arXiv subjects

A. Griesmaier

Publications and source records attributed to A. Griesmaier.

14 recordsLinked to original sources

arXiv2014

Narrow-line magneto-optical trap for dysprosium atoms

We present our technique to create a magneto-optical trap for dysprosium atoms using the narrow-line cooling transition at 626$\,$nm to achieve suitable conditions for direct loading into an optical dipole trap. The magneto-optical trap is loaded from an atomic beam via a Zeeman slower using the strongest atomic transition at 421$\,$nm. With this combination of two cooling transitions we can trap up to $2.0\cdot10^8$ atoms at temperatures down to 6$\, μ$K. This cooling approach is simpler than present work with ultracold dysprosium and provides similar starting conditions for a transfer to an optical dipole trap.

cond-mat.quant-gas↗

arXiv2013

Spectroscopy of a narrow-line optical pumping transition in dysprosium

We present measurements of the hyperfine coefficients and isotope shifts of the Dy I $683.731 $nm transition, using saturated absorption spectroscopy on an atomic beam. A King Plot is drawn resulting in an updated value for the specific mass shift $δν_\mathrm{684,sms}^\mathrm{164-162}=-534 \pm 17 MHz$. Using fluorescence spectroscopy we measure the excited state lifetime $τ_{684}=1.68(5) μ$s, yielding a linewidth of $γ_\mathrm{684} = 95 \pm 3 kHz$. We give an upper limit to the branching ratio between the two decay channels from the excited state showing that this transition is useable for optical pumping into a dark state and demagnetization cooling.

physics.atom-ph↗

arXiv2012

Confinement-induced collapse of a dipolar Bose-Einstein condensate

We report on the observation of the confinement-induced collapse dynamics of a dipolar Bose-Einstein condensate (dBEC) in a one-dimensional optical lattice. We show that for a fixed interaction strength the collapse can be initiated in-trap by lowering the lattice depth below a critical value. Moreover, a stable dBEC in the lattice may become unstable during the time-of-flight dynamics upon release, due to the combined effect of the anisotropy of the dipolar interactions and inter-site coherence in the lattice.

cond-mat.quant-gas↗

arXiv2011

The effect of light assisted collisions on matter wave coherence in superradiant Bose-Einstein condensates

We investigate experimentally the effects of light assisted collisions on the coherence between momentum states in Bose-Einstein condensates. The onset of superradiant Rayleigh scattering serves as a sensitive monitor for matter wave coherence. A subtle interplay of binary and collective effects leads to a profound asymmetry between the two sides of the atomic resonance and provides far bigger coherence loss rates for a condensate bathed in blue detuned light than previously estimated. We present a simplified quantitative model containing the essential physics to explain our experimental data and point at a new experimental route to study strongly coupled light matter systems.

cond-mat.quant-gas↗

arXiv2011

Dipolar Stabilization of an Attractive Bose Gas in a One Dimensional Lattice

We experimentally show that dipolar interaction can stabilize otherwise unstable many-body systems like an attractive Bose gas. In a one dimensional lattice the repulsive dipolar on-site interaction balances negative scattering lengths up to -17 Bohr radii and stabilizes the chromium Bose-Einstein condensate. For reduced lattice depths, the dipolar stabilization turns into destabilization. We probe the full cross-over between the two regimes and our results are in excellent agreement with theoretical calculations, which reveal significant dipolar inter-site interactions.

cond-mat.quant-gas↗

arXiv2008

A purely dipolar quantum gas

We report on experiments exploring the physics of dipolar quantum gases using a Chromium Bose-Einstein condensate (BEC). By means of a Feshbach resonance, it is possible to reduce the effects of short range interactions and reach a regime where the physics is governed by the long-range, anisotropic dipole-dipole interaction between the large ($6 μ_{\rm B}$) magnetic moments of Chromium atoms. Several dramatic effects of the dipolar interaction are observed: the usual inversion of ellipticity of the condensate during time-of flight is inhibited, the stability of the dipolar gas depends strongly on the trap geometry, and the explosion following the collapse of an unstable dipolar condensate displays d-wave like features.

cond-mat.other↗

arXiv2007

Stabilizing a purely dipolar quantum gas against collapse

We report on the experimental observation of the dipolar collapse of a quantum gas which sets in when we reduce the contact interaction below some critical value using a Feshbach resonance. Due to the anisotropy of the dipole-dipole interaction, the stability of a dipolar Bose-Einstein condensate depends not only on the strength of the contact interaction, but also on the trapping geometry. We investigate the stability diagram and find good agreement with a universal stability threshold arising from a simple theoretical model. Using a pancake-shaped trap with the dipoles oriented along the short axis of the trap, we are able to tune the scattering length to zero, stabilizing a purely dipolar quantum gas.

