Testing Anderson's 'Fermi glass' using terahertz photon echoes

Illinois Physics

Illinois Physics Professor Fahad Mahmood in his lab at the Materials Research Laboratory. Photo by Siv Schwink, Illinois Physics
Illinois Physics Professor Fahad Mahmood in his lab at the Materials Research Laboratory. Photo by Siv Schwink, Illinois Physics
Many properties of metals can remarkably be understood in terms of “free” electrons, even though electrons interact with each other through a Coulomb force, which can be both large and long ranged. It turns out that these interactions collectively result in the emergence of “quasiparticles” whose behavior is very similar to that of free, non-interacting electrons. This description forms the basis of the famous Landau Fermi liquid theory. In fact, the notion of finding new non-interacting quasiparticles to describe complicated interacting systems is a key paradigm in condensed matter physics. 

A canonical example of such a complicated interacting system is phosphorus doped silicon near the insulating side of its metal-insulator transition. Here, a complex interplay between the effects of strong interactions and strong disorder results in observable properties markedly different from those of conventional metals or insulators. Perhaps such a system can also be described in terms of non-interacting quasiparticles like “free” electrons in a metal? This conjecture was first put forth by Phil Anderson in 1970 who dubbed such a system as a “Fermi-glass.” This description certainly holds in the absence of interactions as in the case of the celebrated “Anderson localization,” whereby strong disorder localizes electronic states. However, whether this description is possible in materials with both strong interactions and strong disorder has been an open question for nearly 50 years. By developing a unique spectroscopic technique based on strong terahertz frequency light (2D THz spectroscopy), we find the answer to this question is “No.” Interaction effects are always strong enough to preclude any description in terms of non-interacting quasiparticles. 

Using 2D THz spectroscopy, we first observed distinct coherent phenomena, “THz photon echoes,” that allowed us to reliably measure the energy relaxation and decoherence times—typically known as T1 and T2times respectively—in a strongly interacting disordered material. The scaling of these times with temperature and disorder concentration was found to be at odds with Anderson’s conjecture. In our view, this new phenomenology can be best described as a “marginal Fermi glass.”

Our technique of 2D THz spectroscopy can be broadly applied to other complex materials, such as high-temperature superconductors and quantum spin liquids, to understand the fundamental nature of excitations and quasiparticles in these systems.

This research was published in the article, "Observation of a Marginal Fermi Glass," in Nature Physics (2021).

Research was performed by Fahad Mahmood of Illinois Physics; Dipanjan Chaudhuri and Peter Armitage of Johns Hopkins University; Sarang Gopalakrishnan of CUNY College of Staten Island; and Rahul Nandkishore of University of Colorado Boulder

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