�鶹��ý

In the mid-1980s, an unexpected discovery sparked one of the most frenzied episodes in scientific history. The finding in question was of materials that turned into superconductors – materials that conducted electricity with zero resistance – at much higher temperatures than had ever been seen before. Almost overnight, labs the world over shelved their existing research programmes and jumped on the bandwagon to find other examples. Newspapers heralded an impending age of lossless power transmission, floating trains and extraordinary supercomputers. A was handed out within a year.

Amid all the fuss, it was easy to overlook another odd property of the new materials. Even when they were too warm to actually superconduct, they still conducted electricity in an odd way, with an unusual type of resistance that no theory could explain. What became known as strange-metal behaviour was an intriguing mystery for many researchers in the field, but was nonetheless a sideshow to the main drama of resistance disappearing entirely.

Fast forward 40 years, and we still don’t understand high-temperature superconductivity, nor have we managed to find materials that exhibit this property at room temperature and pressure. But the associated strange-metal behaviour has been steadily coming to the fore. Attempts to understand it have forced physicists to question key assumptions about how electricity flows – employing a raft of outlandish concepts along the way, from quantum soups to black holes.

Now, experiments might finally be inching us closer to a resolution. And increasingly, it seems that this will take us beyond strange metals themselves – and that understanding their peculiar conductivity will help us explain superconductivity, too. “There must be something about it that gives the answer,” says theorist at Harvard University.

CERN’s new chief on the gamble that could fix our picture of reality

Mark Thomson has taken the reins at CERN just as particle physics confronts some of its deepest unknowns – and faces hard choices about what comes next

The usual explanation for why metals conduct electricity is that they are full of individual, negatively charged particles known as electrons, which can freely roam. There are a lot of them: just 1 centimetre of ordinary household wire contains roughly as many free-flowing electrons as there are grains of sand on a beach. Attach a battery, and those electrons will be repulsed from the negative terminal and attracted towards the positive one, generating a current.

That is the rough picture many of us learned at school. For physicists, a more nuanced conception of current derives from work by theorist Lev Landau in the 1950s involving the concept of quasiparticles. To imagine these, think of a crowd in a stadium doing a Mexican wave: any one individual is moving only up and down, but, collectively, they create a swoosh that sweeps longways. Landau’s ideas say that the thing that conducts electricity isn’t an electron, pure and simple, but rather an electron-like quasiparticle – an excitation that sweeps through materials thanks to the way all the internal particles interact.

Like their fundamental counterparts, electron quasiparticles can collide and scatter like billiard balls, but calculations involving them are much easier – and phenomenally accurate. For 70 years, quasiparticles have helped us correctly predict pretty much any property of materials we like, from their heat capacity to their electrical conductivity and magnetic susceptibility. Their success has led theorists to believe that all material physics, including conduction and resistance, must boil down to the interactions of individual particle-like objects. “The entire electronics industry – including the iPhone in your pocket – is based on the success of this theory,” says Sachdev.

Quasiparticles don’t travel unimpeded through a material. At room temperature or thereabouts, vibrations in the atomic structure interrupt them, generating resistance, while at lower temperatures, resistance instead mostly comes from the quasiparticles scattering off each other. The exciting discovery in the 1980s was that certain materials could superconduct even at temperatures where quasiparticle scattering should still have been significant.

Strange metals

The strange-metal behaviour was more subtle. In most conductors in cold conditions, resistance rises with the square of the temperature – that is to say, doubling the temperature gives four times the resistance. To the average materials scientist, this seems intuitive because temperature should determine two key factors that influence resistivity: the number of electrons available to collide and the number of places those electrons can end up afterwards. Two temperature dependencies, hence temperature squared.

As such, if you plot a graph of how a metal’s conductivity should vary with temperature, you inevitably get an upward-sweeping curve. Yet in strange metals, the resistance-temperature plot is a straight line (see chart below). There was no obvious quasiparticle-like behaviour that could generate such a trend, and the more physicists dwelled on it, the more mystified they became. There is “no operator or process that would [be able to give] this power of the temperature”, the late theorist Joseph Polchinski . It was, he added, “the conductor from Hell”.

There are some who think the answer isn’t actually that complicated. After all, linear temperature behaviour isn’t totally unheard of: copper exhibits the trend at room temperature, when vibrations running through the metal are by far and away the dominant source of resistance. These vibrations are generally seen as simple attenuators, and the higher the temperature, the more the material vibrates – hence a linear relationship. Last year, at Harvard University and others that these vibrations could be behind strange metals. But most other physicists remain unconvinced: at low temperatures, where strange-metal behaviour persists, the vibrations have long been predicted to freeze out.

A slightly more radical approach to explaining strange metals involves their electrons being caught between different forms of order. This can happen close to a phase transition, when a material reorganises itself – for instance, in such a way that a quantum property of electrons called spins collectively point in one direction, as in a magnet. On the brink of such a transition, when there is no definite preference for one order over another, the electrons are thought to produce fleeting patterns, like the murmurations of starlings in flight. And, crucially, the strength of these critical fluctuations is usually driven by temperature, linearly. “It could be the fluctuations that produce resistance,” says at the University of Bristol, UK.

This year, Hayden and his colleagues used a beam of neutrons at Rutherford Appleton Laboratory in Didcot, UK, to study electron-spin fluctuations in a strange metal. Having angular moments themselves, but no charge to complicate matters, neutrons are excellent probes of electron spin. Hayden’s team found that the – providing some of the strongest evidence to date that critical fluctuations are behind strange-metal behaviour.

