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
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