Within the scope of this thesis, comprehensive experimental studies on interspecies interactions between 87Rb and various isotopes of Yb at temperatures in the µK regime have been performed. The interactions between Yb and 87Rb manifested themselves through interspecies thermalization and - in the case of 174Yb and 87Rb - through phase separation.
The experimental investigations described in this thesis were carried out within a research project, whose long term goal is the creation of ultracold YbRb molecules. The experimental apparatus is designed for combined cooling and trapping of gaseous Yb and Rb atomic samples to the µK and nK regime.
Due to the particular magnetic properties of Rb and Yb, a unique combination of trapping potentials, which allows for independent manipulation of ultracold 87Rb and Yb atoms, was used for the described measurements.
The combined trap setup consisted of a Ioffe-Pritchard-type magnetic trap (MT) for 87Rb and a bichromatic optical dipole trap (BIODT) for Yb. The main scientific result of this PhD work is the observation of phase separation in a thermal mixture of 174Yb and 87Rb in the µK regime and the detailed experimental study and theoretical analysis of this unexpected effect. Analysis of the experimental observations required accurate modeling of the trapping potentials in order to extract quantitative results on the interspecies interaction. These results can be related to the 174Yb 87Rb interspecies s-wave scattering length, using an approach, which not only describes mean field effects but also includes multiple-particle interactions.
This analysis yields a value for the s-wave scattering length of a_(87-174)=4710 (+3970) (-1520) a0, which is extraordinarily large. Indeed, this is to our knowledge the largest interspecies scattering length that has been observed to date at zero magnetic field. In contrast to observations of phase separation in other systems that have been made so far, phase separation between 174Yb and 87Rb as described in this PhD work is observed in purely thermal and not quantum degenerate samples. In this thesis, the study of phase separation in a 174Yb87Rb mixture was supplemented by an investigation of elastic collision rates for several of the Yb isotopes. The most notable finding was that for the combination \Yb{170}87Rb the interspecies scattering length has a value of a_(87-174)=6.6 (+3.5) (-2.9)a0 and is thus extremely small. The results of our studies on interspecies interactions between 87Rb and Yb can be used for model calculations on the YbRb molecular ground state potential: E. Tiesinga and S. Maxwell from the atomic theory group at NIST determined a Lennard-Jones potential, which quantitatively reproduces the present experimental observations. This knowledge will significantly help to provide a strategy for the controlled transfer of excited state YbRb* molecules to a selected ground state.
Within the scope of this thesis, comprehensive experimental studies on interspecies interactions between 87Rb and various isotopes of Yb at temperatures in the µK regime have been performed. The interactions between Yb and 87Rb manifested themselves through interspecies thermalization and - in the case of 174Yb and 87Rb - through phase separation.
The experimental investigations described in this thesis were carried out within a research project, whose long term goal is the creation of ultracold YbRb molecules. The experimental apparatus is designed for combined cooling and trapping of gaseous Yb and Rb atomic samples to the µK and nK regime.
Due to the particular magnetic properties of Rb and Yb, a unique combination of trapping potentials, which allows for independent manipulation of ultracold 87Rb and Yb atoms, was used for the described measurements.
The combined trap setup consisted of a Ioffe-Pritchard-type magnetic trap (MT) for 87Rb and a bichromatic optical dipole trap (BIODT) for Yb. The main scientific result of this PhD work is the observation of phase separation in a thermal mixture of 174Yb and 87Rb in the µK regime and the detailed experimental study and theoretical analysis of this unexpected effect. Analysis of the experimental observations required accurate modeling of the trapping potentials in order to extract quantitative results on the interspecies interaction. These results can be related to the 174Yb 87Rb interspecies s-wave scattering length, using an approach, which not only describes mean field effects but also includes multiple-particle interactions.
This analysis yields a value for the s-wave scattering length of a_(87-174)=4710 (+3970) (-1520) a0, which is extraordinarily large. Indeed, this is to our knowledge the largest interspecies scattering length that has been observed to date at zero magnetic field. In contrast to observations of phase separation in other systems that have been made so far, phase separation between 174Yb and 87Rb as described in this PhD work is observed in purely thermal and not quantum degenerate samples. In this thesis, the study of phase separation in a 174Yb87Rb mixture was supplemented by an investigation of elastic collision rates for several of the Yb isotopes. The most notable finding was that for the combination \Yb{170}87Rb the interspecies scattering length has a value of a_(87-174)=6.6 (+3.5) (-2.9)a0 and is thus extremely small. The results of our studies on interspecies interactions between 87Rb and Yb can be used for model calculations on the YbRb molecular ground state potential: E. Tiesinga and S. Maxwell from the atomic theory group at NIST determined a Lennard-Jones potential, which quantitatively reproduces the present experimental observations. This knowledge will significantly help to provide a strategy for the controlled transfer of excited state YbRb* molecules to a selected ground state.
