Sven Erik Matzen

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Resistance Is Futile: Superconductivity from Onnes to BCS to the Room-Temperature Dream

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Quantum Physics · 2026-07-23

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The Hook: A Current That Never Stops

Picture a ring of metal with an electric current circulating in it. In the world we know, such a current is a fleeting thing: switch off the source and it dies away within fractions of a second, because the metal's electrical resistance relentlessly converts the electrons' motion into heat. Resistance is the friction of electricity, and friction always wins in the end.

Now cool this ring down to a few degrees above absolute zero. At a certain temperature, characteristic of the material, something happens that defies all everyday experience: the resistance does not fall gradually but drops abruptly to exactly zero. Not to "very small," not to "negligible" – to zero. A current once set flowing in such a ring keeps circulating afterward, with no source, without measurable loss. Experiments with superconducting rings have shown that such persistent currents do not decay over years; theoretical estimates speak of timescales that exceed the age of the universe by many orders of magnitude. It is, paradoxical as it sounds, a perpetual motion machine of current – allowed not despite quantum mechanics but because of it.

This state is called superconductivity, and it is one of the few cases in which the strange laws of the quantum world do not remain hidden inside individual atoms but become macroscopically visible – large enough to hold a piece of metal in your hand and watch it hover above a magnet. Superconductivity is at once a triumph of fundamental physics, a treasure trove for engineers (it sits inside every MRI scanner today), and a field in which perhaps the greatest open question of solid-state physics has waited unsolved for almost forty years. And, as we will see, it is also a case study in scientific integrity – about the thin line between the great dream and the great fraud.

This article takes you along the entire arc: from a chance measurement in the Leiden laboratory of 1911, through the elegant explanation of why electrons overcome their own repulsion to form pairs, to the materials that today power fusion reactors and quantum computers – and to the question of whether we will ever have a superconductor for the kitchen table.

Part 1: The Vanishing Resistance – Leiden, 1911

The history of superconductivity begins not with a theory but with a race for the cold. Around 1900, liquefying gases had become the great experimental challenge. One gas after another had been conquered, but helium, the lightest noble gas, resisted every attempt most stubbornly. The Dutchman Heike Kamerlingh Onnes had built the world's most powerful cryogenics laboratory in Leiden, under the motto "Door meten tot weten" – "Through measuring to knowing." On July 10, 1908, he achieved what no one had managed before: he liquefied helium, reaching temperatures of about four degrees above absolute zero (roughly 4 kelvin, i.e. −269 °C). For this feat he received the Nobel Prize in Physics in 1913.

Liquid helium opened up an entirely new temperature regime, and the obvious question was: what does the electrical conductivity of metals do when you cool them that deeply? At the time, physicists argued over three possible answers. Some expected the resistance to keep falling and, in the limit, vanish. Others, among them the great Lord Kelvin, suspected the opposite: at very low temperatures the electrons themselves would "freeze," so the resistance would tend toward infinity. A third camp expected a constant residual resistance from impurities.

Onnes chose mercury as his test object, because it could be made extremely pure by repeated distillation – he wanted to rule out the effect of impurities. On April 8, 1911, his team, which included the technician Gerrit Jan Flim and the student Gilles Holst, measured the resistance of a fine mercury capillary as it slowly warmed up. The result was so unexpected that Onnes at first noted it soberly in his lab book: "Kwik nagenoeg nul" – "Mercury practically zero." Below about 4.2 kelvin the resistance was not small but, to within measurement accuracy, simply gone. It did not fade away gradually but disappeared in a sharp jump that occurred within hundredths of a degree.

Onnes quickly recognized that he was not dealing with a gradual dying-away but with a new, qualitatively distinct state of matter. He soon coined the term "supraconductivity." The characteristic temperature below which the effect sets in is today called the critical temperature or transition temperature (\(T_c\)). For mercury it is 4.2 K, for lead 7.2 K, for niobium – long the most useful element – 9.3 K. Onnes also found two further limits: too strong a magnetic field (the critical field) and too high a current density (the critical current density) also destroy superconductivity. These three quantities – critical temperature, critical field, critical current – define to this day the "existence space" of every superconductor.

