A New Angle on Quantum Impurities

Abstract

Understanding the behavior of molecules interacting with superfluid helium represents a formidable challenge and, in general, requires approaches relying on large-scale numerical simulations. Here, we demonstrate that experimental data collected over the last 20 years provide evidence that molecules immersed in superfluid helium form recently predicted angulon quasiparticles [Phys. Rev. Lett. 114, 203001 (2015)]. Most important, casting the many-body problem in terms of angulons amounts to a drastic simplification and yields effective molecular moments of inertia as straightforward analytic solutions of a simple microscopic Hamiltonian. The outcome of the angulon theory is in good agreement with experiment for a broad range of molecular impurities, from heavy to medium-mass to light species. These results pave the way to understanding molecular rotation in liquid and crystalline phases in terms of the angulon quasiparticle. DOI: 10.1103/PhysRevLett.118.095301 Among its many peculiar properties, superfluid 4 He is quite averse to mixing with impurities which could serve as a microscopic probe of the superfluid phase. As a result, for several decades after the discovery of superfluidity by Allen, Misener, and Kapitza [1,2], only macroscopichydrodynamic-properties of superfluid helium have been studied in the laboratory. In the 1990s, however, it was demonstrated that atoms and molecules can be trapped in superfluid helium if the latter forms little droplets containing on the order of 1000 helium atoms [3][4][5][6][7]. Over the following years, trapping atoms, molecules, and ions inside superfluid helium nanodropletssometimes called "nanocryostats"-emerged as an important tool of molecular spectroscopy [6][7][8][9][10][11][12]. Such nanodroplets allow us to trap single molecules in a cold environment (∼0.4 Kelvin), thereby isolating them from external perturbations. This allows us to record spectra free of collisional and Doppler broadening, as well as to study species that are unstable in the gas phase, such as free radicals.While superfluid helium does not cause a substantial broadening of molecular spectral lines, it affects molecular rotation. In particular, molecules in superfluid helium nanodroplets acquire an effective moment of inertia that is larger compared to its gas-phase value [6,9]. The relative magnitude of the effect increases from lighter to heavier species and is somewhat similar to renormalization of the effective mass for electrons interacting with a crystalline lattice [13][14][15][16].Semiclassically, molecular rotation in helium can be rationalized within the "adiabatic following" model [6,7,[17][18][19][20][21][22]. There, it is assumed that the molecule induces a local density deformation (a "nonsuperfluid shell") of helium which corotates along with the molecule, thereby increasing its moment of inertia. However, such a classical approach does not allow us to get insight into the intriguing aspects of the problem arising from quantum many-body physics. Helium, on the other hand, repres...

Strongly aligned molecules inside helium droplets in the near-adiabatic regime

“…Recently it has been shown that molecules in superfluid helium droplets can be conveniently described as 'angulon' quasiparticles [4,[16][17][18][19][23][24][25][26][27]. Here we use the angulon theory as a simple model to show that under the experimental conditions reported here, the alignment of I 2 , DIB, and DBB in the presence of helium can be approximated as the gas-phase alignment.…”

Fingerprints of angulon instabilities in the spectra of matrix-isolated molecules

“…Recently, it was predicted that a superfluid can acquire angular momentum via a different, microscopic route, which takes effect in the presence of rotating impurities, such as molecules [6][7][8][9][10][11][12][13]. In particular, it was demonstrated that a rotating impurity immersed in a superfluid forms the 'angulon' quasiparticle, which can be thought of as a rigid rotor dressed by a cloud of superfluid excitations carrying angular momentum [14-21].The angulon theory was able to describe, in good agreement with experiment, renormalization of rotational constants [22,23] and laser-induced dynamics [24,25] of molecules in superfluid helium nanodroplets. One of the key predictions of the angulon theory are the socalled 'angulon instabilities ' [14-16] that occur at some critical value of the molecule-superfluid coupling where the angulon quasiparticle becomes unstable and one or a few quanta of angular momentum are resonantly transferred from the impurity to the superfluid.…”

Diagrammatic approach to orbital quantum impurities interacting with a many-particle environment

“…4. We stress, however, that in the case of a molecular impurity within a He nanodroplet, the parameters of the model can also be inferred, in a more physical way, from the impurity-bath potential energy surfaces [55,62].…”

“…A strong evidence was provided that molecules rotating in superfluid 4 He form angulon quasiparticles [55,62].…”