We present measurements of interspecies thermalization between ultracold samples of 133Cs and either 174Yb or 170Yb. The two species are trapped in a far-off-resonance optical dipole trap and 133Cs is sympathetically cooled by Yb. We extract effective interspecies thermalization cross sections by fitting the thermalization measurements to a kinetic model, giving ${\ensuremath{\sigma}}_{\mathrm{Cs}^{174}\mathrm{Yb}}=\left(5\ifmmode\pm\else\textpm\fi{}2\right)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}$ and ${\ensuremath{\sigma}}_{\mathrm{Cs}^{170}\mathrm{Yb}}=\left(18\ifmmode\pm\else\textpm\fi{}8\right)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}$. We perform quantum scattering calculations of the thermalization cross sections and optimize the CsYb interaction potential to reproduce the measurements. We predict scattering lengths for all isotopic combinations of Cs and Yb. We also demonstrate the independent production of 174Yb and 133Cs Bose-Einstein condensates using the same optical dipole trap, an important step toward the realization of a quantum-degenerate mixture of the two species.
This thesis presents the development of an experimental apparatus designed to investigate the ultracold collisional properties for mixtures of Cs and Yb, with a long-term view to the creation of ultracold CsYb molecules via indirect cooling methods. The unpaired electron spin that is inherent to molecules of this form gives rise to a magnetic dipole moment in addition to a ground state electric dipole moment. This enables extra control over molecular interactions and should enable the experimental simulation of spin lattice models. We focus on the implementation of a system designed to controllably laser cool and optically trap Yb. The first step in this system is the production of a magneto-optical trap (MOT) on the triplet 1S0 to 3P1 transition of Yb. With careful control over the cooling beam detunings and power, gravitational-assisted Doppler cooling allows samples of Yb to be prepared at 22 uK. This regime of enhanced Doppler cooling is investigated and proves to be a crucial step to ensuring good transfer of cold Yb to optical traps. The construction and characterisation of single and crossed beam optical dipole traps for Yb are discussed. The single beam optical trap has been used to verify a model for the optical trapping of Yb in its ground state. This trap has also been utilised as a tool for the measurement of the light shift on the 1S0 to 3P1 transition at a wavelength of 1070~nm. In the main experimental sequence, Yb atoms are loaded from the magneto-optical trap into the crossed optical dipole trap, allowing evaporative cooling ramps to quantum degeneracy to be performed. This highly-reproducible system typically forms Bose-Einstein condensates with 2 x 10^5 174Yb atoms. This thesis additionally reports on the progress made towards measurements of the interspecies scattering length for 133Cs and Yb isotopes. We present two approaches that are being developed in tandem: rethermalisation in a conservative trap, and two-photon photoassociation. Progress towards rethermalisation measurements has focussed on developing systems for the efficient transfer of Cs to an optical trap. For photoassociative measurements, a laser system has been developed and tested by producing one-photon photoassociation spectra of Cs2.
This thesis reports the formation of a dual-species Bose-Einstein condensate of 87Rb and 133Cs in the same trapping potential. Quantum degenerate mixtures exhibit rich physics inaccessible to single species experiments and provide an ideal starting point for the creation of ultracold dipolar molecules. These molecules offer a wealth of new research avenues including precision metrology, quantum simulation and computation. The experimental method exploits the efficient sympathetic cooling of 133Cs via elastic collisions with 87Rb, initially in a magnetic quadrupole trap and subsequently in a levitated optical trap. Evaporative cooling in the dipole trap must compete against a high interspecies three-body inelastic collision rate \mbox{∼10−25−10−26~cm6/s}. The two condensates each contain up to \mbox{2×104} atoms and exhibit a striking phase separation, revealing the mixture to be immiscible due to strong repulsive interspecies interactions. Sacrificing all the 87Rb during the cooling leads to the creation of single-species 133Cs condensates of up to \mbox{6×104} atoms. In addition this thesis reports the observation of an interspecies Feshbach resonance at 181.7(5)~G and the creation of a pure sample of Cs2 molecules via magneto-association on the 4(g)4 resonance at 19.8~G. These results represent important steps towards the creation of ultracold polar RbCs molecules.
Within the scope of this thesis, the production of ultracold molecules at a temperature of a few µK with various isotopes of rubidium (Rb) and ytterbium (Yb) was examined by means of photoassociation spectroscopy and magnetic Feshbach resonances in combined conservative traps. The long-term goal of this experiment is the production of ultracold RbYb molecules in the rovibronic ground state.
It was possible to produce electronically excited ^{87}Rb ^{176}Yb molecules in a novel hybrid trap (HT) at a combined temperature of 1.7 µK by means of 1-photon photoassociation close to the Rb D1 line at 795 nm. This HT takes advantage of the different magnetic properties of Rb and Yb and allows for independent trapping and manipulation of the atomic species. It combines an Ioffe-Pritchard type magnetic trap for Rb and a near-resonant optical dipole trap for Yb. The excited molecular ^2 Pi_1/2 state could be characterized further extending previous works in a combined MOT and vibrational levels reaching binding energies up to E_b=-h x 2.2 THz could be assigned by trap-loss spectroscopy.
