The discovery of superconductivity turned 113 on April 8 this year. Respectable age, but the mystery of the phenomenon has not been solved. The first clear theory of superconductivity was created only in 1957, but 50 years later surprises began again. Thus, in 2009, high-temperature superconductivity (HTSC), discovered by that time, presented a new problem – in superconductors above the critical temperature an energy gap unexplained by theory was discovered.
The normal energy gap regime in HTSC is shown on the left; the position of electrons in the pseudogap regime is shown on the right. Image source: Science
A complete understanding of the physics of superconductivity and HTSC in particular will allow us to search for the necessary materials literally with our eyes open, whereas now (and throughout the previous 113 years) scientists are moving mainly at random. Created half a century ago by scientists John Bardeen, Leon Cooper and Robert Schrieffer, the “BCS theory” explains superconductivity by the coherent behavior of pairs of electrons – the so-called Cooper pairs. This is a phenomenon of a quantum order, which makes the analysis and modeling of superconductivity an extremely complex and hardly feasible task. At least that was the case until the new job came along.
A group of scientists from the Paris Polytechnic Institute was able to develop an approach that would make it possible to simulate the behavior of electrons and Cooper pairs in HTSC materials. According to them, this is a breakthrough that is difficult to overestimate. The analysis revealed at least one mystery inherent in HTSCs – how a pseudogap arises in cuprates (copper oxides that discovered superconductivity in 1986). This is an interesting and mysterious thing, which until now has not had a clear explanation (there were only two hypotheses).
In classical superconductors, the energy gap in which quantum entangled Cooper pairs are formed from electrons occurs at temperatures below the critical temperature (when superconductivity becomes possible). For HTSC cases, the energy gap can appear at a temperature above the critical one. This means that superconductivity can occur at high temperatures, including room temperatures. Do I need to explain that understanding the physics of a phenomenon promises wonderful discoveries? But from the point of view of the BCS theory, this gap is inexplicable, which is why it came to be called a pseudogap. Scientists from France and Switzerland are now ready to explain how and why it occurs.
To model the behavior of electrons in HTSCs, the scientists used a simplified Hubbard model, to which they applied the statistical Monte Carlo method. Hubbard’s model imagined electrons as pawns on a chessboard, which can move from one square to another and align themselves on it (in the material) depending on the direction of the spin, and can even be located on the same square in the case of oppositely directed spins. The Monte Carlo method made it possible to analyze the entire “board” at once, which brings the modeling closer to the natural process.
The simulation presented the transition to the pseudogap regime as a rearrangement of electrons on a “chessboard” from a stripe ordered structure to voids arranged as if in a checkerboard pattern. As soon as the strip disappeared, the material acquired the properties of a pseudo-gap.
«Our discovery will help scientists in their quest for superconductivity at room temperature, the Holy Grail of condensed matter physics that would allow lossless energy transfer, faster MRI machines and ultra-fast levitating trains,” the study authors note.