GATE 2025 PH – Question 47
Which of the following option(s) is/are correct for a Type I superconductor?
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Show answer and explanation
Correct answer: (A) The phase transition to the normal state in the absence of a magnetic field is of second order; (C) Below the critical temperature, the entropy in the superconducting state is less than that in the normal state; (D) The phase transition to the normal state in the presence of a magnetic field is of first order
Explanation
Check the statements for a type I superconductor.
- **A. The transition to the normal state in zero field is of second order.** True. There is no latent heat at $T_c$ in zero field, while the specific heat jumps. ✓
- **B. The critical field decreases linearly with temperature.** False. $H_c(T)\approx H_c(0)\left[1-(T/T_c)^2\right]$, a parabola.
- **C. The entropy of the superconducting state is lower than that of the normal state.** True. The superconducting state is more ordered. ✓
- **D. The transition in the presence of a magnetic field is of first order.** True. At $T<T_c$ the transition occurs at $H_c(T)$ with a latent heat $T(S_n-S_s)$. ✓
Answer **A, C and D**.