@article{427,
abstract = {We investigate the quantum interference induced shifts between energetically close states in highly charged ions, with the energy structure being observed by laser spectroscopy. In this work, we focus on hyperfine states of lithiumlike heavy-Z isotopes and quantify how much quantum interference changes the observed transition frequencies. The process of photon excitation and subsequent photon decay for the transition 2s→2p→2s is implemented with fully relativistic and full-multipole frameworks, which are relevant for such relativistic atomic systems. We consider the isotopes Pb79+207 and Bi80+209 due to experimental interest, as well as other examples of isotopes with lower Z, namely Pr56+141 and Ho64+165. We conclude that quantum interference can induce shifts up to 11% of the linewidth in the measurable resonances of the considered isotopes, if interference between resonances is neglected. The inclusion of relativity decreases the cross section by 35%, mainly due to the complete retardation form of the electric dipole multipole. However, the contribution of the next higher multipoles (e.g., magnetic quadrupole) to the cross section is negligible. This makes the contribution of relativity and higher-order multipoles to the quantum interference induced shifts a minor effect, even for heavy-Z elements.},
author = {Amaro, Pedro and Loureiro, Ulisses and Safari, Laleh and Fratini, Filippo and Indelicato, Paul and Stöhlker, Thomas and Santos, José},
journal = { Physical Review A - Atomic, Molecular, and Optical Physics},
number = {2},
publisher = {American Physical Society},
title = {{Quantum interference in laser spectroscopy of highly charged lithiumlike ions}},
doi = {10.1103/PhysRevA.97.022510},
volume = {97},
year = {2018},
}
@article{435,
abstract = {It is shown that two fundamentally different phenomena, the bound states in continuum and the spectral singularity (or time-reversed spectral singularity), can occur simultaneously. This can be achieved in a rectangular core dielectric waveguide with an embedded active (or absorbing) layer. In such a system a two-dimensional bound state in a continuum is created in the plane of a waveguide cross section, and it is emitted or absorbed along the waveguide core. The idea can be used for experimental implementation of a laser or a coherent-perfect-absorber for a photonic bound state that resides in a continuous spectrum.},
author = {Midya, Bikashkali and Konotop, Vladimir},
journal = {Optics Letters},
number = {3},
pages = {607 -- 610},
publisher = {OSA},
title = {{Coherent-perfect-absorber and laser for bound states in a continuum}},
doi = {10.1364/OL.43.000607},
volume = {43},
year = {2018},
}
@article{5794,
abstract = {We present an approach to interacting quantum many-body systems based on the notion of quantum groups, also known as q-deformed Lie algebras. In particular, we show that, if the symmetry of a free quantum particle corresponds to a Lie group G, in the presence of a many-body environment this particle can be described by a deformed group, Gq. Crucially, the single deformation parameter, q, contains all the information about the many-particle interactions in the system. We exemplify our approach by considering a quantum rotor interacting with a bath of bosons, and demonstrate that extracting the value of q from closed-form solutions in the perturbative regime allows one to predict the behavior of the system for arbitrary values of the impurity-bath coupling strength, in good agreement with nonperturbative calculations. Furthermore, the value of the deformation parameter allows one to predict at which coupling strengths rotor-bath interactions result in a formation of a stable quasiparticle. The approach based on quantum groups does not only allow for a drastic simplification of impurity problems, but also provides valuable insights into hidden symmetries of interacting many-particle systems.},
author = {Yakaboylu, Enderalp and Shkolnikov, Mikhail and Lemeshko, Mikhail},
issn = {00319007},
journal = {Physical Review Letters},
number = {25},
publisher = {American Physical Society},
title = {{Quantum groups as hidden symmetries of quantum impurities}},
doi = {10.1103/PhysRevLett.121.255302},
volume = {121},
year = {2018},
}
@article{5983,
abstract = {We study a quantum impurity possessing both translational and internal rotational degrees of freedom interacting with a bosonic bath. Such a system corresponds to a “rotating polaron,” which can be used to model, e.g., a rotating molecule immersed in an ultracold Bose gas or superfluid helium. We derive the Hamiltonian of the rotating polaron and study its spectrum in the weak- and strong-coupling regimes using a combination of variational, diagrammatic, and mean-field approaches. We reveal how the coupling between linear and angular momenta affects stable quasiparticle states, and demonstrate that internal rotation leads to an enhanced self-localization in the translational degrees of freedom.},
author = {Yakaboylu, Enderalp and Midya, Bikashkali and Deuchert, Andreas and Leopold, Nikolai K and Lemeshko, Mikhail},
issn = {2469-9950},
journal = {Physical Review B},
number = {22},
publisher = {American Physical Society},
title = {{Theory of the rotating polaron: Spectrum and self-localization}},
doi = {10.1103/physrevb.98.224506},
volume = {98},