Part 2: More Than Just Zero Resistance – the Meissner Effect (1933)

For twenty-two years, superconductivity was essentially regarded as "resistance equals zero" – as a metal that had become a perfect conductor. But in 1933, Walther Meissner and his collaborator Robert Ochsenfeld in Berlin discovered something that turned this view on its head and, I am of the opinion, first exposed the real core of the phenomenon.

They were studying how the magnetic field behaves around a superconducting tin and lead cylinder, and stumbled on a surprising behavior: a superconductor pushes an externally applied magnetic field completely out of its interior. It becomes a perfect diamagnet. This sounds harmless at first but is a sensation – and the reason becomes clear only when you think through the difference between a superconductor and a merely "perfect conductor."

A hypothetical perfect conductor (only zero resistance, nothing else special) would, according to the laws of electrodynamics, merely oppose changes in the magnetic field: a field already present would remain trapped if you cooled the material inside the field. The real superconductor does more. It expels the field actively, whether it was already there before cooling or applied only afterward. The final state is always the same – a field-free interior – regardless of the order of the steps. It is precisely this path-independence that is decisive: it means that superconductivity is a genuine thermodynamic equilibrium state, a distinct phase of matter like solid, liquid, or gas – and not merely an extreme limiting case of ordinary conduction. This made the powerful tool of thermodynamics applicable to superconductivity.

The Meissner effect is also what you see in the famous levitation experiment: place a small magnet above a superconducting block, and the superconductor expels its field lines, producing a repulsive force that holds the magnet in the air. The levitation is not a trick of ordinary magnetism but the directly visible consequence of a macroscopic quantum state forbidding a magnetic field.

In the years that followed, the brothers Fritz and Heinz London (1935) provided a first mathematical description that derived the Meissner effect from two simple equations, and Vitaly Ginzburg and Lev Landau (1950) offered a deeper theory organized around an "order parameter" – a phenomenological masterpiece that later even helped explain the difference between two classes of superconductors (more on that shortly). All these theories described what happens extraordinarily well. But none explained the why: where does superconductivity come from at the level of individual electrons? That question would take another seven years and one of the sharpest minds of the century.

Part 3: The Riddle of Attraction – the BCS Theory (1957)

The fundamental problem on which generations of theorists broke their teeth – Einstein, Bohr, Heisenberg, and Feynman all tried in vain – can be summed up in one sentence: electrons are negatively charged and repel one another. How, of all things, should they band together into a collective, frictionless flow?

The solution came in 1957 from a trio at the University of Illinois: John Bardeen, his postdoc Leon Cooper, and the doctoral student Robert Schrieffer. After the initials of their names, their theory is still called the BCS theory. They received the Nobel Prize in Physics for it in 1972 – for Bardeen the second, after he had already been honored in 1956 for the invention of the transistor; to this day he is the only person with two Nobel Prizes in Physics.

The Key: the Cooper Pair

The first breakthrough came from Leon Cooper with a seemingly modest calculation. In 1956 he showed: if there is even an arbitrarily small attractive force between two electrons, they form a bound state with lower energy than the two free electrons. The Fermi sea of the remaining electrons makes this possible, because it keeps the pair from drifting apart. Such a bound electron duo has since been called a Cooper pair. But where should the attraction come from, when electrical repulsion is the obvious thing?

The answer lies in the crystal lattice, more precisely in its vibrations. Picture the metal as a lattice of positively charged ions through which the electrons travel. As an electron flies past, it pulls the sluggish positive ions a tiny bit toward itself – it leaves behind a slight concentration of positive charge, a kind of wake. This concentration decays only slowly, because the heavy ions are inert. A second electron passing by shortly afterward "feels" this locally elevated positive charge and is attracted. The two electrons therefore do not attract each other directly – their mutual repulsion persists – but mediated by a lattice vibration. In the language of quantum physics, such a vibrational quantum is called a phonon, and one says the electrons exchange a phonon. Vividly: they communicate through the lattice, the way two people on a sagging mattress inevitably roll toward each other.

