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Collaborations: L. Santos (Hannover) Students : Antoine Reigue, Ariane

Recent results with ultra cold chromium atoms. de Paz (PhD), B. Naylor, A. Sharma (post-doc), A. Chotia (post doc), J. Huckans (visitor), O. Gorceix , E. Maréchal, L. Vernac , P. Pedri, B. Laburthe-Tolra. Collaborations: L. Santos (Hannover) Students : Antoine Reigue, Ariane. Outline.

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Collaborations: L. Santos (Hannover) Students : Antoine Reigue, Ariane

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  1. Recent results with ultra cold chromium atoms • de Paz (PhD), B. Naylor, A. Sharma (post-doc), A. Chotia (post doc), J. Huckans (visitor), O. Gorceix , E. Maréchal, L. Vernac , P. Pedri,B. Laburthe-Tolra Collaborations: L. Santos (Hannover) Students: Antoine Reigue, Ariane

  2. Outline Quantum Magnetism with ultracold bosons Production of a chromium Fermi sea

  3. Quantum Magnetism: what is it about? What is (are) the (quantum) phase(s) of a given crystal at "low" T ? Heisenberg Hamiltonian ferromagnetic anti ferromagnetic Magnetism ie quantum phases not set by ddi but by exchange interactions

  4. Quantum Magnetism with cold atoms tunneling assisted super exchange U

  5. Quantum Magnetism with a dipolar species in a 3D lattice Vdd Exchange term Ising term magnetic dipole moment dipole-dipole interactions direct spin-spin interaction real spin S=3 quantum regime, high filling factor long range = beyond the next neighbor T < 1 nK Vdd = 10-20 Hz to reach ground state Spin dynamics in an out of equilibrium system

  6. Quantum Magnetism with a chromium BEC in a 3D lattice S=3 Cr BEC loaded in a 3D lattice: a Mott state 3 2 1 0 spin preparation, measurement of the evolution of the Zeeman states populations -1 different spin dynamics induced by dipole-dipole interactions -2 -3 spin exchange 3 constant magnetization 3 dipolar relaxation 2 2 1 change of the magnetization 0 1 0 -1 -1 -2 -2 -3 magnetization = -3

  7. Dipolar relaxation in a 3D lattice - observation of resonances nx , ny , nz kHz (Larmor frequency) 1 mG = 2.8 kHz width of the resonances: tunnel effect + B field, lattice fluctuations

  8. Spin exchange dynamics in a 3D lattice 10 mG B 0 first resonance dipolar relaxation suppressed evolution at constant magnetization spin exchange from -2 experimental sequence: -1 -2 Load optical lattice -3 state preparation in -2 vary time Stern Gerlach analysis

  9. Different Spin exchange dynamics in a 3D lattice Contact interaction (intrasite) expected Mott distribution

  10. Different Spin exchange dynamics in a 3D lattice Contact interaction (intrasite) Dipole-dipole interaction (intersite) without spin changing term dipolar relaxation with doublons removed = only singlons expected Mott distribution

  11. Spin exchange dynamics in a 3D lattice: with only singlons the spin populations change! E(ms) = q mS2 comparison with a plaquette model (Pedri, Santos) 3*3 sites , 8 sites containing one atom + 1 hole quadratic light shift and tunneling taken into account measured with interferometry Proof of intersite dipolar coupling Many Body system

  12. Spin exchange dynamics in a 3D lattice with doublons at long time scale intersite dipolar coupling result of a two site model: two sites with two atoms dipolar rate raised (quadratic sum of all couplings) our experiment allows the study of molecular Cr2 magnets with larger magnetic moments than Cr atoms, without the use of a Feshbach resonance not fast enough: the system is many body

  13. Dipolar Spin exchange dynamics with a new playground: a double well trap idea: direct observation of spin exchange with giant spins, "two body physics" compensating the increase in R by the number of atoms realization: load a Cr BEC in a double well trap + selective spin filp R -3 +3 N atoms N atoms frequency of the exchange: precession of one spin in the B field created by N spins at R R = 4 µm j = 3 N = 5000 Hz B fieldcreated by one atom

  14. Spin exchange dynamics in a double well trap: realization realizing a double well spin preparation RF spin flip in a non homogeneous B field +3 -3

  15. Spin exchange dynamics in a double well trap: results No spin exchange dynamics

  16. Inhibition of Spin exchange dynamics in a double well trap: interpretation (1) What happens for classical magnets? evolution in a constant external B field evolution of two coupled magnetic moments q

  17. Inhibition of Spin exchange dynamics in a double well trap: interpretation (2) What happens for quantum magnets in presence of an external B field when S increases? Evolution of two coupled magnetic moments in presence of an external B field no spin changing terms 2S+1 states Ising contribution gives different diagonal terms Ising term Exchange term "half period" of the exchange grows exponentially if no more exchange possible no complete exchange It is as if we had two giant spins interacting

  18. Contact Spin exchange dynamics from a double well trap after merging after merging without merging Spin exchange dynamics due to contact interactions Fit of the data with theory gives an estimate of a0 the unknown scattering length of chromium

  19. Production of a degenerate quantum gas of fermionic chromium Two very different quantum statistics or T<<TF T > Tc T < Tc a quantum gas at T<<TF a quantum gas at T<Tc