year = {2018},
}
@article{294,
abstract = {We developed a method to calculate two-photon processes in quantum mechanics that replaces the infinite summation over the intermediate states by a perturbation expansion. This latter consists of a series of commutators that involve position, momentum, and Hamiltonian quantum operators. We analyzed several single- and many-particle cases for which a closed-form solution to the perturbation expansion exists, as well as more complicated cases for which a solution is found by convergence. Throughout the article, Rayleigh and Raman scattering are taken as examples of two-photon processes. The present method provides a clear distinction between the Thomson scattering, regarded as classical scattering, and quantum contributions. Such a distinction lets us derive general results concerning light scattering. Finally, possible extensions to the developed formalism are discussed.},
author = {Fratini, Filippo and Safari, Laleh and Amaro, Pedro and Santos, José},
journal = {Physical Review A - Atomic, Molecular, and Optical Physics},
number = {4},
publisher = {American Physical Society},
title = {{Two-photon processes based on quantum commutators}},
doi = {10.1103/PhysRevA.97.043842},
volume = {97},
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{195,
abstract = {We demonstrate that identical impurities immersed in a two-dimensional many-particle bath can be viewed as flux-tube-charged-particle composites described by fractional statistics. In particular, we find that the bath manifests itself as an external magnetic flux tube with respect to the impurities, and hence the time-reversal symmetry is broken for the effective Hamiltonian describing the impurities. The emerging flux tube acts as a statistical gauge field after a certain critical coupling. This critical coupling corresponds to the intersection point between the quasiparticle state and the phonon wing, where the angular momentum is transferred from the impurity to the bath. This amounts to a novel configuration with emerging anyons. The proposed setup paves the way to realizing anyons using electrons interacting with superfluid helium or lattice phonons, as well as using atomic impurities in ultracold gases.},
author = {Yakaboylu, Enderalp and Lemeshko, Mikhail},
journal = {Physical Review B - Condensed Matter and Materials Physics},
number = {4},
publisher = {American Physical Society},
title = {{Anyonic statistics of quantum impurities in two dimensions}},
doi = {10.1103/PhysRevB.98.045402},
volume = {98},
year = {2018},
}
@article{6013,
abstract = {The first hundred attoseconds of the electron dynamics during strong field tunneling ionization are investigated. We quantify theoretically how the electron’s classical trajectories in the continuum emerge from the tunneling process and test the results with those achieved in parallel from attoclock measurements. An especially high sensitivity on the tunneling barrier is accomplished here by comparing the momentum distributions of two atomic species of slightly deviating atomic potentials (argon and krypton) being ionized under absolutely identical conditions with near-infrared laser pulses (1300 nm). The agreement between experiment and theory provides clear evidence for a nonzero tunneling time delay and a nonvanishing longitudinal momentum of the electron at the “tunnel exit.”},
author = {Camus, Nicolas and Yakaboylu, Enderalp and Fechner, Lutz and Klaiber, Michael and Laux, Martin and Mi, Yonghao and Hatsagortsyan, Karen Z. and Pfeifer, Thomas and Keitel, Christoph H. and Moshammer, Robert},
issn = {1079-7114},
journal = {Physical Review Letters},
number = {2},
publisher = {American Physical Society},
title = {{Experimental evidence for quantum tunneling time}},
doi = {10.1103/PhysRevLett.119.023201},
volume = {119},
year = {2017},
}
@inbook{604,
abstract = {In several settings of physics and chemistry one has to deal with molecules interacting with some kind of an external environment, be it a gas, a solution, or a crystal surface. Understanding molecular processes in the presence of such a many-particle bath is inherently challenging, and usually requires large-scale numerical computations. Here, we present an alternative approach to the problem, based on the notion of the angulon quasiparticle. We show that molecules rotating inside superfluid helium nanodroplets and Bose–Einstein condensates form angulons, and therefore can be described by straightforward solutions of a simple microscopic Hamiltonian. Casting the problem in the language of angulons allows us not only to greatly simplify it, but also to gain insights into the origins of the observed phenomena and to make predictions for future experimental studies.},
author = {Lemeshko, Mikhail and Schmidt, Richard},
booktitle = {Cold Chemistry: Molecular Scattering and Reactivity Near Absolute Zero },
editor = {Dulieu, Oliver and Osterwalder, Andreas},
issn = {20413181},
pages = {444 -- 495},
publisher = {The Royal Society of Chemistry},
title = {{Molecular impurities interacting with a many-particle environment: From ultracold gases to helium nanodroplets}},
doi = {10.1039/9781782626800-00444},
volume = {11},
year = {2017},
}
@inproceedings{313,
