TY - JOUR
AB - We consider a two-component Bose gas in two dimensions at a low temperature with short-range repulsive interaction. In the coexistence phase where both components are superfluid, interspecies interactions induce a nondissipative drag between the two superfluid flows (Andreev-Bashkin effect). We show that this behavior leads to a modification of the usual Berezinskii-Kosterlitz-Thouless (BKT) transition in two dimensions. We extend the renormalization of the superfluid densities at finite temperature using the renormalization-group approach and find that the vortices of one component have a large influence on the superfluid properties of the other, mediated by the nondissipative drag. The extended BKT flow equations indicate that the occurrence of the vortex unbinding transition in one of the components can induce the breakdown of superfluidity also in the other, leading to a locking phenomenon for the critical temperatures of the two gases.
AU - Karle, Volker
AU - Defenu, Nicolò
AU - Enss, Tilman
ID - 6632
IS - 6
JF - Physical Review A
SN - 24699926
TI - Coupled superfluidity of binary Bose mixtures in two dimensions
VL - 99
ER -
TY - CONF
AB - We demonstrate robust retention of valley coherence and its control via polariton pseudospin precession through the optical TE-TM splitting in bilayer WS2 microcavity exciton polaritons at room temperature.
AU - Khatoniar, Mandeep
AU - Yama, Nicholas
AU - Ghazaryan, Areg
AU - Guddala, Sriram
AU - Ghaemi, Pouyan
AU - Menon, Vinod
ID - 6646
SN - 9781943580576
T2 - CLEO: Applications and Technology
TI - Room temperature control of valley coherence in bilayer WS2 exciton polaritons
ER -
TY - JOUR
AB - Dipolar coupling plays a fundamental role in the interaction between electrically or magnetically polarized species such as magnetic atoms and dipolar molecules in a gas or dipolar excitons in the solid state. Unlike Coulomb or contactlike interactions found in many atomic, molecular, and condensed-matter systems, this interaction is long-ranged and highly anisotropic, as it changes from repulsive to attractive depending on the relative positions and orientation of the dipoles. Because of this unique property, many exotic, symmetry-breaking collective states have been recently predicted for cold dipolar gases, but only a few have been experimentally detected and only in dilute atomic dipolar Bose-Einstein condensates. Here, we report on the first observation of attractive dipolar coupling between excitonic dipoles using a new design of stacked semiconductor bilayers. We show that the presence of a dipolar exciton fluid in one bilayer modifies the spatial distribution and increases the binding energy of excitonic dipoles in a vertically remote layer. The binding energy changes are explained using a many-body polaron model describing the deformation of the exciton cloud due to its interaction with a remote dipolar exciton. The surprising nonmonotonic dependence on the cloud density indicates the important role of dipolar correlations, which is unique to dense, strongly interacting dipolar solid-state systems. Our concept provides a route for the realization of dipolar lattices with strong anisotropic interactions in semiconductor systems, which open the way for the observation of theoretically predicted new and exotic collective phases, as well as for engineering and sensing their collective excitations.
AU - Hubert, Colin
AU - Baruchi, Yifat
AU - Mazuz-Harpaz, Yotam
AU - Cohen, Kobi
AU - Biermann, Klaus
AU - Lemeshko, Mikhail
AU - West, Ken
AU - Pfeiffer, Loren
AU - Rapaport, Ronen
AU - Santos, Paulo
ID - 6786
IS - 2
JF - Physical Review X
TI - Attractive dipolar coupling between stacked exciton fluids
VL - 9
ER -
TY - JOUR
AB - 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.
AU - Bighin, Giacomo
AU - Defenu, Nicolò
AU - Nándori, István
AU - Salasnich, Luca
AU - Trombettoni, Andrea
ID - 6940
IS - 10
JF - Physical Review Letters
SN - 0031-9007
TI - Berezinskii-Kosterlitz-Thouless paired phase in coupled XY models
VL - 123
ER -
TY - JOUR
AB - We study few-body bound states of charged particles subject to attractive zero-range/short-range plus repulsive Coulomb interparticle forces. The characteristic length scales of the system at zero energy are set by the Coulomb length scale D and the Coulomb-modified effective range r eff. We study shallow bound states of charged particles with D >> r eff and show that these systems obey universal scaling laws different from neutral particles. An accurate description of these states requires both the Coulomb-modified scattering length and the effective range unless the Coulomb interaction is very weak (D -> ). Our findings are relevant for bound states whose spatial extent is significantly larger than the range of the attractive potential. These states enjoy universality – their character is independent of the shape of the short-range potential.
AU - Schmickler, C.H.
AU - Hammer, H.-W.
AU - Volosniev, Artem
ID - 6955
JF - Physics Letters B
SN - 0370-2693
TI - Universal physics of bound states of a few charged particles
VL - 798
ER -
TY - JOUR
AB - Recently it was shown that a molecule rotating in a quantum solvent can be described in terms of the “angulon” quasiparticle [M. Lemeshko, Phys. Rev. Lett. 118, 095301 (2017)]. Here we extend the angulon theory to the case of molecules possessing an additional spin-1/2 degree of freedom and study the behavior of the system in the presence of a static magnetic field. We show that exchange of angular momentum between the molecule and the solvent can be altered by the field, even though the solvent itself is non-magnetic. In particular, we demonstrate a possibility to control resonant emission of phonons with a given angular momentum using a magnetic field.
