Spin fluctuations: another piece in the superconducting puzzle
18 Nov 2022
- Rosie de Laune



Researchers have studied the magnetic behaviour of a cuprate superconductor and may have explained some of the unusual properties of the conduction electrons.

​​​Magnetic excitations with a low energy scale in the normal and mixed states of LSCO (p = 0.22). Energy dependence of the magnetic response (χ''Qδ, ω). Left: Normal state (T = 26 K, B = 0, pink) showing the low energy scale. 
Right: Illustration of the magnitude of χ'' (Qδ, ħω ~ 1 meV) (red shading) in the normal state (top) and its enhancement under applied magnetic field (bottom). 

For the potential applications of superconducting materials to be fully realised, developing superconductors that maintain their properties at higher temperatures is crucial. The cuprate superconductors currently exhibit relatively high transition point temperatures and therefore give scientists an opportunity to study what makes higher temperature superconductivity possible.

In this study, published in Nature Physics, a group of researchers focussed on the cuprate superconductor La2-xSrxCuO4 (LSCO). Superconductivity in this system is very sensitive to the exact ratio of Lanthanum (La) to Strontium (Sr) offering the ability to understand which properties are correlated with superconductivity. LSCO is also close to being magnetically ordered and one possibility is that the magnetic fluctuations are what enables its superconductivity.

Inelastic neutron scattering offers an excellent method to study these magnetic fluctuations. By using both the LET and Merlin instruments at ISIS, and IN12 at the ILL, the researchers were able to measure over a wide range of reciprocal space and energy scales. This enabled them to build a full picture of the spin fluctuations and phonons, allowing very low energy spin fluctuations to be isolated.  

Although cuprate superconductors are metals above the temperature where they become superconducting, the electrons that carry current behave very strangely.  As the temperature is increased, their ability to carry current is dramatically reduced. The low-energy spin fluctuations could scatter the conduction electrons and explain this "strange metal behaviour". Furthermore, when the superconductor was cooled and the superconductivity suppressed with a magnetic field' the spin fluctuations became stronger and slows down suggesting the material is close to magnetic order. This could help to explain the unusual electronic properties of the cuprates.

This study has demonstrated the potential importance of spin fluctuations in understanding cuprates, physics which neutron scattering is ideal to study. A deeper understanding of their properties and their relation to superconductivity is another step towards designing materials with higher superconducting temperatures.

"The work relies on the unique instrumentation and sample environment available at ISIS," explains Professor Stephen Hayden, from the University of Bristol, who led the project. 

Further information 

The full paper can be found online at DOl: 10.1038/s41567-022-01825-3​

Contact: Voneshen, David (STFC,RAL,ISIS)