Can electricity flow without electrons?
We all learned that electricity is caused by electrons moving in a metal. Each electron carries a discrete charge. The picture is more complicated because electrons repel each other. Any motion from a single electron may disturb a cloud of neighboring electrons. If the disturbance is mild, electrons end up moving in clumps instead of as solitary entities. Physicists call these clumps electron quasiparticles. In the end, a current is still carried by discrete charges, except that these discrete charges are not technically “free” electrons. This collective motion, called a Fermi liquid, has been the standard theory of metals for six decades. Surprisingly, many new conductors, named “strange metals,” fail to obey this paradigm. In these materials, the electricity is not carried by discrete charges but by something else! Using a measurement technique called shot noise, researchers observed a radical quantum blurring of electrons into a featureless liquid.
If individual electrons do not carry electricity, then what does? The Fermi liquid explanation of electrical transport is one of the foundational successes of condensed matter physics—the study of solid substances. This new research challenges our current understanding and may lead to a new theory of electrical transport. The repercussions might be far reaching. For example, understanding the departure from Fermi liquid behavior might reveal the hidden workings of high temperature superconductors, which in their normal, non-superconducting state behave like strange metals.
Strange metals defy the orthodox understanding of electric transport via discrete charges. In these materials, the resistance changes linearly at low temperature. In contrast, typical metals carriers have a quadratic behavior in resistance change. To determine whether electricity is transported in discrete chunks, researchers used a technique called shot noise. Shot noise measures random fluctuations in a direct electrical current. These random fluctuations occur because the current is a flow of discrete charges and each charge's arrival varies statistically. It’s like when a small number of large drops of rain hit a roof. They do not all hit the roof at the same time, but rather their arrival is distributed. In this case, the shot noise is high. At the other extreme, if the rain is hard enough, there are no drops – the flow of rain is continuous and featureless. In this case the shot noise is zero. This is what seems to happen in strange metals!
Measuring shot noise without external influences is not easy. In a metal, the vibration of the atoms’ lattice can push electrons around and obscure the shot noise. Researchers had to fabricate nanoscale wires so small that electrons pass through them faster than it takes to feel the ripples of the lattice vibrations. The experiments provided strong evidence that in the strange metal YbRh₂Si₂ there are no quasiparticles, and the current is not carried by discrete chunks. It’s like electrons lose their identity and meld into a quantum soup. The claim of absence of quasiparticles is a very strong one and not all physicists are ready to accept it. These results will spark a flurry of high-profile investigations and contribute to the development of new theories of strange metals. Reference Shot noise in a strange metal Liyang Chen, Dale T. Lowder, Emine Bakali, Aaron Maxwell Andrews, Werner Schrenk, Monika Waas, Robert Svagera, Gaku Eguchi, Lukas Prochaska, Yiming Wang, Chandan Setty, Shouvik Sur, Qimiao Si, Silke Paschen, Douglas Natelson https://www.science.org/doi/10.1126/science.abq6100 U.S. Department of Energy
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