Superconductive materials seem miraculous. Their resistanceless flow of electricity has
been exploited in some powerful ways—like for super-strong magnets used in MRIs, particle
accelerators and fusion plants. And then there’s, their bizarre ability to levitate in magnetic
fields. But the broader use of superconductors is limited because they need to be cooled
to extremely low temperatures to work. But what if we could produce superconductivity
at room temperature? It would change the world.
Recently there has been a flurry of activity around the material known as LK-99. A group
of researchers claimed to have discovered a phenomenon as coveted as cold fusion, perpetual
motion machines, and decent frozen pizza. They claim it exhibits superconductivity
at room temperature. Now a superconductive material has zero electrical resistance, but it
normally requires extremely low temperatures to work. A room temperature superconductor would
have incredible implications. Levitating cars on magnetic highways, super powerful magnets
in every-day applications, and overclocking your computer to ridiculous degrees without
melting it and that’s just off the top of my head.
Sadly, LK-99 did not turn out to be a room-temperature
superconductor. It was sort of faking it.
But does that mean the dream is dead? How plausible is room-temperature superconductivity?
How did LK-99 managed to mimic the phenomenon well enough to trick its research team?
Now we talked about the underlying phenomenon behind superconductivity when we talked about
quasi-particles and Bose-Einstein condensates, but we never got to the really cool stuff,
so it’s worth a deeper dive into superconductivity.
But before even that, let’s remind ourselves
about regular, non-super conductivity. So a conductor is just a material in which
some of the electrons are free to move. Normally we think of electrons as being bound into
their atomic orbitals. Sometimes the outer shell electrons are shared between atoms in
molecular bonds. In conductors, there are either extra outer-shell electrons or electron
holes that can jump pretty freely between
adjacent atoms. Add a charge or voltage differential
across the coxnductor and a stream of electric charge—a current—will flow.
In a regular conductor, current doesn’t flow completely freely. The conduction electrons
can interact with each other, or with the non-conducting layers of electrons. All that
bumping around manifests as heat—atoms begin jiggling, electrons move randomly. This
saps energy from the current and results in resistance. In fact, the hotter the material
gets the higher the resistance because the random motion inhibits a nice smooth flow
of electrons.
And cooling a conductor decreases resistance. It might be reasonable, then, to wonder how
low resistance can get. For example, if you get to zero temperature—zero jiggling—do you
get zero resistance. The universe rarely satisfies our most simplistic extrapolations. Except
in this case—near-zero resistance IS possible—although
it’s not for simple reasons.
In 1911, the Dutch physicist Heike Kamerlingh Onnes followed exactly this intuition. He
managed to cool a wire or mercury below 4.2K - roughly the temperature of deep space. At
that temperature all resistance in the mercury wire vanished. This was the first observation
of superconductivity. Onnes also discovered superfluidity in helium on the way to making
temperatures low enough for superconductivity, and scored a Nobel prize for his foundational
work in low temperature physics.
Superconductivity and superfluidity are closely related—both enable resistantless or frictionless
flow. And they can lead to cool things like permanent vortices—whether physical
whirlpools in superfluids or permanent electrical eddies in superconductors.
Zero electrical resistance by itself is quite nice, but it’s not as flashy as the fact
that superconductors can make things fly.
Well, can make them hover. Fast forward to 1933. Lots of
people were messing around with this fun new physics toy, including Walther Meißner and
Robert Ochsenfeld. They were the first to make a superconductor float above a magnet.
This is the Meissner-Ochsenfeld effect, or more commonly just the Meissner effect - sorry
Robert.
OK, so what’s really happening here? We can predict what’ll happen to a superconductor
inside a magnetic field if we know just a few things: 1) electric currents generate
magnetic fields 2) magnetic fields also cause electric currents and 3) the electric current
caused by magnetic field will itself generate another magnetic field that works to counteract
the original field.
If you apply a magnetic field across a normal material, the induced electric current will
somewhat counteract that field, but there’s a limit because the resistance of
the material will limit the strength of the induced electric current.
