The History of Superconductors (Before LK-99)

The History of Superconductors (Before LK-99)

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On July 22th 2023, a paper was posted  claiming the creation of a room temperature,   ambient pressure superconductor. LK-99. The world is now trying to synthesize  the thing and replicate the paper's   findings. The LK-99 Wikipedia  page is keeping a running tally. The quest for a room temperature  superconductor has gone for over a   century. It has seen some amazing highs and lows. In this video let us dive into  the dream and track the 100-year   history of technological progression  towards this mythical substance.

## Superconductivity Resistance is a material's opposition to  an electric current flowing through it. A conductor has low resistance  and so lets electrons easily   move through them. Insulators do not. In previous videos, I have mentioned that all   real world materials have some amount of  resistance. Of course, I was not thinking  

about superconductors - perhaps because I  didn't consider them practical materials. We have known about superconductivity  for over a century now. In 1911,   the Dutch physicist and Nobel prize  winner Heike Kamerlingh-Onnes and his   students started performing experiments at  super low temperatures using liquid helium.

At the time, people speculated  on what might happen to the   electrical resistance of metals  as they approached absolute zero. Sir James Dewar - inventor of  the vacuum flask - theorized   that electrical resistance would  infinitesimally approach zero. Lord Kelvin on the other hand thought that it  would hit some minimum before quickly rising   towards infinity. His reasoning being that the  electrons would stop moving as it gets colder. So Kamerlingh-Onnes did an experiment  with mercury. Why mercury? Because   it was the only metal they had  available in sufficient purity. And unexpectedly, the experiment showed  mercury's resistance falling to zero   just below 4.2 Kelvin. Kamerlingh-Onnes  named this state "superconductivity".

The temperature at which the material achieves  this state is called the "transition temperature". ## Disappointment The first superconductors triggered  a great deal of excitement. It was the electrical engineer's wet dream  - a future free of electrical resistance. In   1913 Kamerlingh-Onnes traveled to Chicago  and spoke of building a helium cooling   plant in pursuit of smaller, more efficient  motors or generators using superconductivity. Optimistic, Kamerlingh-Onnes continued his  work. He discovered that lead's transition   temperature was 7.2 Kelvin -  slightly higher than mercury.

In 1914, he then made a coil of lead wire, froze  it to 7.2 Kelvin and passed an electrical current   through it. But to his disappointment,  the coil lost all of its superconducting   powers when the magnetic field got to be  about as strong as a horseshoe magnet. As it turns out, superconductivity requires not  only a low enough temperature but also a low   enough magnetic field and a low enough amount of  current passing through it - current density. The   material can only maintain their superconductivity  if all three sit within the parameters. Today, pure metal or simple alloy  superconductors are known for this   fundamental weakness. These are not good  for practical industrial use. After all,  

electrical systems encounter  magnetic fields all the time. Kamerlingh-Onnes must have been  crushed. He was one of the first   scientists to have fallen victim to the sirens  of superconductivity. He would not be the last.

## A Curiosity Research on superconductor  materials then slowed for a decade. The need for liquid helium cooling stood as  a formidable financial barrier to continued   commercialization. And there was no  big market need pushing for it either.   Many people worked on the concept  simply because it fascinated them.

Through them, pioneering science was  done in the 1930s. We discovered three   more superconductors - tantalum at  4.4K, thorium at 1.4K, and niobium.   Which has the highest transition  temperature yet known at 9.2 Kelvin. In 1933 we discovered the Meissner Effect.  This is where a superconductor expels all  

magnetic fields as it transitions  into a superconducting state. In other words, the magnetic field  will go around the superconductor   material rather than permeate through  it. This explains why all the videos   you see of these things are  of them floating on magnets. And then a second major discovery that  went under the radar. In the late 1920s,   the Soviet Ukrainian scientist Lev  Shubnikov worked at a cryogenic lab   in the Netherlands. In 1930, he returned to  the Soviet Union and continued his research. There, his experiments discovered what are now   called Type II superconductors. These  are quite different from the pure metal  

superconductors - today called "Type I"  - that so disappointed Kamerlingh-Onnes. Type I superconductors fall back to normal  behavior when they breach their magnetic limit. But Type II superconductors can display a mix  of ordinary and superconducting properties   at certain magnetic limits. So Type IIs can  potentially be made to be far more tolerant of   higher magnetic fields than Type Is. These are  almost always complicated alloys or compounds. Sadly Shubnikov never got to see the benefits  of his work. At the height of the Great Purge   in 1937, the NKVD targeted him for his  foreign connections. They arrested him,  

accused him of espionage and shot him.  It would be many years before the rest   of the superconductor community recognized and  capitalized on the significance of his work. ## Post-War World War II had shown the entire  world the economic and military   value of turning sciences into technologies. Shortly before the war, MIT professor  Samuel Collins invented the Collins helium   liquefier system. Widely introduced after the war,   the Collins made liquid helium more  available for superconductor research. Yet the search for higher temperature  superconductors - perhaps even a room   temperature one - remained quite  difficult. We did not have good  

