Lecture 3 : ultracold Fermi Gases

Transcription

1 Lecture 3 : ultracold Fermi Gases The ideal Fermi gas: a reminder Interacting Fermions BCS theory in a nutshell The BCS-BEC crossover and quantum simulation

2 Many-Body Physics with Cold Gases Diluteness: atom-atom interactions described by 2-body (and 3 body) physics. At low energy: a single parameter, the scattering length a Control of the sign and magnitude of interaction Control of trapping parameters: access to time dependent phenomena, out of equilibrium situations, 1D, 2D, 3D n(k) n(k) 1 1 Simplicity of detection Comparison with quantum Many-Body theories: Gross-Pitaevskii, Bose and Fermi Hubbard models, search for exotic phases, dipolar gases disorder effects, Anderson localization, Sherson et al., MPQ 2010 Link with condensed matter (high Tc superconductors), astrophysics (neutron stars), Nuclear physics, high energy physics (quark-gluon plasma), 1 k/k k/k F 1 F Quantum simulation with cold atoms «a la Feynman»

3 The ideal Fermi gas: a reminder Zero temperature Fermi sea: E(p) E kf k 2m 2 2 F F kt B F (6 n) ~ (particle distance) 2 1/3 1 E F p Fermi pressure: 1 2m /2 E 5/2 F Approximation valid as long as T<<T F

4 Electrons vs cold atoms Electrons in metal Ultra cold atoms Density /m /m 3 Mass kg kg Fermi temperature 10 4 K 1µK T/T F <10-5 ~ 10-2 Lifetime Infinite ~10s Size 1cm 10 µm Particle number

5 Pauli Exclusion Principle and evaporative cooling of ultra-cold Fermi gases Collision between two atoms. Effective potential in the l-wave: ( 1) 2mr 2 eff () () 2 V r V r l >0 Interatomic potential (long range~-1/r 6 ) l=0 centrifugal potential At low temperature, atoms cannot cross the centrifugal barrier: only s-wave collisions. Symmetrization for identical particles: even l-wave collisions forbidden for polarized fermions.

6 Suppression of elastic collisions in a spin polarized Fermi gas Spin mixture (s-wave) Spin polarized (p-wave) B. DeMarco, J. L. Bohn, J.P. Burke, Jr., M. Holland, and D.S. Jin, Phys. Rev. Lett. 82, 4208 (1999). Use spin mixtures or several atomic species (eg 6 Li- 7 Li, K-Rb, different spin states )

7 Quantum gases in in harmonic traps Bose-Einstein statistics (1924) Fermi-Dirac statistics (1926) Bose-Einstein condensate Fermi sea E F Bose enhancement h T = (0.83 N) 1/3 C k B Dilute gases: 1995, JILA, MIT Pauli Exclusion h T << T = (6N) F k B 1/3 Dilute gases: 1999, JILA

8 Absorption imaging in situ: cloud size

9 Bose-Einstein condensate and Fermi sea Lithium ENS Lithium Li 7 atoms, in thermal equilibrium with 10 4 Li 6 atoms in a Fermi sea. Quantum degeneracy: T= 0.28 K = 0.2(1) T C = 0.2 T F Now: T=0.03 T F

10 The non-interacting Fermi gas Gaussian Fit Fermi-Dirac T/T F <0.05 Atom number~10 5

11 Quantum simulation of strongly interacting Fermions

12 Quantum fluids Bose Einstein condensates Superconductivity and helium 3 4 He High Tc and 3He Dilute gas BEC

13 BEC of of strongly bound fermions Connecting the two regimes? Theory since 80 : Leggett, Randéria, Nozières,Schmidt-Rink, Holland, Kokkelmans, Levin, Ohashi, Griffin, Strinati, Falco, Stoof, Bruun, Pethick, Combescot, Giorgini.

14 intermediate regime Near Feshbach resonance, interactions in cold Fermi gas can be enhanced T c ~T F /5

15 Fermions with two spin states with attractive interaction BEC of molecules T c T F e / 2k BCS fermionic superfluid F a Bound state Interaction strength No bound state Dilute gases: Feshbach resonance

16 Bardeen, Cooper, Schrieffer, years of supraconductivity. Cooper Pairing Naive interpretation: Take a homogeneous T=0 Fermi gas, k F, E F Add two fermions, 1 et 2, which display attractive interaction < 0 V( r r ) V ( r r ) avec V < 0 Then these particles will always form at state with an energy lower than E F, a bound state. k, k Pairs at Fermi surface: These pairs form a superfluid phase k kf 2 k a F T 0.3 T e ka1 BCS F F a

17 Many-body hamiltonian: h 0 Hˆ d r ˆ ( r) h ˆ ( r) d rd r' ˆ ( r) ˆ ( r') V( rr') ˆ ( r') ˆ ( r) d r ˆ () r h ˆ () r g d rˆ () r ˆ () r ˆ () r ˆ () r 2 2m Mean-field Theory (BCS) at T=0 In momentum space: Bardeen, Cooper, Schrieffer, b ˆ () r k e i. L 3 aˆ kr k, ˆ g H aˆ aˆ aˆ aˆ aˆ aˆ k b k k k 3 kq k' q k' k L kk, ', q k k 2m 2 2

18 Mean-field Theory (2) Search for a solution in the form: N Ncka a vac k k k c Fourier components of spatial function ( r r ) k c k Assumption: zero momentum for BCS pairs: aˆ aˆ ~ order parameter for a gas of bosonic k k k molecules with zero center of mass momentum. i j N /2 BCS order parameter ˆ * ˆ ˆ ˆ ˆ ˆ ˆ k k k k k k k k k k H cte a a a a a a... g L g b b aˆ aˆ 3 k 3 k k k L k

