Project Activities > Joint Research Activities > JRA2

Ultralow temperature nanorefrigerator

Objectives and expected impact

Description of work

In this joint research activity we propose to develop ultralow temperature nanorefrigerators in which devices can be cooled to milliKelvin and sub-milliKelvin temperatures by nanoelectronic means. We will investigate nanoscale hybrid refrigerators as well as quantum dot based nano-coolers and develop the necessary filtering and thermalization methods to obtain ultralow temperatures in nano-samples. This will make use of innovative ideas, materials and optimized geometries.

Promising micro- and nanoelectronics applications include low temperature devices with unprecedented properties and functionalities as compared to conventional devices operating at room temperature. One of the main challenges of present-day cryogenics is to develop small, low-temperature refrigeration systems that provide targeted microscale cooling.

Hybrid nanostructures combining Superconductors (S) and Normal metals (N) offer a promising possibility in nanocooling. Owing to energy-selective electron tunneling, an N-I-S tunnel junction voltage-biased below the gap features a quasiparticle cooling effect: only electrons with an energy exceeding the gap are effectively removed from N. As a consequence, the normal metal electrons are cooled. Two tunnel junctions arranged in a symmetric S-I-N-I-S configuration routinely give a reduction of the electron temperature from 300 mK to below 100 mK. Innovative materials choices seem appropriate to improve cooler performance, but this still needs to be explored explicitly. Such coolers could provide a platform for experiments on actual quantum devices under ultra-low temperature conditions, which can hardly be reached by other means. In order to ensure a galvanic isolation of the detector from to the cooler, a membrane technology appears necessary.

The feasibility of unexplored nanocooling methods needs to be investigated. For instance, the discrete energy spectrum in semiconductor QDs can be exploited for quantum cooling at ultra-low temperatures. If a two-dimensional electron gas is coupled to two electrodes via QDs, electrons can be transmitted through the sample by resonant tunneling. The QDs quantized energy levels can be adjusted so that the transfer from the gas to one electrode depletes the electron states above the Fermi energy. Similarly, the tunneling from the other electrode to the gas can fill states below the Fermi energy. The quasiparticle distribution function in the electron gas then sharpens, leading to electron cooling.

In order to go beyond the present limitations, an important objective is to fulfill the stringent filtering and thermalization requirements in order to reach low effective electron temperatures in nanodevices. This development will be very beneficial for nanoelectronics in general. Further, to reach microKelvin temperatures in samples regardless of the cooling method (nano-refrigerators in JRA2, demagnetization for nanosamples JRA1) will require even more efficient filtering and thermalization methods, which we aim to develop here.

> Task 1: Thermalizing electrons in nanorefrigerators
> Task 2: Microkelvin nanocooler
> Task 3: Development of a 100 mK, robust, electronically-cooled platform based on a 300 mK 3He bath