PhD students Amin Ghadami and Longji Cui are both recipients of a Rackham Predoctoral Fellowship. The Rackham Predoctoral Fellowship supports outstanding doctoral students who have achieved candidacy and are actively working on dissertation research and writing. It seeks to support students working on dissertations that are unusually creative, ambitious and impactful.
Amin Ghadami’s abstract reads as follows:
Anticipating Bifurcations for Identifying Dynamic Characteristics of Complex Systems
Dramatic changes occur in the dynamics of complex systems, from ecosystems to engineered systems. Forecasting such events is of major importance and would have a significant impact in a variety of fields. In this research, a model-less approach is introduced to forecast critical points and post-critical dynamics of complex systems using measurements of the system response collected only in the pre-transition regime. The method is employed to forecast 1) flutter instability in fluid-structural systems, and 2) collapse of natural populations in ecological systems, as two important classes of complex systems. Our theoretical and experimental results highlight that by monitoring the system’s response to perturbations in the pre-transition regime, it is possible to forecast bifurcation diagrams and gain crucial information about the future system’s safety and stability, such as distance to upcoming transition and future system equilibriums, which makes our method a unique tool for stability analysis of complex systems.
Longji Cui’s abstract reads as follows:
Probing Thermal Transport and Energy Conversion at the Atomic and Molecular Scale
The study of thermal energy transport and conversion at the nanoscale is of fundamental interest, and holds great promise for the development of a variety of technologies, including heat management in nanoelectronics, thermoelectrics and thermophotovoltaics. Although much attention has been directed towards studying nanocale optical and electronic properties, thermal properties from the atomic scale to the realm of a few nanometers has been barely explored due to experimental challenges. To tackle these challenges, a series of novel experimental techniques are developed and leverage to systematically answer how heat is conduct through atomic contacts and single molecules, radiated across nanoscale gaps and converted to electricity in molecular junctions. Specifically, by employing custom-fabricated picowatt- heat-resolution calorimetric scanning probes, quantized thermal transport at room temperature in metallic wires that are only single-atom wide is observed and the Wiedemann-Franz law is validated all the way down to the single atom limit. Moreover, radiative heat transfer in angstrom and nanometer scale gaps is examined and results in heat fluxes that are several orders of magnitude larger than the far-field heat fluxes predicted by Planck’s Black body limit. Finally, thermoelectric energy conversion of organic molecule junctions is measured and interesting molecular-scale refrigeration phenomena due to Peltier effects is demonstrated. These findings set the stage for rational design of thermally-efficient nanoscale devices and are expected to enable future development of environmentally-friendly energy saving solutions.