In a recent set of quantum noise experiments at Rice University, a peculiar “strange metal” quantum material has defied expectations by exhibiting an unexpected tranquility. Published this week in Science, the measurements focused on quantum charge fluctuations, specifically “shot noise,” providing a groundbreaking revelation. The results suggest that electricity may traverse strange metals in a distinct liquid-like form, challenging the conventional understanding framed around quantized packets of charge known as quasiparticles.
Rice's Doug Natelson, the corresponding author of the study, highlighted the significant deviation in noise levels compared to ordinary wires, prompting a reevaluation of the nature of quasiparticles. He pondered whether quasiparticles might lack well-defined characteristics or even be absent, leading to more intricate charge movement mechanisms. The challenge, as Natelson pointed out, lies in developing the appropriate vocabulary to articulate how charge moves collectively through these materials.
The experiments honed in on nanoscale wires composed of a well-studied quantum critical material featuring a precise 1-2-2 ratio of ytterbium, rhodium, and silicon (YbRh2Si2). This material, known for its quantum entanglement, exhibits temperature-dependent behavior. For instance, when cooled below a critical temperature, it swiftly transitions from a non-magnetic to a magnetic state. At temperatures slightly above this critical threshold, YbRh2Si2 manifests as a “heavy fermion” metal, characterized by charge-carrying quasiparticles that are hundreds of times more massive than bare electrons.
Quasiparticles, conceptualized 67 years ago to represent the collective effect of countless electron interactions within metals, are now under scrutiny. Prior theoretical studies hinted that strange metal charge carriers might not conform to the quasiparticle model. The shot noise experiments conducted by Doug Natelson, lead author Liyang Chen, and collaborators from Rice and the Technical University of Vienna presented the first direct empirical evidence challenging the conventional quasiparticle framework.
Shot noise measurements, as explained by Natelson, assess how granular the charge is as it traverses a material. The discrete charge carriers, on average, arrive at a certain rate, but their temporal spacing varies. Applying this technique to crystalline films of the 1-2-2 ratio material presented formidable technical challenges. Achieving near-perfect crystalline films and fashioning wires approximately 5,000 times narrower than a human hair required meticulous work.
Lead theorist Qimiao Si, along with Natelson and lead TU-Wien co-author Silke Paschen, initially discussed the experiment's idea in 2016. Si emphasized that the results align with his quantum criticality theory, published in 2001, which explores the behavior of electrons near localization. Calculations by Si's group ruled out the quasiparticle concept, indicating a departure from conventional understanding.
The broader question raised by Natelson revolves around whether similar unconventional behavior may be observed in other compounds exhibiting strange metal characteristics. The ubiquity of “strange metallicity” across diverse physical systems, each with distinct microscopic physics, prompts scientists to consider underlying commonalities. Natelson mused, “This ‘strange metallicity' shows up in many different physical systems despite the fact that the microscopic, underlying physics is very different.” The study's findings open avenues for further exploration into the intriguing realm of strange metals and their unique quantum behaviors.
Source: Rice University