transition to this state involves the formation of Cooper pairs as described by John Bardeen, Leon Cooper, and John Robert Schrieffer in their BCS theory, which condense into a macroscopic quantum state that cannot support a magnetic field. This phase transition is reversible and occurs below a critical temperature and critical magnetic field strength specific to each material.

Discovery and history

The phenomenon was first observed in 1933 at the Physikalisch-Technische Bundesanstalt in Berlin by Walther Meissner and his assistant Robert Ochsenfeld. They measured the magnetic field distribution outside superconducting tin and lead samples cooled below their transition temperature in the presence of a weak field. Their results, published in the journal Naturwissenschaften, conclusively showed the field was expelled, not merely trapped. This contradicted earlier expectations based on the work of Heike Kamerlingh Onnes, who discovered superconductivity, and the ideas of Felix Bloch. The finding immediately influenced theorists like Fritz London and Heinz London, who formulated the London equations in 1935 to provide the first successful phenomenological explanation.

Types of superconductors

The response to magnetic fields categorizes superconductors into two primary types. Type-I superconductors, which include elements like mercury and aluminium, exhibit a complete Meissner effect up to a critical field, then abruptly transition to a normal state. In contrast, Type-II superconductors, such as niobium-tin and yttrium barium copper oxide, allow magnetic flux to penetrate in quantized tubes called vortices above a lower critical field, creating a mixed state while retaining zero electrical resistance until an upper critical field. This classification was theoretically established by Aleksey Abrikosov building on the Ginzburg–Landau theory developed by Vitaly Ginzburg and Lev Landau.

Experimental observation

Classic demonstrations involve cooling a superconducting material like niobium or YBCO in the presence of a permanent magnet, causing the magnet to levitate due to the repulsive diamagnetic force. Quantitative measurements use techniques such as SQUID magnetometry to precisely track magnetic moment changes during the phase transition. Experiments at facilities like CERN or Brookhaven National Laboratory often study these properties under extreme conditions. The observation of flux expulsion is a definitive test for superconductivity, as performed in studies on novel materials like iron-based superconductors at institutions such as the Max Planck Institute.

Technological applications

The perfect diamagnetism is harnessed in MRI and NMR spectroscopy systems, where superconducting coils generate stable, high-field environments. It is essential for maglev train technologies, such as those developed in Japan by the Central Japan Railway Company, which use superconducting magnets for lift and propulsion. Other applications include sensitive magnetic sensors in geophysical survey equipment, frictionless bearings in flywheel energy storage, and shielding for sensitive electronics. Large-scale projects like the International Thermonuclear Experimental Reactor (ITER) rely on superconducting magnets to confine plasma.

Theoretical models

The first quantitative theory was the London equations, introduced by Fritz London and Heinz London, which predicted exponential field decay. A more general Ginzburg–Landau theory, developed by Vitaly Ginzburg and Lev Landau, provided a phenomenological framework explaining the difference between Type-I and Type-II superconductors. The microscopic BCS theory, formulated by John Bardeen, Leon Cooper, and John Robert Schrieffer, explained the formation of Cooper pairs and the energy gap responsible for the effect. Extensions of these ideas are found in the work of Nikolay Bogoliubov and are applied to unconventional superconductors like those based on copper oxide studied at Bell Labs.

Category:Superconductivity Category:Electromagnetism Category:Condensed matter physics