Question

Which of the following is true regarding superconductivity?

• A. The superconducting state cannot exist in the presence of a magnetic field lesser than a critical value, even at absolute zero
• B. The superconducting state exists in the presence of a magnetic field greater than a critical value, even at absolute zero
• C. The superconducting state exists in the presence of a magnetic field lesser than a critical value, even at absolute zero
• D. The superconducting state cannot exist in the presence of a magnetic field greater than a critical value, even at absolute zero

Answer 1

Answer: (D)

$1.$ This critical magnetic field is strongly correlated with the critical temperature for the superconductor which in turn correlated with the bandgap.

$2.$ As it is the nature of superconductor to exclude magnetic fields so long as the applied field does not exceed their critical magnetic field.

$3.$ This critical magnetic field is tabulated for $0K$ and decrease from the magnitude with increasing temperature, reaches zero at the critical temperature for superconductivity.

$4.$ The critical magnetic field at any temperature below the critical temperature is given by the relationship,

$Bc\approx Bc\left(0\right)[1-{\left(\frac{T}{Tc}\right)}^2]$

where,

$Bc\to$ Critical Magnetic Field

$T\to$ Temperature

$Tc\to$ Critical Temperature

So, the superconductivity state cannot exist in the presence of a magnetic field greater than a critical value, even at absolute zero.