theory to tell us where to look. So most  people started with metals with already   low amounts of resistivity -  copper, gold, silver so on. In the early 1950s, a pair of scientists  - B.T. Matthias and John Hulm - took a   different approach. They embarked on a  systematic search via experimentation. The two pioneered the method of finding  superconductors via the Meissner Effect - the   superconductor’s tendency to expel magnetic  fields. They applied an external magnetic  

field to a sample as it was being cooled and  measured whether it expelled that magnetic field. Matthias and Hulm systematically  moved through the periodic table,   eventually testing over 5,000 compounds. This was how they discovered the  A3B superconductors - 3 niobium   atoms and 1 atom of either silicon, tin, or  aluminum. Yes, it is a terrible name. Anyway  

these would hold the record for the highest  transition temperature - 23 K for 28 years. Matthias eventually formulated the "Matthias  Rules" for finding a superconductor - which   like the Pirate Code - are more what  you'd call guidelines than actual rules. Rule number six was "Stay away from theorists!" ## BCS As you might expect, Matthias  looked down on superconducting   theory - calling them just "descriptions". But the physics world wanted a theory to  help guide their way forward. And in 1957,   three scientists proposed a  theory that did just that. John Robert Schrieffer, Leon Cooper one of the  namesakes of Sheldon Cooper, and John Bardeen.

Recognize the name of that last guy?  Bardeen won his first Nobel prize in   Physics as one of the three inventors of the  transistor. This theory won him his second. The theory was named BCS -  named after its three inventors.   BCS also stands for Bowl Championship  Series. It was a system that helped   select the match-ups in top tier  American football college divisions. Cal hasn't won a football championship  since before World War II so I don't   know why I cared to mention this. Let's move on.

Anyway, BCS theory suggests that when a  material enters a superconducting state,   their electrons pair up. This is weird  because normally they repel each other. But in that superconducting state, we  have the influence of a new force called   phonons. Phonons are measurable, energized  vibrations in the material's crystal lattice. Thanks to the attractive  forces from these phonons,   the electron pairs create a brand  new form called "Cooper Pairs".

Cooper Pairs are exempt from  the Pauli Exclusion Principle,   which explains why two solid objects can't be  in same place at the same time. And also why   all matter does not simply collapse into  a single point despite everything being   mostly empty space. I think I asked  my mom that once when I was a kid. Thusly, Cooper Pairs allow these  electrons to act differently than   they usually do - traveling through the crystal  lattice without resistance. This part of the   theory has generally held over the years -  Cooper Pairs are key to superconductivity. The energy bonds holding these Cooper Pairs   are quite weak. So they can be easily  shaken apart by thermal energy. Thus,  

BCS theory does a good job of explaining  superconductivity at low temperatures. Superconducting materials whose behavior is   explainable by BCS theory are known  as "conventional superconductors". By 1960, we knew about 35 elements and  a thousand different alloys to have   shown superconductivity under certain conditions. ## Superconducting Wire In 1961, scientists discovered a  series of niobium-based alloys that   retained their superconductivity in the  presence of a stronger magnetic field.

These were called high-field superconductors.  And they theoretically allowed us to make   large superconducting wires that performed  far better than traditional copper ones.   Such wires can produce very powerful  magnetic fields using very little power. High-field superconductors achieved the original  dream of Kamerlingh-Onnes many decades ago. But  

even so, it took the industry until early 1970 to  reliably and viably produce superconducting wire. Such superconducting wires eventually became the   widely accepted method for producing  MRIs. MRIs with superconducting wire   remain the single biggest and most important  commercial use for superconductor materials.

## The Breakthrough In 1986, two researchers at an IBM lab in Zurich,   Switzerland were collaborating  on a study on superconductors. Alex Muller was a senior research fellow at IBM  who received the job basically as a stopover   before retirement. He had gotten interested in  the field after spending 20 months in New York. There, he observed IBM's massive  Josephson Computer Technology   project. That was a $300 million effort  to build an ultra-fast computer based   on superconducting digital logic  switches. It ultimately failed.

I will talk about Josephson and his  learnings in a future video about   superconducting circuits and their computers. Upon returning to Zurich in 1983, Muller  recruited a colleague Georg Bednorz to   look for superconductors in oxides. After a  couple frustrating years searching around, they came across an article from a team  of French scientists describing metal-like   electrical conductivity at a rather high  temperature. Like 900 degrees Celsius high. That was interesting. Ceramics are traditionally  known to be electrical insulators. After further  

experiments, the duo produced a ceramic compound  of barium, lanthanum, copper, and oxygen that   achieved superconductivity at 35 Kelvin (-238  degrees Celsius) - a legit breakthrough. As is so often in the land of superconductors,  this discovery came as a total surprise. Bednorz   and Muller first kept their results away from  even their colleagues and their employer. They decided to first publish their results  at a small but reputable German physics   journal (Zeitschrift fur Physik) apparently  because they personally knew its editor. In April 1986 they submitted the paper, which  was deliberately given a very disarming title:   "Possible High Tc Superconductivity  in the Ba-La-Cu-O System".  

