Collisional properties of a dipolar gas of ultracold chromium atoms

Temperature dependence of the cross-section for elastic ground-state collisions of ⁵²Cr- and ⁵⁰Cr-atoms using the method of cross-dimensional relaxation, respectively is determined. The corresponding absolute values of the scattering length are on the order of 100a₀ for ⁵²Cr and ⁵⁰a₀ for ⁵⁰Cr, respectively. However, to use evaporative cooling to increase the phase space density and achieve Bose-Einstein condensation with chromium atoms, have been limited to values below 10^-2. Unusuually high inelastic collisio rates due to dipolar relaxation cause atom loss and heating of the sample. These losses are investigated and found similar magnetic-field-dependent rate coefficients (≈10^-11 cm³/s @ 50 G offset field) for the two isotopes ⁵⁰Cr and ⁵²Cr indicating that the strongly enhanced dipolar relaxation is a generic feature of the dipole-dipole interaction. This conclusion is supported by theoretical calculations. To circumvent these losses, an optical dipole trap is currently being set up. Here, chromium can be trapped in the energetically lowest state where dipolar relaxation is absent.