Fermionic Suppression of Dipolar Relaxation

Abstract

We observe the suppression of inelastic dipolar scattering in ultracold Fermi gases of the highly magnetic atom dysprosium: the more energy that is released, the less frequently these exothermic reactions take place, and only quantum spin statistics can explain this counterintuitive effect. Inelastic dipolar scattering in non-zero magnetic fields leads to heating or to loss of the trapped population, both detrimental to experiments intended to study quantum many-body physics with strongly dipolar gases. Fermi statistics, however, is predicted to lead to a kinematic suppression of these harmful reactions. Indeed, we observe a 120-fold suppression of dipolar relaxation in fermionic versus bosonic Dy, as expected from theory describing universal inelastic dipolar scattering, though never before experimentally confirmed. Similarly low inelastic cross sections are observed in spin mixtures, also with striking correspondence to universal dipolar scattering predictions. The suppression of relaxation opens the possibility of employing fermionic dipolar species-atoms or molecules-in studies of quantum many-body physics involving, e.g., synthetic gauge fields and pairing. Spin-statistics play a prominent role in determining the character and rate of elastic collisions among ultracold atoms or molecules [1][2][3], often leading to the enhancement or suppression of thermalization. For example, elastic collisions mediated by short-range interactions between spin-polarized fermions are suppressed at low velocity. The reason lies in the requirement that the total two-particle state-the tensor product of spin and orbital-must be antisymmetric both before and after a collision [4]. Because the orbital wavefunction must be of odd parity for spin-polarized fermions, collisions between two such atoms are inhibited by the p-wave centrifugal energy barrier [5]. For van der Waals interactions, this leads to a kinematic suppression of the elastic cross section as k i → 0, where the wavevector k i is proportional to the relative incoming momentum. The fermionic suppression of thermalizing elastic collisions has an important, well-known consequence: inefficient evaporative cooling near quantum degeneracy [6].This unfavorable scaling is modified in the case of 3D dipolar interactions. The long-range, r −3 nature of the dipolar interaction leads to an elastic cross section independent of k i and proportional to the fourth power of the magnetic dipole moment µ regardless of quantum statistics in the limit k i → 0 [7][8][9][10]. This manifestation of universal dipolar scattering implies that sufficiently strong dipolar interactions allow spin-polarized fermions to evaporatively cool even at energies comparable to and below the Fermi temperature T F . Here "universal" means short-range physics plays no role; scattering only depends on atomic parameters through µ and mass [8] and not on, e.g., the difficult-to-calculate phaseshifts of partial-waves at short range [11]. Indeed, recent experiments employing the highly dipolar fermionic gases KRb...

Bilayer fractional quantum Hall states with dipoles

“…1(a), we consider loading 161 Dy one atom per site in a square lattice in the X-Y plane with nearestneighbor spacing of λ lat /2 = 266 nm created with offresonant light of λ lat = 532 nm wavelength. We further assume that the lattice is so deep that tunneling is negligible, which allows us to avoid dipolar relaxation [39] and light-assisted collisions [40]. No static magnetic fields are applied.…”

Low-lying excitation modes of trapped dipolar Fermi gases: From the collisionless to the hydrodynamic regime

“…In this case, the radial part can be evaluated with he help of the identity [90, (6.574.2)], while the angular part corresponds to the definition of the anisotropy function in Eq. (22). Then, the dipolar interaction energy can be cast in the final form…”

“…To this end we substitute u i = k ′ i /K ı and switch to spherical coordinates. The integrals over the angular variables lead to the anisotropy function (22), and the radial and t-integrals can be solved in an elementary way, thus leading to the final result…”

Interaction-driven Lifshitz transition with dipolar fermions in optical lattices

“…1. Furthermore, ultracold atomic gases of fully polarized fermions are readily available in experiment [45][46][47] , and allow to avoid dealing with complex spin preparation protocols and dipolar relaxation effects between spin components that modifies the initial spin preparation [68][69][70][71][72][73][74][75][76] . The model's Hamiltonian is given by:…”