19 Excitation Gap equations Gap In the ground state, no excitation of the Bogoliubov modes E k E k 2 2 k D x k k k 2m 2 2 BCS wavefunction: i BCS ( k k ) k k k u v e a a vac 2 2 uk vk 1

20 Tuning interactions in in Fermi gases Lithium 6 scattering length [nm] a>0 a<0 BEC-BCS Crossover Bound state 2 E B ma 0,0 0,5 1,0 1,5 2,0 Magnetic field [kg] condensate of molecules 2 No bound state BCS phase

21 The Equation of State of a Fermi Gas with Tunable Interactions Cold atoms, Spin ½ Dilute gas : at/cm 3, T=100nK BEC-BCS crossover Spin imbalance, exotic phases Neutron star, Spin ½ a = fm, n~ cm -3 T c =10 10 K, T=T F /100 k F a ~ -4,-10, k F r e <<1

22 Experimental sequence - Loading of 6 Li in the optical trap - Tune magnetic field to Feshbach resonance - Evaporation of 6 Li - Image of 6 Li in-situ

23 Suppression of of vibrational relaxation for fermions W(r) a a E B r R e C 6 / r 6 R e Binding energy: E B = h 2 /ma 2 Momentum of each atom: h/a Pauli exclusion principle Inhibition by factor (a/r e ) 2 >>1 G~ 1/a s with s = 2.55 for dimer-dimer coll for dimer-atom coll. D. Petrov, C. Salomon, G. Shlyapnikov, 04

24 Unitary Fermi Gas : a Optical Density (a.u) Pixels

25 Rotating classical gas Direct proof of superfluidity: classical vs. quantum rotation velocity field of a rigid body Rotating a quantum macroscopic object macroscopic wave function: In a place where, irrotational velocity field: The only possibility to generate a non-trivial rotating motion is to nucleate quantized vortices (points in 2D or lines in 3D) with quantized circulation around vortex core. Feynman, Onsager to keep single-valued Vortices now all have the same sign, imposed by the external rotation

26 MIT 2005: Vortex lattices in the BEC-BCS Crossover Energy [MHz] 1 Direct proof of superfluidity BEC side BCS side M. Zwierlein A.. Schirotzek C. Stan C. Schunk P. Zarth W. Ketterle Science 05 Magnetic Field [G]

27 Direct proof of superfluidity Critical SF temperature = 0.19 T F MIT 2006

28 Superflow in fermionic superfluids Landau criterion: dissipation for V>w k /k Theory: Combescot, Kagan and Stringari. Experiment: MIT, 2008.

29 strongly interacting Fermions thermodynamic properties

30 n(k) n(k) 1 1 Thermodynamics PV Nk B Is a useful but incomplete equation of state! Complete information is given by thermodynamic potentials: Grand potential PV Pressure Volume E T TS Temperature Entropy N Chemical potential Atom number Internal energy We have measured the grand potential of a tunable Fermi gas S. Nascimbène et al., Nature, 463, 1057, (2010), arxiv N. Navon et al., Science 328, 729 (2010) S. Nascimbène et al., 1 NJP (2010) k/k k/k F S. Nascimbène et al., 1arXiv : normal F phase, to appear PRL 2011 M. Horikoshi et al., Science, 327, 442 (2010)

31 The ENS Fermi Gas Team S. Nascimbène, N. Navon, F. Chevy, K. Gunther, B. Rem, A. Grier, I Ferrier-barbut, U. Eismann, K. Magalhães, K. Jiang A. Ridinger, T. Salez, S. Chaudhuri, D. Wilkowski, F. Chevy, F. Werner, Y. Castin, M. Antezza, D. Fernandes Theory collaborators: D. Petrov, G. Shlyapnikov, D. Papoular, J. Dalibard, R. Combescot, C. Mora C. Lobo, S. Stringari, I. Carusotto, L. Dao, O. Parcollet, C. Kollath, J.S. Bernier, L. De Leo, M. Köhl, A. Georges

32 Gibbs Duhem relation: Density: Useful thermodynamic quantities Entropy density: P S / V T Free energy in canonical ensemble: Isothermal Compressibility: 1/ T 2 F Specific heat: CV T 2 T VdP SdT Nd P n NV, F( T,, V) P V 2 F V 2 V T, N Legendre transform between grand canonical and canonical variables

33 Thermodynamics of a Fermi gas The Equation of State of a uniform Fermi gas can be written as: (, T; a) E TS N P(, T; a) V Pressure contains all the thermodynamic information Variables : scattering length temperature a T chemical potential µ We build the dimensionless parameters : Canonical analogs Interaction parameter Fugacity (inverse)

34 Measuring the EoS of the Homogeneous Gas Local density approximation: gas locally homogeneous at

35 Measuring the EoS of the Homogeneous Gas Extraction of the pressure from in situ images Ho, T.L. & Zhou, Q., Nature Physics, 09 S. K Yip, 07 doubly-integrated density profiles equation of state measured for all values of

36 LDA: Derivation 1 1 ( x, yz, ) m ( x y) m z 2 2 Cylindrical symmetry Gibbs-Duhem: r z dp SdT nd nd Integrating over x and y between 0 and z r r mr P( ) m n( x, y, z)2rdr m P( z, T) n( x, y, z) dxdy n( z) at constant temperature Directly relates the pressure at abscissa z to the doubly integrated absorption signal at z

37 Unitary Fermi Gas a Doubly integrated Density Pressure of the locally homogeneous gas Position / Chemical potential z S. Nascimbène et al., Nature, 463, 1057, (2010)

38 Next lecture: measurement of the Equation of State P(, T )