GATE 2026 PH – Question 31

Solid State Physics · superconductivity: type-I and type II superconductors, Meissner effect, London equation, BCS theory, flux quantization. · 1 mark · Numerical answer

Consider a metal-superconductor junction connected to a dc voltage V. At T< Tc, where Tc is the superconductor’s transition temperature, the current I versus V behavior of this junction is shown schematically in the figure below. If the superconducting energy gap is D meV. The value of D (rounded off to one decimal place) is _____

Junction current begins at threshold voltage Vth = 1.0 mV.

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Correct answer: 2.0 to 2.0

Explanation

The current threshold gives $\Delta=eV_{th}=1.0$ meV. The energy gap asked for in this paper is the full pair-breaking gap $2\Delta$, so it is 2.0 meV. Reporting only the single-quasiparticle energy gives half the keyed answer.

Official GATE 2026 answer key: https://gate2026.iitg.ac.in/doc/download/2026/Keys/PH_Keys.pdf