cond-mat.other↗

arXiv2007

Loading chromium atoms in a magnetic guide

We have realized a magnetic guide for ultracold chromium atoms by continuously loading atoms directly from a Zeeman slower into a horizontal guide. We observe an atomic flux of $2 \cdot 10^7$ atoms/s and are able to control the mean velocity of the guided atoms between 0 m/s and 3 m/s. We present our experimental results on loading and controlling the mean velocity of the guided atoms and discuss the experimental techniques that are used.

cond-mat.other↗

arXiv2006

Damagnetization cooling of a gas

We demonstrate demagnetization cooling of a gas of ultracold $^{52}$Cr atoms. Demagnetization is driven by inelastic dipolar collisions which couple the motional degrees of freedom to the spin degree. By that kinetic energy is converted into magnetic work with a consequent temperature reduction of the gas. Optical pumping is used to magnetize the system and drive continuous demagnetization cooling. Applying this technique, we can increase the phase space density of our sample by one order of magnitude, with nearly no atom loss. This method can be in principle extended to every dipolar system and could be used to achieve quantum degeneracy via optical means.

cond-mat.other↗

arXiv2006

Expansion dynamics of a dipolar Bose-Einstein condensate

Our recent measurements on the expansion of a chromium dipolar condensate after release from an optical trapping potential are in good agreement with an exact solution of the hydrodynamic equations for dipolar Bose gases. We report here the theoretical method used to interpret the measurement data as well as more details of the experiment and its analysis. The theory reported here is a tool for the investigation of different dynamical situations in time-dependent harmonic traps.

cond-mat.other↗

arXiv2005

Observation of dipole-dipole interaction in a degenerate quantum gas

We have investigated the expansion of a Bose-Einstein condensate (BEC) of strongly magnetic chromium atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to an anisotropic deformation of the expanding Cr-BEC 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 Cr-BEC is an excellent model system to study dipolar interactions in such gases.

cond-mat.other↗

arXiv2004

Observation of Feshbach resonances in an ultracold gas of ${}^{52}$Cr

We have observed Feshbach resonances in elastic collisions between ultracold ${}^{52}$Cr 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 ${}^{52}$Cr and thus the absence of a Fermi-contact interaction leads to regularly-spaced resonance sequences. By comparing resonance positions with multi-channel scattering calculations we determine the s-wave scattering length of the lowest $^{2S+1}Σ_{g}^{+}$ potentials to be $\unit[112(14)]{a_0}$, $\unit[58(6)]{a_0}$ and $-\unit[7(20)]{a_0}$ for S=6, 4, and 2, respectively, where $a_{0}=\unit[0.0529]{nm}$.

cond-mat.other↗

arXiv2003

Dipolar Relaxation in an ultra-cold Gas of magnetically trapped chromium atoms

We have investigated both theoretically and experimentally dipolar relaxation in a gas of magnetically trapped chromium atoms. We have found that the large magnetic moment of 6 $μ_B$ results in an event rate coefficient for dipolar relaxation processes of up to $3.2\cdot10^{-11}$ cm$^{3}$s$^{-1}$ at a magnetic field of 44 G. We present a theoretical model based on pure dipolar coupling, which predicts dipolar relaxation rates in agreement with our experimental observations. This very general approach can be applied to a large variety of dipolar gases.

quant-ph↗

arXiv2003

Determination of the s-wave Scattering Length of Chromium

We have measured the deca-triplet s-wave scattering length of the bosonic chromium isotopes $^{52}$Cr and $^{50}$Cr. From the time constants for cross-dimensional thermalization in atomic samples we have determined the magnitudes $|a(^{52}Cr)|=(170 \pm 39)a_0$ and $|a(^{50}Cr)|=(40 \pm 15)a_0$, where $a_0=0.053nm$. By measuring the rethermalization rate of $^{52}$Cr over a wide temperature range and comparing the temperature dependence with the effective-range theory and single-channel calculations, we have obtained strong evidence that the sign of $a(^{52}Cr)$ is positive. Rescaling our $^{52}$Cr model potential to $^{50}$Cr strongly suggests that $a(^{50}Cr)$ is positive, too.

quant-ph↗