(Abbildung dient nur zur Veranschaulichung)
Discovered in 1933 by Walther Meissner and Robert Ochsenfeld, the Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. When a material is cooled below its kritische Temperatur ([latex]T_c[/latex]) in the presence of a weak external magnetic field, it actively cancels all magnetic flux inside it, becoming a perfect diamagnet.
The Meissner effect is a defining characteristic of Supraleitung, distinguishing it from a hypothetical perfect conductor. A perfect conductor, according to Lenz’s law, would trap any existing magnetic field inside it as it cools, because changing the flux would induce currents to oppose the change. In contrast, a superconductor actively expels the field. This expulsion is achieved by the generation of screening currents on the surface of the material. These currents create a magnetic field that perfectly cancels the external field within the bulk of the superconductor. The magnetic field penetrates only a small distance into the surface, known as the London penetration depth ([latex]\lambda[/latex]). This discovery was crucial because it showed that superconductivity is a true thermodynamic phase transition, not just a case of infinite conductivity. It provided a key piece of the puzzle that any microscopic theory of superconductivity would have to explain. The effect is the basis for the dramatic levitation demonstrations where a magnet floats above a high-temperature superconductor cooled with liquid nitrogen. The expulsion of the magnet’s field by the superconductor creates a repulsive force strong enough to counteract gravity.
Der Meißner-Effekt versagt, wenn das angelegte Magnetfeld zu stark ist. Bei Supraleitern vom Typ I wird die Supraleitung oberhalb eines kritischen Feldes [latex]H_c[/latex] abrupt zerstört. Bei Supraleitern vom Typ II dringt das Feld oberhalb eines unteren kritischen Feldes [latex]H_{c1}[/latex] in Form von quantisierten Flusswirbeln in das Material ein, während das Material bis zu einem deutlich höheren oberen kritischen Feld [latex]H_{c2}[/latex] supraleitend bleibt.
Meissner-Effekt
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