CAREER: Revealing the Dynamics of Charge Carriers in Strongly Correlated Materials with Scanning Tunneling Potentiometry

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Non-technical abstract: The way in which electrons and heat move through a material is often assumed to resemble billiard balls or diffusive spreading, but in many materials this is not the case and the movement is unknown. This project aims to develop new microscopy techniques that can be used to directly visualize the flow of electrons and heat in a material at the microscopic level. Understanding that motion allows materials to be produced that have less resistance, better heat conductivity, and that can model fluid and air flows in ways that are less costly than current methods. A significant aspect of this project is to develop touch-based learning tools for understanding the physics of materials, where students use tactile models and haptic-feedback systems to ‘feel’ the crystal lattice of a material in the same way an electron or molecule would. These lessons are being developed for advanced undergraduate labs and for K-12 outreach programs, with an aim of explaining the properties of materials in an intuitive and memorable way. A travelling class is also being developed for older adults and differently sighted individuals, so that they can learn about materials in an engaging tactile manner; and a class in being developed for Grandparents University at the University of Wisconsin, where older adults and their grandchildren learn about scientific concepts together. Technical abstract: This project aims to advance and utilize two scanned probe techniques – scanning tunneling potentiometry (STP) and scanning tunneling superconducting thermometry (STST) – to separately image the nanoscale flow of charge and heat in materials, such as graphene, with hydrodynamic and quantum hall phases. Hydrodynamic material systems exhibit novel thermoelectric properties that violate the Wiedemann-Franz law, and have been predicted to display vortical or even turbulent electron and heat flow. Combined STP and STST measurements are used in this project to determine the degree to which heat and charge flow becomes uncorrelated in such systems, and this project also utilizes engineered potential barriers that affect heat and charge flow differently, with an aim of further decoupling the two. Moreover, by probing how heat is dissipated in viscous electronic phases, this project explores the mechanisms that limit the conductivity of such phases and provides data that allows the complete problem of thermoelectric flow to be solved self-consistently in hydrodynamic materials. STP measurements of charge motion in magnetic fields, meanwhile, explore how weakly bound states and snake states can perturb the values of quantum hall conductance plateaus and precipitate the formation of vorticies of circularly flowing charge as the viscosity of the system is increased. These measurements also test predictions that electron viscosity itself should become quantized in quantum hall systems. This project has broad implications for creating improved thermoelectric devices from hydrodynamic materials, and for understanding the effect of hydrodynamics on other material systems. 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.