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Journal of the Optical Society of America B

Enhanced inelastic scattering rates of cold atomic chromium

Robert deCarvalho, Cindy Hancox, John M. Doyle

Published May 1, 2003• 12 citations
Publisher

Abstract

Inelastic scattering rates for magnetically trapped atomic chromium are determined for temperatures of 2 mK and higher. It is found that the rate is very large (2×10-9 cm3/s at 4 mK) and has a strong dependence on temperature. The origin of this structure in the inelastic rate remains for theoretical explanation. These experiments show that, even with bad inelastic rates, the efficacy of buffer-gas loading in producing large numbers of cold atoms is quite good.

DOI
10.1364/josab.20.001131

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Enhanced inelastic scattering rates of cold atomic chromium

Robert deCarvalho, Cindy Hancox, John M. Doyle

Inelastic scattering rates for magnetically trapped atomic chromium are determined for temperatures of 2 mK and higher. It is found that the rate is very large (2×10-9 cm3/s at 4 mK) and has a strong dependence on temperature. The origin of this structure in the inelastic rate remains for theoretical explanation. These experiments show that, even with bad inelastic rates, the efficacy of buffer-gas loading in producing large numbers of cold atoms is quite good.

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Physical Review A•Jun 15, 2007•9 citations
Inelastic scattering rates for magnetically trapped atomic chromium are determined for temperatures of 2 mK and higher. It is found that the rate is very large (2×10-9 cm3/s at 4 mK) and has a strong dependence on temperature. The origin of this structure in the inelastic rate remains for theoretical explanation. These experiments show that, even with bad inelastic rates, the efficacy of buffer-gas loading in producing large numbers of cold atoms is quite good.
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.
Atomic manganese 55Mn^{55}\mathrm{Mn}55Mn and chromium 52Cr^{52}\mathrm{Cr}52Cr are simultaneously loaded and confined in a magnetic trap. Using a cryogenic 3He^{3}\mathrm{He}3He buffer gas, 1011{10}^{11}1011 manganese and 1012{10}^{12}1012 chromium atoms are trapped at an initial temperature of 600 mK. The buffer gas is then pumped away, thermally isolating the sample. The Mn-Cr interspecies inelastic rate constant is measured to be ${\ensuremath{\Gamma}}_{\mathrm{Mn},\mathrm{Cr}}=1.5(\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$.
We report the magnetic trapping and evaporative cooling of bosonic and fermionic isotopes of atomic chromium. Using a cryogenic helium buffer gas, 10 1 2 chromium atoms are trapped at an initial temperature of ∼ 1 K. The chromium atoms are then cooled adiabatically and evaporatively to temperatures as low as ∼10 mK. Elastic and inelastic 5 2 Cr collisional cross sections are measured over this temperature range. Prospects for simultaneously creating a 5 2 Cr Bose-Einstein condensate and 5 3 Cr Fermi degenerate gas will he discussed.
We report the magnetic trapping and evaporative cooling of bosonic and fermionic isotopes of atomic chromium. Using a cryogenic helium buffer gas, 1012{10}^{12}1012 chromium atoms are trapped at an initial temperature of \ensuremath{\sim}1 K. The chromium atoms are then cooled adiabatically and evaporatively to temperatures as low as \ensuremath{\sim}10 mK. Elastic and inelastic 52Cr{}^{52}\mathrm{Cr}52Cr collisional cross sections are measured over this temperature range. Prospects for simultaneously creating a 52Cr{}^{52}\mathrm{Cr}52Cr Bose-Einstein condensate and 53Cr{}^{53}\mathrm{Cr}53Cr Fermi degenerate gas will be discussed.
Elastic and inelastic collision-rate constants of 52Cr^{52}\mathrm{Cr}52Cr in the temperature range of 20mKto1K20\phantom{\rule{0.3em}{0ex}}\mathrm{mK}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}20mKto1K are inferred from the evaporative cooling of buffer gas loaded atomic chromium. Using a model that describes the dynamics of the trapped chromium cloud during evaporation, we find ${g}_{\mathrm{el}}=2.15(+2.5,\ensuremath{-}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$ and ${g}_{\mathrm{in}}=1.36(+1.2,\ensuremath{-}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$, consistent with theory but in disagreement with previously reported measurements.