Bose-Einstein Condensation of Chromium

Abstract

We report on the generation of a Bose-Einstein condensate in a gas of chromium atoms, which will make studies of the effects of anisotropic long-range interactions in degenerate quantum gases possible. The preparation of the chromium condensate requires novel cooling strategies that are adapted to its special electronic and magnetic properties. The final step to reach quantum degeneracy is forced evaporative cooling of 52 Cr atoms within a crossed optical dipole trap. At a critical temperature of Tc ≈700 nK, we observe Bose-Einstein condensation by the appearance of a two-component velocity distribution. Released from an anisotropic trap, the condensate expands with an inversion of the aspect ratio. We observe critical behavior of the condensate fraction as a function of temperature and more than 50,000 condensed 52 Cr atoms. The essential properties of degenerate quantum gases depend on range, strength and symmetry of the present interactions. Since the first observation of Bose-Einstein condensation in weakly interacting atomic gases, eight different elements have been Bose-Einstein condensed [1,2,3,4,5,6,7,8]. All these elements, mainly alkali atoms, interact dominantly via short-range isotropic potentials. Based on this effective contact interaction, many exciting phenomena have been studied [9,10]. Examples are the realization of four-wave mixing with matter waves [11] as well as the observation of vortices [12,13] and solitons [14,15,16] in degenerate quantum gases. Bose-Einstein condensates (BECs) with contact interaction have also been used to investigate solid-state physics problems like the Mott-metal-insulator transition [17,18]. Tuning the contact interaction, the collapse and explosion ("Bosenova") of Bose-Einstein condensates has been studied [19] and new types of quantum matter like a Tonks-Girardeau gas have been realized [20]. In a chromium Bose-Einstein condensate, one can not only tune the short-range contact interaction using one of the recently observed Feshbach resonances [21] but also investigate effects of the longrange and anisotropic dipole-dipole interaction. This becomes possible because, compared to other Bose-condensed elements, the transition metal chromium has a unique electronic structure. The valence shell of its ground state contains six electrons with parallel spin alignment (electronic configuration: [Ar]3d 5 4s 1 ). For the bosonic chromium isotopes, which have no nuclear spin, this gives rise to a total electronic spin quantum number of 3 and a very high magnetic moment of 6 µ B (µ B is the Bohr magneton) in its ground state 7 S 3 . Since the magnetic dipole-dipole interaction (MDDI) scales with the square of the magnetic moment, it is a factor of 36 higher for chromium than for alkali atoms. For this reason, dipole-dipole interactions which have not yet been investigated experimentally in degenerate quantum gases will become observable in chromium BEC. For example, it was shown in [22] that the MDDI in chromium is strong enough to manifest itself in a well pronounced modif...

Dynamical probing of a topological phase of bosons in one dimension

“…This model applies to atoms and molecules with fixed electric or magnetic dipoles 17,18,24 , and to Rydberg atoms with induced dipoles 23 . For simplicity, we will assume that only the nearest-neighbor term V 1 = V is non zero.…”

“…This phase arises in models of interacting bosons on a one-dimensional lattice, which are relevant to ultracold dipolar atoms 17 or molecules 18 and Rydberg atoms 19 , in optical lattices 20 . These systems support additional phases that are topologically trivial, such as the Mott insulator (MI) and the density-wave (DW) phase.…”

Cold Dipolar Gases in Quasi-One-Dimensional Geometries

“…Interestingly, new experiments on atoms with large magnetic moment [10] cold molecules [11] and Rydberg atoms [12] have recently opened a fascinating new research area, namely that of dipolar gases. In these gases, the dipole-dipole interaction (DDI), which is long-range and anisotropic, plays a significant or even dominant role when compared to the short-range isotropic interaction.…”

Dipolar Bose–Einstein condensates in triple-well potentials

“…BECs have been experimentally realised with atoms sustaining a large magnetic dipole moment such as 52 Cr [1][2][3] and, more recently, 164 Dy [4,5] and 168 Er [6]. Very recently, fast progress towards the creation of BECs of polar molecules [7], which sustain large electric dipole moments, has been made.…”