Unraveling the Enigma of Strange Metals: New Insights from Quantum Noise Experiments

Strange metals, a class of quantum materials that defy conventional understanding, have long been a subject of fascination for scientists. In a recent breakthrough, researchers at Rice University have conducted quantum noise experiments shedding light on these enigmatic materials. Published in Science, the study provides compelling evidence that electricity flows through strange metals in an unconventional liquid-like form, challenging the prevailing notion of quantized charge packets called quasiparticles.

Traditionally, it was believed that charge carriers in metals were well-defined quasiparticles, which were the result of intricate interactions between individual electrons. However, the Rice University study suggests that quasiparticles may not be as well-defined as previously thought or may not even exist. The quantum charge fluctuations, known as “shot noise,” observed in the experiments displayed a significant suppression compared to regular wires. This observation implies that the movement of charge in strange metals occurs through more complex and collective means, necessitating the development of a new theoretical framework to describe this phenomenon accurately.

The researchers conducted the experiments on nanoscale wires made from a quantum critical material called ytterbium, rhodium, and silicon (YbRh2Si2). This compound exhibits unique temperature-dependent behavior due to its high degree of quantum entanglement. As the material is cooled below a critical temperature, it undergoes a rapid transition from a non-magnetic to a magnetic state. Above this threshold, YbRh2Si2 displays characteristics of a “heavy fermion” metal, with quasiparticles carrying charges hundreds of times greater than bare electrons.

The measurement of shot noise provides valuable insights into the granularity of charge carriers as they traverse a material. By analyzing the temporal distribution of discrete charge carriers, researchers can discern patterns of their arrival and spacing. In the case of strange metals, the experiment confirmed that the charge carriers arriving at an average rate can occasionally be closely packed or more distantly spaced due to the non-quasiparticle nature of charge flow.

Performing the shot noise experiments on crystals composed of YbRh2Si2 presented numerous technical hurdles. The crystalline films used needed to be nearly flawless, necessitating considerable effort in their growth. Additionally, the wires fashioned from these crystals had to be extraordinarily narrow, approximately 5,000 times thinner than a human hair. Overcoming these challenges required the collaboration of experts in materials science, quantum physics, and engineering to achieve reliable and accurate results.

The lead theorist of the study, Qimiao Si, along with Doug Natelson and Silke Paschen, first conceived the idea while Paschen was a visiting scholar at Rice University in 2016. Their experiments align with Si’s 2001 theory of quantum criticality, which proposed that electrons, driven to the border of localization, lose their quasiparticle characteristics everywhere on the Fermi surface. The calculations performed by Si’s team further supported the notion that the observed shot noise results disprove the existence of quasiparticles in strange metals.

This groundbreaking research raises a broader question regarding the prevalence of similar behavior in other compounds exhibiting strange metal behavior. Despite the distinct microscopic physics at play, the phenomenon of “strange metallicity” transcends different physical systems. Even in copper-oxide superconductors, which differ drastically from the heavy-fermion system investigated by the researchers, the strange metal behavior persists. This suggests a fundamental and universal aspect of strange metals waiting to be unraveled.

The recent quantum noise experiments conducted by researchers at Rice University have provided significant insights into the mysterious nature of strange metals. The observed suppression of shot noise offers a tantalizing glimpse into the unconventional flow of charge in these materials, challenging the existing understanding of quasiparticles. With further investigations and theoretical advancements, scientists hope to unlock the secrets of strange metals and uncover a new paradigm for understanding the behavior of complex quantum systems.

Science

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