Many-body aspects of coherent atom-molecule oscillations

We study the many-body effects on coherent atom-molecule oscillations by means of an effective quantum field theory that describes Feshbach-resonant interactions in Bose gases in terms of an atom-molecule Hamiltonian. We determine numerically the many-body corrections to the oscillation frequency for various densities of the atomic condensate. We also derive an analytic expression that approximately describes both the density and magnetic-field dependence of this frequency near the resonance. We find excellent agreement with experiment.     

Effective theory of excitations in a Feshbach-resonant superfluid

A strongly interacting Fermi gas, such as that of cold atoms operative near a Feshbach resonance, is difficult to study by perturbative many-body theory to go beyond mean-field approximation. Here I develop an effective field theory for the resonant superfluid based on broken symmetry. The theory retains both fermionic quasiparticles and superfluid phonons, the interaction between them being derived nonperturbatively. The theory converges and can be improved order by order, in a manner governed by a low energy expansion rather than by a coupling constant. I apply the effective theory to calculate the specific heat and discuss the theory with a recent heat capacity experiment.     

Sum rule for the optical absorption of an interacting many-polaron gas

A sum rule for the first frequency moment of the optical absorption of a many-polaron system is derived, taking into account many-body effects in the system of constituent charge carriers of the many-polaron system. In our expression for the sum rule, the electron-phonon coupling and the many-body effects in the electron (or hole) system formally decouple, so that the many-body effects can be treated to the desired level of approximation by the choice of the dynamical structure factor of the electron (hole) gas. We calculate correction factors to take into account both low and high experimental cutoff frequencies. Received 26 April 2000 and Received in final form 5 December 2000     

Entangling Lattice-Trapped Bosons with a Free Impurity: Impact on Stationary and Dynamical Properties

We address the interplay of few lattice trapped bosons interacting with an impurity atom in a box potential. For the ground state, a classification is performed based on the fidelity allowing to quantify the susceptibility of the composite system to structural changes due to the intercomponent coupling. We analyze the overall response at the many-body level and contrast it to the single-particle level. By inspecting different entropy measures we capture the degree of entanglement and intraspecies correlations for a wide range of intra- and intercomponent interactions and lattice depths. We also spatially resolve the imprint of the entanglement on the one- and two-body density distributions showcasing that it accelerates the phase separation process or acts against spatial localization for repulsive and attractive intercomponent interactions, respectively. The many-body effects on the tunneling dynamics of the individual components, resulting from their counterflow, are also discussed. The tunneling period of the impurity is very sensitive to the value of the impurity-medium coupling due to its effective dressing by the few-body medium. Our work provides implications for engineering localized structures in correlated impurity settings using species selective optical potentials.     

BCS-BEC crossover in 2D Fermi gases with Rashba spin-orbit coupling

We present a systematic theoretical study of the BCS-BEC crossover in two-dimensional Fermi gases with Rashba spin-orbit coupling (SOC). By solving the exact two-body problem in the presence of an attractive short-range interaction we show that the SOC enhances the formation of the bound state: the binding energy E(B) and effective mass m(B) of the bound state grows along with the increase of the SOC. For the many-body problem, even at weak attraction, a dilute Fermi gas can evolve from a BCS superfluid state to a Bose condensation of molecules when the SOC becomes comparable to the Fermi momentum. The ground-state properties and the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature are studied, and analytical results are obtained in various limits. For large SOC, the BKT transition temperature recovers that for a Bose gas with an effective mass m(B). We find that the condensate and superfluid densities have distinct behaviors in the presence of SOC: the condensate density is generally enhanced by the SOC due to the increase of the molecule binding; the superfluid density is suppressed because of the nontrivial molecule effective mass m(B).     

Two-body physics in quasi-low-dimensional atomic gases under spin–orbit coupling

One of the most dynamic directions in ultracold atomic gas research is the study of low-dimensional physics in quasi-low-dimensional geometries, where atoms are confined in strongly anisotropic traps. Recently, interest has significantly intensified with the realization of synthetic spin–orbit coupling (SOC). As a first step toward understanding the SOC effect in quasi-low-dimensional systems, the solution of two-body problems in different trapping geometries and different types of SOC has attracted great attention in the past few years. In this review, we discuss both the scattering-state and the bound-state solutions of two-body problems in quasi-one and quasi-two dimensions. We show that the degrees of freedom in tightly confined dimensions, in particular with the presence of SOC, may significantly affect system properties. Specifically, in a quasi-one-dimensional atomic gas, a one-dimensional SOC can shift the positions of confinement-induced resonances whereas, in quasitwo-dimensional gases, a Rashba-type SOC tends to increase the two-body binding energy, such that more excited states in the tightly confined direction are occupied and the system is driven further away from a purely two-dimensional gas. The effects of the excited states can be incorporated by adopting an effective low-dimensional Hamiltonian having the form of a two-channel model. With the bare parameters fixed by two-body solutions, this effective Hamiltonian leads to qualitatively different many-body properties compared to a purely low-dimensional model.     