They chose this title because the two were  not sure about their result. For instance,   the Meissner effect had not  yet been measured in it. After some time, the paper was finally  published in September 1986. But rumors  

had been circulating since July, and they shook  the scientific community like an earthquake.   Not only because ceramics like as I said  before were not seen to be good conductors. But also Bednorz and Muller were seen as outsiders  to the superconductor community - working in a   backwater of the IBM research machine. One  prominent member of the community - jaded   by false claims over the years - was tempted to  simply throw the paper away when he first saw it. These new ceramics also violated the previously  established rules of superconductors. It did  

not follow the Matthias Rules. And not only  that, it highlighted a hole in BCS Theory. By then, physicists had largely adopted  BCS as a reasonably rigorous explanation   of superconductivity. And because  BCS only worked at low temperatures,   superconductivity in general was regarded as  an exclusively low-temperature phenomenon. This discovery heralded a new class  of ceramic oxide superconductors with   higher transition temperatures. Higher  than ever before seen - referred to as  

"High Temperature Superconductors" or High-T. Research teams around the world raced to  replicate the results. In late November 1986,   some 2 months after publication, the Asahi Shinbun  newspaper reported that a team led by Shoji Tanaka   of the University of Tokyo had successfully  repeated the experiment, confirming the results. ## Cuprates A few weeks after that, Paul Chu of the  University of Houston replicated the results.

Chu's results made the  headlines. More impressively,   he believed that there were other materials  with even higher transition temperatures. A race for new superconductors soon  emerged, with the big line being   77 Kelvin, the temperature of liquid  nitrogen. Various groups in Beijing,  

Tokyo, and the United States  traded papers back and forth. Then in January 1987, Dr. Chu  discovered a superconductor with   a transition temperature above 90 Kelvin.  Breaking the so-called nitrogen barrier.

Chu held off publishing for as long as possible  as he tried to first patent the material. Finally in February 1987, he announced  his marvelous technical breakthrough. This class of high temperature superconductors  is sometimes called 1-2-3 superconductors   because they have 1 atom of Yttrium, 2  atoms of Barium, and 3 atoms of copper.   There is also some oxygen attached but  whatever it messes with the cool naming. These things are also more generally  referred to as YBCO superconductors,   cuprate superconductors or  rare-earth cuprate superconductors. Oh hey, more rare earths stuff. Wonder  who’s the world leader in those?

Anyway, all cuprate superconductors share the same  structure. They are big layered compounds kind of   like cakes. Layers of copper-oxygen separated  from one another by insulating oxide layers. The superconductivity happens within  those thin copper-oxygen layers. We  

aren't exactly sure how it works.  Cooper pairs are probably involved,   but differently from how it is in BCS theory  - making them unconventional superconductors. Cuprates broke the nitrogen barrier, which  meant that we can use liquid nitrogen to   cool them rather than liquid helium - which is  what we used before. This has significant cost   benefits. Liquid nitrogen costs about 50 cents  per gallon while liquid helium, $24 per gallon.

On March 15th, the American Physical  Society held a special symposium   on high temperature superconductors  specifically to talk about the results. Several thousand physicists gathered at the Hilton  at what they now call the Woodstock of Physics. The highlight was a special evening  session dedicated to the discovery - held   on Wednesday night in the Hilton Grand  Ballroom. It was filled to the brink.   The session started out at 7 pm and ended  at around 3 am - a magical 8 hours long. ## A Gold Rush A "gold rush" was on for new  materials based on this breakthrough.

Superconductor related headlines hit the media  like the New York Times. BusinessWeek cried out,   "Superconductors! More important than  the light bulb and the transistor". The fever swept over Japan. Multiple ministries   rapidly directed R&D spending  into this new category. By 1988,   MITI's single biggest R&D expenditure  was on high temperature superconductors. The United States too. The White  House even held a Presidential  

Conference on superconductivity in July 1987. Reagan proposed an 11-point plan to build  new industries around this new technology.   Tens of millions of dollars of federal  R&D were funneled into superconductors. On the corporate front, IBM and Bell Labs  faced off with one another. They joined the   fray alongside a bunch of new startups with buzzy  names like American Superconductor Corporation,   Conductus, Illinois Superconductor Incorporated,  and Superconducting Technologies Incorporated.

