Science & Technology (Commonwealth Union) – The discovery of superconductors by Dutch scientist, Kamerlingh Onnes was a key development in physics.
Superconductivity is one of the most remarkable collective phenomena in quantum materials. Certain materials, when cooled below a certain transition temperature, have their electrical resistance drop to zero, and current can flow without losing energy as heat. This happens when electrons pair up in correlated units known as Cooper pairs and move together through the material as a coherent quantum state. Superconductors could therefore be useful in applications such as high-powered magnets, very sensitive detectors and quantum circuits.
However, superconductivity cannot carry unlimited electrical current. All superconductors have a critical current; the maximum current they can carry before resistance and energy dissipation begin.
In type-II superconductors the critical current measured experimentally is often not so much determined by the microscopic superconducting properties of the material, but by the dynamics of magnetic vortices. These little vortex structures allow the magnetic flux to penetrate the material. If the current is strong enough the vortices can start to move through the superconductor. Their motion generates electrical resistance and heat, which can cause the material to lose its superconducting state.
But there is a more fundamental limit, known as the depairing current. This is the maximum current a superconducting state can theoretically sustain before the Cooper pairs themselves begin to break apart.
Eryin Wang, the study’s lead author indicated that they are winding up a spring. In effect, the flow of current twists the phase of the coherent quantum state of the superconductor. If that twist is too large, then the superconducting condensate becomes unstable and the Cooper pairs begin to break apart.
This intrinsic limit is hard to achieve in standard DC measurements, since the vortex motion and the associated heat destroy the superconductivity first. Hence these dissipative effects conceal the intrinsic properties of the superconducting state.
Wang pointed out that their strategy is to outrun the vortex dynamics.
Vortices typically move at tens of kilometres per second so in a picosecond they move only tens of nanometres.
The study demonstrates the need for further research into this area. Recent advances in technology on new methods of analyzing vast amounts of data demonstrate to us the advances in a wide area of science that had was previously unable to process and scrutinize data and results. This gives us a promising future in the area of superconductivity. For physics researchers these new developments are area to take note and apply in related areas that could possibly be linked to their research and unanswered questions.
“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,” explained Andrea Cavalleri, who is the lead of the study.
