@article{8170,
abstract = {Alignment of OCS, CS2, and I2 molecules embedded in helium nanodroplets is measured as a function
of time following rotational excitation by a nonresonant, comparatively weak ps laser pulse. The distinct
peaks in the power spectra, obtained by Fourier analysis, are used to determine the rotational, B, and
centrifugal distortion, D, constants. For OCS, B and D match the values known from IR spectroscopy. For
CS2 and I2, they are the first experimental results reported. The alignment dynamics calculated from the
gas-phase rotational Schrödinger equation, using the experimental in-droplet B and D values, agree in
detail with the measurement for all three molecules. The rotational spectroscopy technique for molecules in
helium droplets introduced here should apply to a range of molecules and complexes.},
author = {Chatterley, Adam S. and Christiansen, Lars and Schouder, Constant A. and Jørgensen, Anders V. and Shepperson, Benjamin and Cherepanov, Igor and Bighin, Giacomo and Zillich, Robert E. and Lemeshko, Mikhail and Stapelfeldt, Henrik},
issn = {10797114},
journal = {Physical Review Letters},
number = {1},
publisher = {American Physical Society},
title = {{Rotational coherence spectroscopy of molecules in Helium nanodroplets: Reconciling the time and the frequency domains}},
doi = {10.1103/PhysRevLett.125.013001},
volume = {125},
year = {2020},
}
@article{5886,
abstract = {Problems involving quantum impurities, in which one or a few particles are interacting with a macroscopic environment, represent a pervasive paradigm, spanning across atomic, molecular, and condensed-matter physics. In this paper we introduce new variational approaches to quantum impurities and apply them to the Fröhlich polaron–a quasiparticle formed out of an electron (or other point-like impurity) in a polar medium, and to the angulon–a quasiparticle formed out of a rotating molecule in a bosonic bath. We benchmark these approaches against established theories, evaluating their accuracy as a function of the impurity-bath coupling.},
author = {Li, Xiang and Bighin, Giacomo and Yakaboylu, Enderalp and Lemeshko, Mikhail},
issn = {00268976},
journal = {Molecular Physics},
publisher = {Taylor and Francis},
title = {{Variational approaches to quantum impurities: from the Fröhlich polaron to the angulon}},
doi = {10.1080/00268976.2019.1567852},
year = {2019},
}
@article{6940,
abstract = {We study the effect of a linear tunneling coupling between two-dimensional systems, each separately
exhibiting the topological Berezinskii-Kosterlitz-Thouless (BKT) transition. In the uncoupled limit, there
are two phases: one where the one-body correlation functions are algebraically decaying and the other with
exponential decay. When the linear coupling is turned on, a third BKT-paired phase emerges, in which one-body correlations are exponentially decaying, while two-body correlation functions exhibit power-law
decay. We perform numerical simulations in the paradigmatic case of two coupled XY models at finite
temperature, finding evidences that for any finite value of the interlayer coupling, the BKT-paired phase is
present. We provide a picture of the phase diagram using a renormalization group approach.},
author = {Bighin, Giacomo and Defenu, Nicolò and Nándori, István and Salasnich, Luca and Trombettoni, Andrea},
issn = {1079-7114},
journal = {Physical Review Letters},
number = {10},
publisher = {American Physical Society (APS)},
title = {{Berezinskii-Kosterlitz-Thouless paired phase in coupled XY models}},
doi = {10.1103/physrevlett.123.100601},
volume = {123},
year = {2019},
}
@article{417,
abstract = {We introduce a Diagrammatic Monte Carlo (DiagMC) approach to complex molecular impurities with rotational degrees of freedom interacting with a many-particle environment. The treatment is based on the diagrammatic expansion that merges the usual Feynman diagrams with the angular momentum diagrams known from atomic and nuclear structure theory, thereby incorporating the non-Abelian algebra inherent to quantum rotations. Our approach works at arbitrary coupling, is free of systematic errors and of finite size effects, and naturally provides access to the impurity Green function. We exemplify the technique by obtaining an all-coupling solution of the angulon model, however, the method is quite general and can be applied to a broad variety of quantum impurities possessing angular momentum degrees of freedom. },
author = {Bighin, Giacomo and Tscherbul, Timur and Lemeshko, Mikhail},
journal = {Physical Review Letters},
