Isotope dependent interactions in a mixture of ultracold atoms [Elektronische Ressource] / vorgelegt von Florian Baumer

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• Physik

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• Publié par: universitat_zu_koln
• Publié le: 01 janvier 2010
• Nombre de lectures: 11
• Langue: English
• Poids de l'ouvrage: 5 Mo

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Isotope dependent interactions
in a mixture of ultracold atoms
Inaugural-Dissertation
zur
Erlangung des Doktorgrades der
Mathematisch-Naturwissenschaftlichen Fakult¨at
der Heinrich-Heine-Universit¨at Du¨sseldorf
vorgelegt von
Florian Baumer
aus Lauingen a.d Donau
M¨arz 2010Aus dem Institut fur¨ Experimentalphysik
der Heinrich-Heine-Universit¨at Dus¨ seldorf
Gedruckt mit der Genehmigung der
Mathematisch-Naturwissenschaftlichen Fakult¨at der
Heinrich-Heine-Universit¨at Dus¨ seldorf
Referent: Prof. Dr. Axel G¨orlitz
Koreferent: Prof. Dr. Georg Pretzler
Tag der mundlichen¨ Prufung:¨ 29.04.2010Summary
Within the scope of this thesis, comprehensive experimental studies on interspecies inter-
87actions between Rb and various isotopes of Yb at temperatures in the K regime have
87been performed. The interactions between Yb and Rb manifested themselves through
174 87interspeciesthermalizationand–inthecaseof Yband Rb–throughphaseseparation.
Both, Yb and Rb, are well suited for the application of laser cooling, and quantum
degeneracy has been reached for Yb as well as for Rb [1, 2]. In the context of mixtures
of ultracold atoms, the combination of Yb and Rb is an interesting system, which has
not been investigated so far. Due to the availability of several stable Yb isotopes, Bose-
Bose mixtures as well as Bose-Fermi mixtures can be realized. The goal of the research
project within which this PhD thesis has been carried out is the production of ultracold
heteronuclearYbRbmoleculesinsuchaquantumdegeneratemixture. YbRbmoleculesare
an interesting system with significantly different physical properties than the alkali-alkali
combinations, which are investigated in most experiments [3, 4, 5, 6]: YbRb molecules
posses an electric as well as a magnetic dipole moment, which can for example be exploited
for the study of spin lattice models [7].
Experiments described within this thesis were carried out at an apparatus, which has
been used in previous studies on mixtures of Yb and Rb [8, 9, 10, 11]. However, the
experiments presented here, required a significant improvement in stability and precision
of the existing setup. Hence, in the course of this work, a number of components were
changed or added to the existing system: Large parts of the laser systems were redesigned
and improved detection methods were introduced to the experimental setup.
Due to the particular magnetic properties of Rb and Yb, a unique combination of trap-
87ping potentials, which allows for independent trapping and manipulation of ultracold Rb
and Yb atoms was used. The combined trap setup consisted of a Ioffe-Pritchard-type mag-
87netic trap (MT) for Rb and a bichromatic optical dipole trap (BIODT) for Yb. In the
experimental sequence, Yb was pre-cooled in a MOT and transferred to the conservative
87BIODT.Subsequently, RbwaspreparedandevaporativelycooledintheMT.Asaresult,
7 87atomic samples consisting of typically≈ 10 Rb atoms at a temperature of≈ 1K and,
5depending on the isotope, (0.3...1.5)×10 Yb atoms at temperatures of 5...8K, were
produced.
After the two atom clouds were brought into contact, thermalization of Yb with the
87colder Rb cloud was observed. Measurements of the thermalization rate were performed
87 170 172 173 174 176using Rb and the Yb isotopes Yb, Yb, Yb, Yb and Yb, allowing the com-
parison of their relativescatteringproperties. Two Yb isotopes exhibited exceptional char-
87acteristics in combination with Rb: On the one hand, almost no thermalization between
170 87Yband Rbcouldbeobserved. Basedonaquantitativeanalysisofthermalizationdata,
the interspecies scattering length for this isotope combination could be determined to beiv Summary
+3.5 174|a | = 6.6 a . Ontheotherhand, Ybatomsthermalizedalmostinstantaneously87−170 0−2.9
87with the colder Rb atoms, indicating a large elastic interspecies cross section.
174 87In addition, we have observed phase separation in a mixture of Yb and Rb for
certain trap geometries.
Phase separation has been studied before in mixtures of ultracold gases [12, 13, 14].
However, in contrast to those experiments, which are performed with degenerate quantum
174 87gases, phase separation between Yb and Rb as described in this PhD work is observed
with purely thermal samples at temperatures in the K regime.
174For a quantitative understanding of the observed phase separation between Yb and
