11/14/2007NSU, Singapore Dipolar Quantum Gases: Bosons and Fermions Han Pu 浦晗 Rice University, Houston, TX, USA Dipolar interaction in quantum gases Dipolar BEC Rotons and charge density wave Dipolar Fermions

Current cold atom research at Rice Univ. Hulet group (exp.) BEC (Anderson localization, soliton) Fermi superfluid (Feshbach resonance, BEC-BCS crossover) Strongly correlated fermions in optical lattice Killian group (exp.) Ultracold plasma Neutral Sr (photoassociation, condensation) Pu group (th.) BEC (vortex and vortex lattice, spinor condensate, dipolar condensate, coupled atom-molecule condensate) Fermi gas (dipolar, BEC-BCS crossover, Fermi superfluid) Bose-Fermi mixture Quantum simulation (optical lattice systems) Quantum optics (interaction between light and cold atoms)

Two-body collisions at ultracold temperature (current paradigm) Dominated by the s-wave (isotropic) Short ranged (~1/R 6 ) Effective interaction --- pseudopotential Gross-Pitaevskii Equation

Dipolar system Atoms: magnetic dipole moment Polar molecules: large electric dipole moment Besides the short-range interaction, atoms or molecules may interact with each other via long-range anisotropic dipole-dipole interaction.

Dipolar interaction between two atoms Long ranged (~1/R 3 ) Anisotropic (Polarized dipole)

Computational method Two problems: 1) How to calculate efficiently the convolution integrals. 2) Dipolar interaction looks singular. One solution: Convolution theorem → Fourier Transform (FFT). Fourier transform of dipolar interaction is non-singular.

Chromium BEC realized Griesmaier et al., Phys. Rev. Lett. 95, (2005) Stuhler et al., Phys. Rev. Lett. 95, (2005) Dipolar effects on expansion dynamics

Vortex States in Dipolar condensate (and its connection with roton) Yi and Pu, PRA 73, (R) (2006)

Vortex studies in BEC Formation and decay Multiply charged vortices Tkachenko and Kelvin mode excitation Vortex state in combined harmonic and quartic trap Vortex in spinor condensate Vortex rings Scissors mode excitation Vortex lattice Bragg spectroscopy Quantum melting Vortex pinning (Ketterle, MIT)

Quasi-2D dipolar condensate Trapping potential Wave function decomposition Trap aspect ratio: Axial wave function:

Effective 2D dipolar interaction potential Dipoles polarized along z-axis Isotropic (azimuthal symmetry in the xy-plane x y

Vortex state for z-polarized dipoles Single vortex state: What causes the density ripples? Higher rotating freq. Vortex lattice: hexagonal

Vortex line as roton emitter? calculated vortex core structure for He II, at different pressure Dalfovo, PRB 46, 5482 (1992) Excitation spectrum of He II

Vortex structure vs. roton excitation in dipolar BEC Single vortex state: Excitation spectrum (homogeneous system) Santos et al., PRL (2003)

Rotons can be ‘emitted’ by other impurities Vortex line, alien atoms, container wall Difficult to observe in superfluid helium as the roton wavelength is only a few angstrons. A qualitative plot of superfluid density of HeII confined between two rigid walls. --- Rasetti and Regge, 1978

Quasi-2D BEC in a box Like the vortex line, here the wall may become the roton emitter. Roton instability induces charge density wave: Square CDW → triangular CDW → collapse

Beyond quasi-2D Charge density wave damped inside the bulk.

Dipolar BEC in 3D harmonic trap Ronen, Bortolotti, Bohn PRL 98, (2007)

Conclusion for dipolar BEC Dipolar interaction: long-range, anisotropic, New ground state structure Roton excitation and CDW (crystallization, supersolid?)

Dipolar fermions (spin polarized) Miyakawa, Sogo and Pu, arXiv:

Fermions vs. Bosons (T=0) Bosons Condensed (all atoms occupy the same state, macroscopic wave function) Symmetric many-body wave function: Hartree direct energy Real space and momentum space distribution: Fourier transform Fermions Pauli principle Anti-symmetric many-body wave function: Hartree direct and Fock exchange energy Real space and momentum space distribution: no direct relation Trapped non-interacting fermions: isotropic momentum distribution.

Model N spin polarized (along z-axis) fermions Interacting with each other via the dipolar interaction

Total energy Goal: minimize the total energy Strategy: treat the Wigner function variationally

Homogeneous case: Wigner function

Homogeneous case: energies Kinetic energy favors an isotropic Fermi surface (α =1); Fock energy tends to stretch the Fermi surface along z-axis (α =0); Competition b/w the two results in a prolate Fermi surface (0< α <1).

Inhomogeneous case: Wigner function

Inhomogeneous case: energies Interaction energy is not bounded from below (dipolar interaction is partially attractive). The system is not absolutely stable against collapse (λ → ∞). A local minimum may exist: the system may sustain a metastable state.

Results (I): density profiles Real space: Momentum space:

Results (II): density profiles in real space Solid lines: interacting Dashed lines: non-interacting

Results (III): deformation Momentum spaceReal space

Results (IV): stability Sufficiently large dipolar strength will collapse the system.

Conclusion and outlook Dipolar interaction deforms the quantum Fermi gas in both real and momentum space. Dipolar effects can be observed in TOF image. Ideal gas: isotropic expansion Dipolar gas: anisotropic expansion Importance of the Fock exchange term. Near future: Collective excitation Dipolar induced superfluid pairing

Collaborators: Hong Lu (Rice) Su Yi (ITP, CAS) Takahiko Miyakawa (Tokyo Univ. of Sci.) Takaaki Sogo (Kyoto Univ.)