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One of the main concerns in the field of condensed-matter physics is the nature of strongly interacting particles. For context, most metals are described as systems containing weakly-interacting electrons, despite electron interactions being usually strong.

In physics, the Landau Fermi liquid theory is a theoretical model that describes interacting fermions and, subsequently, the normal state of metals at low temperatures. It also helps explain how electrons interact with other electrons and get affected in return. In metals, these interactions affect and alter the metals' characteristics but supposedly have no effect on the structure of the system, remaining similar to its state when it still had free electrons.

Now, researchers from the University of Illinois, Johns Hopkins University, the City University of New York (CUNY) College of Staten Island, and the University of Colorado Boulder used a novel technique to inquire about the possibility of a "strongly disordered and highly correlated and disordered electron system" could be mapped to a system, leading to a unique phenomenon called marginal Fermi glass.

Finding an Exception to the Anderson Localization

Building on Philip W. Anderson's work, an American theoretical physicist and 1977 Nobel laureate for Physics who demonstrated that waves do not propagate in strongly random systems. This absence of wave diffusion in a disordered medium is now known as the Anderson localization and applies to acoustic, electromagnetic, and neutral matter waves. There have also been theories that the Anderson localization also applies to electronic waves or the waves where electrons propagate and have profound applications in the field of quantum mechanics.

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"Strongly interacting electron waves can certainly be localized by disorder, but whether they do it in a fashion consistent with Anderson localization is unclear," explains Peter Armitage, one of the authors behind the new study, in an interview with Phys.org. He adds that while interactions are strong in insulator materials, the important question is whether or not these interactions "effectively irrelevant, as in many metals," noting that their work is the first instance that demonstrates that these interactions are actually irrelevant.

Basically, researchers were able to demonstrate that Anderson localization, indeed, does not apply to electron waves. To arrive at this groundbreaking observation, researchers used a technology called THz 2-D coherent spectroscopy. It allowed the generation of very large electric fields in the context of radio frequencies in the Terahertz (THz) range. The electric fields generated allowed researchers to observe optical nonlinearities at that level, looking for interaction signatures between electrons based on THz photons' interactions in the study.

Marginal Fermi Glass

"When a physical system is excited, some rate of that energy always leaves the system," Armitage explains. Because interactions in most metals are "only weakly felt," the rate of this energy leaving the system is too small. He adds that through the THz 2-D spectroscopy, they were able to show that not only the rate is small, but it is also actually proportional to the frequency used to excite the system.

Armitage explains that the phenomenon can be called "marginal Fermi liquid," a little-known state predicted to exist in materials similar to the normal state of cuprate superconductors.

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