We have investigated the role of momentum dependent s-wave and higher partial wave strong interactions to determine the ground state properties and the column densities in the Bose-Einstein condensate (BEC) for large scattering length (a) such that ka >>1 even for small values of momentum where the momentum p=(h/2pi)k and k is the wave number. Since the scattering length is large we have included the first correction (Lee-Huang-Yang correction) both for the k-dependent (s-wave + higher partial wave) interactions and k-independent contact interactions (s-wave). We have derived the time-independent equations from the corresponding energy functionals and found that the ground state properties and the column densities differ significantly for these two types of interactions even for moderate values of scattering length (a = 3000 a_0) in BEC of cylindrically trapped 85Rb atoms at 100 nK. The effect of higher partial wave (d-wave) increases with increase in a and it is > 20% for peak density at a= 8700 a_0 which can be experimentally detected. Dependence of column density on particle number density has been studied. Column densities have been compared with experimental results.
We have investigated the role of momentum dependent s-wave and higher partial wave strong interactions to determine the ground state properties and the column densities in the Bose-Einstein condensate (BEC) for large scattering length (a) such that ka >>1 even for small values of momentum where the momentum p=(h/2pi)k and k is the wave number. Since the scattering length is large we have included the first correction (Lee-Huang-Yang correction) both for the k-dependent (s-wave + higher partial wave) interactions and k-independent contact interactions (s-wave). We have derived the time-independent equations from the corresponding energy functionals and found that the ground state properties and the column densities differ significantly for these two types of interactions even for moderate values of scattering length (a = 3000 a_0) in BEC of cylindrically trapped 85Rb atoms at 100 nK. The effect of higher partial wave (d-wave) increases with increase in a and it is > 20% for peak density at a= 8700 a_0 which can be experimentally detected. Dependence of column density on particle number density has been studied. Column densities have been compared with experimental results.
We investigate the role of momentum-dependent interactions to determine the ground-state properties in the Bose-Einstein condensate for large scattering lengths (a) such that ka $\ensuremath{\ge}1$, even for small momentum (p) where $p=\ensuremath{\hbar}k$ and k is the wave number. The results for momentum-dependent and momentum-independent interactions differ significantly, even for moderate values of a, and the effect of higher partial-wave interaction increases with the increase in a. We have made an attempt to compare the theoretical column density with experimental results at different magnetic fields [Cornish et al., Phys. Rev. Lett. 85, 1795 (2000)]. Since the initial number of atoms changes while swapping the magnetic field and the actual value of the number of atoms at different magnetic fields is not known, we studied the dependence of column densities on the value of N at different magnetic fields. We present here the results for those values of N for which the theoretical column densities are comparable with the experimental results. It is shown that the column density for 85Rb atoms at 100 nK with momentum-dependent interaction is in fairly good agreement with the experimental values for different values of N at two different values of magnetic fields (B=157.2 and 156.4 G).
Using the density-functional theory, the Ginzburg Pitaevskii Gross (GPG) equation for Bose-Einstein (BE) condensate, confined in a magnetic trap, is modified to include contribution from three-body collisions in the strongly interacting regime a>>l, 'a' is the scattering length and 'l' being the characteristic low energy length scale. This generalized GPG equation has been solved numerically using the analytically derived Thomas-Fermi order parameter, which also includes three-body interactions. The order parameter, chemical potential, extent of correlation and other ground state properties are computed when the aspect ratio, λ, is varied from1.0 to 0.05 (λ represents the anisotropy of the magnetic trap). As λ is varied from 1.0 to 0.05, the condensate shape changes from isotropic three-dimensional (3-D) to highly anisotropic quasi one-dimensional (1-D). The stability of the BE condensate increases with decrease in λ, which is also borne out by the behavior of chemical potential and the total energy per particle, as there is a decrease of about four times for a=5000 a0 as well as for a=7000 a0, 'a0' being the Bohr radius. The extent of correlations, however, increases by more than five folds, showing that quasi 1-D BE condensate is highly correlated. Both two- and three-body interaction energies show a decrease with decrease in λ: three-body interaction energy staying below two-body interaction energy for a=5000 a0 while for a=7000 a0, a cross-over occurs between the two at λ ~ 0.35. As one goes from 3-D to quasi 1-D, the percentage difference for various physical quantities, computed between only two-body interactions and when both two- and three-body interactions are considered, shows a decrease, suggesting that the effect of three-body collisions become increasingly less significant in agreement with the recent study.
