Introduction and Properties of Superconducting Materials: 19 MCQs with answers

Exactly zero electrical resistance

A superconductor has exactly zero resistance below $T_c$, not merely a small resistance.

Zero resistance and the Meissner effect

Zero resistance and the Meissner effect are the two defining signatures of superconductivity.

Critical temperature $T_c$

The critical temperature $T_c$ marks the phase transition into the superconducting state.

Mercury

Mercury was the first superconductor discovered, with $T_c \approx 4.2\,\text{K}$.

$9\,\text{K}$

Niobium has $T_c \approx 9\,\text{K}$.

Two electrons bound via lattice vibrations

Cooper pairs are two electrons bound together through phonon-mediated interaction.

True

A superconductor has **exactly zero electrical resistance** below its critical temperature $T_c$, and this is a distinct phase of matter, not an extreme of ordinary conduction.

Its resistance is identically zero, a distinct phase of matter

Even the best normal metals retain residual resistance; a superconductor's resistance is exactly zero, making it a distinct phase.

Cooper pairs condense into a state with no scattering channel

Cooper pairs condense into a coherent quantum state where no scattering channel exists, so resistance is zero.

The field is actively expelled from the interior

The Meissner effect actively expels the field, rather than freezing it in place as a perfect conductor would.

Persistent surface currents

Persistent surface currents generate an opposing magnetic field that expels the applied field.

Type I has one critical field; Type II has two

Type I has a single critical field $H_c$; Type II has $H_{c1}$ and $H_{c2}$ and allows flux penetration as vortices.

They survive much stronger magnetic fields

Type II superconductors allow flux penetration as vortices, letting them survive much stronger fields, which is why practical magnets use them.

Low-temperature superconductivity via phonon-mediated pairing

BCS theory explains low-temperature superconductivity via phonon-mediated electron pairing, predicting $T_c$ up to roughly $30\,\text{K}$.

It may snap back to normal conduction above the critical field

Superconductivity requires being below $T_c$, below the critical magnetic field, and below the critical current density simultaneously.

Liquid nitrogen cooling is far cheaper than liquid helium

Crossing $77\,\text{K}$ means liquid nitrogen, far cheaper than liquid helium, can be used for cooling.

It will transition to a normal conducting state and lose its zero resistance.

When a superconductor is exposed to a magnetic field greater than its critical field $H_c$, it cannot maintain its superconducting state and transitions back to a normal conductor, losing its zero resistance.

The magnetic field will penetrate the material in quantized vortices, allowing superconductivity to persist.

In a Type II superconductor, when subjected to a magnetic field between $H_{c1}$ and $H_{c2}$, the magnetic field penetrates the material in the form of vortices, allowing it to retain its superconducting properties.

The material will exhibit zero resistance and expel the magnetic field.

Since the material is cooled below its critical temperature $T_c$ and the magnetic field is below its critical field $H_c$, it will exhibit zero resistance and actively expel the magnetic field, confirming its superconducting state.