Question

Type-II superconductors exhibit a 'mixed state' between \( H_{c1} \) (lower critical magnetic field) and \( H_{c2} \) (upper critical magnetic field), which means:

• A. The material alternates between superconducting and normal conducting regions
• B. Cooper pairs break down entirely
• C. Magnetic flux partially penetrates the material in the form of quantized vortices
• D. The material's critical temperature (Tc) fluctuates

Hint:

In Type-II superconductors, think of the magnetic field slipping in through narrow “tubes” — those are the quantized vortices that define the mixed state!

Answer 1

The Correct Option is C

Type-II superconductors, unlike Type-I, do not completely expel magnetic fields above a certain point. Instead, they enter what is called a mixed state or vortex state between the lower and upper critical magnetic field limits, denoted as \( H_{c1} \) and \( H_{c2} \). Key Features of the Mixed State:

• When the external magnetic field is between \( H_{c1} \) and \( H_{c2} \), the magnetic field begins to penetrate the material in the form of quantized magnetic flux lines, also called flux vortices.
• Each vortex carries one quantum of magnetic flux and is surrounded by circulating superconducting currents.
• The rest of the material remains superconducting, creating a unique state where magnetic and superconducting regions coexist.
• This makes Type-II superconductors very useful in high-field applications, such as in MRI machines and particle accelerators.

Why the Other Options Are Incorrect:

• (A) There is no alternation between regions — rather, magnetic flux smoothly enters in vortex form.
• (B) Cooper pairs do not completely break down in this state; they remain intact in the regions between vortices.
• (D) The critical temperature remains constant for a given material — it is not affected by the magnetic field in this manner.

Thus, the correct answer is: Magnetic flux partially penetrates the material in the form of quantized vortices.