Quark structure and nuclear effective forces

We formulate the quark meson coupling model as a many-body effective Hamiltonian. This leads naturally to the appearance of many-body forces. We investigate the zero range limit of the model and compare its Hartree-Fock Hamiltonian to that corresponding to the Skyrme effective force. By fixing the three parameters of the model to reproduce the binding and symmetry energy of nuclear matter, we find that it allows a very satisfactory interpretation of the Skyrme force.     

Molecule production via Feshbach resonance in bosonic systems

In this article, we review our recent theoretical works on producing ultracold molecules from ultracold bosonic atoms via magnetically tunable Feshbach resonances. Our analysis relies on a two-channel quantum microscopic model that accounts for many-body effects in the association process. We show that the picture of two-body molecular production depicted by the Landau-Zener model is significantly altered due to many-body effects. We derive an analytic expression for molecular conversion efficiency for the nonadiabatic linearly swept Feshbach resonance, that explains the discrepancy between the prediction of the Landau-Zener formula and the experimental data. With including the thermal dephasing effects in the oscillating magnetic field modulation Feshbach resoance, we reproduce the Lorentzian resonance lineshape and explain the maximum conversion efficiency observed in experiment.     

Many-body effects on intersubband resonances in narrow InAs/AlSb quantum wells

Intersubband polarization couples to collective excitations of the interacting electron gas confined in a semiconductor quantum well (QW) structure. Such excitations include correlated pair excitations (repellons) and intersubband plasmons. The oscillator strength of intersubband resonances (ISBRs) strongly varies with QW parameters and electron density because of this coupling. Using the intersubband semiconductor Bloch equations for a two-conduction-subband model, we show that intersubband absorption spectra for narrow wells are dominated by the Fermi-edge singularity (via coupling to repellons) when the electron gas becomes degenerate and in the presence of large nonparabolicity. Thus the resonance peak position appears at the Fermi edge and the peak is greatly narrowed, enhanced, and red shifted as compared to the free particle result. Our results uncover a new perspective for ISBRs and indicate the necessity of proper many-body theoretical treatment in order for modeling and prediction of ISBR line shape.     

Coherent Zeeman resonance from electron spin coherence in a mixed-type GaAs/AlAs quantum well

Coherent Zeeman resonance from electron spin coherence is demonstrated in a Lambda-type three-level system, coupling electron spin states via trions. The optical control of electron density that is characteristic of a mixed-type quantum-well facilitates the study of trion formation as well as the effects of many-body interactions on the manifestation of electron spin coherence in the nonlinear optical response.     

Four-body problem and BEC-BCS crossover in a quasi-one-dimensional cold fermion gas

The four-body problem for an interacting two-species Fermi gas is solved analytically in a confined quasi-one-dimensional geometry, where the two-body atom-atom scattering length a(aa) displays a confinement-induced resonance. We compute the dimer-dimer scattering length a(dd) and show that this quantity completely determines the many-body solution of the associated BEC-BCS crossover phenomenon in terms of bosonic dimers.     

Dynamical exchange effects in a two-dimensional many-polaron gas

We calculate the influence of dynamical exchange effects on the response properties and the static properties of a two-dimensional many-polaron gas. These effects are not manifested in the random-phase approximation which is widely used in the analysis of the many-polaron system. Here they are taken into account by using a dielectric function derived in the time-dependent Hartree-Fock formalism. At weak electron-phonon coupling, we find that dynamical exchange effects lead to substantial corrections to the random-phase approximation results for the ground state energy, the effective mass, and the optical conductivity of the polaron system. Furthermore, we show that the reduction of the spectral weight of the optical absorption spectrum at frequencies above the longitudinal optical phonon frequency, due to many-body effects, is overestimated by the random-phase approximation.Received: 24 December 2003, Published online: 9 April 2004PACS: 71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons - 71.10.Pm Fermions in reduced dimensions (anyons, composite fermions, Luttinger liquid, etc.) - 71.38.Fp Large or Fröhlich polarons     

Δ-isobar dynamics in finite nuclei: Pion elastic scattering

Pion elastic scattering from finite nuclei in the 3,3 resonance region is described in terms of the coupling of a pion wave to correlated isobar-hole states. This is first carried out in an optical potential picture and then treated as a many-body response problem within the framework of RPA. Results are presented for the example of pion scattering from ⁴He; they show appreciable effects of isobar self-energy interactions and isobar-hole correlations other than one-pion exchange.     

