Science

Superconductivity at room temperature finally understood

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For 40 years, a family of crystals has puzzled physicists with their ability to “superconduct” — conduct electrical current without resistance — at much higher temperatures than other materials. Recently, researchers have developed new microscopy techniques to unlock the mystery of these superconductors. Their discovery could pave the way for the development of materials that work at room temperature and revolutionize the power supply.

Superconductivity is an amazing state of matter in quantum physics. The current flows through the superconductor without any dissipation, and the resistance is completely zero. The absence of dissipation means that there is no loss of energy, and the “overcurrent”, once established in a closed circuit of a superconductor, can last indefinitely. At very low temperatures, the electrons inside come together in pairs, Cooper pairs.

It should be remembered that pairs of electrons usually repel each other, as they have the same negative charges. In a superconductor, the interaction between electrons can become attractive, according to another rule of quantum mechanics. Understanding why electrons choose to pair in some materials and not others is a big challenge for creating new superconducting materials at room temperature.

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It is true that copper oxide superconductors were discovered in 1986, as pointed out by Professor Amalia Koldea, who leads the Quantum Matter in High Magnetic Fields group at Oxford, but it is only now that some of these cuprates have reached their production potential for use in strong magnets. They consist of sheets of copper and oxygen sandwiched between layers of other elements. She adds: “Room temperature superconductivity is the holy grail, and when it becomes possible at scale, it will have many practical applications.”

Recently, an international team of researchers led by Seamus Davis, professor of quantum physics at University College Cork and the University of Oxford, announced results revealing the atomic mechanism behind the emergence of high-temperature superconductors. The results are published in the journal PNAS.

Ambient temperature, the key to widespread use

As mentioned earlier, superconductors are materials that can conduct electricity with zero resistance, so the electric current can be maintained indefinitely without being dissipated. They are already used in various applications, including MRI scanners, but require extremely low temperatures.

However, some superconductors can operate at higher temperatures, such as copper oxides (cuprates). To study them, the research team first created a special cuprate called bismuth-strontium-calcium-copper oxide (BSCCO), whose layers of copper and oxygen atoms are compressed into a wavy structure, changing the distances between the atoms.

Secondly, they developed two new microscopy methods. The first measures the energy difference between the orbitals of copper and oxygen atoms, depending on their location.

You should know that, unlike the planets orbiting the Sun, electrons cannot be at an arbitrary distance from the nucleus; they can only exist in certain specific places, called allowed orbits. Thus, in quantum mechanics, an atomic orbital is a mathematical function that describes the wave-like behavior of an electron or a pair of electrons around the nucleus of an atom, giving the probability of its presence in a certain place.

The measurement of the difference between these orbitals comes from a theory related to superconductors, a quantum phenomenon called superexchange. This is the force resulting from the ability of electrons to jump between atomic orbitals to have a lower energy state. Some point down and some point up while remaining around the core. Thus, superexchange establishes a regular pattern of electrons in certain materials, causing them to stay a certain distance apart, but not too far apart. It is this effective attraction that could form strong Cooper pairs.

The second method measures the magnitude of the electron pair wave function (superconductivity strength) of each oxygen atom and each copper atom.

Professor Davies said in a statement: “By visualizing the strength of superconductivity as a function of differences between orbital energies, we have for the first time been able to accurately measure the relationship needed to confirm or disprove one of the main theories of superconductivity.” temperature superconductivity on the atomic scale”.

As predicted by theory, the results showed a quantitative inverse relationship between the difference in charge transfer energy between neighboring oxygen and copper atoms and the strength of superconductivity. The easier it is for electrons to jump from one place to another, between the copper and oxygen atoms in a given cuprate, the higher its critical temperature and its superconductivity.

Atom-by-atom scanning of the BSCCO crystal. In regions where electrons require more energy to jump between neighboring atoms (bright pink bands 2.6 nanometers apart, left), the electrons form fewer superconducting Cooper pairs (dark bands, right). © S. Davis et al., 2022

Applications for a prosperous energy future

According to a research team led by Shane O’Mahony of University College Cork, the discovery could be a historic step towards the development of room-temperature superconductors.

Indeed, they may have large scale applications such as maglev trains (or maglev trains). The latter use a method of movement based on magnets rather than wheels, axles and bearings. From a practical point of view, the vehicle is levitated a short distance from the track using magnets to generate both lift and thrust, since, like conventional magnets, superconducting magnets repel each other when their respective poles are facing each other.

This discovery can also be used for nuclear fusion reactors, quantum computers and high-energy particle accelerators, as well as at the level of ultra-efficient energy transfer and storage.

The authors explain that electrical resistance is minimized in superconducting materials because the electrons carrying the current are bound in stable Cooper pairs. In low-temperature superconductors, Cooper pairs are held together by thermal vibrations, but at higher temperatures they become too unstable.

These new results demonstrate that in high-temperature superconductors, the Cooper pairs are instead held together by magnetic interactions, and the electron pairs are bound by quantum bonding through an intermediate oxygen atom.

Professor Davis concludes: “This has been one of the holy grails of physics research for almost 40 years. Many believe that cheap and readily available superconductors operating at room temperature will be as revolutionary for human civilization as the advent of electricity itself.

PNAS

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