Question

The critical temperature of a superconductor is the temperature at which

• A. \( \text{Resistance drops to zero} \)
• B. \( \text{Magnetic susceptibility becomes zero} \)
• C. \( \text{Dielectric constant diverges} \)
• D. \( \text{Electron mobility decreases} \)

Hint:

Critical Temperature (\( T_c \)) = Absolute Zero Resistance threshold. Below \( T_c \), a current can circulate in a closed loop indefinitely without any voltage source or power loss!

Answer 1

The Correct Option is A

Concept: Superconductivity is a quantum mechanical phase transition that occurs in certain materials when cooled below a characteristic threshold temperature known as the critical temperature (\( T_c \)).

Step 1: Identifying the Hallmarks of Superconductivity
At and below \( T_c \), two defining transformations happen simultaneously: 1. The electrical DC resistance of the material drops abruptly and completely to exactly zero. 2. The material expels all internal magnetic fields via the Meissner effect, causing its internal magnetic induction \( B \) to become zero. This implies its magnetic susceptibility (\( \chi_m \)) drops to \( -1 \) (perfect diamagnetism), not zero.

Step 2: Analysis of the Options
- Option (1) accurately defines the primary electrical hallmark used to discover and characterize the critical temperature. - Option (2) is false; susceptibility becomes \( -1 \) (perfectly diamagnetic), not zero. - Option (3) relates to ferroelectric transitions, not superconducting phase shifts. - Option (4) is incorrect because electron mobility conceptually approaches infinity when resistance vanishes entirely.