Ultracold LiCr: a new pathway to quantum gases of paramagnetic polar molecules

Quantum gases of doubly-polar molecules represent appealing frameworks for a variety of cross-disciplinary applications, encompassing quantum simulation and computation, controlled quantum chemistry and precision measurements. Through a joint experimental and theoretical study, here we explore a novel class of ultracold paramagnetic polar molecules combining lithium alkali and chromium transition metal elements. Focusing on the specific bosonic isotopologue $^{6}$Li$^{53}$Cr, leveraging on the Fermi statistics of the parent atomic mixture and on suitable Feshbach resonances recently discovered, we produce up to $50\times10^3$ ultracold LiCr molecules at peak phase-space densities exceeding 0.1, prepared within the least-bound rotationless level of the LiCr electronic $sextet$ ground state $X^6\Sigma^+$. We thoroughly characterize the molecular gas, demonstrating the paramagnetic nature of LiCr dimers and the precise control of their quantum state. We investigate their stability against inelastic processes and identify a parameter region where pure LiCr samples exhibit lifetimes exceeding 0.2 s. Parallel to this, we employ state-of-the-art quantum-chemical calculations to predict the properties of LiCr ground and excited electronic states. We identify efficient paths to coherently transfer weakly-bound LiCr dimers to their absolute ground state, to deliver ultracold gases of doubly-polar molecules with significant electric (3.3 D) and magnetic ($5\,\mu_\text{B}$) dipole moments.