Researchers at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, Germany, have successfully measured the ultimate "depairing limit" of superconductors by using ultrafast laser pulses to bypass disruptive quantum processes. The breakthrough study, published in Nature Physics, allowed physicists to observe the exact point where the entangled electrons that enable superconductivity, known as Cooper pairs, are ripped apart.
Outrunning Quantum Vortices
Superconductors are foundational to technologies like levitating trains, particle colliders, and medical imaging machines. However, these materials must be cooled to extreme temperatures to conduct electricity without resistance. In type II superconductors, internal magnetic fields propagate as tiny tornadoes of electricity called quantum vortices. When an electric current becomes too strong, it causes these vortices to move, generating resistance and heat that stops the superconductive state.
To measure the depairing limit before these disruptions could occur, the researchers had to outpace the velocity of the vortices, which move at tens of kilometers per second. Lead author Eryin Wang explains that the team's strategy was to outrun this vortex dynamics. They developed an optical-electric circuit that fired green laser pulses lasting just 300 femtoseconds (quadrillionths of a second) at a switch. This converted the optical pulse into an electric current pulse lasting picoseconds (trillionths of a second), successfully bypassing the slower vortex dynamics that cause resistance.
Comparing NbN and YBCO Materials
The team tested two different superconducting materials: niobium nitride (NbN) and a high-temperature superconductor from the yttrium barium copper oxide (YBCO) family. These materials were chosen for their different gap symmetries, which dictate how easily their paired electrons can be ripped apart. NbN features an s-wave gap symmetry, where the energy needed to disentangle Cooper pairs is the same in all directions of momentum space. In contrast, YBCO has a d-wave gap symmetry, meaning its energy gap varies significantly and disappears in certain directions.
During testing, the NbN material maintained its superconductive state until the applied electric current reached a well-defined threshold, at which point its Cooper pairs fell apart and superconductivity abruptly stopped. The YBCO material displayed a gradual weakening of its superconductive state instead.
Our results suggest that picosecond transport can provide access to microscopic properties of superconductors, including their gap symmetry, that are not directly available from conventional DC transport.
Andrea Cavalleri, leader of the Condensed Matter Dynamics group, MPSD
A New Framework for Superconductor Testing
The novel approach provides a framework for testing the key properties of superconducting materials on their fundamental timescales. By applying these ultrafast current pulses to a wider variety of superconductors, researchers can reveal intrinsic properties like gap symmetries and Cooper pair stability. Jianqiao Meng, a professor of physics at Central South University who was not involved in the study, notes that the work highlights the potential of ultrafast current pulses as a powerful new tool for accessing these intrinsic properties.
This article was written by AI from reporting by sciencealert.com.
