Imaging Correlated Electron States in Single-layer Field-Effect Transistors

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Nontechnical abstract: This project is aimed at exploring materials where the repulsion that exists between electrons (referred to by physicists as “electron-electron interactions”) is strong and leads to interesting new behavior. Such phenomena play an important role in materials that confine electrons to two dimensions (2D), such as planar materials only a single atom thick. A central activity of this project is the characterization of such behavior in 2D materials using an instrument called a scanning tunneling microscope (STM), which is essentially a sharp metal needle used to directly image the behavior of electrons at very high spatial resolution (even down to the size of single atoms). The phenomena of interest are referred to as “many-body” behavior because if one electron moves in a material with strong electron-electron interactions then all the other electrons have to move to make way for it (like people moving in a crowded room). This leads to new electronic and magnetic behavior that could potentially be exploited to create useful new technologies. A novel aspect of this project is that delicate 2D materials are synthesized in a new way that allows the density of electrons in the materials to be varied continuously using an electrical contact called a gate while simultaneously imaging the electrons with an STM held at very low temperature. This enables the research team to experimentally test various scientific hypotheses regarding how electrons behave in 2D materials under different conditions. The team disseminates the project results through journal publications, international conference talks, news releases, and educational videos targeting a broad audience. The project also provides high-level scientific training in a strongly interdisciplinary area to postdocs, graduate students, and undergraduates, as well as high school students through different outreach programs, thus preparing them for careers in STEM fields.Technical abstract: This project focuses on the characterization and manipulation of the local electronic structure of highly correlated single-layer two-dimensional (2D) materials by combining gate-tunable field effect transistors (FETs) with scanning tunneling microscopy (STM). Small-bandgap single-layer systems are explored whose properties cannot easily be probed by STM in exfoliated samples, but rather must be accessed via samples grown by molecular beam epitaxy (MBE). A newly developed experimental technique is used to incorporate single-layer MBE-grown materials into gate-tunable field effect transistors that can be imaged by cryogenic STM. The additional gate-tunability of carrier density for these samples enables the exploration of new physical regimes that are typically not accessible to STM, thus enabling unique atomically-resolved studies of highly correlated low-dimensional electronic behavior. The controllable addition and manipulation of impurities in these material systems creates new opportunities for exploring low-dimensional impurity physics and for testing long-standing theories developed for highly interacting materials. Success at these tasks will provide important new physical insight into fundamental questions, such as the interplay between correlation and topology, magnetic interactions and Kondo physics in 1D, as well as the Mott transition and quantum spin liquid behavior in systems of reduced dimensionality.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.