87Rb, it was essential, to have detailed knowledge of the characteristic parameters of the
involved trapping potentials. With the knowledge of measured of trap frequencies for the
MT and the BIODT, it was possible to simulate the potentials corresponding to the exper-
imental situation. Interspecies interaction was accounted for by an additional interaction
87potential, which, in a first approach, was assumed to have a linear dependence on the Rb
density.
Thequantitativeresultsontheinteractionpotentialareinterpretedintermsoftwo-body
scattering properties of the respective atoms. A ”naive“ mean field approach and thermal
averaging of the energy dependent scattering length lead to conclusions inconsistent with
experimental observations.
In the light of this discrepancy between observations and theory, possible experimental
issues, which may lead to incorrect results for the interspecies interaction potential were
carefully analyzed. Subsequently, the validity of the standard mean field theory in the
present case was discussed on the basis of a simple comparison of relevant length scales. A
more sophisticated theoretical model, which involves nonlinear density dependent correc-
tions to the interaction potential, was introduced. Based on this approach, a low energy
[phone]s-wave interspecies scattering length for Yb and Rb, a = 4710 a could87−174 0−1520
be inferred. Furthermore, the description of the observed phase separation by diffusion
dynamics and three-body recombination, as proposed by E. Tiesinga and S. Maxwell from
the atomic theory group at NIST [15], is discussed.
The experimental results presented in this PhD thesis, allow conclusions on the YbRb
ground state molecular potential and the energetic positions of weakly bound levels. In
this context, E. Tiesinga and S. Maxwell [15] determined a Lennard-Jones potential, which
quantitatively reproduces the experimental observations.
The experimental studies on interspecies interaction between different Yb isotopes and
87Rb, which are described in the present PhD work, have greatly increased the knowledge
of low energy properties of the respective mixtures. This is a major step towards the
long term goal of this research project, which is the production of ultracold rovibronic
ground state YbRb molecules. The future developments required to achieve this goal can
be divided into two areas: One will be the continuation of studies on the YbRb molecular
potentials, which already resulted in the successful production of excited state YbRb*
molecules by photoassociation [16]. The other will be to realize a quantum degenerate
mixture of both species, which would be an ideal starting point for the creation of ground
state YbRb molecules. Based on new insights on the low energy scattering properties ofSummary v
87 87Yb and Rb, a promising pathway towards a combined degenerate mixture of Rb and
Yb has been developed.Zusammenfassung
Im Rahmen dieser Doktorarbeit wurden umfassende experimentelle Untersuchungen zu
87heteronuklearen Wechselwirkungen zwischen Rb und verschiedenen Yb Isotopen im
K-Temperaturbereich durchgefuhr¨ t. Die Wechselwirkung zwischen beiden Atomsorten
174 87¨außerte sich dabei durch Thermalisierung und – im Fall von Yb und Rb – durch
Phasenseparation beider Spezies.
SowohlYbalsauchRbsindgutgeeinetfur¨ dieAnwendungvonLaserkuhl¨ verfahren. Das
Regime der Quantenentartung wurde daher mit beiden Elementen bereits realisiert. [1, 2].
Das bislang noch nicht n¨aher untersuchte System Yb-Rb is eine interessante Elementkom-
bination mit vielseitigen Perspektiven im Bereich der ultrakalten gemischten Gase. Auf-
grund der Verfugbar¨ keit 7 stabiler Ytterbiumisotope sind sowohl Boson-Boson- als auch
Fermion-Boson-Gemische realisierbar. Zudem stellen ultrakalte YbRb Molekul¨ e ein viel
versprechendesSystemmitdeutlichunterschiedlichenphysikalischenEigenschaftenimVer-
gleich zu den in bisherigen in Experimenten untersuchten Alkali-Alkali-Kombinationen dar
[3, 4, 5, 6]: YbRb Molekul¨ e besitzen sowohl ein elektrisches als auch ein magnetisches
Dipolmoment, was z.B. Untersuchungen zu Spin-Gitter-Modellen erlaubt.
Die in dieser Arbeit beschriebenen Experimente wurden an einer bereits fruhe¨ r eingeset-
ztenApparaturdurchgefuhr¨ t[8,9,10,11], diejedochinBezugaufStabilitat¨ undPr¨azision
signifikant verbessert wurde. In diesem Zusammenhang wurden w¨ahrend dieser Arbeit
zahlreiche Komponenten, wie etwa große Teile des Lasersystems neu aufgebaut.
Eine Besonderheit des Systems Yb-Rb ist die Kombination einer diamagnetischen (Yb)
mit einer paramagnetischen (Rb) Spezies. Die unterschiedlichen magnetischen Eigen-
schaften erlauben eine Kombination von Fallenpotentialen, die eine (weitgehend) un-
abh¨angige Kontrolle ub¨ er beide Atomsorten erm¨oglicht. Die kombinierte Falle besteht aus
87einer Ioffe-Pritchard Magnetfalle fur¨ Rb un