abstract = {Tunneling of a particle through a potential barrier remains one of the most remarkable quantum phenomena. Owing to advances in laser technology, electric fields comparable to those electrons experience in atoms are readily generated and open opportunities to dynamically investigate the process of electron tunneling through the potential barrier formed by the superposition of both laser and atomic fields. Attosecond-time and angstrom-space resolution of the strong laser-field technique allow to address fundamental questions related to tunneling, which are still open and debated: Which time is spent under the barrier and what momentum is picked up by the particle in the meantime? In this combined experimental and theoretical study we demonstrate that for strong-field ionization the leading quantum mechanical Wigner treatment for the time resolved description of tunneling is valid. We achieve a high sensitivity on the tunneling barrier and unambiguously isolate its effects by performing a differential study of two systems with almost identical tunneling geometry. Moreover, working with a low frequency laser, we essentially limit the non-adiabaticity of the process as a major source of uncertainty. The agreement between experiment and theory implies two substantial corrections with respect to the widely employed quasiclassical treatment: In addition to a non-vanishing longitudinal momentum along the laser field-direction we provide clear evidence for a non-zero tunneling time delay. This addresses also the fundamental question how the transition occurs from the tunnel barrier to free space classical evolution of the ejected electron.},
author = {Camus, Nicolas and Yakaboylu, Enderalp and Fechner, Lutz and Klaiber, Michael and Laux, Martin and Mi, Yonghao and Hatsagortsyan, Karen and Pfeifer, Thomas and Keitel, Cristoph and Moshammer, Robert},
issn = {17426588},
location = {Kazan, Russian Federation},
number = {1},
publisher = {American Physical Society},
title = {{Experimental evidence for Wigner's tunneling time}},
doi = {10.1088/1742-6596/999/1/012004},
volume = {999},
year = {2017},
}
@article{939,
abstract = {We reveal the existence of continuous families of guided single-mode solitons in planar waveguides with weakly nonlinear active core and absorbing boundaries. Stable propagation of TE and TM-polarized solitons is accompanied by attenuation of all other modes, i.e., the waveguide features properties of conservative and dissipative systems. If the linear spectrum of the waveguide possesses exceptional points, which occurs in the case of TM polarization, an originally focusing (defocusing) material nonlinearity may become effectively defocusing (focusing). This occurs due to the geometric phase of the carried eigenmode when the surface impedance encircles the exceptional point. In its turn, the change of the effective nonlinearity ensures the existence of dark (bright) solitons in spite of focusing (defocusing) Kerr nonlinearity of the core. The existence of an exceptional point can also result in anomalous enhancement of the effective nonlinearity. In terms of practical applications, the nonlinearity of the reported waveguide can be manipulated by controlling the properties of the absorbing cladding.},
author = {Midya, Bikashkali and Konotop, Vladimir},
issn = {00319007},
journal = {Physical Review Letters},
number = {3},
publisher = {American Physical Society},
title = {{Waveguides with absorbing boundaries: Nonlinearity controlled by an exceptional point and solitons}},
doi = {10.1103/PhysRevLett.119.033905},
volume = {119},
year = {2017},
}
@article{994,
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 ℏ 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.},
author = {Cherepanov, Igor and Lemeshko, Mikhail},
journal = {Physical Review Materials},
number = {3},
publisher = {American Physical Society},
title = {{Fingerprints of angulon instabilities in the spectra of matrix-isolated molecules}},
doi = {10.1103/PhysRevMaterials.1.035602},
volume = {1},
year = {2017},
}
@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{996,
abstract = {Iodine (I 2 ) molecules embedded in He nanodroplets are aligned by a 160 ps long laser pulse. The highest degree of alignment, occurring at the peak of the pulse and quantified by ⟨cos 2 θ 2D ⟩ , is measured as a function of the laser intensity. The results are well described by ⟨cos 2 θ 2D ⟩ calculated for a gas of isolated molecules each with an effective rotational constant of 0.6 times the gas-phase value, and at a temperature of 0.4 K. Theoretical analysis using the angulon quasiparticle to describe rotating molecules in superfluid helium rationalizes why the alignment mechanism is similar to that of isolated molecules with an effective rotational constant. A major advantage of molecules in He droplets is that their 0.4 K temperature leads to stronger alignment than what can generally be achieved for gas phase molecules -- here demonstrated by a direct comparison of the droplet results to measurements on a ∼ 1 K supersonic beam of isolated molecules. This point is further illustrated for more complex system by measurements on 1,4-diiodobenzene and 1,4-dibromobenzene. For all three molecular species studied the highest values of ⟨cos 2 θ 2D ⟩ achieved in He droplets exceed 0.96. },
author = {Shepperson, Benjamin and Chatterley, Adam and Søndergaard, Anders and Christiansen, Lars and Lemeshko, Mikhail and Stapelfeldt, Henrik},
