Effect of magnetic field on superconducting transition temperature Tc Consider a superconductor cooled near its transition temperature. What is the general effect of applying an external magnetic field on the observed transition temperature?

Introduction / Context:
Superconductivity is suppressed by magnetic fields. Engineers must understand how fields reduce the temperature margin for superconducting magnets, MRI systems, and power devices.

Given Data / Assumptions:

• Type I or type II superconductor in the vicinity of its transition temperature.
• External magnetic field is applied steadily.
• We consider the general trend, not special cases like strong pinning or mixed states far below Tc.

Concept / Approach:
In type I superconductors, the thermodynamic critical field Hc decreases with temperature, often approximated by Hc(T) ≈ Hc(0) * [1 − (T/Tc(0))^2]. Equivalently, at fixed H, the transition occurs at a lower temperature than Tc(0). In type II materials, Hc2(T) shows a similar suppression trend: higher fields push the transition to lower temperatures.

Step-by-Step Solution:
Recognize that superconductivity is a delicate state destabilized by magnetic fields.At fixed H, solve the critical condition H = Hc(T) (type I) or H = Hc2(T) (type II).Because Hc(T) and Hc2(T) decrease with T, a nonzero H forces the loss of superconductivity at a temperature below the zero-field Tc.

Verification / Alternative check:
Experimental phase diagrams T–H for materials like Pb (type I) and NbTi or Nb3Sn (type II) show Tc(H) < Tc(0) for H > 0.

Why Other Options Are Wrong:

• Increase or no effect contradicts standard phase boundaries.
• Unpredictable behavior is incorrect; the trend is systematic.
• Room-temperature superconductivity is not achieved by applying a magnetic field.

Common Pitfalls:

• Confusing critical current or critical field with Tc; all three are interrelated but distinct.
• Ignoring mixed states in type II; despite vortices, Tc still decreases with applied field.

Final Answer:
It decreases the transition temperature