mK Coolers

Supervisor: Dr David Leadley

This project is motivated by a new era in astronomical detection, space-based applications and in quantum computing which critically depends on a "turn-switch" technology for milliKelvin (mK) operation. It uses an approach to mK refrigeration based on electronic cooling, and involving Si-Ge semiconductor layers, to realise a miniaturised and highly integrated solid state cooler. The current techniques to realize mK temperatures rely on liquid cyrogens or adiabatic demagnetisation and are neither compact or lightweight and certainly not "black-box". On the other hand, electronic refrigerators have been demonstrated that cool the electrons to below 100mK by selective removal of energetic electrons using Normal metal/Insulator/Superconductor (NIS) tunnel junctions. To date rather conventional materials are used in their construction, so there is considerable scope for improvement.  We plan to develop microfabricated electronic mK refrigerators of enhanced performance using Si and MEMS processing techniques. The miniaturised coolers will have the great advantage of high level of integration with Si CMOS.

1. Superconducting tunnel refrigerator (STR): The normal metal of a NIS structure can be replaced by a heavily doped semiconductor and high mobility Si-Ge materials will be employed here, to tailor the Schottky barrier and the electron phonon coupling which ultimately determines the cooling of the lattice. Si-Ge superlattices will be used as phonon blocking structures to enhance the cooling efficiency.
2. Anderson Transition Refrigerator: At temperatures below 1K where the phonon thermal conductivity is small it is predicted that the power factor will be a maximum at an Anderson disorder induced metal insulator transition, which bodes well for refrigeration. Si-Ge-C heterostructures will be used for controlled amounts of disorder.
   

The PhD student on this project would be involved in the growth, and characterisation of the structural, electrical and thermal properties of such layers and would receive training in a wide variety of techniques of relevance to semiconductor research and gain an appreciation of a wide range of applications. They could also spend some time in the laboratories of our collaborators in Cardiff and Helsinki. 

To discuss this project further contact:
d.r.leadley@warwick.ac.uk or EHCParker@warwick.ac.uk

Design of possible mK cooler – cooling key part of system to below 100mK using just electronic cooling.

The payload to be cooled is a Transition Edge Sensor (TES) that gives unsurpassed ability to detect black holes or early stage cancer!