Efficient adiabatic demagnetization refrigeration to below 50 mK with UHV compatible Ytterbium diphosphates $A$YbP$_2$O$_7$ ($A=$Na, K)

Attaining milli-Kelvin temperatures is often a prerequisite for the study of novel quantum phenomena and the operation of quantum devices. Adiabatic demagnetization refrigeration (ADR) is an effective, easy and sustainable alternative to evaporation or dilution cooling with the rare and super-expensive $^3$He. Paramagnetic salts, traditionally used for mK-ADR, suffer from chemical instability related to water of crystallization. We report synthesis, characterization as well as low-temperature magnetization and specific heat measurements of two new UHV compatible candidate materials NaYbP$_2$O$_7$ and KYbP$_2$O$_7$. Utilizing the PPMS at 2 K, the ADR of sintered pellets with Ag powder admixture starting at 5 T yields base temperatures (warm-up times) of 45 mK (55 min) and 37 mK (35 min) for NaYbP$_2$O$_7$ and KYbP$_2$O$_7$, respectively, slightly advantageous to KBaYb(BO$_3$)$_2$ (45 mK and 40 min) studied under similar conditions.