We experimentally investigate and analyze the rich dynamics in F=2 spinor Bose-Einstein condensates of 87 Rb. An interplay between mean-field driven spin dynamics and hyperfine-changing losses in addition to interactions with the thermal component is observed. In particular we measure conversion rates in the range of 10 −12 cm 3 s −1 for spin changing collisions within the F=2 manifold and spin-dependent loss rates in the range of 10 −13 cm 3 s −1 for hyperfine-changing collisions. From our data we observe a polar behavior in the F=2 ground state of 87 Rb, while we measure the F=1 ground state to be ferromagnetic. Furthermore we see a magnetization for condensates prepared with non-zero total spin.PACS numbers: 03.75. Mn, 34.50.Pi, 03.75.Hh The investigation of atomic spin systems is central for the understanding of magnetism and a highly active area of research e.g. with respect to magnetic nanosystems, spintronics and magnetic interactions in high T c superconductors. In addition entangled spin systems in atomic quantum gases show intriguing prospects for quantum optics and quantum computation [1,2,3,4,5]. Bose-Einstein condensates (BEC) of ultra-cold atoms offer new regimes for studies of collective spin phenomena [6,7,8,9,10,11,12,13]. BECs with spin degree of freedom are special in the sense that their order parameter is a vector in contrast to the "common" BEC where it is a scalar. Recent extensive studies have been made in optically trapped 23 Na in the F=1 state [10,11,12,13]. In addition evidence of spin dynamics was demonstrated in optically trapped 87 Rb in the F=1 state [14]. There is current interest in extending the systems under investigation to F=2 spinor condensates [15,16,17,18,19,20], which add significant new physics. F=2 spinor condensates offer richer dynamics, an additional magnetic phase, the so-called cyclic phase [16,18], as well as intrinsic connections to d-wave superconductors [21].In this letter we present first studies of optically trapped 87 Rb F=2 spinor condensates. We measure rates for spin changing collisions for different channels within the F=2 manifold and discuss the steady state for various initial conditions. Additionally we observe and discuss the thermalization of dynamically populated m F condensates. We also present measurements of spin-dependent hyperfine decay rates of the F=2 state in 87 Rb, as a key to further understanding the intensively studied collisional properties of 87 Rb [22,23].Our experimental setup consists of a compact double MOT apparatus which produces magnetically trapped 87 Rb Bose-Einstein condensates containing 10 6 atoms in the F=2, m F = 2 state. To confine the atoms independently of their spin state they are subsequently transferred into a far detuned optical dipole trap. It is operated at 1064 nm generating trapping frequencies of typically 2π × 891 Hz vertically, 2π × 155 Hz horizontally and 2π × 21.1 Hz along the beam direction. After transfer we further cool the ensemble for 500 ms by selective parametric excitation [24] resulting in ...

The dipole response of the N = 50 nucleus 90 Zr was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 7.9, 9.0, and 13.2 MeV. We identified 189 levels up to an excitation energy of 12.9 MeV. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. In this way we derived the photoabsorption cross section up to the neutron-separation energy. This cross section matches well the photoabsorption cross section obtained from (γ, n) data and thus provides information about the extension of the dipole-strength distribution toward energies below the neutron-separation energy. An enhancement of E1 strength has been found in the range of 6 MeV to 11 MeV. Calculations within the framework of the quasiparticle-phonon model ascribe this strength to a vibration of the excessive neutrons against the N = Z neutron-proton core, giving rise to a pygmy dipole resonance.

The dipole response of the magic N = 50 nucleus 88 Sr was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 9.0, 13.2, and 16.0 MeV. We identified 160 levels up to an excitation energy of 12 MeV. By using polarized photons linear polarizations of about 50 γ transitions were measured that enabled parity assignments to the corresponding states. In the energy range of 6-12 MeV we identified only one M1 transition; all other transitions have E1 character. Thus, E1 character was proven for 63% of the total dipole strength of the observed levels in the given energy range. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. In this way we derived the photoabsorption cross section up to the neutron-separation energy. This cross section matches well the photoabsorption cross section obtained from (γ, n) data and thus provides information about the extension of the dipole-strength distribution toward energies below the neutron-separation energy. An enhancement of E1 strength at 6-11 MeV may be considered as an indication for a pygmy dipole resonance.

Recent observations of (6)Li in metal poor stars suggest a large production of this isotope during big bang nucleosynthesis (BBN). In standard BBN calculations, the (2)H(α,γ)(6)Li reaction dominates (6)Li production. This reaction has never been measured inside the BBN energy region because its cross section drops exponentially at low energy and because the electric dipole transition is strongly suppressed for the isoscalar particles (2)H and α at energies below the Coulomb barrier. Indirect measurements using the Coulomb dissociation of (6)Li only give upper limits owing to the dominance of nuclear breakup processes. Here, we report on the results of the first measurement of the (2)H(α,γ)(6)Li cross section at big bang energies. The experiment was performed deep underground at the LUNA 400 kV accelerator in Gran Sasso, Italy. The primordial (6)Li/(7)Li isotopic abundance ratio has been determined to be (1.5 ± 0.3) × 10(-5), from our experimental data and standard BBN theory. The much higher (6)Li/(7)Li values reported for halo stars will likely require a nonstandard physics explanation, as discussed in the literature.

Photoexcitation of the N = 50 nucleus 89 Y has been performed at the bremsstrahlung facility at the superconducting electron accelerator ELBE at electron energies of E kin e = 9.5 and 13.2 MeV. About 250 levels up to the neutron-separation energy were identified. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section derived in this way up to the neutron-separation energy is combined with the photoabsorption cross section obtained from (γ, n) data and provides information about the extension of the giant dipole resonance toward energies below the neutron-separation energy. An enhancement of E1 strength has been found in the range from about 6 to 11 MeV. The experimental photoabsorption cross sections of 89 Y and of the neighboring N = 50 isotones 88 Sr and 90 Zr are compared with predictions of the quasiparticle-random-phase approximation.