## Hype Fades The discoveries kept coming throughout 1987. By playing around with the  original barium, lanthanum,   copper, and oxygen ceramic formula,  researchers were able to find cuprate   superconductors with increasingly  higher transition temperatures. But the gaps between science and technology can  be quite large and take a lot of time to overcome.   As it turns out, transition temperature was  only just one of the factors that engineers   have to consider when building  commercially viable systems. For instance, take superconducting wires  for MRI machines. The original promise of  

High-Temperature superconductors was  that we can swap out more expensive   liquid helium cooling systems for cheaper  liquid nitrogen ones. Should be Easy right? But as Kamerlingh-Onnes found out long before,   superconducting wires need to also be able  to carry high current densities and withstand   very high magnetic fields. In other words,  they need to meet the critical parameters. The cuprate high temperature superconductors do  indeed have much higher transition temperatures   but their other critical parameters  are not as dramatically higher. There was also a substantial  materials engineering problem.   Cuprate superconductors are ceramics  - complicated materials that are very   brittle to work with. This by itself  limited their usefulness as a material.

They also work differently. If you  recall, the cuprates are layered cakes,   and all the superconducting happens within  those single flat layers of copper and oxygen. This imposes big challenges on how we might  achieve this superconductivity. If we want   to produce a wire using this material  - it needs to be done in a specific way   in order to maintain good alignment  over potentially hundreds of meters.

So it took a substantial amount of time  to eventually engineer wire using High   Temperature superconductors. They do exist and are   used for things like cables for power  usage, but that took years to develop. And there are certain applications where the  High-T cuprate superconductors still fall short   of established technologies. Niobium Titanium -  established all the way back in 1970 - continues   to dominate superconducting wires for MRIs. Mostly  due to their superior mechanical properties. The slow speed of commercialization  dampened enthusiasm. And unfortunately,   high temperature superconductors could not  uncover that golden "killer app" that would   stimulate more investment into the space.  Over time the hype receded as it became  

clear that the cuprates did not present a way  towards room temperature superconductivity. The scientific community did not forget  Bednorz and Muller though. Just 16 months   after their discovery, the two won the Nobel Prize   in Physics - the record for the fastest  recognition in Nobel physics history. ## Iron & Beyond Superconductor research continued for another   high-temperature alternative  to the cuprate superconductors. In 2001, a Japanese team discovered an interesting  non-cuprate superconductor - magnesium diboride.  

Its transition temperature was 39 Kelvin. Far below the cuprates, but at least  much higher than the traditional low   temperature superconductors. Unfortunately its  behavior was conventional BCS, so a dead end. The next significant high temperature  discovery came in February 2008,   when Dr. Hideo Hosono reported a Fluorine-doped   iron-based superconductor with a  transition temperature of 26 Kelvin. This opened up a new branch of  high temperature superconductors   separate from the cuprates. At the  outset, there was a lot of hope. A few months, another iron superconductor was  found with a transition temperature of 56 Kelvin.  

But progress there has since stalled. And then recently in 2015 a team in  Germany published a paper claiming   superconductivity in sulfur hydrides  at over 200 Kelvin and high pressure. Interesting, but again nothing that  has real world industrial implications. And there have been some … questions about  superconductivity in these sulfide hydrides.  

One recent controversial room temperature claim  was recently retracted by Nature Magazine. ## Conclusion The search continues for a room  temperature superconductor. Or even a relatively high temperature  superconductor with fewer engineering   compromises than what we already have. Is LK-99 really it? I don't know. But will  it really change everything if it was? Is   it really the next transistor? It  is hard for me to believe that. When we dream of room temperature  superconductors, we talk about things   like levitating trains, compact MRIs,  economical fusion energy, and the like.

Well, they have been citing these things  since 1986. Many applications are already   technically possible with existing  low-temperature superconductors.   But there are real economic  reasons why we don't have these. For instance, magnets for Maglev trains. The  magnets are not a significant portion of the   financial cost for these machines. It is  tied to the cost of the land, the labor,  

the extensive planning, and the construction. And then there is always the  chance that a room temperature   superconductor - whether LK-99 is it or not  - cannot compete with existing solutions. Look at the MRI. Intuitively, you might think this  new generation of High-Temperature cuprates would  

sweep out the old superconducting wire tech  used for those MRIs like yesterday's trash. But it turned out to be far more subtle  than that. Even today 30 years later,   80-90% of today's MRI machines are still  made with good old Niobium Titanium. Just working these things into  wires might take years. Remember,   these things are layered cakes. Imagine  making a cake hundreds of miles long.  

Real world applications might be  years and millions of dollars away. But hey, I hope I am wrong. I love the  excitement around LK-99. No matter the   outcome, the fascinating science of  superconductivity is having another   one of its unique moments. This only happens  ever so often in the world. Let's enjoy it.

2023-08-08 16:14

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