Quantum-interference-enhanced deep sub-Doppler cooling of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mn>39</mml:mn></mml:msup></mml:math>K atoms in gray molasses

We report enhanced sub-Doppler cooling of the bosonic atoms of ${}^{39}$K facilitated by formation of dark states with the cooling and repumping lasers tuned to the Raman resonance in $\ensuremath{\Lambda}$ configuration near the ${D}_{1}$ transition. A temperature of about 12 $\ensuremath{\mu}$K and phase-space density $&gt;$$2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ is achieved in the two-stage ${D}_{2}$-${D}_{1}$ molasses and spans a very large parameter region where quantum interference persists robustly. We also present results on enhanced radiation heating with a subnatural linewidth (0.07$\ensuremath{\Gamma}$) and a signature Fano-like profile of a coherently driven three-level atomic system. The optical Bloch equations relevant for the three-level atom in a bichromatic light field are solved with the method of continued fractions to show that cooling occurs only for a small velocity class of atoms, emphasizing the need for precooling in the ${D}_{2}$ molasses stage.