Supercurrents seem like the stuff of science fiction. But they’re real, and they’re what happen when particles move without any resistance, so they don’t lose any energy. They’re usually only possible at very low temperatures of below -150°C, but now a group of scientists think they might have gone one step further, and produced a supercurrent at room temperature.
According to the paper, which was published by an international team in Nature Physics, a supercurrent is “a macroscopic effect of a phase-induced collective motion of a quantum condensate”. Put simply, it’s a quantum effect seen on a much larger scale than the tiny world of quantum.
It is only possible to set up a supercurrent in Bose-Einstein condensates (or BECs). These are collections of bosons which can be described by a single wavefunction, and include systems like charged particles moving in a superconductor, or particles moving in superfluid helium. The particles’ continuous motion isn’t maintained by a force (since no energy is lost, no force is needed), but by a gradient in the phase of the BEC’s wavefunction. Until now, supercurrents have only been set up at very low temperatures, because heat excites the particles into quantum states above the ground state. This requires more than one wavefunction to fully describe it, so the BEC is destroyed.
In their research, the team used a condensate of magnons, a type of quasiparticle (packets called quanta of energy in a crystal lattice which can be regarded as particles) for the BEC. As had been done previously in 2006, they set up the BEC by injecting magnons into the ground state of a crystal film made of yttrium iron, using a process called parametric pumping, so that the concentration of magnons was high. Then, by shining a laser pulse at a point on the BEC, they created a temperature difference between that spot and the rest of the material. Heating the BEC causes the magnetic properties and, more importantly, the phase of the BEC, to change. This is what caused the phase gradient in the wavefunction, crucial to the formation of the supercurrent.
The team observed the magnons moving away from the heated region, with a flow magnitude which increased when the temperature difference increased. They say that it is highly likely that this flow of particles is a supercurrent. Theoreticians in Israel and Ukraine have also constructed mathematical models of the observed magnon flow, and say a supercurrent is the only explanation.
Other scientists disagree though, saying that the phenomenon could be explained by less novel ideas. Demokritov, another researcher in the field, says that the parametric pumping may be adding energy to the system, which could mean that the magnons gain enough energy to overcome friction rather than moving without it. He says that his own team had better evidence for a supercurrent than this, but that they didn’t call it that because they couldn’t conclusively show that energy wasn’t being dissipated by friction.
If it is a supercurrent though, it could be used in devices using macroscopic quantum states, meaning big things for information storage and processing.
D. A. Bozhko, A. A. Serga, P. Clausen, V. I. Vasyuchka, F. Heussner, G. A. Melkov, A. Pomyalov, V. S. L’voc, B. Hillbrands, Supercurrent in a room-temperature Bose-Einstain magnon condensate, 1st August 2016, Nature Physics, http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3838.html (accessed 08/08/2016)
Tim Wogan, First every supercurrent observed at room temperature, 2nd August 2016, IOP Physics World, http://physicsworld.com/cws/article/news/2016/aug/02/first-ever-supercurrent-observed-at-room-temperature (accessed 05/02/2016)