Question

Type-II superconductors are characterized by

• A. Single critical field
• B. No mixed state
• C. Two critical magnetic fields
• D. Complete flux expulsion

Hint:

- Type-I: 1 critical field, exhibits a sharp transition, perfect flux expulsion. - Type-II: 2 critical fields (\(H_{c1}\) and \(H_{c2}\)), features a "mixed state" where magnetic flux leaks through as quantized vortices. Most commercial high-temperature superconductors are Type-II.

Answer 1

The Correct Option is C

Concept: Superconductors are categorized into two primary classes (Type-I and Type-II) based on how they respond to external magnetic fields. Type-I superconductors display an abrupt breakdown of superconductivity at a single threshold field, whereas Type-II superconductors undergo a gradual transition over an extended range bounded by two distinct critical values.

Step 1: Behavior under an increasing magnetic field \(H\).
Let the external magnetic field be \(H\). In a Type-II superconductor, the magnetic behavior transitions through three distinct phases:
• Meissner State (\(H < H_{c1}\)): At low magnetic fields below the lower critical field (\(H_{c1}\)), the material functions as a perfect diamagnet. It completely expels all internal magnetic flux lines (\(B = 0\)).
• Mixed or Vortex State (\(H_{c1} \le H \le H_{c2}\)): When the applied field exceeds \(H_{c1}\), the magnetic field begins to partially penetrate the material in the form of quantized tubes of flux called vortices or fluxoids. Within each vortex core, the material is normal, but the surrounding matrix remains superconducting. This state is known as the *mixed state*.
• Normal State (\(H > H_{c2}\)): Once the field crosses the upper critical field (\(H_{c2}\)), the density of vortices becomes so high that they overlap completely. Superconductivity is destroyed, and the material reverts entirely to a normal conducting state.

Step 2: Checking the given options.

• Single critical field No mixed state Complete flux expulsion: These options describe a Type-I superconductor, which loses its superconductivity instantly at a single value \(H_c\).
• Two critical magnetic fields: This is the exact signature property of Type-II materials, defined by the distinct values \(H_{c1}\) and \(H_{c2}\). Thus, Type-II superconductors are uniquely characterized by two critical magnetic fields.