Title: Fingerprints of angulon instabilities in the spectra of matrix-isolated molecules,
Abstract: The formation of vortices is usually considered to be the main mechanism of
angular momentum disposal in superfluids. Recently, it was predicted that a
superfluid can acquire angular momentum via an alternative, microscopic route
-- namely, through interaction with rotating impurities, forming so-called
`angulon quasiparticles' [Phys. Rev. Lett. 114, 203001 (2015)]. The angulon
instabilities correspond to transfer of a small number of angular momentum
quanta from the impurity to the superfluid, as opposed to vortex instabilities,
where angular momentum is quantized in units of $\hbar$ per atom. Furthermore,
since conventional impurities (such as molecules) represent three-dimensional
(3D) rotors, the angular momentum transferred is intrinsically 3D as well, as
opposed to a merely planar rotation which is inherent to vortices. Herein we
show that the angulon theory can explain the anomalous broadening of the
spectroscopic lines observed for CH$_3$ and NH$_3$ molecules in superfluid
helium nanodroplets, thereby providing a fingerprint of the emerging angulon
instabilities in experiment.