Physicists have identified the microscopic mechanism that allows electric current to flow through electron-doped high-temperature copper oxides without losing power. A new study by a team led by the University of Bristol confirms that fluctuating electron spins enable electrons to pair up and transmit electricity with zero resistance.To isolate the mechanism, researchers from the UK, the Netherlands, China, and the U.S. built an artificial thin-film sample out of lanthanum cerium copper oxide. They deposited the material onto a base layer by atomic layer. Along the strip, they gradually replaced lanthanum with cerium. This substitution introduced extra conduction electrons and formed a smooth gradient of critical transition temperatures. This shifted the critical superconducting point from its highest value down to zero across a single sample.Suppressing the superconducting state“Using lithography, the researchers then patterned a series of electrical contacts along the film,” explained a media release. This design allowed them to test areas with different critical temperatures while keeping material disorder identical throughout the sample.Next, the researchers placed the film inside high-powered magnets at the European Magnetic Field Laboratory in the Netherlands. They used strong magnetic fields to suppress the superconducting state and recorded how resistance changed across temperatures, field strengths, and critical points.“The processes which facilitate higher temperature superconductivity, however, have remained a source of mystery and fascination,” said study co-lead author Nigel Hussey, Professor of Physics at the University of Bristol.“Through measurements carried out at the European Magnetic Field Laboratory in the Netherlands and subsequent careful analysis, we have found the smoking gun which solves part of the equation and explains what causes this perplexing state.”Uncovering source of frictionAnalysis of this resistance data uncovered the primary source of electrical friction. The electrons were scattering directly off fluctuating electron spins. Because the strength of this interaction closely matched the critical transition temperature along the strip, the team confirmed that spin fluctuations cause the superconducting state in this material.However, the team emphasizes that this discovery solves only one part of the wider problem.Parent copper-oxide materials can be turned into superconductors in two separate ways. The first method adds extra electrons, which the researchers achieved here by using cerium. The second method removes electrons, leaving behind empty spaces called holes.The highest transition temperatures occur exclusively in hole-doped materials. When the Bristol-led team compared their new data with earlier high-field studies on hole-doped samples, they found that spin interactions alone could not account for the higher performance. Although the two materials share the same chemical base, their internal arrangements differ. The researchers concluded that a second, unknown factor must work alongside spin fluctuations to produce the higher operating temperatures on the hole-doped side.Critical for next-gen technologiesThe finding marks a clear break from older, conventional superconductors. In those low-temperature metals, the pairing of electrons relies on slow vibrations of ions in the crystalline lattice. High-temperature copper oxides require much faster interactions, which electron spins provide.Resolving the full mechanism remains critical for advancing technologies that rely on zero resistance. These systems are used to build powerful magnets for medical imaging scans, efficient power cables, and ultra-sensitive measuring equipment. “While we have found the first key ingredient, the task now is to go and find the second,” concluded Prof Hussey. “Only then can theorists develop the full story for high-temperature superconductivity.”The study was published in the journal Nature Physics.