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

Bighin, Giacomo; Lemeshko, Mikhail

doi:10.1103/physrevb.96.085410

Cited by 17 publications

(13 citation statements)

References 130 publications

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“…The model tends to underestimate the line broadening by a few GHz, which we attribute to the fact that only single-phonon excitations are included into the ansatz (2). Using more involved, diagrammatic approaches [21] to the Hamiltonian (1) is expected to further improve the agreement. …”

Section: Matrix Elements For Spectroscopic Transitionsmentioning

confidence: 99%

“…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][15][16][17][18][19][20][21].…”

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confidence: 99%

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

Cherepanov

Lemeshko

2017

Phys. Rev. Materials

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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 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 CH3 and NH3 molecules in superfluid helium nanodroplets, thereby providing a fingerprint of the emerging angulon instabilities in experiment.One of the distinct features of the superfluid phase is the formation of vortices -topological defects carrying quantized angular momentum, which arise if the bulk of the superfluid rotates faster than some critical angular velocity [1,2]. Vortex nucleation has been considered to be the main mechanism angular momentum disposal in superfluids [1][2][3][4][5]. 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. These instabilities are fundamentally different from the vortex instabilities, associated with the transfer of angular momentum quantised in units of per atom of the superfluid. Furthermore, vortices can be thought of as planar rotors, i.e., the eigenstates of theL z operator. Angulons, on the other hand, are the eigenstates of the total angular momentum operator,L 2 , and therefore the transferred angular momentum is three-dimensional. While vortex instabilities have been subject to several experimental studies in the context of superfluid helium [4,5,[26][27][28][29][30][31], ultracold quantum gases [32][33][34][35][36]...

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Section: Matrix Elements For Spectroscopic Transitionsmentioning

confidence: 99%

mentioning

confidence: 99%

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

Cherepanov

Lemeshko

2017

Phys. Rev. Materials

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“…It can be easily generalized to analyze other far-from-equilibrium problems. A system closely related to the quantum impurity problem is the so-called angulon, i.e., a problem of rotational excitations of molecules immersed in a quantum fluid [86][87][88][89]. This system was shown to exhibit quasiparticles similar to polarons, but with a conserved total angular momentum and a cloud of angular excitations renormalizing the moments of inertia.…”

Section: Discussionmentioning

confidence: 99%

Strong-coupling Bose polarons out of equilibrium: Dynamical renormalization-group approach

et al. 2018

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When a mobile impurity interacts with a surrounding bath of bosons, it forms a polaron. Numerous methods have been developed to calculate how the energy and the effective mass of the polaron are renormalized by the medium for equilibrium situations. Here we address the much less studied non-equilibrium regime and investigate how polarons form dynamically in time. To this end, we develop a time-dependent renormalization group approach which allows calculations of all dynamical properties of the system and takes into account the effects of quantum fluctuations in the polaron cloud. We apply this method to calculate trajectories of polarons following a sudden quench of the impurity-boson interaction strength, revealing how the polaronic cloud around the impurity forms in time. Such trajectories provide additional information about the polaron's properties which are challenging to extract directly from the spectral function measured experimentally using ultracold atoms. At strong couplings, our calculations predict the appearance of trajectories where the impurity wavers back at intermediate times as a result of quantum fluctuations. Our method is applicable to a broader class of non-equilibrium problems. As a check, we also apply it to calculate the spectral function and find good agreement with experimental results. At very strong couplings, we predict that quantum fluctuations lead to the appearance of a dark continuum with strongly suppressed spectral weight at low energies. While our calculations start from an effective Fröhlich Hamiltonian describing impurities in a three-dimensional Bose-Einstein condensate, we also calculate the effects of additional terms in the Hamiltonian beyond the Fröhlich paradigm. We demonstrate that the main effect of these additional terms on the attractive side of a Feshbach resonance is to renormalize the coupling strength of the effective Fröhlich model.

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“…(2). They involve perturbations on the top of a microscopic deformation of the helium bath, i. e. an infinite number of bosonic excitations [25][26][27] . In the course of the paper, however, we aim to demonstrate that the solutions including up to triple excitations are able to catch changes in molecular spectra for broad range of species measured in helium.…”

Section: A the Angulon Hamiltonianmentioning

confidence: 99%

A simple model for high rotational excitations of molecules in a superfluid

Cherepanov¹,

Bighin²,

Schouder³

et al. 2022

Preprint

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We present a simple quantum mechanical model describing excited rotational states of molecules in superfluid helium nanodroplets, as recently studied in non-adiabatic molecular alignment experiments [Cherepanov et al., Phys. Rev. A 104, L061303 (2021)]. We show that a linear molecule immersed in a superfluid can be seen as an effective symmetric top, similar to the rotational structure of radicals, such as OH or NO, but with the angular momentum of the superfluid playing the role of the electronic angular momentum in free molecules. The model allows to evaluate the effective rotational and centrifugal distortion constants for a broad range of species and to explain the crossover between light and heavy molecules in superfluid 4 He in terms of the many-body wavefunction structure. Most important, the simple theory allows to answer the question as to what happens when the rotational angular momentum of the molecule increases beyond the lowest excited states accessible by infrared spectroscopy. Some of the above mentioned insights can be acquired by analyzing a simple 2 × 2 matrix.

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

Cited by 17 publications

References 130 publications

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

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

Strong-coupling Bose polarons out of equilibrium: Dynamical renormalization-group approach

A simple model for high rotational excitations of molecules in a superfluid

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