But if you apply a magnetic field across a medium with no resistance, the induced electric
current can get strong enough to completely cancel out the initial magnetic field. In
fact, all free electrons in the superconductor will quickly spiral along the magnetic field
lines and set up vortices of current at the surface—persistent currents. Those little
current loops don’t take any energy to maintain. They act like little electromagnets that
exactly cancel the incoming magnetic field
within the material and we call these shielding currents.
OK, so far so good. But why all the floating? Well, push together the same pole of a pair
of bar magnets and they’ll repel, and we can think of the surface of the superconductor
as being layered with electromagnets. Another way to think about it is that the magnetic
field lines expelled from the superconductor pile up on the outside, causing an intense
magnetic pressure.
This effect is nicely described by the London equations, figured out by the German brothers
Fritz and Heinz London in 1935. They found that in a material with zero resistance, magnetic
field had to exponentially decay inside it. These guys were able to explain the Meissner
effect based on the assumption of perfect conductivity, without having any idea why
superconductors have no resistance.
One other subtle assumption in the London brothers’ theory is that the system should be in
an equilibrium state. They figured out how things should look after the electrons finish
rearranging themselves into whatever final configuration of currents the magnetic field
induced. They took time out of the equation. The next advance was made by putting it back
in. In particular, by thinking about the transition
into and out of the superconducting state.
That change is what we call a phase transition—closely
related to the more familiar state transitions
between solids, liquids and gases. Russian physicists Vitaly Ginzburg and Lev Landau
realised that there are certain general rules that all systems undergoing a phase transition
have to obey, even if we still don’t know the actual nature of the post-transition state.
So, in 1950, still not knowing what caused superconductivity, we got the Ginzburg-Landau
theory. This new mathematical description of superconductivity predicted everything
that the London theory predicted, but also some new weird stuff. And this new way
of looking at superconductors yielded another Nobel prize, shared between a few contributors.
To start with, they predicted that there are two types of superconductor that behave very
differently around the phase transition. For regular states of matter, the phase transition
point depends on both temperature and pressure—water
boils at a lower temperature on a mountain
top. But the phase transition point into superconductivity
depends on temperature, pressure AND magnetic
field. They have to be below a certain critical temperature, above a critical pressure, and
below a critical magnetic field. The old London theory allowed any amount of magnetic field
to be expelled by the superconductor, but the new Ginzberg-Landau theory said that if
the field gets too strong then superconductivity will break down. And fair enough—any material
only has so many free electrons, so there’s a limit to the strength of the shielding current.
With this new understanding came the prediction of brand new superconductor behaviours. For
example, that there should be two broad types of superconductor—Type I and Type II.
Type 1s are the delicate snowflakes of superconductor
world. If you put one in a magnetic field
and crank the field just a tiny bit over the
critical value for that material, its superconductivity
will vanish almost entirely and instantaneously. Type IIs on the other hand are stubborn and
have trouble letting go of the past. When you crank their magnetic field beyond the
critical value they cling to some of their superconductivity by entering an intermediate
state called a vortex state. Remember the vortices of electrons that made up the shield
current on the surface of a superconductor? These
descend into the material again, threaded by growing magnetic
field lines. This leads to a cool new phenomenon called flux
pinning.
In this vortex state, the magnetic field can
penetrate the material, but only in very restricted
ways because it has to line up the vortices within the material. A superconductor in a
vortex state will both levitate and be fixed in a particular orientation in a magnetic
field, as if suspended by wire in an art gallery.
It’s worth taking a second to appreciate that all of these insights and predictions
about the behaviour of superconductors came without knowing why superconductivity happens
in the first place. They came from thinking about how the universe must behave in order
to be internally consistent. But I guess that sort of describes all of physics. It’s incredible
how far we can get by just insisting that the universe behave itself.
But still, it sure would be nice to know how conductivity actually gets super. Because
if we know that maybe we can figure out whether room temperature versions are possible.