Bose-Einstein condensation in trapped gases of 85 Rb atoms can now be obtained in the large gas-parameter region in presence of Feshbach resonance and the gas density which is characterized by peak gas parameter x pk can attain the value ~ 10 -2 . For such large gas-parameter, the first correction term to the Bogoliubov energy functional is considered to give modified Gross-Pitaevskii (MGP) equation. In this work analytical expressions for the chemical potential, the density of particles and the excitation frequency of the monopole mode have been derived in Thomas-Fermi(TF) approximated approach to the MGP equation (modified TF approach) following method of higher order iteration for Bose-Einstein condensate (BEC) of atoms in spherically symmetric trap. We have also solved the MGP equation by using Runge-Kutta method for integration of nonlinear differential equation considering spherically symmetric trap. The analytical results for chemical potential and density of particles for BEC of 10 3 and 10 4 85 Rb atoms in a spherically symmetric trap are in very good agreement with the numerical results from MGP equation at low as well as at large scattering length i.e, at large gas-parameter region. The frequency of monopole mode calculated using the density of particles in the ground state (found out in higher order iterative method) are also in very good agreement with the existing numerical results. The difference between the analytical and numerical results decreases as the value of scattering length i.e, gas-parameter is increased.
Bose-Einstein condensates of 10485Rb atoms in a cylindrical trap are studied using a recently proposed approach based on a local-density approximation. Since the existence of a Feshbach resonance allows for widely tuning the scattering length of the atoms, values of the peak gas parameter xpk of the order of ${10}^{\ensuremath{-}2}$ may be attained and the standard analysis based on Gross-Pitaevskii and/or Thomas-Fermi equations may result in being questionable. Energy functionals derived from the correlated basis functions theory and the low-density expansion for a homogeneous hard-spheres gas are used to estimate corrections to the Gross-Pitaevskii equation. The two functionals give similar results, both showing large differences with respect to the Thomas-Fermi and Gross-Pitaevskii ones. The column densities at z=0 may differ by as much as $\ensuremath{\sim}30%$ and the half maximum radius by $\ensuremath{\sim}20%.$ The scattering lengths estimated by fitting the half maximum radius may differ by $\ensuremath{\sim}40%$ from those given by a Thomas-Fermi-based analysis of the experimental data.
We evaluate the zero temperature properties and Bose-Einstein condensation (BEC) of hard sphere bosons in a harmonic trap using Quantum Monte Carlo methods. The chief purpose of this work is to go beyond the dilute limit, to test the limits of the Gross-Pitaevskii equation and related mean field approximations and to explore the role of interactions in determining the zero temperature properties of the trapped Bose gas. The density is increased by adjusting both the number of trapped bosons, N , and the s-wave scattering length, a, to study systems from the highly dilute region corresponding to early experiments in trapped BEC [2, 3] up to liquid He densities and beyond. Rather than relying on the perturbative Bogoliubov approximation (which assumes a large condensate fraction) to describe the condensate, we use the one-body density matrix (OBDM) formulation of BEC so that the properties of the condensate for systems with arbitrarily large interactions may be studied. In this formulation of BEC, condensate properties are obtained by diagonalizing the OBDM and obtaining the corresponding single particle “natural orbitals” and their occupation numbers for the system. The condensate wave function and condensate fraction are then obtained from the single particle orbital(s) with macroscopic occupation (N0 >> 1). Within this framework, we calculate the effects of interactions and increased density on the ground state energy, the density profile, the momentum distribution, the condensate fraction and condensate “wave-function” and several other properties.
This thesis discusses a series of studies that investigate the effects of interaction – essentially the s-wave scattering – in the various properties of Bose-Einstein condensates (BEC). The phonon wavefunction in a BEC was measured using Bragg spectroscopy and compared with the well-known Bogoliubov theory. Phonons were first excited in a BEC of 3× 10 condensed Na atoms via small-angle two-photon Bragg scattering. Large angle Bragg scattering was then used to probe the momentum distribution. We found reasonable agreement with the theory. With the same technique of Bragg diffraction, we studied the four-wave mixing process for matter waves. The BEC was split into two strong source waves and a weak seed wave. The s-wave scattering coherently mixed pairs of atoms from the sources into the seed and its conjugate wave, creating a pair-correlated atomic beams with “squeezed” number difference. A Feshbach resonance was used to produce ultracold Na2 molecules with initial phase-space density in excess of 20. Starting from an atomic BEC, a magnetic field ramp shifted a bound state from above the threshold of the unbound continuum to below, creating a molecular population with almost zero center-of-mass motion. A reverse field ramp dissociated the cold molecules into free atom pairs carrying kinetic energy dependent on the ramp speed. This dependence provided a measure of the coupling strength between the bound state and the continuum. Condensates were loaded into optical lattices formed with retro-reflected single frequency lasers. Quantum phase transition from the superfluid state to Mott-insulator state was observed in a three dimensional lattice. The increased interaction and flattened dispersion relation led to strongly enhanced quantum depletion in the superfluid state. Thesis Supervisor: Wolfgang Ketterle Title: John D. MacAurthur Professor of Physics