It may have been a telling result, but this case is far from closed. “The big question for us is whether it can be put into a theory,” says Hayden. This is where it gets tricky – and indeed where strange-metal behaviour challenges our deepest notions of what electricity and electrical resistance should be. A murmuration isn’t about individual starlings; it is about the behaviour of the flock as a whole. Likewise, if fluctuations are driving resistance in strange metals, the crucial actors are no longer quasiparticles – or, for that matter, anything particle-like – but collective patterns involving all the electrons at once. What, then, is electrical resistance if not individual collisions of some sort?

Sachdev’s attempts to answer that question with an alternative theory originate in the early 1990s, in collaboration with , who is now at Mississippi State University. The two theorists imagined a deliberately simplified system with no spatiality, no atomic structure at all – basically just a dot, in which every electron is connected to every other. In their model, any electrical disturbance fades at a rate proportional to temperature, despite there being no individually acting particles, or indeed any space for them to travel through. But it hardly resembled a real metal, and Sachdev recalls the idea being met with some scepticism: “My colleagues would think, is this just some curious thing Subir is amusing himself with?”

While Sachdev and Ye’s model languished, the strange-metal problem became ever more urgent. In the beginning, it was limited to “cuprates”, or copper-oxide materials. Then, in 2009, at the University of Sherbrooke in Canada and his colleagues . Ten years later, a team led by at the University of California, Santa Barbara, and at Columbia University in New York spotted it in . Then, just a few years ago, compounds called by at the SLAC National Accelerator Laboratory in California and his colleagues. But theorists were still struggling to come up with descriptions of resistance that didn’t involve particles.

Then, a hint of progress came from an unexpected corner of theoretical physics. In the late 1990s, string theorists discovered a mathematical trick that allows everything in a certain volume of space to be perfectly described by the physics taking place on a shell enclosing it. This “holography” was a strange idea, but it provided a new window onto some very difficult problems, including the nature of black holes. According to holography, everything happening inside a black hole can be completely encoded on its event horizon – the threshold within which even light is sucked in.

Black holes are a far cry from strands of metal in the lab, yet theorists such as at the University of Cambridge believed there could be a link. Beginning in the late 2000s, he and others showed that in certain holographic models, an electric current inside a strange metal could be , steadily losing some of its momentum to the inner black hole.

Nobody thought strange metals were black holes, but all this did suggest that holography might give us a foothold in terms of finding out how they work. In 2015, , a theorist at the California Institute of Technology, presented a talk about one particular holographic model that, others later realised, looked remarkably similar to Sachdev and Ye’s early work. Now taking that work more seriously, theorists built on it, creating a family of “SYK” models – after the initials of Sachdev, Ye and Kitaev – that increasingly resembled real materials. “Suddenly, our original paper started getting several hundred citations every year,” says Sachdev.

Intriguingly, SYK models didn’t only predict a resistance that rises linearly with temperature. At a deeper level, they suggested that in strange metals, electrical current somehow loses momentum at a rate depending merely on temperature and Planck’s constant, the fundamental quantity that sets the scale of quantum effects. It was as though resistance was butting up against a universal quantum speed limit. The chemistry of a particular strange metal didn’t seem to matter at all.

The discomfiting implication of this is that there may not be an easily interpretable answer to what electricity is in strange metals. At best, the SYK model requires physicists to be content to think of it as a kind of “quantum soup”, beholden only to distant, universal laws that don’t involve individual particles. In a way, this is a return to a 19th-century picture of conduction, when scientists thought that electricity was like a fluid. In time, atomic theory filled in the details of that view, showing how to predict certain key properties such as viscosity; since then, physicists have grown accustomed to all macroscopic behaviour resting on a clear and intuitive foundation of microscopic physics. Removing the latter now, says Sachdev, “is like the rug being pulled out from under our feet”.

The question is whether conduction in strange metals really is indifferent to individual particles, or whether holographic physics is a mask for something more fundamental and particle-like that we have yet to uncover. And it turns out there may be a way to know for sure.

Typically, when a small current passes through a conductor, it should create a certain level of electrical crackle, or “shot noise”. The idea is that each charge-carrying particle arrives like the pitter-patter of raindrops on a window – if the current is indeed carried by particles. If not, “you’ve got a very soupy situation,” says experimentalist at Rice University in Texas. “You should basically get no shot noise at all.”

In 2023, Natelson’s group tried to measure shot noise in very pure wires of a well-known strange metal. However, the result was slightly ambiguous: the shot noise was . A quantum soup with the occasional electron crouton, you might say. “It’s really interesting,” says Natelson. Other research groups, such as at the Indian Institute of Science and his colleagues, are currently for different materials. Theorists remain divided over what it means. Some, such as Sachdev, see the suppressed shot noise as evidence that strange metals . Others argue that it can still be , as implied by Hayden’s experiments.

And then there is the bigger question, the one that had scientists so excited back in the 1980s. If we are reaching for a new picture of conduction in strange metals, can that tell us anything about how to obtain room-temperature superconductivity?

Maybe. A few years ago, Sachdev and colleagues found that a was able to predict both strange-metal resistance and a colder superconducting phase in the same system. Sachdev is now trying to , such as cuprates, in the hope that it will show that strange-metal behaviour and superconductivity are two manifestations of the same underlying quantum soup. But, crucially, his model doesn’t yet predict at what temperature superconductivity will kick in, or what sort of material will exhibit it closest to room temperature.

Still, 40 years after the discovery of high-temperature superconductivity and strange metals, physicists have a raft of ways to think about them. From collective fluctuations to holography, the mystery no longer looks quite as impenetrable as it once did. “Hopefully, some combination of these, put together in the right way, will ultimately shed some light on what is going on,” says Hartnoll.