Libration of Strongly‐Oriented Polar Molecules inside a Superfluid

Redchenko, E. S.; Lemeshko, Mikhail

doi:10.1002/cphc.201601042

Cited by 11 publications

(16 citation statements)

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“…As a result, a detailed quantum mechanical understanding of molecular impurities in helium requires first-principles approaches based on extensive numerical simulations [7]. During last years, several numerical studies, based mainly on pathintegral, variational, and diffusion quantum Monte-Carlo (MC), have been performed for molecules embedded in finite-size He n clusters with n 100 [22,.In this Letter we show that such an involved manyparticle problem simplifies tremendously, if one assumes that molecules in helium droplets form angulons -recently introduced quasiparticles consisting of a quantum rotor dressed by a field of many-body excitations [47][48][49][50][51][52][53][54]. The angulon theory is inherently many-body and describes interactions between a molecule and an infinite number of helium atoms.…”

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confidence: 93%

“…In this Letter we show that such an involved manyparticle problem simplifies tremendously, if one assumes that molecules in helium droplets form angulons -recently introduced quasiparticles consisting of a quantum rotor dressed by a field of many-body excitations [47][48][49][50][51][52][53][54]. The angulon theory is inherently many-body and describes interactions between a molecule and an infinite number of helium atoms.…”

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confidence: 96%

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Quasiparticle Approach to Molecules Interacting with Quantum Solvents

Lemeshko

2017

Phys. Rev. Lett.

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105

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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. Among its many peculiar properties, superfluid4 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 macroscopic -hydrodynamic -properties of superfluid helium have been studied in the laboratory. In the 1990's, 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 a thousand helium atoms [3][4][5][6][7]. Over the following years, trapping atoms, molecules, and ions inside superfluid helium nanodroplets -sometimes called 'nanocryostats' -emerged as an important tool of molecular spectroscopy [6][7][8][9][10][11][12]. Such nanodroplets allow to trap single molecules in a cold environment (∼ 0.4 Kelvin), thereby isolating them from external perturbations. This allows 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 'non superfluid shell') of helium which co-rotates along with the molecule, thereby increasing its moment of inertia. However, such a classical approach does not allow to get insight into the intriguing aspects of the problem arising from quantum many-body physics. Helium, on the other hand, represents a dense, strongly-interactin...

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confidence: 93%

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confidence: 96%

Quasiparticle Approach to Molecules Interacting with Quantum Solvents

Lemeshko

2017

Phys. Rev. Lett.

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105

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“…Recently, it was shown that interaction of such orbital impurities with a many-particle environment can be rationalized by using the concept of the angulon quasiparticle [52][53][54][55][56][57][58]. While in the case of polarons the bath degrees of freedom couple to the impurity's translational motion, in angulons the orbital angular momentum is redistributed between the impurity and the many-particle environment.…”

Section: Introductionmentioning

confidence: 99%

“…The concept of angulons has been used to study a variety of physical systems, ranging from molecular ions rotating in a BEC [61] to molecules in superfluid helium nanodroplets [55,57], using variational approaches in either the strong- [53,58] or weak-coupling [52,56,57] regimes. A strong evidence was provided that molecules rotating in superfluid 4 He form angulon quasiparticles [55,62].…”

Section: Introductionmentioning

confidence: 99%

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

Bighin

Lemeshko

2017

Phys. Rev. B

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Recently it was shown that an impurity exchanging orbital angular momentum with a surrounding bath can be described in terms of the angulon quasiparticle [Phys. Rev. Lett. 118, 095301 (2017)]. The angulon consists of a quantum rotor dressed by a many-particle field of boson excitations, and can be formed out of, for example, a molecule or a nonspherical atom in superfluid helium, or out of an electron coupled to lattice phonons or a Bose condensate. Here we develop an approach to the angulon based on the path-integral formalism, which sets the ground for a systematic, perturbative treatment of the angulon problem. The resulting perturbation series can be interpreted in terms of Feynman diagrams, from which, in turn, one can derive a set of diagrammatic rules. These rules extend the machinery of the graphical theory of angular momentum -well known from theoretical atomic spectroscopy -to the case where an environment with an infinite number of degrees of freedom is present. In particular, we show that each diagram can be interpreted as a 'skeleton,' which enforces angular momentum conservation, dressed by an additional many-body contribution. This connection between the angulon theory and the graphical theory of angular momentum is particularly important as it allows to systematically and substantially simplify the analytical representation of each diagram. In order to exemplify the technique, we calculate the 1− and 2−loop contributions to the angulon self-energy, the spectral function, and the quasiparticle weight. The diagrammatic theory we develop paves the way to investigate next-to-leading order quantities in a more compact way compared to the variational approaches.