Interaction enhanced imaging of individual Rydberg atoms in dense gases

We propose a new all-optical method to image individual Rydberg atoms embedded within dense gases of ground state atoms. The scheme exploits interaction-induced shifts on highly polarizable excited states of probe atoms, which can be spatially resolved via an electromagnetically induced transparency resonance. Using a realistic model, we show that it is possible to image individual Rydberg atoms with enhanced sensitivity and high resolution despite photon-shot noise and atomic density fluctuations. This new imaging scheme could be extended to other impurities such as ions, and is ideally suited to equilibrium and dynamical studies of complex many-body phenomena involving strongly interacting particles. As an example we study blockade effects and correlations in the distribution of Rydberg atoms optically excited from a dense gas.     

Distribution of resonance widths and dynamics of continuum coupling

We analyze the statistics of resonance widths in a many-body Fermi system with open decay channels. Depending on the strength of continuum coupling, such a system reveals growing deviations from the standard chi-square (Porter-Thomas) width distribution. The deviations emerge from the process of increasing interaction of intrinsic states through common decay channels; in the limit of perfect coupling this process leads to the superradiance phase transition. The width distribution depends also on the intrinsic dynamics (chaotic versus regular). The results presented here are important for understanding the recent experimental data concerning the width distribution for neutron resonances in nuclei.     

Test for pairing symmetry based on spin fluctuations in the organic superconductor kappa-(BEDT-TTF)2X

We propose that the superconducting pairing symmetry of organic superconductors kappa-(BEDT-TTF)2X can be determined by measuring the position in momentum space of the incommensurate peaks of the spin susceptibility. Using the weak coupling BCS theory and including the many-body effects via the random-phase approximation for the Hubbard model on an anisotropic triangular lattice, we show that the position of these peaks is uniquely determined by the pairing symmetry of the superconducting state and the geometry of the Fermi surface. We demonstrate the different incommensurate patterns of spin responses for d(x(2)-y(2-)) and d(xy)-like pairing states. In addition, we find that there is no spin resonance mode in the reasonable range of parameters discussed.     

Microscopic Many-Body Effects on Intersubband Resonance in Quantum Well

Considering the Coulomb many-body interactions, we investigate the intersubband optical processes of the quantum well by using the semiconductor Bloch equations. We calculate the evolution of intersubband absorption spectral line shape as a function of lattice temperature and electron density. It is found that the coupling of intersubband plasmons can reduce and red-shift the lower energy resonance, simultaneously enhance and blue-shift the higher energy resonance. The dependence of cascading resonances on temperature and electron density is also discussed.     

Virial theorem and universality in a unitary fermi gas

Unitary Fermi gases, where the scattering length is large compared to the interparticle spacing, can have universal properties, which are independent of the details of the interparticle interactions when the range of the scattering potential is negligible. We prepare an optically trapped, unitary Fermi gas of 6Li, tuned just above the center of a broad Feshbach resonance. In agreement with the universal hypothesis, we observe that this strongly interacting many-body system obeys the virial theorem for an ideal gas over a wide range of temperatures. Based on this result, we suggest a simple volume thermometry method for unitary gases. We also show that the observed breathing mode frequency, which is close to the unitary hydrodynamic value over a wide range of temperature, is consistent with a universal hydrodynamic gas with nearly isentropic dynamics.     

Many-body braiding phases in a rotating strongly correlated photon gas

We present a theoretical study of fractional quantum Hall physics in a rotating gas of strongly interacting photons in a single cavity with a large optical nonlinearity. Photons are injected into the cavity by a Laguerre–Gauss laser beam with a non-zero orbital angular momentum. The Laughlin-like few-photon eigenstates appear as sharp resonances in the transmission spectra. Using additional localized repulsive potentials, quasi-holes can be created in the photon gas and then braided around in space: an unambiguous signature of the many-body Berry phase under exchange of two quasi-holes is observed as a spectral shift of the corresponding transmission resonance.