Spin-injection Spectroscopy of a Spin-orbit coupled Fermi Gas

Transcription

1 Spin-injection Spectroscopy of a Spin-orbit coupled Fermi Gas Tarik Yefsah Lawrence Cheuk, Ariel Sommer, Zoran Hadzibabic, Waseem Bakr and Martin Zwierlein July 20, 2012 ENS

2 Why spin-orbit coupling? A little bit of History Last century : classification of quantum states in terms of spontaneous symmetry breaking [Anderson 1997] In 1980 : Quantum Hall state. First topological state characterized by a topological invariant [von Klitzing et al. 1980] : a new topological class predicted and discovered, where timereversal symmetry is preserved. Spin-Orbit coupling plays a crucial role [M.Z. Hasan and C.L. Kane, RMP, 2010] Also : Modified interactions, unconventional pairing, Majorana fermions Cold atoms : (very often) constitute optimal system thanks to purity and controle

3 Spin-orbit Hamiltonian Electron moving in an electric field creates a momentum-dependent magnetic fields in the moving frame In 2D semiconductor electric field can arises from structure Rashba Dresselhaus Provides a good description of 2D SOC in solids

4 Engineering SO coupling Laser 1 Laser 2,, How does the Hamiltonian look like? Reminder 2-level system + electric field e g RWA approx. : By adiabatic elimination of the excited state Raman Beams the Raman process can be described as the interaction of a 2-level system with Flip spin + imparts momentum a field

5 Engineering SO coupling Local pseudo-spin rotation of angle around the z-axis Global rotation Momentum dependent Zeeman field equal Rashba and Dresselhaus contributions Y. J. Lin et al. Nature 471, (2011)

6 Engineering SO coupling,, Define quasi-momentum q quasi-momentum real space

7 Engineering SO coupling 2 (q -Q / 2) 2,, Define quasi-momentum q 2m + d 2 2 (q +Q / 2) 2 2m - d 2 0 quasi-momentum space

8 Experimental Setup Fermionic 6 Li atoms sympathetically cooled by 23 Na Relevant states are 2 nd and 3 rd lowest states at 11G Interactions are negligible ( )

9 Coupling spin and momentum via Raman Fermion Laser 1 Laser 2 2 v/ v Vary detuning Short pulse 1 v/ State-selective imaging after TOF provides spin and momentum information

10 Image analysis Start with a mixture of and, and apply a Raman pulse for a given Check for the linear dependence of the transfer with momentum q (Doppler shift )

11 Pulsing on Raman Beams Start with state

12 Pulsing on Raman Beams Start with state Atomic system is coherent over many cycles Momentum-dependent Rabi oscillations Probability of transfer : with

13 Start with state Set large initial detuning ( Adiabatic Sweep ) and then sweep

14 Start with state Set large initial detuning ( Adiabatic Sweep ) and then sweep

15 Spin-injection spectroscopy How to characterize Hamiltonian? Can topology be measured? Condensed matter: transport, (spin-)arpes, STM Cold atom analog: momentum resolved RF (Jin, Koehl) (=photoemission spectroscopy) Photoemission Spectroscopy probes dispersion E(k)

16 What has been done so far Y. J. Lin et al. Nature 471, (2011) Ian Spielman s group P. Wang et al arxiv: (Jing Zhang s group)

17 Spin-injection spectroscopy Can Topology be measured? Spin-injection spectroscopy: Measures spin, energy, momentum 1. Inject atoms from reservoir 2. Project into free space 3. Spin-selective imaging Reconstruct E(k) along with color of band

18 Experimental Setup 1 st and 4 th states used as reservoir states R R

19 Spin-injection spectroscopy

20 Spin-injection spectroscopy

21 Spin-injection spectroscopy

22 Spin-injection spectroscopy

23 Spin-injection spectroscopy

24 Spin-injection spectroscopy

25 Spin-injection spectroscopy

26 Spin-injection spectroscopy

27 Spin-injection spectroscopy

28 Spin-injection spectroscopy

29 Spin-injection spectroscopy

30 Spin-injection spectroscopy

31 Spin-injection spectroscopy Increasing Raman Intensity

32 Spin-injection spectroscopy Increasing Raman Intensity

33 Creating a Spinful Lattice Add RF coupling -> lattice system with full bandgaps and spinful bands K. Jimenez-Garcia et al PRL 108, (2012)

34 Creating a Spinful Lattice Add RF coupling -> lattice system with full bandgaps and spinful bands The Spin-Orbit band structure is periodically repeated K. Jimenez-Garcia et al PRL 108, (2012)

