Physical Review Letters 100(11), 113002 (2007).
Laser cooling on weak transitions is a useful technique for reaching ultracold temperatures in atoms with multiple valence electrons. However,
for strongly magnetic atoms a conventional narrow-line magneto-optical trap (MOT) is destabilized by competition between optical and magnetic forces.
We overcome this difficulty in Er by developing an unusual narrow-line MOT that balances optical and magnetic forces using laser light tuned to the
blue side of a narrow (8 kHz) transition. The trap population is spin polarized with temperatures reaching below 2 µK. Our results constitute an
alternative method for laser cooling on weak transitions, applicable to rare-earth-metal and metastable alkaline earth elements.
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