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arXiv:quant-ph/0307184 · Applied Physics B

Dipolar relaxation in an ultra-cold gas of magnetically trapped chromium atoms

S. Hensler, J. Werner, Axel Griesmaier, Piet O. Schmidt, Axel Görlitz, Tilman Pfau, S. Giovanazzi, Kazimierz Rza̧żewski

Published Dec 1, 2003• 109 citations
arXivHTMLPDF
DOI
10.1007/s00340-003-1334-0

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Scattering properties of ultra-cold chromium atoms

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In this work a gas of ultra-cold chromium atoms in a magnetic trap has been prepared and its elastic and inelastic scattering properties have been investigated with regard to Bose-Einstein condensation. Bose-Einstein condensation of dilute atomic gases is achieved using different cooling and trapping techniques. Deviating from the standard way we were able to devise a continuous loading mechanism for a magnetic trap as a result of the spectroscopic properties and the large magnetic dipole moment of chromium. It not only allows us to trap more atoms but also facilitates the subsequent preparation steps. After loading, the magnetic trap is compressed to increase the atomic density. In doing so we also increases the temperature of the atomic cloud. Doppler cooling of the atoms in the compressed trap reduces the temperature and increases the density. Subsequent evaporative cooling further reduces the temperature. The efficiency of the cooling mechanism is determined by the elastic and inelastic scattering properties of atomic species. Ultra-cold collisions between ground state atoms are dominated by s-wave collisions. They are characterized by a single parameter, the scattering length a. We were able to determine the temperature dependence of the elastic collision rate for the two bosonic isotopes 52Cr and 50Cr in a relaxation experiment. Comparing our results with the effective range theory allowed us not only to extract the magnitude of the scattering length, but also its sign. The sign of the scattering length is important since only condensates with a positive scattering length are stable. Our efforts to achieve Bose-Einstein condensation in chromium by evaporative cooling in a magnetic trap resulted in a maximum phase space density of 0.04. Further cooling reduced the phase space density due to increased loss of atoms from dipolar relaxation. We were able to obtain preliminary results on the magnetic field dependence of dipolar relaxation. Our experimental data is in excellent agreement with theory. These experiments confirm the theoretical prediction that the dipolar relaxation rate is independent from the details of the interaction potential but rather scales with the magnetic offset field and the magnetic dipole moment of the atoms. The findings of this work mark an important step towards the realization of a Bose-Einstein condensate with chromium atoms. Especially the knowledge of the elastic and inelastic scattering properties allow now to devise a successful strategy.

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Collisional properties of a dipolar gas of ultracold chromium atoms

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Temperature dependence of the cross-section for elastic ground-state collisions of /sup 52/Cr- and /sup 50/Cr-atoms using the method of cross-dimensional relaxation, respectively is determined. The corresponding absolute values of the scattering length are on the order of 100a/sub 0/ for /sup 52/Cr and /sup 50/a/sub 0/ for /sup 50/Cr, 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/sup -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 (/spl ap/10/sup -11/ cm/sup 3//s @ 50 G offset field) for the two isotopes /sup 50/Cr and /sup 52/Cr 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.

European Quantum Electronics Conference (EQEC)•Jan 1, 2003•0 citations
In this work a gas of ultra-cold chromium atoms in a magnetic trap has been prepared and its elastic and inelastic scattering properties have been investigated with regard to Bose-Einstein condensation. Bose-Einstein condensation of dilute atomic gases is achieved using different cooling and trapping techniques. Deviating from the standard way we were able to devise a continuous loading mechanism for a magnetic trap as a result of the spectroscopic properties and the large magnetic dipole moment of chromium. It not only allows us to trap more atoms but also facilitates the subsequent preparation steps. After loading, the magnetic trap is compressed to increase the atomic density. In doing so we also increases the temperature of the atomic cloud. Doppler cooling of the atoms in the compressed trap reduces the temperature and increases the density. Subsequent evaporative cooling further reduces the temperature. The efficiency of the cooling mechanism is determined by the elastic and inelastic scattering properties of atomic species. Ultra-cold collisions between ground state atoms are dominated by s-wave collisions. They are characterized by a single parameter, the scattering length a. We were able to determine the temperature dependence of the elastic collision rate for the two bosonic isotopes 52Cr and 50Cr in a relaxation experiment. Comparing our results with the effective range theory allowed us not only to extract the magnitude of the scattering length, but also its sign. The sign of the scattering length is important since only condensates with a positive scattering length are stable. Our efforts to achieve Bose-Einstein condensation in chromium by evaporative cooling in a magnetic trap resulted in a maximum phase space density of 0.04. Further cooling reduced the phase space density due to increased loss of atoms from dipolar relaxation. We were able to obtain preliminary results on the magnetic field dependence of dipolar relaxation. Our experimental data is in excellent agreement with theory. These experiments confirm the theoretical prediction that the dipolar relaxation rate is independent from the details of the interaction potential but rather scales with the magnetic offset field and the magnetic dipole moment of the atoms. The findings of this work mark an important step towards the realization of a Bose-Einstein condensate with chromium atoms. Especially the knowledge of the elastic and inelastic scattering properties allow now to devise a successful strategy.
We create Bose-Einstein Condensates of52Cr atoms in an optical trap. Intense radio-frequency fields are used to induce degeneracy between the Zeeman sub-states of the ground state atoms in the BEC despite an ambient static field of 100 mG. We demonstrate this effect by analyzing atom trajectories under the influence of dressed magnetic potentials. We investigate the problem of adiabaticity in the dressing process. In a further set of experiments, we transfer the atoms from the absolute ground state into the excited Zeeman sub-state and study the dipolar relaxation in ultra-cold gases of chromium atoms above and below the condensation threshold. We observe a minimum in the dipolar relaxation rate for a finite magnetic field value which enables a new determination of the scattering length.
Temperature dependence of the cross-section for elastic ground-state collisions of /sup 52/Cr- and /sup 50/Cr-atoms using the method of cross-dimensional relaxation, respectively is determined. The corresponding absolute values of the scattering length are on the order of 100a/sub 0/ for /sup 52/Cr and /sup 50/a/sub 0/ for /sup 50/Cr, 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/sup -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 (/spl ap/10/sup -11/ cm/sup 3//s @ 50 G offset field) for the two isotopes /sup 50/Cr and /sup 52/Cr 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.