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Low-temperature refrigeration is of crucial importance in fundamentalresearch of condensed matter physics, as the investigations of fascinatingquantum phenomena, such as superconductivity, superfluidity and quantumcriticality, often require refrigeration down to very low temperatures.Currently, cryogenic refrigerators with $^3$He gas are widely used for coolingbelow 1 Kelvin. However, usage of the gas is being increasingly difficult dueto the current world-wide shortage. Therefore, it is important to consideralternative methods of refrigeration. Here, we show that a new type ofrefrigerant, super-heavy electron metal, YbCo$_2$Zn$_{20}$, can be used foradiabatic demagnetization refrigeration, which does not require 3He gas. Anumber of advantages includes much better metallic thermal conductivitycompared to the conventional insulating refrigerants. We also demonstrate thatthe cooling performance is optimized in Yb$_{1-x}$Sc$_x$Co$_2$Zn$_{20}$ bypartial Sc substitution with $x\sim$0.19. The substitution induces chemicalpressure which drives the materials close to a zero-field quantum criticalpoint. This leads to an additional enhancement of the magnetocaloric effect inlow fields and low temperatures enabling final temperatures well below 100 mK.Such performance has up to now been restricted to insulators. Since nearly acentury the same principle of using local magnetic moments has been applied foradiabatic demagnetization cooling. This study opens new possibilities of usingitinerant magnetic moments for the cryogen-free refrigeration.

Super-heavy fermion material as metallic refrigerant for adiabatic demagnetization cooling