UTe₂ superconductor at 2 K keeps memory of current pulses, Charles University and Cambridge find
A team from Charles University and the University of Cambridge showed that current pulses switch UTe₂ between low and high critical current density states, and the state persists for hours.
Researchers from Charles University in Prague and the University of Cambridge have found a memory effect in an ultraclean crystal of the uranium–tellurium compound UTe₂. The work was published in Nature nature.com
Naturenature.comOpen source ↗. The effect was observed at a temperature of about two kelvins, and the crystal retained information about the current that had passed through it for several hours.
How the switching works
UTe₂ is a candidate p-wave superconductor. The authors applied a magnetic field to reach an intermediate regime between two distinct superconducting phases. In this regime, direct current pulses push the material in and out of a metastable state with an enhanced critical current density, J_c.
The switching is controlled by the strength and duration of the pulses. According to the authors, the system "remembers" whether it is in the high or low J_c state for extended periods. Michal Vališka of the Faculty of Mathematics and Physics, Charles University, a co-author, said that what matters is not only the magnitude of the current but also the rate at which the change is made. In magnetic memories, information is stored in the orientation of magnetic domains, whereas here the states are set by the time course of the current.
Vortex glass as the explanation
The authors attribute the effect to magnetic vortices inside the superconductor. They interpret it as quenching of a disordered, out-of-equilibrium glassy vortex state, which has stronger pinning forces and therefore a higher J_c nature.com
Naturenature.comOpen source ↗. A sharp change in current can trap the vortices in this disordered configuration, while a gradual change lets them settle into another arrangement. The equilibrium vortex lattice is restored by annealing the system with a gradual current ramp, which returns it to the original state.
The authors state that the memory does not require adjacent magnetic or semiconducting interfaces. They consider it an intrinsic property of UTe₂, rooted in the superconducting order itself. Multiphase superconductors are rare; the paper names CeRh₂As₂, UPt₃ and URhGe among the known examples.
Crystal quality played a key role. Vališka notes that impurities can suppress the properties of unconventional superconductors, so growing ultraclean crystals is a necessary first step. UTe₂ first appeared in research in 2016. Whether similar memory occurs in other materials remains an open question, and the practical use of this particular compound has not been estimated.
