Many complex compounds that contain oxygen and copper – known as cuprates – become superconducting at temperatures much higher than most other materials. It is still not understood why this is the case and if these compounds can be tweaked to further increase the superconducting transition temperatures. In this respect, an outstanding open question is the nature of the superconducting precursor regime at temperatures above the bulk transition. In this regime, the materials are not fully-developed superconductors, but traces of superconductivity are still observable. Over the years, many experimental probes have been applied and theoretical ideas developed to study the precursor regime, but even basic questions – such as how far the regime extends above the bulk transition temperature – have remained unsettled.
In order to address these long-standing issues, a team of scientists from the University of Minnesota, Vienna University of Technology, Austria, and University of Zagreb, Croatia, developed a new approaches to detect the elusive superconducting precursor. They employ the fact that superconductivity is easily influenced by an external magnetic or electric field, and measured the resulting nontrivial response with several distinct experimental setups in a number of representative cuprates. The team discovered unsuspected universal behavior of the precursor that is at odds with conventional theory. However, the observations are explained within a simple model that invokes inhomogeneity: superconducting puddles with a distribution of local transition temperatures exist even above the bulk transition temperature, and they give rise to the unconventional precursor regime. Signatures of the puddles were then found in conventional conductivity measurements, as well as in various previous experiments by other groups, confirming the model. These results thus resolve a pivotal, three-decade old scientific question in the field of superconductivity, and they demonstrate that these materials are inherently inhomogeneous at the nanoscale. The new insights can potentially be exploited to further enhance the bulk transition temperature of these fascinating quantum materials.