Materials ranging from heavy fermion metals to high-temperature superconductors have been shown to exhibit strange-metal behavior. Quasiparticles carry current in ordinary metals; direct experimental data supporting the absence of quasiparticles in strange metals is missing.
In recent quantum noise experiments at Rice University, physicists found that a strange metal quantum material exhibits strangely quiet behavior. Shot noise, or quantum charge fluctuations, measurements offer the first concrete proof that electricity appears to flow through strange metals in a peculiar liquid-like form that defies easy explanation in terms of quasiparticles, which are quantized packets of charge.
The experiments were conducted on nanoscale wires of a quantum critical material (YbRh2Si2), precisely a 1-2-2 ratio of ytterbium, rhodium, and silicon. High levels of quantum entanglement in the material result in a very peculiar (or “strange”) temperature-dependent behavior that differs significantly from common metals like silver or gold.
Significant technical obstacles were encountered when using the procedure in YbRh2Si2 crystals. Shot noise investigations require samples that are nanoscopic in size rather than single macroscopic crystals.
Consequently, it was necessary to grow incredibly thin but flawlessly crystalline films, which Silke Paschen, Maxwell Andrews, and their associates at TU Wien accomplished after nearly ten years of arduous struggle. Chen then had to figure out how to make wires out of these thin films, roughly 5,000 times thinner than a human hair, while keeping that degree of precision.
Rice co-author Qimiao Si, the study’s lead theorist, said, “The low shot noise brought about fresh new insights into how the charge-current carriers entwine with the other agents of the quantum criticality that underlies the strange metallicity. In this theory of quantum criticality, the electrons are pushed to the verge of localization, and the quasiparticles are lost everywhere on the Fermi surface.”
Rice’s Doug Natelson, the study’s corresponding author, said, “The noise is greatly suppressed compared to ordinary wires. Maybe this is evidence that quasiparticles are not well-defined things or are just not there, and the charge moves in more complicated ways. We have to find the right vocabulary to talk about how charge can move collectively.”
“Sometimes, you kind of feel like nature is telling you something. This ‘strange metallicity’ shows up in many different physical systems, despite the fact that the microscopic underlying physics is very different. In copper-oxide superconductors, for example, the microscopic physics is very, very different than in the heavy-fermion system we’re looking at.”
“They all seem to have this linear-in-temperature resistivity characteristic of strange metals, and you have to wonder if there is something generic going on that is independent of whatever the microscopic building blocks are inside them.”
Journal Reference:
- Liyang Chen, Dale T. Lowder, Emine Bakali, Aaron M. Andrews, Werner Schrenk, Monika Waas, Robert Svagera, Gaku Eguchi, Lukas Prochaska, Yiming Wang, Chandan Setty, Shouvik Sur, Qimiao Si, Silke Paschen and Douglas Natelson. Shot noise in a strange metal. Science. DOI: 10.1126/science.abq6100