What is a Cooper pair?

A Cooper pair refers specifically to a pair of electrons that interact in such a way that they can move through a superconductor without experiencing resistance. This phenomenon occurs at low temperatures where the electron pairs form due to an attractive interaction facilitated by lattice vibrations, known as phonons.

In the superconducting state, these paired electrons can condense into a ground state, leading to the remarkable property of zero electrical resistance. This pairing is crucial for the formation of the superconducting state and explains many aspects of superconductivity, including the expulsion of magnetic fields, known as the Meissner effect.

The other options do not pertain to the concept of a Cooper pair. For instance, protons in a hydrogen molecule, positive ions in gas, and photons in electron transitions each relate to different areas of physics, such as chemical bonding and atomic transitions, rather than superconductivity and the unique existence of Cooper pairs.