PBS Space Time

Superconductive materials seem miraculous.

Their resistanceless flow of electricity has been exploited in some powerful wayslike for super-strong magnets used in MRIs, particle accelerators and fusion plants.

And then theres, 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 thats 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 its worth a deeper dive into superconductivity.

But before even that, lets 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 chargea currentwill flow.

In a regular conductor, current doesnt 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 heatatoms 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 temperaturezero jigglingdo you get zero resistance.

The universe rarely satisfies our most simplistic extrapolations.

Except in this casenear-zero resistance IS possiblealthough its 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 relatedboth enable resistantless or frictionless flow.

And they can lead to cool things like permanent vorticeswhether physical whirlpools in superfluids or permanent electrical eddies in superconductors.

Zero electrical resistance by itself is quite nice, but its 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 Meiner 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 whats really happening here?

We can predict whatll 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 theres 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 surfacepersistent currents.

Those little current loops dont 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 theyll 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 transitionclosely 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 dont 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 pressurewater 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 enoughany material only has so many free electrons, so theres 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 superconductorType 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.

Its 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.

Its 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 were 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 werent counting, this is the 3rd superconductor-related Nobel.

Remember that resistance is caused by electrons being jostled out of a nice streamline flowrandom 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 materials crystal lattice.

The electrons sort of ride these waves in pairswhat 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 spinany half-integer spin particles is a Fermion, so that covers the spin-half electrons But Cooper pairs typically have spins of 0 or one: + or - , 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 arent energetic enough to allow that transition.

In quantum mechanics you cant excite a particle only halfway to a new energy stateit is all or nothing.

In the case of the superconductors its 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 doesnt 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 Mller , 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 heliuma 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.

Weve 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 dont actually know, because we dont fully understand how any high-temperature superconductivity works.

Its 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 hasnt been settled on yet.

That means its 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 its 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 includethat 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 thisthey found something cool and reported it.

If anyone is to blame for the hysteria its the media, who went crazy for a paper that hadnt 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.