Introduction and Properties of Superconducting Materials

C · Exactly zero electrical resistance

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

C · Zero resistance and the Meissner effect

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

B · Critical temperature $T_c$

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

D · Mercury

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

C · $9\,\text{K}$

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

B · Two electrons bound via lattice vibrations

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

A · 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.

C · 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.

A · 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.

C · 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.

C · Persistent surface currents

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

A · 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.

C · 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.

A · Low-temperature superconductivity via phonon-mediated pairing

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

B · 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.

D · 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.

C · 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.

D · 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.

B · 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.