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Saturday, 16 November 2019

Discovery of a new state of matter: metallic Cooper pairs

Described for the first time in 1956, the Cooper pairs are low temperature electron bound states responsible for the superconductivity phenomenon. In today's quantum models, these pairs can be either zero electrical resistance or electrical insulation. However, physicists have discovered a new state of matter in which Cooper pairs conduct electricity while generating some resistance, just like ordinary metal. A phenomenon not foreseen by the physics of condensed matter and which could lead to the development of new electronic devices.

For years, physicists have assumed that the Cooper pairs, the electron pairs that allow superconductors to drive electricity without resistance, were binary systems. The pairs slide freely, creating a superconducting state, or an insulating state by getting stuck in a material, unable to move.

Pairs of Cooper: they can combine conductivity and electrical resistance

But in a new article published in the journal Science , a team of researchers has shown that Cooper's pairs can also conduct electricity with some resistance, as do ordinary metals. The results describe a new state of matter, according to the researchers, which will require a new theoretical explanation.

" It had been proven that this metallic state would form in thin-film superconductors as they cooled down to their superconducting temperature, but the question of whether this state involved Cooper pairs is an open question, " says Jim. Valles, professor of physics at Brown University.

" We have developed a technique that allows us to answer this question and we have shown that, as a result, Cooper pairs are responsible for carrying the charge in this metallic state. What's interesting is that no one is really sure how it works. This discovery will therefore require more theoretical and experimental work to understand exactly what is happening."

Cooper pairs and superconductivity

Cooper's pairs are named after Leon Cooper, professor of physics at Brown, who won the Nobel Prize in 1972 for describing their role in superconductivity. Resistance is created when electrons vibrate in the atomic network of a material as they move. But when the electrons unite to become Cooper pairs, they undergo a remarkable transformation.

The BCS theory explains the phenomenon of superconductivity by the appearance of Cooper pairs. At very low temperatures, the electrons pair together (b), forming many Cooper pairs within the material (c). These pairs occupy the same fundamental quantum state and form a single quantum wave (d). Cooper pairs are treated as bosons, they obey Bose-Einstein statistics and are not subject to the Pauli exclusion principle. Credits: CNRS

The electrons themselves are fermion s, particles that obey the Pauli exclusion principle, which means that each electron tends to maintain its own quantum state. The Cooper pairs, however, act as bosons, which can share with the same state. This bosonic behavior allows Cooper pairs to coordinate their movements with other sets of Cooper pairs, so as to reduce electrical resistance to zero.

A new bosonic state of Cooper pairs

In 2007, Valles, in collaboration with Jimmy Xu, Professor of Engineering and Physics at Brown, showed that Cooper's pairs could also produce insulating states as well as superconductivity. In very thin materials, rather than moving together, couples stay in place, grouped by islands without means to join those around.

Scanning electron microscope observation of the material used for the experiment: a YBCO superconductor with a network of tiny holes to study the dynamics of the Cooper pairs. Credits: Brown University

For this new study, Valles, Xu and his colleagues searched for non-superconducting metallic Cooper pairs using a technique similar to that which revealed insulators of Cooper pairs. This technique consists of modeling a thin-film superconductor, in this case a high-temperature superconducting yttrium, barium and copper oxide (YBCO), with networks of tiny holes.

When the material is traversed by a current and is exposed to a magnetic field, the charge carriers of the material gravitate around the holes like water surrounding a drain. " We can measure the frequency with which these charges revolve around. In this case, we found that the frequency is compatible with the fact that two electrons circulate at once instead of one. So we can conclude that the charge carriers in this state are Cooper pairs and not electrons, "says Valles.

Towards the potential development of new electronics

The fact that this phenomenon has been detected in a high temperature superconductor will make future research more practical. The YBCO type begins its superconductivity at about -181 ° C and the metal phase begins at a temperature just above that. This higher temperature facilitates the use of spectroscopy and other techniques to better understand what is happening in this metallic phase.

According to the researchers, it may be possible to exploit this bosonic metal state for new types of electronic devices. " The problem with bosons is that they tend to be in a wave state more like electrons. We are talking about a phase and interference similar to that of light. So there may be new ways to move the load in the devices by playing with interference between the bosons "concludes Valles.


Intermediate bosonic metallic state in the superconductor-insulator transition

Science 14 Nov 2019:
DOI: 10.1126/science.aax5798 



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