EAGER: CRYO: Engineering Charge and Energy Transfer in Superconducting Tunnel Junctions to Achieve Solid-State Refrigeration to Sub-Kelvin Temperatures

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Accessing sub-Kelvin temperatures is critical for exploring interesting quantum phenomena as well as for the development of novel quantum computing and quantum optics-based technologies. Currently available approaches to refrigeration to sub-1 K temperatures are expensive, complicated to build and maintain as well as bulky, and therefore are restricted largely to specialized research labs. Establishing solid-state refrigeration below 1K will be truly transformative, because it will impact a broad range of emerging, high-impact technologies that all hinge on the availability of compact, efficient and low-cost technologies for ultra-low temperature cryogenic cooling. Successful demonstration of high cooling rates and temperatures as low as 100 mK will have a dramatic impact on quantum technologies, such as quantum computing. Further, the sub-Kelvin solid-state low temperature refrigeration can also dramatically improve and simplify the use of ultrasensitive detectors for a number of applications ranging from astrophysics to detection of hazardous materials. This project will establish a novel approach to solid state refrigeration to address these challenges. In this EArly-concept Grant for Exploratory Research (EAGER) project the team seeks to develop superconducting tunnel junction-based solid-state refrigeration that will enable cooling from 1 K to 100 mK without a dilution refrigerator. Superconducting tunnel junction-based refrigeration is, in principle, very similar to cooling via a thermionic device as it relies on the selective transmission of high energy carriers from one electrode to a second superconducting electrode featuring a small bandgap. Such selective transmission of high energy carriers results in cooling of the semiconductor/normal metal and heating in the superconductor. A key challenge to achieve superconducting tunnel junction-based refrigeration is the thermal coupling between the hot and cold sides, via phonons, which prevents realization of high cooling powers and the required low temperatures. The intellectual merit of this work is in establishing completely novel approaches to solid-state refrigeration and is organized into thrusts aimed at understanding: 1) How the thermal resistance between the hot and the cold sides of the superconducting tunnel junction can be increased by engineering the phonon density of states via nanostructuring or by introducing molecular moieties at interfaces, 2) How molecular junctions can be engineered to provide excellent electronic transparency, cooling and thermal resistance to achieve record cooling rates, and 3) How superconducting tunnel junctions based on different materials and structures can be cascaded to achieve cooling all the way from 1 K to 100 mK? Successfully addressing these important challenges will enable development of both superconducting tunnel junction based solid-state refrigeration and novel refrigeration devices that can be readily integrated with various quantum technologies.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.