Tunneling Cooling By A Normal-vacuum-superconductor Junction

Thanks to the great progress in cryogenics, a variety of commercial products are currently available, allowing different samples to be cooled to any temperature from room temperature down to milliKelvin (mK) range. However, while a simple cryocooler can cover the temperature range from 300K to around 4K, elaborate setups—for example, ~3H_e and ~4H_e Vapour Pressure Refrigerator, ~3H_e Isentropic Pressure Refrigerator, Dilution Refrigerator, Isentropic Demagnetization Refrigerator and so on—are usually needed to reach the ultralow temperatures below 1K. To be able to reach this temperature range with a simple setup has been an attractive goal of the cryogenics community for a long time.The principle of NVS refrigeration is simple: a normal metal (N) is separated by an vacuum(V) barrier from a superconductor(S). Electrons can pass across the insulating barrier by quantum tunnelling—a consequence of the uncertainty in quantum mechanics that means there is a finite probability of finding a particle on the other side of a barrier. Cooling occurs as a consequence of the different electron configurations in the N and S regions: in the normal metal, electrons occupy states with an almost constant density over the whole range of relevant energies, whereas in the superconductor there is a gap in which no electron energy states exist. Electrons in the normal metal at energies corresponding to the gap in the superconductor are forbidden from tunnelling through the vacuum barrier.The possibility of cooling a system from liquid helium temperature, 4.2 K, using a tunnel junction refrigerator is analyzed. Calculations show that the device can be used over a wide temperature range from 4K down to well below 1mK with necessary cooling power. An additional advantage of the method over the traditional methods is that the electron subsystem is directly cooled in the method, which is very important when the electronic properties are interested. However, several serious difficulties must be overcome before the method can be used in low temperature laboratories.