Scientists are one step closer to solving a superconducting puzzle with applications in medicine, transportation and power transmission

Spin and phonon fluctuations in La2-xSrxCuO4 (x = 0.22) near Qδ. S(Q, ω) as a function of energy and wave vector along a path passing through two incommensurable wave vectors Qδ = (0.5-δ, 0.5, L) and (0 .5, 0.5-δ, L) (see inset in panel a). The integration ranges are a L ∈[ − 1, 1]and b L ∈[3.8, 4.2]. Strong phonons are observed (panel b) for L ≈ 4, but these are not visible near L = 0 (panel a) where spin fluctuations are observed. Data were collected on LET (panel a) and MERLIN (panel b). Credit: Natural Physics (2022). DOI: 10.1038/s41567-022-01825-3

Researchers studying the magnetic behavior of a cuprate superconductor may have explained some of the unusual properties of their conduction electrons.

Cuprate superconductors are used in levitating trains, quantum computing and power transmission. They belong to a family of materials made up of layers of copper oxides alternating with layers of other metal oxides, which act as charge reservoirs.

The greatest use of superconductors today is for the manufacture of superconducting magnets used for MRI medical devices and for scientific applications such as particle accelerators.

For the potential applications of superconducting materials to be fully realized, the development of superconductors that retain their properties at higher temperatures is crucial for scientists. Cuprate superconductors currently exhibit relatively high transition point temperatures and therefore provide scientists with an opportunity to investigate what makes higher temperature superconductivity possible.

In this study published in Natural Physicsa collaboration involving the University of Bristol and ISIS Pulsed Neutron and Muon Source, they focused on the cuprate superconductor La_2-xsr_XCuO₄ (LSCO). Superconductivity in this system is very sensitive to the exact ratio of lanthanum (La) to strontium (Sr) offering the possibility of understanding which properties are correlated with superconductivity. LSCO is also close to being magnetically ordered and one possibility is that magnetic fluctuations are what allow its superconductivity.

Inelastic neutron scattering offers an excellent method to study these magnetic fluctuations. The researchers were able to measure on a wide range of reciprocal space and energy scales. This allowed them to build a complete picture of spin fluctuations and phonons, allowing very low energy spin fluctuations to be isolated.

Although cuprate superconductors are metals above the temperature at which they become superconductors, the current-carrying electrons behave in very strange ways. As the temperature rises, their ability to carry current is greatly reduced. Low-energy spin fluctuations could scatter the conduction electrons and explain this strange behavior of the metal.

Moreover, when the superconductor was cooled and the superconductivity suppressed with a magnetic field, the spin fluctuations became stronger and slowed down, suggesting that the material is close to magnetic order. This could help explain the unusual electronic properties of cuprates.

Professor Stephen Hayden from the Bristol School of Physics said: “This study has demonstrated the potential importance of spin fluctuations in understanding cuprates. A deeper understanding of their properties and their relationship to superconductivity is another step towards designing materials with higher superconducting temperatures.

“In the future, they are expected to be used for quantum computing, transportation, including levitating trains and compact motors, as well as power transmission. There are already demonstration projects for these.

“The work leverages the unique instrumentation and sampling environment available at ISIS.”