The Meissner effect is one of the two defining characteristics of superconductivity, a state of matter where a material exhibits a complete loss of electrical resistance below a specific critical temperature (\(T_c\)). While zero resistance allows current to flow indefinitely without energy loss, the Meissner effect relates to the material’s magnetic behavior. This magnetic expulsion fundamentally distinguishes a superconductor from a merely hypothetical perfect electrical conductor. Discovered by Walther Meissner and Robert Ochsenfeld in 1933, the effect shows the superconducting state is defined magnetically as well as electrically.
Defining the Meissner Effect
The Meissner effect describes the phenomenon where a superconductor, when cooled below its critical temperature in the presence of an external magnetic field, completely expels all magnetic flux lines from its interior. This expulsion is a bulk property, meaning it occurs throughout the volume of the material, not just on the surface. This complete flux exclusion effectively creates a region of zero magnetic field inside the superconductor.
The most vivid demonstration of the Meissner effect is magnetic levitation, where a small magnet is suspended above the material. The magnetic field from the external magnet is repelled by the superconductor, creating a repulsive force strong enough to counter gravity. This illustrates the powerful magnetic opposition that arises when the material enters the superconducting state.
The Mechanism of Magnetic Field Expulsion
Magnetic field expulsion is achieved through the spontaneous formation of electric currents that flow only along the material’s surface, often called “screening” or “shielding” currents. These currents are generated to oppose the external magnetic field attempting to penetrate the material. Since the material is in a superconducting state, these surface currents flow without resistance and do not decay over time.
The shielding currents work by generating their own internal magnetic field. This internally generated field is equal in magnitude and opposite in direction to the external magnetic field. The net result is that the two fields perfectly cancel each other out within the bulk of the superconductor, making the total magnetic field inside zero.
This magnetic cancellation does not occur instantaneously throughout the entire volume. The external magnetic field decays exponentially from the surface inward over a short distance, known as the London penetration depth. This depth is typically on the order of 20 to 40 nanometers in many superconductors. The surface currents flow within this layer to maintain the internal magnetic nullification.
Why the Meissner Effect is Crucial for Superconductivity
The Meissner effect separates a superconductor from a theoretical “perfect conductor,” a material with zero electrical resistance but no special magnetic properties. If a normal conductor were made “perfect” while a magnetic field was already passing through it, the laws of electromagnetism would dictate that the magnetic flux would be trapped inside. This trapping occurs because, without a change in the applied magnetic field, there is no voltage to drive currents that could expel the existing flux, even with zero resistance.
In contrast, a superconductor expels any pre-existing magnetic field when it transitions into the superconducting state. This expulsion occurs regardless of whether the material was cooled before or after the magnetic field was applied. The Meissner effect shows that the superconducting state is not merely a condition of zero resistance, but rather a distinct thermodynamic phase of matter.
The magnetic expulsion signifies that the superconducting state is characterized by perfect diamagnetism. This means the material’s magnetic susceptibility is exactly negative one, a characteristic of a material that opposes an applied magnetic field. This perfect magnetic opposition complements the zero electrical resistance.
The Boundaries of the Effect
The Meissner effect is not absolute and will break down if the external magnetic field becomes too strong. Every superconducting material possesses a characteristic limit known as the critical magnetic field (\(H_c\)). If the applied magnetic field exceeds this critical threshold, the material’s superconductivity is destroyed, and it reverts to its normal, resistive state.
Superconductors are broadly categorized into two types based on how they behave near this critical field. Type I superconductors, typically pure metals like lead or aluminum, exhibit a complete Meissner effect up to a single critical field (\(H_c\)). At this point, the transition to the normal state is abrupt, and these materials generally have relatively low critical fields.
Type II superconductors, which include alloys and high-temperature ceramic materials, behave differently and are characterized by two critical fields: a lower critical field (\(H_{c1}\)) and an upper critical field (\(H_{c2}\)). Between these two fields, the material enters a “mixed state” where the magnetic flux partially penetrates the material in the form of quantized magnetic vortices. In this mixed state, the Meissner effect is incomplete, as some magnetic field is allowed to pass through the bulk of the material.