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

TL;DR

This paper introduces a novel metallic refrigerant, YbCo$_2$Zn$_{20}$, for adiabatic demagnetization cooling, offering an alternative to helium-3 gas with enhanced thermal conductivity and improved low-temperature performance near quantum criticality.

Contribution

It demonstrates the use of super-heavy electron metal as a metallic refrigerant for cryogen-free cooling, expanding the scope of materials beyond traditional insulators.

Findings

YbCo$_2$Zn$_{20}$ effectively cools below 100 mK without helium-3.

Partial Sc substitution enhances magnetocaloric effect near quantum critical point.

Metallic refrigerant shows advantages over conventional insulating refrigerants.

Abstract

Low-temperature refrigeration is of crucial importance in fundamental research of condensed matter physics, as the investigations of fascinating quantum phenomena, such as superconductivity, superfluidity and quantum criticality, often require refrigeration down to very low temperatures. Currently, cryogenic refrigerators with $^3$He gas are widely used for cooling below 1 Kelvin. However, usage of the gas is being increasingly difficult due to the current world-wide shortage. Therefore, it is important to consider alternative methods of refrigeration. Here, we show that a new type of refrigerant, super-heavy electron metal, YbCo$_2$Zn$_{20}$, can be used for adiabatic demagnetization refrigeration, which does not require 3He gas. A number of advantages includes much better metallic thermal conductivity compared to the conventional insulating refrigerants. We also demonstrate that the cooling performance is optimized in Yb$_{1-x}$Sc$_x$Co$_2$Zn$_{20}$ by partial Sc substitution with $x\sim$0.19. The substitution induces chemical pressure which drives the materials close to a zero-field quantum critical point. This leads to an additional enhancement of the magnetocaloric effect in low fields and low temperatures enabling final temperatures well below 100 mK. Such performance has up to now been restricted to insulators. Since nearly a century the same principle of using local magnetic moments has been applied for adiabatic demagnetization cooling. This study opens new possibilities of using itinerant magnetic moments for the cryogen-free refrigeration.