Session M3: Dipolar Matter

Electric-field-induced inelastic collisions between magnetically trapped hydroxyl radicals

We have observed inelastic collisions between magnetically trapped hydroxyl (OH$^\bullet$) molecules at a temperature of 45 mK, induced by the presence of an electric field. We measured the collisional loss rate over electric field strengths of 0.2-10 kV/cm. Accurate measurements of the loss rate also required recognition and understanding of a new form of single-body trap loss, arising from non-adiabatic transitions between Zeeman levels at avoided crossings created by the presence of a transverse electric field.   

Optical depletion spectroscopy for probing evaporatively cooled OH

Pulsed laser induced fluorescence (PLIF) measurements provide a sensitive probe for the detection of molecular species. However, the broad linewidth of the pulsed lasers used for molecular excitation obscures spectral information useful for determination of molecule temperature. This limitation can be overcome by the use of a second, narrowband source of radiation that can deplete a subset of the molecules detected using PLIF, and a high resolution depletion spectrum can be obtained. In the past, we have demonstrated the use of microwave depletion spectroscopy to measure magnetically trapped, evaporatively cooled OH temperatures in the range of 5-50 mK. The lower limit of 5 mK is set by the details of the microwave transition. Here we present temperature measurements of trapped OH using an optical depletion technique, which is in principle capable of probing temperatures as low as 50 microkelvin.   

The ground state OH molecule in combined electric and magnetic fields: exact solution of the effective Hamiltonian

The OH molecule is currently of great interest from the perspective of ultracold chemistry, quantum fluids, precision measurement and quantum computation. Crucial to these applications are the slowing, guiding, confinement and state control of OH using electric and magnetic fields. In this talk, we will show that the corresponding eight-dimensional effective ground state Stark-Zeeman Hamiltonian is exactly solvable. We will discuss our results in the context of current experiments. Our analytical solution opens the way to insightful characterization of the magnetoelectrostatic manipulation of the OH molecule and is also immediately applicable to other diatomic free radicals such as ClO, SF, NS, OD and SrF.   

Quantum degenerate Bose and Fermi dipolar gases of dysprosium

Advances in the quantum manipulation of ultracold atomic gases are opening a new frontier in the quest to better understand strongly correlated matter. By exploiting the long-range and anisotropic character of the dipole-dipole interaction, we hope to create novel forms of soft quantum matter, phases intermediate between canonical states of order and disorder. Our group recently created Bose and Fermi quantum degenerate gases of the most magnetic element, dysprosium, which should allow investigations of quantum liquid crystals. We present details of recent experiments that created the first degenerate dipolar Fermi gas as well as the first strongly dipolar BEC in low field.   

Stability Spectroscopy of Rotons in a Dipolar Bose Gas

We study the stability of a quasi-one-dimensional dipolar Bose-Einstein condensate that is perturbed by a weak lattice potential along its axis. Our numerical simulations demonstrate that systems exhibiting a roton-maxon structure destabilize readily when the lattice wavelength equals either half the roton wavelength or a low roton subharmonic. We apply perturbation theory to the Gross-Pitaevskii and Bogoliubov de Gennes equations to illustrate the mechanisms behind the instability threshold. The features of our stability diagram are a novel signature of roton physics, and their experimental observation would constitute a direct measurement of the roton wavelength for quasi-one-dimensional geometries.   

Pattern Formation in Dipolar Bose Mixtures

We consider the effects of dipole-dipole interactions on the immiscibility, or spatial separation of a binary Bose condensate. In free space, such interactions introduce anisotropic immiscibility thresholds, while the effects are still richer in trapped systems. In particular, we discuss the transition to immiscibility in a quasi-two dimensional geometry, created by the presence of a strong trapping potential in one direction. In contrast to the case of purely short-range interactions, the dipolar interactions introduce novel bistabilities and patterned phases in these mixtures. Further, we find that such features persist when the in-plane interactions are tuned to be anisotropic, resulting in stripe phases that can be accessed by simply tilting the dipole polarization. In addition to a composite Bogoliubov analysis, which provides a clear and qualitatively enlightening analysis of such physics, we present full numeric simulations that demonstrate the patterning and dynamics of these phases.   