That the lattice really is the mediator had already been hinted at experimentally in 1950: in the so-called isotope effect, the critical temperature was found to depend on the mass of the lattice ions (heavier isotopes, lower \(T_c\)). The ions and their vibrations therefore had to be involved – a decisive clue for Bardeen.

From Pair to Condensate

The second, deeper step was Schrieffer's and Bardeen's work: what happens when not one pair but all the conduction electrons pair up like this? Here a subtle quantum magic comes into play. Individual electrons are fermions – they are extreme individualists and, by the Pauli principle, tolerate no second electron in the same state. But a Cooper pair, composed of two electrons, behaves in sum like a boson – and bosons are herd animals: they not only may but want to all be in the same, lowest quantum state. The countless Cooper pairs therefore condense into a single, shared quantum-mechanical state, described by a single macroscopic wave function. Millions of pairs from then on march in lockstep, as if they were a single object.

And here it also becomes clear why there is no more resistance. Electrical resistance arises because individual electrons are scattered off defects and lattice vibrations, losing energy in the process. But a Cooper pair can no longer be scattered out individually: to break a pair you have to supply a certain minimum energy, the energy gap (\(\Delta\)). As long as the thermal energy of the environment is smaller than this gap, no pair can be scattered – the entire condensate train glides on unhindered. Only when you warm the material above \(T_c\) does the thermal energy suffice to break the pairs, and normal resistance returns. The energy gap was a central, experimentally confirmed prediction of the BCS theory and one of the reasons for its resounding success.

Remarkable about a Cooper pair, by the way, is its size. The two partners are not close neighbors but bound to each other over a distance of about a hundred nanometers – an enormous span by atomic standards, within which millions of other Cooper pairs mill about. The pairs fully interpenetrate one another; the image of two firmly linked spheres is misleading. It is this large-scale, overlapping entanglement that produces the collective, coherent character of the superconducting state.

Part 4: Two Kinds of Superconductor and the Josephson Effect

Two practically enormous refinements still belong to the picture. The first is the distinction between Type I and Type II. Type-I superconductors – mostly pure metals like lead or mercury – lose their superconductivity abruptly as soon as the magnetic field exceeds a single critical value; they tolerate only weak fields and are of little technical use. Type-II superconductors, by contrast, predicted by Alexei Abrikosov on the basis of Ginzburg-Landau theory (Nobel Prize 2003), let the magnetic field partially penetrate in the form of tiny, quantized flux tubes (vortices) at stronger fields, but otherwise remain superconducting. This allows them to withstand enormously high fields – the prerequisite for all powerful superconducting magnets. Niobium-titanium and niobium-tin, the workhorses of MRI and accelerator technology, are Type-II superconductors.

The second refinement is the Josephson effect, predicted in 1962 by the then 22-year-old doctoral student Brian Josephson (Nobel Prize 1973). He showed: if you bring two superconductors almost, but not quite, into contact through a wafer-thin insulating layer, Cooper pairs can tunnel through this barrier – a purely quantum-mechanical process. This Josephson junction is today the single most important superconducting device. It sits inside the SQUIDs, the most sensitive magnetic-field sensors in the world (they measure fields down to \(10^{-15}\) tesla and are used, for example, in magnetoencephalography to capture the brain's tiny magnetic fields), it defines the international voltage standard – and, particularly significant for our time, it forms the heart of the superconducting qubits with which Google, IBM, and others build their quantum computers. A transmon qubit is, at its core, nothing but a cleverly wired Josephson junction.

Part 5: The Earthquake of 1986 – High-Temperature Superconductors

For decades the record for the critical temperature crept upward only laboriously. In 1973, niobium-germanium reached 23 kelvin, and many physicists believed a natural limit had been reached here: BCS theory seemed to suggest that phonon-mediated superconductivity above about 30–40 kelvin was hardly possible. Superconductivity thus remained an expensive niche phenomenon, trapped in the realm of liquid helium.

