Manifestations of the Roton in Dipolar Bose-Einstein Condensates

<p>Today, sixteen years after the realization of the first Bose-Einstein condensate (BEC), the field of ultracold many-body physics is booming. In particular, much excitement has been generated by the prospect of creating a degenerate quantum gas of dipolar atoms or molecules. Already, some experimental groups have succeeded in Bose-condensing atomic 52Cr and 164Dy, while other groups have made significant progress towards achieving degeneracy of heteronuclear molecules, such as fermionic 40K87Rb and bosonic 87Rb133Cs, where the strength of the dipolar interaction promises to be much greater than that of the already rich 52Cr condensate. Just as the creation of BEC launched a whole new field of research, dipolar BECs are likely to do the same. However, such systems present a theoretical challenge due to the long-range, anisotropic nature of the dipolar interaction. In this thesis, I present a theoretical investigation of ultracold Bose gases with dipolar interactions.</p> <p>The first part of this thesis is dedicated to the field theoretical treatment of a quantum Bose fluid with dipolar interactions in the ultracold, dilute regime, where the system is well-described by a classical condensate field with quasiparticle excitations. The set of nonlinear integrodifferential equations that describe these objects are derived and novel methods for solving them are presented that, in general, require intricate numerical treatment. Of particular importance is the emergence of a roton mode, reminiscent of that in superfluid 4He. In the second part of this thesis, I show how the roton plays a critical role in the ground state structure and dynamics of a dipolar BEC. Full numerical simulations show that the roton can, for example, be seen in the radial density profile of a quantized vortex state or in the angular collapse and explosion of a dipolar BEC. Additionally, I show the crucial role that this roton plays in determining the transition to superfluidity in thesesystems. Thus, a set of novel phenomena in ultracold dipolar Bose gases is explained by the presence of the roton, and experimental signatures of these phenomena are made clear.</p>
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University of Colorado Boulder
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