High-Temperature Superconductivity Understood at Last


Physicists have been baffled by a family of crystals' perplexing ability to superconduct, or transport an electric current without any resistance, at temperatures much higher than those of ordinary materials, for decades.

Finally identifying the origin of the phenomena to virtually everyone's satisfaction, an experiment that has been years in the making has now directly visualised superconductivity on the atomic scale in one of these crystals. A venerable idea that is almost as ancient as the riddle itself first proposed that electrons appear to nudge each other into a frictionless flow.

Subir Sachdev, a physicist at Harvard University who develops theories of the crystals, known as cuprates, and who was not engaged in the experiment, said, "This evidence is extremely beautiful and direct."

J. C. Séamus Davis, the professor in charge of the new experiment at the University of Oxford, said, "I've worked on this problem for 25 years, and I hope I have solved it. "I'm overjoyed," you said.

The theory, which ascribes cuprate superconductivity to a quantum phenomena known as superexchange, makes a prediction based on the new observation. André-Marie Tremblay, a physicist at the University of Sherbrooke in Canada and the head of the team that made the forecast last year, expressed his amazement at the quantitative consistency.

The study advances the field's long-standing goal of enhancing cuprate superconductivity's underlying mechanism in order to create revolutionary new materials that can conduct electricity at even greater temperatures. Although it is still a long way off, room-temperature superconductivity would make everyday electronics, power lines, and more completely efficient.

It should be able to describe synthetic materials with different atoms in different positions for which the critical temperature is higher, according to Davis, who was referring to the superexchange theory.

Three Glues

Superconductivity has been a source of controversy for physicists ever since it was discovered in 1911. The electrical resistance of a mercury wire dropped to zero when the Dutch scientist Heike Kamerlingh Onnes and her team cooled it to roughly 4 kelvins, or 4 degrees above absolute zero. Resistance was created when electrons carefully navigated past the wire's atoms without creating heat in the process. To figure it out would require "a lifetime of work," according to Davis.

John Bardeen, Leon Cooper, and John Robert Schrieffer published their Nobel Prize-winning theory of this common type of superconductivity in 1957, building on significant experimental discoveries from the middle of the 1950s. According to the "BCS theory," as it is known now, vibrations passing along atomic rows "bind" electrons together. A ripple is created when a negatively charged electron travels between positively charged atomic nuclei, pulling them toward it. Another electron is attracted by that ripple. The two electrons overcome their strong electrical attraction to create a "Cooper pair."

Jörg Schmalian, a physicist at the Karlsruhe Institute of Technology in Germany, described it as "real cunning of nature." This Cooper couple is not expected to exist.

Superconductivity becomes inevitable when electrons pair together due to additional quantum entanglement. Cooper pairs behave like light particles, which can accumulate in any number on a pin's head, breaking a different quantum mechanical law from the one that states that electrons cannot overlap. A large number of Cooper pairs combine to form a single quantum mechanical state known as a "superfluid" that loses awareness of the atoms it travels through.

The superconducting behaviour of mercury and the majority of other metallic elements is explained by BCS theory, which also explains why this behaviour ends above a few kelvins. The weakest of glues are created by atomic ripples. Increasing the heat causes the atoms to vibrate and removes the lattice vibrations.

Then, in 1986, IBM scientists Georg Bednorz and Alex Müller discovered cuprates, crystals made of copper and oxygen sheets sandwiched between layers of other elements, which served as a stronger electron bond. Researchers first discovered a cuprate superconductor at 30 kelvins, then quickly discovered others superconducting at 100, then over 130 kelvins.

The discovery spurred a massive research effort to identify the more durable glue causing this "high-temperature" superconductivity. Perhaps clumps of electrons formed rippling, uneven concentrations of charge. They might have communicated by exchanging spin, an intrinsic feature of the electron that directs it in a specific direction like a magnet the size of a quantum.

This proof is very elegant and clear. Subir Sachdev, University of Harvard

Just a few months after the discovery of high-temperature superconductivity, the late Philip Anderson, an American Nobel laureate and condensed matter physics superstar, proposed a theory. He asserted that the superexchange quantum phenomenon, which results from the ability of electrons to hop, lies at the core of the adhesive. When electrons can move quickly between different places, their position at any one time is unclear while their momentum is well-defined. Particles will typically gravitate toward a condition with a sharper momentum because it has a lower momentum and, hence, a lower energy.

The end result is that electrons look for opportunities to hop. For instance, an electron prefers to point downward when its partner does so upward since this permits the two electrons to go amongst the same atoms. In some materials, superexchange creates a consistent up-down-up-down pattern of electron spins. Additionally, it pushes electrons to maintain a specific separation. (If it's too far, they can't hop.) Anderson was of the opinion that strong Cooper pairs may be formed by this powerful attraction.