number = {16},
publisher = {APS Physics},
title = {{Diagrammatic Monte Carlo approach to rotating molecular impurities}},
doi = {10.1103/PhysRevLett.121.165301},
volume = {121},
year = {2018},
}
@article{420,
abstract = {We analyze the theoretical derivation of the beyond-mean-field equation of state for two-dimensional gas of dilute, ultracold alkali-metal atoms in the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein condensate (BEC) crossover. We show that at zero temperature our theory — considering Gaussian fluctuations on top of the mean-field equation of state — is in very good agreement with experimental data. Subsequently, we investigate the superfluid density at finite temperature and its renormalization due to the proliferation of vortex–antivortex pairs. By doing so, we determine the Berezinskii–Kosterlitz–Thouless (BKT) critical temperature — at which the renormalized superfluid density jumps to zero — as a function of the inter-atomic potential strength. We find that the Nelson–Kosterlitz criterion overestimates the BKT temperature with respect to the renormalization group equations, this effect being particularly relevant in the intermediate regime of the crossover.},
author = {Bighin, Giacomo and Salasnich, Luca},
journal = {International Journal of Modern Physics B},
number = {17},
pages = {1840022},
publisher = {World Scientific Publishing},
title = {{Renormalization of the superfluid density in the two-dimensional BCS-BEC crossover}},
doi = {10.1142/S0217979218400222},
volume = {32},
year = {2018},
}
@article{6339,
abstract = {We introduce a diagrammatic Monte Carlo approach to angular momentum properties of quantum many-particle systems possessing a macroscopic number of degrees of freedom. The treatment is based on a diagrammatic expansion that merges the usual Feynman diagrams with the angular momentum diagrams known from atomic and nuclear structure theory, thereby incorporating the non-Abelian algebra inherent to quantum rotations. Our approach is applicable at arbitrary coupling, is free of systematic errors and of finite-size effects, and naturally provides access to the impurity Green function. We exemplify the technique by obtaining an all-coupling solution of the angulon model; however, the method is quite general and can be applied to a broad variety of systems in which particles exchange quantum angular momentum with their many-body environment.},
author = {Bighin, Giacomo and Tscherbul, Timur and Lemeshko, Mikhail},
journal = {Physical Review Letters},
number = {16},
publisher = {APS},
title = {{Diagrammatic Monte Carlo approach to angular momentum in quantum many-particle systems}},
doi = {10.1103/physrevlett.121.165301},
volume = {121},
year = {2018},
}
@article{995,
abstract = {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.},
author = {Bighin, Giacomo and Lemeshko, Mikhail},
issn = {24699950},
journal = {Physical Review B - Condensed Matter and Materials Physics},
number = {8},
publisher = {American Physical Society},
title = {{Diagrammatic approach to orbital quantum impurities interacting with a many-particle environment}},
doi = {10.1103/PhysRevB.96.085410},
volume = {96},
year = {2017},
}
@article{1015,
abstract = {Vortices are commonly observed in the context of classical hydrodynamics: from whirlpools after stirring the coffee in a cup to a violent atmospheric phenomenon such as a tornado, all classical vortices are characterized by an arbitrary circulation value of the local velocity field. On the other hand the appearance of vortices with quantized circulation represents one of the fundamental signatures of macroscopic quantum phenomena. In two-dimensional superfluids quantized vortices play a key role in determining finite-temperature properties, as the superfluid phase and the normal state are separated by a vortex unbinding transition, the Berezinskii-Kosterlitz-Thouless transition. Very recent experiments with two-dimensional superfluid fermions motivate the present work: we present theoretical results based on the renormalization group showing that the universal jump of the superfluid density and the critical temperature crucially depend on the interaction strength, providing a strong benchmark for forthcoming investigations.},
author = {Bighin, Giacomo and Salasnich, Luca},
issn = {20452322},
journal = {Scientific Reports},
publisher = {Nature Publishing Group},
title = {{Vortices and antivortices in two-dimensional ultracold Fermi gases}},
doi = {10.1038/srep45702},
volume = {7},
year = {2017},
}