How Does It
Work?
The property of superconductivity can seem
baffling at the macroscopic scale but is, in
reality, quite a simple phenomenon.
Bardeen–Cooper–Schrieffer theory (BCS
theory), which is currently the most widely
accepted explanation for superconductivity,
states that the lack of
resistance of superconductors arises from
Cooper Pairs; two electrons with a special
bond.
A Touch of Quantum
Mechanics
Let’s take a slight detour to
understand this further. In the
world of the subatomic, there
are two main groups of
particles: fermions and
bosons. We can decide which
category a particle falls
under based on one of its
quantum numbers (a
quantized property of the
In order to understand them, imagine a close-
particle) known as ‘spin’.
up view of a superconductor. You would
Those who are familiar with
notice a structured lattice of cations
the strange names given to
(positively charged ions) surrounded by a
quantum numbers won’t be
number of free-flowing electrons. As the
surprised that spin doesn’t
actually refer to how a
electrons move, they would be attracted to
particle physically spins, spin
any nearby cations wherever they went,
describes one of the intrinsic
creating a sort of constant positive entourage
qualities of particles. That
bunched around the electron that disrupts the
being said, the use of angular
uniform lattice shape. This disruption that
momentum and spin as an
continually follows the electron, also known
analogy is quite useful as we
as a phonon, begins to attract other nearby
will find out later).
electrons with its positive charge.
If a particle has a spin value
of 1⁄2, it is classed as a
fermion, and if it has an
integer spin value, it is
classed as a boson. Our
friends electrons have a spin
value of 1⁄2, so they are
fermions.
Fermions obey a special rule known as the Pauli Exclusion Principle. This rule states
that two fermions in the same quantum system cannot occupy the same quantum state
at the same time.
Electrons in an atom are defined by four different quantum numbers:
Principal quantum number, Azimuthal quantum number, Magnetic quantum number,
and Spin quantum number
According to the Pauli Exclusion Principle, these quantum numbers cannot take the
same values for two electrons in the same quantum system. In the case of two
electrons coexisting in an atomic orbital, the first three of these quantum numbers
would be the same, thus forcing the two electrons to have different values for their
spin. But how is this possible given that electrons must have a spin value of 1⁄2?
It turns out that this is where the aforementioned angular momentum analogy comes
into play. We know that in classical physics, angular momentum is a vector, and thus
has a direction. This same idea is applied to quantum mechanics; particles can ‘spin’
clockwise and anticlockwise. In the case of the electron, spin values both +1⁄2 and -1⁄2
are possible, denoting two different directions of spin. If we go back to our atomic
orbital example, this means an orbital containing two electrons is possible; one has a
+1⁄2 spin, and the other has a -1⁄2 spin.