Ultracold dynamics of magnetic dipolar molecules

Recently, Bose-Einstein condensation of magnetic polar atoms of Er have been achieved [1]. This was soon followed by the formation of Er$_2$ Feshbach molecules [2], made of magneto-associated ultracold Er atoms, each of them having a strong magnetic dipole moment ($7 \mu_B$). As these molecules are weakly bound, they can be highly sensitive to collisions in the ultracold gas and further lost from the trap. When the atom-atom scattering length (which characterizes the size of the Feshbach molecules) becomes comparable to the dipole-dipole length, the atoms of the molecules are well separated in the collision and the molecular dynamics is mainly mediated by the atomic magnetic dipolar interactions. Thus the anisotropy of each individual atomic dipole-dipole interactions might prevail in the molecular collision and this is what we will explore in our presentation. We will compare our findings with ongoing experiments performed in Innsbruck [2]. \\[4pt] [1] K. Aikawa, A. Frisch, M. Mark, S. Baier, A. Rietzler, R. Grimm, F. Ferlaino, Phys. Rev. Lett. {\bf 108}, 210401 (2012).\\[0pt] [2] A. Frisch, M. Mark, K. Aikawa, S. Baier, R. Grimm , and F. Ferlaino, private communication (2012).   

Quantum Phases of Soft-Core Dipolar Bosons in Optical Lattices

We perform quantum Monte Carlo simulations of a system of soft-core ultracold bosonic atoms with dipolar interactions, confined in a two dimensional optical lattice. We consider long range isotropic repulsive interactions which refers to dipoles are alligned perpendicular to the plane. We calculate the ground state phase diagram for a parameter range that exhibits various solids, superfluid and supersolid phases. We also present finite temperature results and discuss the experimental feasibility of such phases.   

Chaotic Dynamics of a Dipolar Bose-Einstein Condensate

Recent theoretical work has shown that short-range contact and long-range dipole-dipole interactions of a Bose-Einstein condensate provide the stability needed for the formation of a two-dimensional soliton in a chaotic optical-dipole potential. However, inclusion of the dipole-dipole interaction causes the BEC soliton to deviate from the classical trajectories of the trapping potential and to instead follow trajectories given by an effective potential; the effective potential is the convolution of the trapping potential with the BEC density. Here, we study the transition from purely Schrodinger evolution in the trapping potential to soliton evolution in the effective potential. This transition from Schrodinger to soliton behavior can be viewed as an alternative kind of classical transition, distinct from the usual Ehrenfest wavepacket approach in which hbar goes to zero. We also explore the increase in fractal resolution in the escape-time data as the soliton limit is approached.   

Rydberg Wavepacket Evolution in an Ensemble of Cold Dipole-Dipole Coupled Atoms

Abstract: We have studied the evolution of Rydberg wavepackets in the presence of interatomic dipole-dipole interactions in a cold Rb gas. In the experiments, Rb atoms in a MOT are laser excited to 32s states. The Rydberg atoms are then exposed to a picosecond mm-wave pulse that creates a coherent superposition of states, predominantly 32s$+$32p, within individual atoms. The wavepackets are allowed to evolve for a variable time, T \textless 15 ns, before their exposure to a second mm-wave pulse. The second pulse coherently transfers population among the Rydberg levels, resulting in a delay-dependent interference in the probability for detecting atoms (via selective field ionization) in the constituent states in the wavepacket. In the independent atom picture, the probability amplitudes of the superimposed states are continually modified by the dipole-dipole interactions between atoms. The predominant interactions result in excitation exchange, sp$\to $ps, between atom pairs during the evolution time T. Experimentally we observe a suppression of the delay-dependent interference oscillations in the population in different Rydberg states. The degree of suppression for a given T increases with increasing atom density. Numerical simulations suggest that the dipole-dipole coupling leads to complex interferences which dephase due to the variation in the separations between neighboring atoms in the MOT.