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“…The second term of Eq. (1), corresponds to the many-body part of the Hamiltonian, which includes the kinetic energy of the bath and the impurity-bath interactions that depend on r. Furthermore,Ĥ mb (r) can include any external potential such as that due to an electromagnetic field [53][54][55]. The eigenvalue equation for Hamiltonian (1) can be written asĤ (r)|Ψ α (r) = E α |Ψ α (r) ,where |Ψ α (r) ≡ r|Ψ α is the eigenstate in the coordinate space of the impurity, and α is the quantum number labeling the eigenstate.…”

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Emergence of Non-Abelian Magnetic Monopoles in a Quantum Impurity Problem

2017

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Recently it was shown that molecules rotating in superfluid helium can be described in terms of the angulon quasiparticles [Phys. Rev. Lett. 118, 095301 (2017)]. Here we demonstrate that in the experimentally realized regime the angulon can be seen as a point charge on a 2-sphere interacting with a gauge field of a non-abelian magnetic monopole. Unlike in several other settings, the gauge fields of the angulon problem emerge in the real coordinate space, as opposed to the momentum space or some effective parameter space. Furthermore, we find a topological transition associated with making the monopole abelian, which takes place in the vicinity of the previously reported angulon instabilities. These results pave the way for studying topological phenomena in experiments on molecules trapped in superfluid helium nanodroplets, as well as on other realizations of orbital impurity problems.In Maxwell's unification of electricity and magnetism, there was one piece missing that would make the electric and magnetic forces perfectly symmetric with respect to each otherthe magnetic monopoles. As demonstrated by Dirac [1], the existence of a single magnetic monopole would explain quantisation of electric charge everywhere in the Universe. Since Dirac's work, the existence of magnetic monopoles -as real elementary particles or effective quasiparticles -has preoccupied physicists working in several different fields. In high-energy physics, 't Hooft [2] and Polyakov [3] demonstrated the existence of non-abelian magnetic monopoles in the context of the unification of the fundamental interactions [4]. Despite the lack of experimental evidence for elementary monopoles in nature [5], collective phenomena exhibiting the behavior of magnetic monopoles have been predicted to emerge in condensed matter systems [6][7][8][9][10], and subsequently observed in experiment [11][12][13][14].As shown by Berry [15], magnetic monopoles can also emerge in an external parameter space of a simple quantum mechanical problem [16,17]. Moreover, the parameter space can be further generalized to coordinates of a particle interacting with another particle [18][19][20]. For example, Moody et al. [18] showed that the effective Hamiltonian describing nuclear rotation of a diatomic molecule can be rewritten as that of a charged particle interacting with a gauge field of a magnetic monopole. In follow-up studies the emerging gauge fields have been studied in various contexts, from different types of atomic problems [21][22][23][24] to spinor Bose-Fermi mixtures [25], to the fractional quantum Hall effect [26], to Bose-Einstein Condensates [8][9][10]. Finally, emergence of monopole-like gauge fields represents an important tool for computing topological invariants of quantum systems using Chern numbers [27], and thereby classifying the topology of the problem [8,[28][29][30][31]. Such a topological classification of quantum states is particularly relevant in the context of current research on topological states of matter [32][33][34][35][36][37].In this Lette...

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Libration of Strongly‐Oriented Polar Molecules inside a Superfluid

Cited by 11 publications

References 36 publications

Quasiparticle Approach to Molecules Interacting with Quantum Solvents

Quasiparticle Approach to Molecules Interacting with Quantum Solvents

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

Emergence of Non-Abelian Magnetic Monopoles in a Quantum Impurity Problem

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