Almost every detected vibrational state consists of two resonances that could be assigned to the molecular analogue of the hyperfine structure of ^{87}Rb. An important experimental observation is a decrease in hyperfine splitting with increasing binding energy of a vibrational level. For the deepest found vibrational state the hyperfine splitting amounts only 70 % of the atomic value (817 MHz) which emphasizes a gradual passage from weakly to tightly bound molecules.
Furthermore, detailed attempts were undertaken to induce magnetic Feshbach resonances in ^{85}Rb and different Yb isotopes, especially ^{171}Yb in a crossed optical dipole trap at 1064 nm at temperatures of 10 µK. For this purpose, a homogeneous magnetic field was applied and scanned in small steps over the range of 495 G... 640 G. Unfortunately, our efforts were without success. Additionally, well known as well as not yet reported Rb_2 Feshbach resonances in ^{85}Rb and ^{87}Rb were detected. These resonances allowed for a detailed analysis of the typical behaviour of Feshbach resonances.
The first experimental realization of photoassociation spectroscopy in a conservative is also a major step towards the ultimate project goal of producing ultracold RbYb molecules in the electronic ground state. A spatial confinement, which is in principle possible in the HT, is a prerequisite for transferring the molecules to the rovibronic ground state, e.g. with the help of a STIRAP process.
This thesis reports the observation of interspecies Feshbach resonances in an ultracold mixture of Rb and Cs atoms. A versatile combined magnetic and optical potential has been designed and constructed which is capable of bringing both 87Rb and 133Cs to degeneracy, and reaching high phase-space density in 85Rb. High phase-space density mixtures are the first step required in the production of ultracold polar molecules, the topic of much current research. The apparatus capitalises on the efficient capture of atoms by a magnetic trap from a magneto-optical trap, and the efficient sympathetic cooling of Cs by Rb therein. Upon transfer to the crossed optical dipole trap condensates in excess of 1×10687Rb atoms and approximately 1×105133Cs atoms are produced after direct evaporation and gravito-magnetic tilting of the potential. The observation of six interspecies 87Rb-133Cs Feshbach resonances are reported, three of which had only been predicted theoretically, allowing testing and development of the theoretical model. Furthermore, the extrapolation of this model has predicted numerous Feshbach resonances between 85Rb and 133Cs, none of which have been experimentally observed prior to this work. The versatile nature of this apparatus is discussed, including the application of the current system to cooling of 85Rb. Initial experiments observed seven interspecies resonances, including a broad s-wave resonance at a magnetic field of (644±2) G which is in excellent agreement with the theoretical prediction. Further work has revealed that fourteen Feshbach resonances exist in the 0-700 G magnetic field range between 85Rb and 133Cs atoms in the ∣2,+2⟩ and ∣3,+3⟩ states, respectively. Several of these resonances would be ideal for magneto-association of RbCs molecules, prior to transfer to the rovibrational ground-state.
This thesis reports the investigation of the scattering properties of a mixture of Rb and Cs and the formation of ultracold Feshbach molecules. The production of Feshbach molecules is a crucial step towards the production of ultracold polar molecules, which is of significant interest for a wide range of potential applications. We have investigated the scattering properties of a mixture of 85Rb and 133Cs in their lowest spin channel in a magnetic field range from 0 to 700\,G. Furthermore, we explored the Feshbach spectrum of 85Rb alone in both, the (f=2,mf=−2) and (2,+2) states up to a magnetic field of 1000\,G. Additionally a Feshbach resonance in a (2,+2)+(3,+3) spin mixture was experimentally confirmed. We associated Cs2 Feshbach molecules using a Feshbach resonance at 19.9\,G. \mbox{2.1(1)×104} molecules with a temperature of ∼60\,nK were produced from a sample of 3.28(2)×105 Cs atoms with a PSD of 0.20(1). Due to a magnetic field gradient, the molecules `bounce' at an avoided crossing between two states at 13.5\,G. This gradient field was also used to produce multiple molecular clouds from one atomic sample. A combination of both techniques led to a `collision' between two Cs2 clouds. Furthermore, we associated up to ∼5000 heteronuclear 87RbCs Feshbach molecules using an interspecies resonance at 197.1\,G. Confined in the dipole trap the molecules have a lifetime of 0.21(1)\,s. We have measured the magnetic moment of the molecules in different internal states in a magnetic field range from 181 to 185\,G. Molecular loss spectroscopy on electronically excited states was performed to identify candidates for the intermediate state of a STIRAP transfer of the molecules into their rovibrational ground state. Subsequently, the binding energy of the rovibrational ground state was measured to be 3811.574(1)\,cm−1, using two-photon spectroscopy.