35 Bandstructure of Raman + RF lattice In repeated scheme

36 Bandstructure of Raman + RF lattice Degenerate point inside spin orbit gap

37 Bandstructure of Raman + RF lattice Bandgap opens between 2 nd and 3 rd band

38 Bandstructure of Raman + RF lattice Larger RF, gap between lowest bands

39 Increasing RF Intensity Spin-injection Spectra Increasing Raman Intensity

40 Increasing RF Intensity Spin-injection Spectra Increasing Raman Intensity

41 Increasing RF Intensity Spin-injection Spectra Increasing Raman Intensity

42 Increasing RF Intensity Spin-injection Spectra Increasing Raman Intensity

43 Increasing RF Intensity Spin-injection Spectra Increasing Raman Intensity

44 Increasing RF Intensity Spin-injection Spectra Increasing Raman Intensity

45 Increasing RF Intensity Spin-injection Spectra Increasing Raman Intensity

46 Increasing RF Intensity Spin-injection Spectra Increasing Raman Intensity

47 Spin-injection Spectra

48 Spin-injection Spectra

49 Spin-injection Spectra Raman gap RF gap Raman + RF gap

50 Reconstructing the Bandstructure In addition to dispersion, can reconstruct eigenstates TOF gives eigenstate in the basis of free space spin/momentum states

51 Reconstructing the Bandstructure In addition to dispersion, can reconstruct eigenstates TOF gives eigenstate in the basis of free space spin/momentum states

52 Reconstructing the Bandstructure In addition to dispersion, can reconstruct eigenstates TOF gives eigenstate in the basis of free space spin/momentum states

53 Reconstructing the Bandstructure In addition to dispersion, can reconstruct eigenstates TOF gives eigenstate in the basis of free space spin/momentum states

54 Reconstructing the Bandstructure In addition to dispersion, can reconstruct eigenstates TOF gives eigenstate in the basis of free space spin/momentum states

55 Summary and Outlook Summary: SO-coupled Fermi gas Spinful lattice Spin-injection spectroscopy Band and eigenstate reconstruction Future: Interactions : p-wave Pairing in 1D tubes : Majorana edge mode? For details see: L. W. Cheuk, A. T. Sommer, Z. Hadzibabic, T. Y, W. Bakr, M. W. Zwierlein, arxiv:

56 Collaborators Lawrence Cheuk Ariel Sommer We thank these organizations for their support: DARPA, NSF, ONR, AFOSR, Sloan Foundation Zoran Hadzibabic Waseem Bakr Martin Zwierlein

57

58 Experimental Setup Fermionic 6 Li atoms sympathetically cooled by 23 Na Relevant states are 2 nd and 3 rd lowest states at 11G

59 Spin-injection spectroscopy Experiment Theory

60 SO-coupling in a Fermi gas Direct demonstration of SO-coupling through Rabi oscillations Controlled adiabatic loading of SO-coupled bands. Reversibility of loading shows adiabaticity.

61 The spin-orbit Hamiltonian The SO Hamiltonian Raman Coupling Hamiltonian maps to 1D spin-orbit Hamiltonian with

62 Spin-injection spectroscopy Spin-injection spectroscopy on a spinful lattice

63 The spin-orbit Hamiltonian Raman coupled atomic system maps to SO Hamiltonian. Rotating-Frame approximation: Write in terms of COM momentum q (spindependent transformation):

64 The spin-orbit Hamiltonian Raman coupled atomic system maps to SO Hamiltonian. Rotating-Frame approximation:

65 The spin-orbit Hamiltonian Raman coupled atomic system maps to SO Hamiltonian. Rotating-Frame approximation: Write in terms of COM momentum q (spindependent transformation): Amplitude of Raman beams give splitting Detuning imbalances the two wells

66 The spin-orbit Hamiltonian Raman coupled atomic system maps to SO Hamiltonian. Rotating-Frame approximation: Write in terms of COM momentum q (spindependent transformation): Amplitude of Raman beams give splitting Detuning imbalances the two wells

67 SO-coupling in a Fermi gas When SO coupling is ramped slowly: Spin composition follows effective magnetic field Process is reversible By changing detuning, either upper band or lower band

68 Spin-injection spectroscopy How to characterize Hamiltonian? Can we measure topology? Condensed matter: transport, ARPES, STM Cold atom analog: photoemission spectroscopy (PES) has been PES probes E(k) Transfer to hyperfine states outside system with RF Measure momentum in TOF Use RF frequency, free particle dispersion and momentum to reconstruct E(k)

69 Creating a Spinful Lattice Add RF coupling -> lattice system with full bandgaps and spinful bands K. Jimenez-Garcia et al PRL 108, (2012)

70 Detecting Spin Texture Image Sequence: TOF + state-selective imaging

71 Detecting Spin Texture Image Sequence: TOF + state-selective imaging

72 Detecting Spin Texture Image Sequence: TOF + state-selective imaging Some parameter

73 Engineering SO coupling Raman Beams couple two hyperfine states SO coupling along one direction Recoil momentum: Q Recoil energy : E R = 2 Q 2 2m

74 SO-coupling in Ultracold Atoms Realized in bosons: Modified dispersion Synthetic higher-order partial waves Synthetic magnetic field Y. J. Lin et al. Nature 471, (2011) R. A. Williams et al. Science 335, (2011) Y. J. Lin et al Nature (2009). Recently realized in fermions P. Wang et al arxiv: L. W. Cheuk at el arxiv:

75 Bandstructure of Raman + RF lattice

76 Bandstructure of Raman + RF lattice

77 Bandstructure of Raman + RF lattice

78 A Spin Diode Spin diode when the Fermi level is inside the spin gap

79 Experiment vs Simulation

80 Why spin-orbit coupling?