And if you watched our quasi-particle episode you may remember the answer, but let me give
you a refresher. So we’re now in 1957— John Bardeen, Leon Cooper, and John Robert Schrieffer
came along and changed everything. Together these three developed what is now known as
the BCS theory of superconductivity, for which they won the 1972 Nobel prize and if you weren’t
counting, this is the 3rd superconductor-related Nobel.
Remember that resistance is caused by electrons being jostled out of a nice streamline flow—random
collisions leading to exchanges of energy. Superconductors are in a strange state that
makes those energy exchanges impossible. As the material is cooled down, traveling electrons
set up these patterns of oscillations in the nuclei of the material’s crystal lattice.
The electrons sort of ride these waves in pairs—what we call Cooper pairs. Now electrons
are of the Fermion particle type which means no two electrons can occupy the same quantum
state. That type is determined by the electron spin—any
half-integer spin particles is a Fermion,
so that covers the spin-half electrons
But Cooper pairs typically have spins of 0 or one: ½ + ½ or ½ - ½, which makes
them Bosons, and bosons behave very differently to fermions. Any number of bosons can occupy
the same quantum state. So our material gets cold enough and these coherent oscillations
allow Cooper pairs to form,
all of the Cooper pairs drop down to the lowest energy level. Now from
there it should be possible to excite a Cooper pair back
up to a higher energy state through a collision.
But in a cold material, collisions aren’t energetic enough to allow that transition.
In quantum mechanics you can’t excite a particle only halfway to a new energy state—it is
all or nothing. In the case of the superconductors it’s nothing. No excitations are possible,
so no collisions happen. Hence, no resistance.
So finally we understand how superconductors behave, and why they behave that way. Maybe
we can figure out whether room temperature superconductors are even possible. At first
glance it doesn’t look good. Coldness seems necessary to get electrons into a Bose-Einstein
condensate of Cooper pairs and to avoid knocking them out of that state. And for a while it
was thought that superconductivity could only occur in pretty extreme cold.
But then in 1986 a couple of IBM researchers, Georg Bednorz and K. Alex Müller , managed
to create superconductivity in a ceramic material at 35 Kelvin, and others soon managed to get
the critical temperature up to 93K. That was a game-changer because it meant superconductivity
could be produced using liquid nitrogen rather than liquid helium—a much cheaper option
that opened up the technology to industrial use. Bednorz and Muller got the Nobel for
their demonstration of the newly emerging field of high-temperature superconductivity.
And we've actully stopped counting superconductor nobel prizes and this wasn't even the last.
We’ve now managed to induce superconductivity in exotic ceramics with temperatures as high
as 133 K — and higher still for materials under extreme pressure. But this is way colder than
the room temperature superconductivity claimed by the recent result, so it still requires
expensive and cumbersome cooling apparatus. So is room temperature superconductivity
possible? We don’t actually know, because we don’t fully understand how any high-temperature
superconductivity works.
It’s certainly not quite as simple as Cooper pairs riding waves of oscillating nuclei.
For example, a common class of high temperature superconductor involves thin layers of copper
oxides interspersed with other materials that provide free electrons. There are different
stories for how the Cooper pairs form, and a single mechanism hasn’t been settled on yet.
That means it’s very difficult to know the maximum temperature for which superconductivity
is possible.
OK, so you and I now know quite a lot about superconductors.
Maybe it’s time to debunk the recent claim of
this phenomenom in LK99.
So the team claimed a massive drop in resistance below a temperature of
127 Celsius —so actually quite a bit hotter than room temperature.
And the material also appeared to partially
levitate in a magnetic field. But since then, a number of teams have synthesized the same
material and report no evidence of superconductivity.
Speculations about why the original team thought
otherwise include—that their sample had
an impurity that made it just very conductive (regular conductive),
but not superconductive.
And that the levitation was probably just regular ferromagnetism, not the Meissner
effect. Not that the original researchers should be criticized for this—they found
something cool and reported it. If anyone is to blame for the hysteria it’s the media,
who went crazy for a paper that hadn’t even been peer reviewed. But overall this was a
good thing, because it gave us a change to
talk about superconductivity— which is surely one of the
coolest-no pun-phenomena in our generally phenomenal space time.
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