Collaborative Research: One-Dimensional Correlated and Topological Electronic States in Ultra-Clean Carbon Nanotubes

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Non-Technical: By integrating metals and semiconductors into circuits, humans have learned to control the flow of electrons to process information. Despite enormous successes in this field, there are still unsolved challenges. Existing methods to control electron flow produce imprecise levels of electrical current, and often lead to energy wasted as heat. In this project, the team explores a new way of controlling electric current using a novel method for pumping electrons. The pump is built from a single carbon nanotube that is integrated with a microscopic pattern of wires that controls a time-varying electric field. The pump is predicted to carry an electrical current without dissipation of energy. It is also predicted to be useful as a metrological standard for generating a precise current from a frequency source. Additionally, the system unravels new knowledge about the behavior of interacting electrons which is fundamentally different in such reduced-dimensional systems. The project trains students including under-represented minorities and strengthens the pipeline that feeds the science and engineering workforce. Outreach activities for high-school students from under-represented minorities include an annual science summer camp. Technical: One dimensional (1D) electronic systems are naturally correlated, in contrast to the Fermi liquids that usually form in higher (two and three) dimensions. While topological order has been studied intensely in 2D and 3D, it has received relatively little attention in 1D where the combination of topology and correlations leads to fascinating possibilities. In the early 80s, Thouless predicted that the same topological invariant as in the quantum Hall effect arises in 1D systems, when the magnetic field is replaced by a time-varying periodic potential. This system, known as an adiabatic charge pump or Thouless pump, has not been realized cleanly in condensed matter, due to problems of single-electron charging and sample disorder. The research team’s preliminary results demonstrate the suitability of their long, suspended, ultra-clean carbon nanotubes (CNTs) for realizing the Thouless pump, and related exotic phenomena. Electrons in a long, ultra-clean CNT form either a Wigner crystal, Luttinger liquid, or a correlated insulator state. Upon application of the periodic potential, the system may then evolve into an integer Thouless pump, a fractional Thouless pump, an artificial Mott insulator or other exotic states. This project focuses on the rich physics that can be found in, and between, these exotic states. To access and control the various phenomena, the team will tune (i) the electron density in the CNT, (ii) the wavelength of the external potential, and (iii) the Coulomb interaction strength (a function of CNT band gap). The specific aims of the project are: 1. Map the parameter space of 1D correlated states in the CNT system without an external periodic potential. 2. Introduce topological order by generating an external periodic potential with surface acoustic waves and a precision flip-chip geometry. 3. Study the interplay of correlations and topological order in 1D.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.