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1957년 알렉세이 아브리코소프가 이론적으로 예측한 바에 따르면 Ginzburg-Landau theory, Type II superconductors are characterized by two critical magnetic fields, [latex]H_{c1}[/latex] and [latex]H_{c2}[/latex]. Between these fields, they enter a mixed or “vortex” state, allowing partial magnetic field penetration through quantized flux tubes called Abrikosov vortices. This allows them to remain superconducting in much higher magnetic fields than Type I superconductors.
The existence of Type II superconductors is crucial for most high-field applications of superconductivity. A Type I superconductor completely expels magnetic fields up to a critical field [latex]H_c[/latex], above which it abruptly transitions to the normal state. This [latex]H_c[/latex] is generally too low for building powerful magnets. Abrikosov showed that for materials where the 긴즈버그-란다우 parameter [latex]\kappa > 1/\sqrt{2}[/latex], it is energetically favorable for the material to allow magnetic flux to penetrate in a quantized manner rather than becoming fully normal. This penetration occurs above a lower critical field [latex]H_{c1}[/latex]. The flux enters in the form of cylindrical filaments called vortices or fluxons. Within the core of each vortex, the material is in the normal state, but the surrounding bulk remains superconducting. Each vortex carries a single quantum of magnetic flux, [latex]\Phi_0 = h/2e[/latex]. As the external field increases, more vortices enter the material, forming a regular triangular lattice known as an Abrikosov vortex lattice. The material remains superconducting, with zero resistance, until an upper critical field [latex]H_{c2}[/latex] is reached, at which point the vortex cores overlap and the entire material becomes normal. Since [latex]H_{c2}[/latex] can be hundreds of times larger than [latex]H_{c1}[/latex], Type II materials are essential for generating strong magnetic fields. All high-temperature superconductors and many alloys like Niobium-titanium (NbTi) and Niobium-tin (Nb3Sn) are Type II.
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