Since material characteristics that could be measured, such as reflectance or resistance, only provided rough descriptions of the aggregate behaviour of trillions of electrons rather than pairs, experimentalists struggled for a long time to test theories like Anderson's.

No conventional condensed matter physics method has ever been created to address an issue like this, according to Davis.


Davis, an Irish physicist who maintains laboratories at Cornell University, University College Cork, Oxford University, and the International Max Planck Research School for Chemistry and Physics of Quantum Materials in Dresden, has gradually created methods to examine cuprates at the atomic level. In earlier investigations, the critical temperature at which superconductivity commenced was used to measure the strength of a material's superconductivity; warmer temperatures indicated stronger glue. However, Davis' team has improved a technique for poking the glue that surrounds certain atoms during the past ten years.

They altered the scanning tunnelling microscopy method, which involves dragging a needle across a surface to measure the current of electrons hopping between them. They measured an electron pair current rather than an individual electron current by changing the needle's regular metallic tip for a superconducting tip and sweeping it across a cuprate. They were able to map the density of Cooper pairs that are present around each atom, which is a direct indicator of superconductivity. In 2016, they released the first picture of Cooper pair swarms in Nature.

A key piece of evidence in favour of Anderson's superexchange theory came from an experiment conducted by Chinese physicists in the same year, which demonstrated that the critical temperature of a given cuprate increases with the ease with which electrons can move between copper and oxygen atoms in the cuprate (and thus the stronger its glue). In order to more clearly identify the composition of the glue, Davis and his colleagues attempted to integrate the two techniques in a single cuprate crystal.

In 2020, he claimed, the "aha" moment occurred at a group discussion through Zoom. The scientists discovered a remarkable property in a cuprate known as bismuth strontium calcium copper oxide (BSCCO, or "bisko," for short), which enabled them to conduct their ideal experiment. In BSCCO, the surrounding sheets of atoms press the layers of copper and oxygen atoms into a wavy pattern. This alters the separations between specific atoms, which impacts how much energy is needed to hop. The variance gives theorists, who prefer neat lattices, headaches, but it provided experimentalists with just what they needed: a variety of hopping energies in a single sample.

They mapped the hopping energies across the cuprate by attaching electrons to some atoms and removing them from others using a conventional scanning microscope with a metal tip. The density of Cooper pairs surrounding each atom was then measured by switching in a cuprate tip.

The two maps were aligned. Superconductivity was weak where electrons found it difficult to hop. Superconductivity was strong where hopping was simple. Hoping energy and Cooper pair density have a strong correlation, which closely resembles a complex numerical forecast made in 2021 by Tremblay and colleagues, who suggested that Anderson's theory should predict this correlation.

Superglue Superexchange

The fact that Davis' research on the relationship between hopping energy and superconductivity strength was just published in the Proceedings of the National Academy of Sciences suggests that superexchange is what holds high-temperature superconducting together.

According to Ali Yazdani, a Princeton University physicist who has developed comparable techniques to study cuprates and other unusual manifestations of superconductivity concurrently with Davis' group, "it's a nice piece of work because it brings a new technique to further show that this idea has legs."

Yazdani and other researchers warn that the field is falling victim to the well-known correlation-equals-causes fallacy and that there is still a chance, however remote, that adhesive strength and ease of hopping move in lockstep for some other reason. Yazdani believes that using superexchange to create some eye-catching new superconductors will be the best method to demonstrate a causal connection.

The critical temperature was mentioned, and he responded, "If it's finished, let's increase Tc."

Since superexchange is not a novel concept, several researchers have already considered ways to strengthen it, maybe by further compressing the copper and oxygen lattice or by experimenting with additional elemental combinations. Tremblay stated, "Predictions are already on the table."

Of course, drawing atomic blueprints and creating materials that satisfy researchers' needs takes time and effort. Furthermore, there is no assurance that even custom cuprates will reach critical temperatures that are significantly higher than those of the cuprates we are already familiar with. Similar to how atomic vibrations appear to have a hard ceiling, superexchange might also have one. Some scientists are looking into prospective possibilities for completely distinct and possibly much more powerful forms of glue. Some people use otherworldly pressures to support the standard atomic vibrations.

However, Davis' finding might inspire and direct the work of chemists and materials scientists who want to advance cuprate superconductors.

Schmalian stated that "the ingenuity of those who develop materials is boundless." "It is more natural to invest farther in this one the more certain we are that a process is correct."

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