Then, in 1986, came the earthquake. Johannes Georg Bednorz and Karl Alexander Müller at the IBM research lab in Rüschlikon near Zurich did something unorthodox: instead of in metals, they searched in a class of materials normally regarded as electrical insulators – ceramic copper oxides, the cuprates. In February 1986 they found superconductivity in a lanthanum-barium copper oxide at 35 kelvin, well above anything known until then. The field reacted first with skepticism, then euphoria. As early as 1987, yttrium-barium copper oxide (YBCO) achieved the decisive jump to about 92 kelvin – and thus crossed a magic threshold: 77 kelvin, the boiling point of liquid nitrogen. From then on, superconductivity could be cooled with liquid nitrogen, which is dirt-cheap and easy to handle, instead of with expensive, laborious helium. Bednorz and Müller received the Nobel Prize as early as 1987, just one year after their discovery – one of the fastest awards in Nobel history.

The record for cuprates today stands at about 133–138 kelvin (in a mercury-barium-calcium copper oxide, even higher under pressure). But the real shock was not just the high temperature; it was a deep theoretical riddle: BCS theory does not explain the cuprates. The attraction that pairs the electrons there cannot be the ordinary phonon interaction – the temperatures are too high and the material properties too peculiar for that. In the cuprates the electrons are so strongly correlated that the usual approximations of solid-state physics break down. A different pairing mechanism is suspected (the pairs have a "d-wave" symmetry, and magnetic fluctuations probably play the role that phonons play in BCS), but a complete, generally accepted microscopic theory is still lacking.

This is remarkable: almost forty years after the discovery, the mechanism of high-temperature superconductivity is one of the most significant unsolved problems in physics. Tens of thousands of papers, entire careers, and several Nobel Prizes for adjacent methods have failed to crack the core riddle. I am of the opinion that this very stubbornness is instructive: it shows that "an effect can be technically exploited" and "an effect is understood" are two completely different things. We have been building with materials for decades whose functioning we basically cannot explain.

Part 6: The Race for Room Temperature – Pressure, Hydrides, and a Scandal

The dream that has accompanied superconductivity from the very beginning is a superconductor that works at room temperature and normal pressure. It would revolutionize technology: lossless power grids, everyday-viable levitating trains, compact magnets, revolutionary electronics. Two paths are currently being pursued, and both must be viewed with caution.

The first, scientifically solid path leads through hydrogen-rich compounds under extreme pressure. The idea goes back to an old conjecture: metallic hydrogen, so light and with such high lattice-vibration frequencies, should be an excellent (BCS-like, phonon-mediated) superconductor with a very high \(T_c\) – only it can hardly be produced. The way around this is hydrides, in which other elements "pre-compress" the hydrogen, so to speak. In 2015, Mikhail Eremets and colleagues achieved the breakthrough: hydrogen sulfide (H₃S) became superconducting at about 150 gigapascals (1.5 million times atmospheric pressure) at 203 kelvin (−70 °C) – an enormous leap. This was followed by lanthanum decahydride (LaH₁₀) at about 250 kelvin (−23 °C) and yttrium hydrides with similar values. These results are widely regarded as reproduced and real; they show that superconductivity near room temperature is physically possible. The catch is the pressure: it arises only between the tips of a diamond anvil cell, in samples the size of a speck of dust – utterly useless for any practical application. The great open question is: can such a material be found that retains its superconductivity at normal pressure too?

The second path is the more spectacular one – and here the story becomes a cautionary tale. In the years 2020 to 2023, two affairs shook the field. One concerned the US physicist Ranga Dias, who repeatedly reported room-temperature superconductivity in Nature – in 2020 in a carbon-sulfur hydride at 15 °C (but under high pressure), in 2023 in a lutetium-hydrogen-nitrogen material even at 21 °C and considerably lower pressure. Both sensational papers were retracted: the first in 2022, the second in November 2023. Eight of the eleven authors of the 2023 paper – all except Dias – themselves requested the retraction, because the work "does not accurately reflect the provenance of the investigated materials"; independent investigations called the doubts about the resistance data "credible, substantial, and unresolved." An investigation commissioned by his university found data manipulation.