issn = {00219606},
journal = {The Journal of Chemical Physics},
number = {1},
publisher = {AIP},
title = {{Strongly aligned molecules inside helium droplets in the near-adiabatic regime}},
doi = {10.1063/1.4983703},
volume = {147},
year = {2017},
}
@article{997,
abstract = {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.},
author = {Yakaboylu, Enderalp and Deuchert, Andreas and Lemeshko, Mikhail},
issn = {00319007},
journal = {APS Physics, Physical Review Letters},
number = {23},
publisher = {American Physiological Society},
title = {{Emergence of non-abelian magnetic monopoles in a quantum impurity problem}},
doi = {10.1103/PhysRevLett.119.235301},
volume = {119},
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},
}
@article{1076,
abstract = {Signatures of the Coulomb corrections in the photoelectron momentum distribution during laser-induced ionization of atoms or ions in tunneling and multiphoton regimes are investigated analytically in the case of a one-dimensional problem. A high-order Coulomb-corrected strong-field approximation is applied, where the exact continuum state in the S matrix is approximated by the eikonal Coulomb-Volkov state including the second-order corrections to the eikonal. Although without high-order corrections our theory coincides with the known analytical R-matrix (ARM) theory, we propose a simplified procedure for the matrix element derivation. Rather than matching the eikonal Coulomb-Volkov wave function with the bound state as in the ARM theory to remove the Coulomb singularity, we calculate the matrix element via the saddle-point integration method by time as well as by coordinate, and in this way avoiding the Coulomb singularity. The momentum shift in the photoelectron momentum distribution with respect to the ARM theory due to high-order corrections is analyzed for tunneling and multiphoton regimes. The relation of the quantum corrections to the tunneling delay time is discussed.},
author = {Klaiber, Michael and Daněk, Jiří and Yakaboylu, Enderalp and Hatsagortsyan, Karen and Keitel, Christoph},
issn = {24699926},
journal = { Physical Review A - Atomic, Molecular, and Optical Physics},
number = {2},
publisher = {American Physical Society},
title = {{Strong-field ionization via a high-order Coulomb-corrected strong-field approximation}},
doi = {10.1103/PhysRevA.95.023403},
volume = {95},
year = {2017},
}
@article{1109,
abstract = {Rotation of molecules embedded in He nanodroplets is explored by a combination of fs laser-induced alignment experiments and angulon quasiparticle theory. We demonstrate that at low fluence of the fs alignment pulse, the molecule and its solvation shell can be set into coherent collective rotation lasting long enough to form revivals. With increasing fluence, however, the revivals disappear -- instead, rotational dynamics as rapid as for an isolated molecule is observed during the first few picoseconds. Classical calculations trace this phenomenon to transient decoupling of the molecule from its He shell. Our results open novel opportunities for studying non-equilibrium solute-solvent dynamics and quantum thermalization. },
author = {Shepperson, Benjamin and Søndergaard, Anders and Christiansen, Lars and Kaczmarczyk, Jan and Zillich, Robert and Lemeshko, Mikhail and Stapelfeldt, Henrik},
journal = {Physical Review Letters},
number = {20},
publisher = {American Physical Society},
title = {{Laser-induced rotation of iodine molecules in helium nanodroplets: Revivals and breaking-free}},
doi = {10.1103/PhysRevLett.118.203203},
volume = {118},
year = {2017},
}
@article{1119,
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 importantly, 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.},
author = {Lemeshko, Mikhail},
issn = {00319007},
journal = {Physical Review Letters},
number = {9},
publisher = {American Physical Society},
title = {{Quasiparticle approach to molecules interacting with quantum solvents}},
doi = {10.1103/PhysRevLett.118.095301},
volume = {118},
year = {2017},
}
@article{1120,
abstract = {The existence of a self-localization transition in the polaron problem has been under an active debate ever since Landau suggested it 83 years ago. Here we reveal the self-localization transition for the rotational analogue of the polaron -- the angulon quasiparticle. We show that, unlike for the polarons, self-localization of angulons occurs at finite impurity-bath coupling already at the mean-field level. The transition is accompanied by the spherical-symmetry breaking of the angulon ground state and a discontinuity in the first derivative of the ground-state energy. Moreover, the type of the symmetry breaking is dictated by the symmetry of the microscopic impurity-bath interaction, which leads to a number of distinct self-localized states. The predicted effects can potentially be addressed in experiments on cold molecules trapped in superfluid helium droplets and ultracold quantum gases, as well as on electronic excitations in solids and Bose-Einstein condensates. },
author = {Li, Xiang and Seiringer, Robert and Lemeshko, Mikhail},
issn = {24699926},
journal = {Physical Review A},
number = {3},
publisher = {American Physical Society},
title = {{Angular self-localization of impurities rotating in a bosonic bath}},
doi = {10.1103/PhysRevA.95.033608},
volume = {95},
year = {2017},
}