Magnetic trapping of atomic chromium

Recently we demonstrated the technique of buffer-gas loading of neutral species by using it to load a magnetic trap with europium atoms @1,2#. The atom density achieved in that experiment ( n;10 12 cm 23 ! is comparable to the highest densities achieved in magneto-optical traps, and the number trapped ( N;10 12 ) is an order of magnitude higher than can currently be obtained with light-force traps @3#. The buffer-gas method promises the extension of magnetic trapping and evaporative cooling to species other than alkali atoms, and should open the way for producing ultracold, dense samples in a variety of atomic and molecular species. Herein, we report the magnetic trapping of atomic chromium via buffer-gas loading. Chromium has many properties of interest. It is a metal dissimilar to europium; trapping it further demonstrates the generality of buffer-gas loading. Chromium’s large magnetic moment of 6 m B ~Bohr magneton! allows for trapping at elevated buffer-gas temperatures, opening up the possibility to trap it with a simple pumped liquid helium cryostat. Study of chromium is also motivated by its applications in atom lithography @4,5#. As it has four naturally occurring isotopes, three bosons and one fermion, trapping could open the door to the study of chromium BoseEinstein condensates or degenerate Fermi gases @6‐9#. The principles of buffer-gas loading and the specifics of our apparatus are outlined in previous papers @1,2#. We give a brief description below. Our magnetic trap is a linear quadrupole field formed by two anti-Helmholtz coils. The trapping region is filled with helium buffer gas. The buffer gas is maintained at cryogenic temperatures by a dilution refrigerator. The species of interest ~atomic chromium in this case! is introduced into the trap, where it diffuses through the helium gas and quickly thermalizes with it via elastic collisions. The atoms in the weak-field-seeking magnetic states are contained by the magnetic fields, but as the atoms are thermally distributed, they evaporate from the trap at a rate determined by h, the ratio of the trap depth to the temperature of the atoms. For a sufficiently large h, this evaporation is slow enough that a large fraction of the atoms may be held for a long enough time that the ~nonmagnetic! helium gas can be ~cryo-!pumped out of the trapping region. This leaves a thermally isolated, trapped sample. Because this technique relies only on elastic collisions with the buffer gas and on the magnetic state of the species, it should be applicable to any magnetic species that can be cleanly introduced into the cryogenic environment. In our experiment, Cr atoms are produced, thermalized, and trapped within a copper cell. There is a fused silica window on the bottom of the cell to permit optical access ~for detection and ablation! and a mirror on the top of the cell to retroreflect the probe beam for absorption spectroscopy of the trapped sample. Resistance thermometry is used to determine the cell temperature. Additional resistors attached to the cell are used as heaters. The cell is filled with either 3 He or 4 He buffer gas. A sufficient amount of 3 He ( 4 He) is present so that at temperatures above 0.3 K ~0.9 K! the density is approximately 10 17 cm 23 . Below these temperatures, the density is determined by the helium vapor pressure. The Cr atoms are brought into the gas phase by single pulse laser ablation of a solid sample of isotopically pure 52 Cr. The solid 52 Cr is positioned at the edge of the trapping region inside the cell. A doubled yttrium-aluminum-garnet laser with typical ablation pulse energy of 25 mJ at 532 nm is used for ablation. The atoms are detected by laser absorption spectroscopy on the a 7 S3$z 7 P3 transition at 23 386 cm 21