The second affair was LK-99. In July 2023, a South Korean group claimed that a lead-based material called LK-99 was superconducting at room temperature and normal pressure – the ultimate sensation. Videos of a levitating sample went viral, and labs around the world dropped everything to reproduce it. Within a few weeks the matter was settled: LK-99 is not a superconductor. The observed partial "levitation" and drop in resistance could be explained by magnetic impurities (in particular copper sulfide and traces of iron) in the sloppily produced material. Even so, this episode ran more honorably than the Dias affair: it was open, fast, self-correcting science – a bold claim, publicly tested and refuted within weeks.

What do these cases teach? I am of the opinion that they vividly demonstrate the value of the central scientific virtues: extraordinary claims demand extraordinary evidence, independent reproduction is not optional, and a true Meissner effect (the complete field expulsion, not just some levitation) is the real gold standard against which every superconductivity claim must be measured. The lure of the room-temperature superconductor is so great that it produces both honest exuberance and outright fraud.

Part 7: What It's All Good For – Superconductivity in Action

For all the fascination of the fundamentals, superconductivity has long been everyday technology. By far the largest application sits inside every MRI scanner. Its strong, highly stable magnetic field is generated by a superconducting coil of niobium-titanium, in which a once-injected current circulates practically without loss. Without superconducting magnets, today's medical imaging and the nuclear magnetic resonance (NMR) spectroscopy of chemistry would be unthinkable. The same technology steers and focuses the particle beams in accelerators like the Large Hadron Collider at CERN, whose thousands of superconducting magnets keep the protons on their circular orbit.

Levitating trains (maglev) use superconducting magnets to glide frictionlessly above the track; a Japanese maglev train has exceeded 600 km/h. The SQUIDs already mentioned measure the smallest magnetic fields in medicine, geophysics, and materials research.

Two future fields are especially exciting. The first is nuclear fusion. To confine a plasma of over 100 million degrees, you need gigantic magnetic fields, and the fusion power rises extremely steeply with the field strength. Modern high-temperature superconductors in tape form (REBCO tapes, a cuprate) allow considerably stronger and more compact magnets than the old niobium alloys. The SPARC project of Commonwealth Fusion Systems and MIT relies on REBCO coils with peak fields around 20 tesla to make fusion possible in a much smaller, more cost-effective plant – one of the reasons for the current optimism in fusion research.

The second is quantum computing. As mentioned, the qubits of the leading quantum computers from Google and IBM are based on Josephson junctions – tiny superconducting circuits that operate only at a few millikelvin. Here a circle closes back to quantum error correction and to the most fragile computing units engineering has ever tried to master: without the macroscopic quantum coherence of superconductivity, these qubits would not exist at all.

The Central Takeaway

The deepest lesson of superconductivity reaches beyond physics and reads: collective behavior can produce properties that no individual building block possesses – and that cannot be guessed from the behavior of the individual. A single electron knows nothing of resistance-free flow; only the coherent condensation of billions of Cooper pairs into a shared quantum state produces a behavior that is qualitatively new. The technical term for this is emergence, and superconductivity is one of its most beautiful examples: from repelling individual particles arises a frictionless whole.

Practically, this means two things. First, for evaluating claims – whether in physics, in technology, or elsewhere: measure against the strictest possible criterion, not the most impressive appearance. For a superconductor that is the complete Meissner effect and independent reproduction, not the levitating video. Second, for understanding complex systems: an effect that can be technically exploited need not be theoretically understood. We build high-temperature superconductors into devices without knowing their mechanism – proof that engineering and fundamental understanding are different fronts that do not always move in lockstep.

And the next time you lie in an MRI tube or read about a quantum-computing breakthrough: remember that deep inside these machines a current circulates that never stops – a piece of quantum mechanics large enough to touch.

A Question to Reflect On

The cuprates show that we can exploit a technology on an industrial scale whose fundamental mechanism we still do not understand after forty years. Where in your own field – whether software architecture, AI, or security – do you rely daily on a system that works reliably without anyone being able to explain fully why; and at what point does this lack of understanding turn from an acceptable trade-off into a genuine risk?

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