  20. Production of a degenerate quantum gas of fermionic chromium A quantum gas ? 3D harmonic trap Degeneracy criteria Chemical Potential

  21. Production of a degenerate quantum gas of fermionic chromium So many lasers… 7P4 53Cr MOT : Trapping beams sketch 53Cr MOT : laser frequencies production 7S3 Lock of Ti:Sa 2 is done with an ultrastable cavity

  22. Production of a degenerate quantum gas of fermionic chromium Loading a one beam Optical Trap with ultra cold chromium atoms direct accumulation of atoms from the MOT in mestastable states RF sweep to cancel the magnetic force of the MOT coils crossed dipole trap for 53Cr : finding repumping lines

  23. Production of a degenerate quantum gas of fermionic chromium Spectroscopy and isotopic shifts isotopic shifts unknown 5D J=3 →7P° J=3 for the 52    //  5D J=3 F=9/2 →7P° J=3 F=9/2   for the 53 Shift between the 53 and the 52 line: 1244 +/-10 MHz Deduced value for the isotopic shift: Center value = 1244 -156.7 + 8 = 1095.3 MHz Uncertainty: +/-(10+10) MHz (our experiment) +/-8 MHz (HFS of 7P3) • isotopic shift: • mass term • orbital term

  24. Production of a degenerate quantum gas of fermionic chromium Strategy to start sympathetic cooling make a fermionic MOT, load the IR trap with 53Cr more than 10553Cr about 10652Cr make a bosonic MOT, load the IR trap with 52Cr + 6.10552Cr 3.10453Cr inelastic interspecies collisions limits to not great, we tried anyway…

  25. Production of a degenerate quantum gas of fermionic chromium Evaporation

  26. Production of a degenerate quantum gas of fermionic chromium Why such a good surprise? Maybe we reach the hydrodynamic regime for the fermions… then fermions are trapped by collisions If collisions with bosons set the mean free path of fermions below the trap radius How to measure Fermion-Boson collision cross section? By heating selectively and quickly the bosons and then measure fermions thermalization very preliminary measurements + analysis support this interpretation

  27. Production of a degenerate quantum gas of fermionic chromium Results In situ images Expansion analysis Nat parametric excitation of the trap trap frequencies Temperature slightly degenerated

  28. Production of a degenerate quantum gas of fermionic chromium What can we study with our gas? 9/2 Fermionic magnetism 7/2 5/2 3 Phase separation very different from bosonic magnetism ! 3/2 2 1/2 Picture at T= 0 and no interactions 1 -1/2 0 -3/2 -1 -5/2 -2 -7/2 -3 -9/2 Boltzmann Population in mF=-9/2 T=10 nK requires good in situ imaging T=50 nK Fermi T=0 T=200 nK minimize Etot Larmor frequency (kHz)

  29. thank you for your attention!

  30. R Dipolar Quantum gases van-der-Waals Interactions dipole – dipole interactions BEC Tc= few 100 nK Isotropic Short range Anisotropic Long Range comparison of the interaction strength for the BEC can become unstable polar molecules alcaline chromium dysprosium for 87Rb erbium

  31. mS = -2 Preparation in an atomic excited state -3 energy creation of a quadratic light shift Raman transition s- p -3 -2 -1 0 1 2 3 -1 A s- polarized laser Close to a JJ transition (100 mW 427.8 nm) -2 -1 -3 1 quadratic effect (laser power) -2 -3 laser power 0 -1 -3 -2 -2 transfer adiabatic transfer in -2 ~ 80% -3

  32. Dipolar Relaxation in a 3D lattice kinetic energy gain Ec is quantized 3 dipolar relaxation is possible if: 2 1 0 -1 + selection rules -2 -3 If the atoms in doubly occupied sites are expelled

  33. Spin exchange dynamics in a 3D lattice with doublons at short time scale initial spin state onsite contact interaction: spin oscillations with the expected period strong damping contact spin exchange in 3D lattice: Bloch PRL 2005, Sengstock Nature Physics 2012

  34. Different Spin exchange dynamics with a dipolar quantum gas in a 3D lattice -1 -2 -3 intrasite contact intersite dipolar expected Mott distribution Heisenberg like hamiltonian quantum magnetism with S=3 bosons and true dipole-dipole interactions doublons removed = only singlons intersite dipolar de Paz et al, Arxiv (2013)

  35. Spin changing collisions V' V -V -V' from the ground state from the highest energy Zeeman state -1 +3 dipolar relaxation -2 +2 -3 +1 after an RF transfer to ms=+3 study of the transfer to the others mS spin changing collisions become possible at low B field dipole-dipole interactionsinduce a spin-orbitcoupling rotation induced the Cr BEC can depolarizeat low B fields At low B field the Cr BEC is a S=3 spinor BEC Cr BEC in a 3D optical lattice: coupling between magnetic and band excitations

  36. Spin changing collisions V' V -V -V' from the ground state -1 1 mG (a) -2 0.5 mG (b) 0.25 mG -3 (c) « 0 mG » (d) spin changing collisions become possible at low B field -3 -2 -1 0 1 2 3 As a6 > a4 , it costs no energy at Bc to go from mS=-3 to mS=-2 : stabilization in interaction energy compensates for the Zeeman excitation the Cr BEC can depolarizeat low B fields At low B field the Cr BEC is a S=3 spinor BEC

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