Efficient Adiabatic Demagnetization Refrigeration to below 50 mK with Ultrahigh-Vacuum-Compatible Ytterbium Diphosphates AYbP2O7 (

Attaining millikelvin (mK) temperatures is often a prerequisite for the study of 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 superexpensive ${}^{3}\text{He}$. 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 alternative UHV-compatible candidate materials ${\text{NaYbP}}_{2}{\text{O}}_{7}$ and ${\text{KYbP}}_{2}{\text{O}}_{7}$. Utilizing the physical property measurement system 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 ${\text{NaYbP}}_{2}{\text{O}}_{7}$ and ${\text{KYbP}}_{2}{\text{O}}_{7}$, respectively, slightly advantageous to $\text{KBaYb}({\text{BO}}_{3}{)}_{2}$ (45 mK and 40 min) studied under similar conditions.