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Energy dependent scattering of ultracold atoms about a Feshbach resonance

Milena S. J. Horvath

Published Jan 1, 2016• 1 citation
OpenAlex

Abstract

In this thesis we explore ultracold atomic collisions in the vicinity of a magnetic Feshbach resonance located at 9.1 G, between 87Rb atoms in the |F = 2, mF = 1> and the |F =1, mF = 1> hyperfine states. We experimentally investigate the energy dependence of the enhancement of inelastic interaction in the neighborhood of this Feshbach resonance by analyzing the temperature dependence of a loss spectroscopy feature. An asymmetric widening of the resonance feature with respect to temperature is observed. We develop a simple model which captures these asymmetric line shapes through thermal averaging over the collision energy dependence of the Feshbach resonance. The energy dependence of the Feshbach interactions is further explored by colliding ultracold atomic samples at a fixed energy significantly above threshold using an optical trapping system; the elastic scattering enhancement analyzed by colliding the samples in free space, while for the inelastic measurements the collisions were carried out in trap. No conclusive resonant behavior could obtained for the elastic component of the 9.1 G resonance, however the inelastic scattering enhancement exhibited an anticipated shift.

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Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.Studies on energy-dependent scattering of ultracold atoms were previously carried out near zero collision energies. Here, the authors observe a magnetic Feshbach resonance in ultracold Rb collisions for above-threshold energies and their method can also be used to detect higher partial wave resonances.

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In this thesis we explore ultracold atomic collisions in the vicinity of a magnetic Feshbach resonance located at 9.1 G, between 87Rb atoms in the |F = 2, mF = 1> and the |F =1, mF = 1> hyperfine states. We experimentally investigate the energy dependence of the enhancement of inelastic interaction in the neighborhood of this Feshbach resonance by analyzing the temperature dependence of a loss spectroscopy feature. An asymmetric widening of the resonance feature with respect to temperature is observed. We develop a simple model which captures these asymmetric line shapes through thermal averaging over the collision energy dependence of the Feshbach resonance. The energy dependence of the Feshbach interactions is further explored by colliding ultracold atomic samples at a fixed energy significantly above threshold using an optical trapping system; the elastic scattering enhancement analyzed by colliding the samples in free space, while for the inelastic measurements the collisions were carried out in trap. No conclusive resonant behavior could obtained for the elastic component of the 9.1 G resonance, however the inelastic scattering enhancement exhibited an anticipated shift.
Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.Studies on energy-dependent scattering of ultracold atoms were previously carried out near zero collision energies. Here, the authors observe a magnetic Feshbach resonance in ultracold Rb collisions for above-threshold energies and their method can also be used to detect higher partial wave resonances.
Abstract Ultracold atomic gases have realized numerous paradigms of condensed matter physics, where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterization of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here, we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the center-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.
This thesis presents a simple theoretical model of a Feshbach resonance between two colliding ultracold atoms and experimental progress towards the observation of such a resonance in Rb85. On the theoretical side, the inter-atomic scattering potential is approximated by a spherical square well potential. The potential supports a number of bounds states whose energy depends on the width and depth of the well. In the low energy limit, only s-wave scattering is present, and the associated scattering length for the process diverges if there is a bound state at the dissociation limit: this is the hallmark of a Feshbach resonance. The model also shows that the Feshbach resonance can be shifted to higher scattering energies by adjusting the well depth, indicating the presence of a quasi-bound state above the dissociation limit. On the the experimental side, a crossed dipole trap has been developed and used to trap ultracold rubidium atoms. The performance of the trap is characterized, and the thesis describes work towards upgrading it for an experiment to observe a Feshbach resonance in the F=2, m_F=-2 hyperfine ground state of Rb85.
This thesis describes experiments investigating the collisions of alkali metal atoms at energies between 10 - 2000 uK, measured in units of the Boltzmann constant. The atoms are accelerated towards each other using a purpose-built collider comprised of a crossed-beam optical dipole trap, which enables us to collide dense ensembles of ultracold atoms in any internal state at relatively high energies. I present the results of two experiments centered around resonant enhancement of the collisions. The first is between homonuclear 40K atoms near a shape resonance where the fermionic nature of the atoms determines the character of multiple scattering effects. The second experiment involves heteronuclear 40K87Rb collisions near a magnetically-tunable Feshbach resonance where we measure parameters describing the resonance as a function of collision energy. Theoretical models are developed that let us describe the collisions using published empirical interaction potentials, and we find good agreement between these models and the experiment.
We report on experiments investigating the collisional properties of atoms at ultralow collision energies using an all-optical atom collider. By using a pair of optical tweezers, we can manipulate two ultracold atom clouds and collide them together at energies up to three orders of magnitude larger than their thermal energy. Our experiments measure the scattering of 87 Rb, 40 K, and 40 K- 87 Rb collisions. The versatility of our collider allows us to probe both shape resonances and Feshbach resonances in any partial wave. As examples, we present experiments demonstrating p-wave scattering with indistinguishable fermions, inelastic scattering at non-zero energies near a homonuclear Feshbach resonance, and partial wave interference in heteronuclear collisions.
We study collisions in an optically trapped, pure sample of ultracold Cs 2 molecules in various internal states. The molecular gas is created by Feshbach association from a near-degenerate atomic gas, with adjustable temperatures in the nanokelvin range. We identify several narrow loss resonances, which point to the coupling to more complex molecular states and may be interpreted as Feshbach resonances in dimerdimer interactions. Moreover, in some molecular states we observe a surprising temperature dependence in collisional loss. This shows that the situation cannot be understood in terms of the usual simple threshold behavior for inelastic two-body collisions. We interpret this observation as further evidence for a more complex molecular structure beyond the well-understood dimer physics.