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...
We have investigated the expansion of a Bose-Einstein condensate of strongly magnetic chromium atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to an anisotropic deformation of the expanding chromium condensate which depends on the orientation of the atomic dipole moments. Our measurements are consistent with the theory of dipolar quantum gases and show that a chromium condensate is an excellent model system to study dipolar interactions in such gases.
We have observed Feshbach resonances in collisions between ultracold 52Cr atoms. This is the first observation of collisional Feshbach resonances in an atomic species with more than one valence electron. The zero nuclear spin of 52Cr and thus the absence of a Fermi-contact interaction leads to regularly spaced resonance sequences. By comparing resonance positions with multichannel scattering calculations we determine the s-wave scattering length of the lowest (2S+1)Sigma(+)(g) potentials to be 112(14) a(0), 58(6) a(0), and -7(20) a(0) for S=6, 4, and 2, respectively, where a(0)=0.0529 nm.
We developed a gravity-gradiometer based on atom interferometry for the determination of the Newtonian gravitational constant G. The apparatus, combining a Rb fountain, Raman interferometry and a juggling scheme for fast launch of two atomic clouds, was specifically designed to reduce possible systematic effects. We present instrument performances and show that the sensor is able to detect the gravitational field induced by source masses. A discussion of projected accuracy for G measurement using this new scheme shows that the results of the experiment will be significant to discriminate between previous inconsistent values.
We have realized a scheme for continuous loading of a magnetic trap (MT).52 Cr atoms are continuously captured and cooled in a magneto-optical trap (MOT). Optical pumping to a metastable state decouples atoms from the cooling light. Due to their high magnetic moment (6 µB), low-field seeking metastable atoms are trapped in the magnetic quadrupole field provided by the MOT. Limited by inelastic collisions between atoms in the MOT and in the MT, we load 10 8 metastable atoms at a rate of 10 8 atoms/s below 100 µK into the MT. Optical repumping after the loading allows us to realize a MT of ground state chromium atoms. Although multiply loading of a MT has been achieved [10] experiments so far suffer from the absence of a method for efficient cw loading of atoms into a BEC. Hence to date a matter wave analogon to the continuous wave optical laser has not been realized. Alternatively, a cw atom laser based on magnetic guiding in combination with atomic collisions was suggested [11]. In addition, cw loading of low-dimensional optical traps with laser cooled atoms was proposed [12] and recently demonstrated [13].In this letter we report on the cw loading of a three dimensional conservative trap with laser cooled atoms that are decoupled from all light fields present. We show that atoms can be optically pumped within a chromium magneto-optical trap (MOT) [14,15] into metastable "dark" states and stored in a MT built up by the quadrupole magnetic field of the MOT. We present results of systematic studies on the loading process and on the lifetime of the MT. Using the cw loading mechanism and a final repumping process we obtain good starting conditions for experiments towards degenerate quantum gases with ground state chromium atoms.Our cw loading scheme consists of an atomic reservoir and a conservative trap overlapped in space and time. The reservoir is prepared by cooling atoms in a MOT on a transition |g → |e (FIG. 1). A weak decay channel |e → |d allows the transfer of reservoir atoms into an additional long lived and trapped state |d in which atoms can be accumulated. In our realization low field seeking Zeeman substates of |d are trapped in the magnetic quadrupole field of the MOT. The loading can be very efficient if |d atoms are decoupled from the MOT light and if their kinetic energy is smaller than the conservative trap depth. A large decay rate branching ratio (Γ eg /Γ ed ≫ 1) assures a steady state MOT in thermal equilibrium and is expected to greatly reduce reabsorption of transfer photons by atoms in the MT [16]. MT FIG. 1. Relevant part of the52 Cr level scheme. The MOT involves all levels and transitions, the continuous loading process of the magnetic trap (MT) relies on the Λ-system depicted in black (levels |g , |e , |d ).Chromium combines the desired Λ-like level scheme (FIG. 1, black levels and transitions) with a high magnetic moment of up to 6 µ B (µ B =Bohr's magneton). Due to its isotopic composition (3 bosons:52 Cr (84%), 50 Cr (4%), 54 Cr (2%), and one fermion: 53 Cr (10%) ) it is a promising e...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.