Next generation energy efficiency: the potential of superconductivity
Climate change and the energy crisis have highlighted the need to rapidly accelerate progress on global energy efficiency. While superconductivity holds immense potential to revolutionise energy storage and transmission, the complexity of the domain requires multidisciplinary research in order to understand the physics of superconductors and how they can be enhanced. These next-generation materials and technology are now a step closer as a result of international collaborative research combining the capabilities and expertise of a number of large-scale research facilities (Institut Laue-Langevin, Diamond Light Source, the European Spallation Source and Paul Scherrer Institut) and universities (Technical University of Denmark , Chalmers University of Technology, Université Grenoble Alpes, Hokkaido University, Muroran Institute of Technology KTH Royal Institute of Technology and the Universität Zürich).
Superconductivity describes the ability of certain materials to conduct an electric current with zero resistance and thus extremely low energy losses. First observed in mercury in 1911, approximately one third of chemical elements have now been found to superconduct when cooled close to 0 K (-273.15oC). Today, superconducting magnets are used to create powerful magnetic fields with applications from MRI machines to scientific facilities such as the Large Hadron Collider at CERN. The ultimate ambition, however, is to achieve superconductivity at room-temperature, enabling an energy-efficiency revolution through the lossless transmission and storage of electrical energy.
A major breakthrough came in 1986 with the discovery of high-temperature superconductivity in cuprates – a new class of material made of layers of copper and oxygen atoms separated by layers of other elements. Though an explosion of research followed, high-temperature superconductivity remains a challenge for theorists in the field. “It’s a very complex research area,” explains Martin Boehm, instrument scientist and head of the Spectroscopy group at the Institut Laue-Langevin (ILL). “Many different scientists are involved, using an array of techniques and materials and proposing a number of contrasting ideas and theories to try to understand how these materials work at the microscopic level.”
One key question is whether charge density waves (CDW) and spin density waves (SDW) simply coexist or are directly coupled in cuprate materials and how their fluctuations may give rise to high-temperature superconductivity. Recent x-ray scattering experiments demonstrated that the population of CDW domains can be controlled by the application of a small uniaxial strain along the copper-oxygen (Cu-O) bond direction . If the CDW and SDW orders are coupled, it is expected that SDW domains should respond to uniaxial pressure in a similar manner. “Neutrons are the only technique capable of acquiring the specific information necessary to answer this question,” explains Boehm. “Spin is a magnetic property and neutrons have the unique ability to provide a global vision of the magnetic moments and thus information on the generation and dynamics of the pattern.”
The first step involved the preparation of a crystal of the cuprate superconductor La1.88Sr0.12CuO4 (LSCO). “The sample preparation is very difficult and there are only a few research groups in the world capable of producing such high-quality homogeneous crystals,” explains Boehm. A comprehensive characterisation of the sample was thus required: the magnetic response was checked using muon spin rotation at the Paul Scherrer Institut (PSI), the surface crystallinity was confirmed using x-ray Laue diffraction and the bulk crystallinity was verified using the ILL’s Laue diffractometer Orient Express.
In order to achieve a sufficiently high uniaxial strain, the size of the LSCO crystal was much smaller than is generally considered suitable for neutron experiments due to the fact that the scattered neutron intensity scales linearly with the crystal mass. Detecting the subtle SDW order in the weak signal from a small crystal was further complicated by the increased background scattering caused by the sample environment required to create the pressure conditions. A pressure cell made from high purity aluminium and masked with neutron-absorbing cadmium was specially designed by collaborators at PSI who then employed advanced neutron-ray-tracing simulations to ensure a sufficient signal-to-noise ratio (SNR) was achievable.
The high neutron intensity – essential in order to achieve that SNR – was obtained by combining the high flux of neutrons with the unique focusing capabilities of the low-energy spectrometer ThALES at the ILL. Reconstructed in 2015 with funding from the Czech Ministry of Science and Education, “ThALES is equipped with a double-focusing silicon monochromator that enables the instrument to concentrate the ILL’s high flux neutron beam onto a sample measuring several millimetres in dimension, thus providing the instrument with a unique capability to address such science cases,” explains Boehm.
Neutron scattering experiments were first performed at ambient pressure and at a temperature of 2 K. A compressive strain along the Cu-O bond direction was then applied and the measurements repeated at temperatures of 2 and 40 K. The results, recently published in Communications Physics, show that the application of uniaxial pressure leads to a dramatic redistribution of magnetic peak intensity, without altering the other characteristics of the magnetic order . “The x-ray and neutron scattering results together provide direct experimental evidence that the SDW and CDW order in LSCO respond to uniaxial pressure in the same manner, strongly suggesting that spin and charge degrees of freedom are intertwined,” explains Boehm. “A charge-spin stripe arrangement thus appears to be the suitable model for high-temperature superconductors, excluding more complex structures previously proposed.”
The project demonstrated the unique capability of ThALES to generate an exceptionally strong and focused neutron flux in order to successfully probe strongly correlated electron materials involving very small samples and the significant sample environments needed to create extreme conditions such as high pressure or very low temperature. “We are capable of carrying out exceptional experiments here at the ILL that can’t be done elsewhere due to the unique combination of the facility’s high neutron flux and the powerful focusing capability of ThALES,” explains Boehm.
 Unveiling Unequivocal Charge Stripe Order in a Prototypical Cuprate Superconductor, Physical Review Letters, 2022
 Single-domain stripe order in a high-temperature superconductor, Communications Physics, 2022
ILL instrument used: ThALES, the three-axis low energy spectrometer