AU - Rzadkowski, Wojciech
AU - Lemeshko, Mikhail
ID - 415
IS - 10
JF - The Journal of Chemical Physics
TI - Effect of a magnetic field on molecule–solvent angular momentum transfer
VL - 148
ER -
TY - JOUR
AB - 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.
AU - Bighin, Giacomo
AU - Tscherbul, Timur
AU - Lemeshko, Mikhail
ID - 417
IS - 16
JF - Physical Review Letters
TI - Diagrammatic Monte Carlo approach to rotating molecular impurities
VL - 121
ER -
TY - JOUR
AB - 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.
AU - Bighin, Giacomo
AU - Salasnich, Luca
ID - 420
IS - 17
JF - International Journal of Modern Physics B
TI - Renormalization of the superfluid density in the two-dimensional BCS-BEC crossover
VL - 32
ER -
TY - JOUR
AB - 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.
AU - Amaro, Pedro
AU - Loureiro, Ulisses
AU - Safari, Laleh
AU - Fratini, Filippo
AU - Indelicato, Paul
AU - Stöhlker, Thomas
AU - Santos, José
ID - 427
IS - 2
JF - Physical Review A - Atomic, Molecular, and Optical Physics
TI - Quantum interference in laser spectroscopy of highly charged lithiumlike ions
VL - 97
ER -
TY - JOUR
AB - 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.
AU - Midya, Bikashkali
AU - Konotop, Vladimir
ID - 435
IS - 3
JF - Optics Letters
TI - Coherent-perfect-absorber and laser for bound states in a continuum
VL - 43
ER -
TY - JOUR
AB - 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.
AU - Yakaboylu, Enderalp
AU - Shkolnikov, Mikhail
AU - Lemeshko, Mikhail
ID - 5794
IS - 25
JF - Physical Review Letters
SN - 00319007
TI - Quantum groups as hidden symmetries of quantum impurities
VL - 121
ER -
TY - JOUR
AB - 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.
AU - Yakaboylu, Enderalp
AU - Midya, Bikashkali
AU - Deuchert, Andreas
AU - Leopold, Nikolai K
AU - Lemeshko, Mikhail
ID - 5983
IS - 22
JF - Physical Review B
SN - 2469-9950
TI - Theory of the rotating polaron: Spectrum and self-localization
VL - 98
ER -
TY - JOUR
AB - 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.
AU - Fratini, Filippo
AU - Safari, Laleh
AU - Amaro, Pedro
AU - Santos, José
ID - 294
IS - 4
JF - Physical Review A - Atomic, Molecular, and Optical Physics
TI - Two-photon processes based on quantum commutators
VL - 97
ER -
TY - JOUR
AB - 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.
AU - Bighin, Giacomo
AU - Tscherbul, Timur
AU - Lemeshko, Mikhail
ID - 6339
IS - 16
JF - Physical Review Letters
TI - Diagrammatic Monte Carlo approach to angular momentum in quantum many-particle systems
VL - 121
ER -
TY - JOUR
AB - 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.
AU - Yakaboylu, Enderalp
AU - Lemeshko, Mikhail
ID - 195
IS - 4
JF - Physical Review B - Condensed Matter and Materials Physics
TI - Anyonic statistics of quantum impurities in two dimensions
VL - 98
ER -
TY - JOUR
AB - 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.”
AU - Camus, Nicolas
AU - Yakaboylu, Enderalp
AU - Fechner, Lutz
AU - Klaiber, Michael
AU - Laux, Martin
AU - Mi, Yonghao
AU - Hatsagortsyan, Karen Z.
AU - Pfeifer, Thomas
AU - Keitel, Christoph H.
AU - Moshammer, Robert
ID - 6013
IS - 2
JF - Physical Review Letters
SN - 0031-9007
TI - Experimental evidence for quantum tunneling time
VL - 119
ER -
TY - CHAP
AB - 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.
AU - Lemeshko, Mikhail
AU - Schmidt, Richard
ED - Dulieu, Oliver
ED - Osterwalder, Andreas
ID - 604
SN - 20413181
T2 - Cold Chemistry: Molecular Scattering and Reactivity Near Absolute Zero
TI - Molecular impurities interacting with a many-particle environment: From ultracold gases to helium nanodroplets
VL - 11
ER -
TY - CONF
AB - 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.
AU - Camus, Nicolas
AU - Yakaboylu, Enderalp
AU - Fechner, Lutz
AU - Klaiber, Michael
AU - Laux, Martin
AU - Mi, Yonghao
AU - Hatsagortsyan, Karen
AU - Pfeifer, Thomas
AU - Keitel, Cristoph
AU - Moshammer, Robert
ID - 313
IS - 1
SN - 17426588
TI - Experimental evidence for Wigner's tunneling time
VL - 999
ER -
TY - JOUR
AB - 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.
AU - Midya, Bikashkali
AU - Konotop, Vladimir
ID - 939
IS - 3
JF - Physical Review Letters
SN - 00319007
TI - Waveguides with absorbing boundaries: Nonlinearity controlled by an exceptional point and solitons
VL - 119
ER -
TY - JOUR
AB - 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.
AU - Cherepanov, Igor
AU - Lemeshko, Mikhail
ID - 994
IS - 3
JF - Physical Review Materials
TI - Fingerprints of angulon instabilities in the spectra of matrix-isolated molecules
VL - 1
ER -