DYNAMICS OF TRAPPED BOSE AND FERMI GASES

1. IHP Paris, June 2007 DYNAMICS OF TRAPPED BOSE AND FERMI GASES Sandro Stringari Lecture 2 University of Trento CNR-INFM

2. Lecture 1:- Hydrodynamic theory of superfluid gases - expansion - collective oscillations and equation of state - collective vs s.p. excitations :Landau critical velocity - Fermi superfluids with unequal masses - example of non HD behavior: normal phase of spin polarized Fermi gas Lecture 2:- Dynamics in rotating superfluid gases - Scissors mode - Expansion of rotating gas - vortex lattices and rotational hydrodynamics - Tkachenko oscillations

3. Hydrodynamics and superfluidity • Is the measurement of anisotropic expansion and • collective frequencies a proof of superfluidity? • These measurement probe validity of hydrodynamic theory • and predictions for equation of state • More direct proofs of superfluidity concern • - absence of viscosity (Landau critical velocity) • - rotations (response to transverse field) This talk

4. Rotating superfluids behave very differently from classical fluids - In classical fluid, due to viscosity, the velocity field of steady rotation is given by the rigid value and is characterized by uniform vorticity - Moment of inertia takes rigid value - Superfluids are characterized by irrotationality constraint for velocity field. Vorticity is hence vanishing ( ) except along lines of singularity (quantized vorticies) - Without vortices moment of inertia takes irrotational value where is deformation of the cloud

5. Moment of inertia Direct measurement of moment of inertia is difficult because images of atomic cloud probe density distribution (not velocity distribution) In deformed traps rotation is however coupled to density oscillations. Exact relation, holding also in the presence of 2-body forces: angular momentum quadrupole operator Response to transverse probe measurable thorugh densityresponse function !! In principle measurement of quadrupole excitations and strengths in deformed trap permits to determine moment of inertia !! Exact sum rules

6. Scissors mode Example of coupling between angular momentum and quadrupole excitations is provided by scissor mode. If confining (deformed) trap is suddenly rotated by angle the gas is no longer in equilibrium. Behaviour of resulting oscillation depends on value of moment of inertia (irrotational vs rigid)

7. Scissors frequencies (Guery-Odelin and Stringari, 1999) Superfluid (T=0) With the irrotational ansatz one finds exact solution of HD equations for the scissor mode. If trap is deformed ( ) the solution corresponds to rotation of the gas around the principal axis in x,y plane Result is independent of equation of state (surface mode) Collisionless normal gas (above ). Gas is dilute and interactions can be ignored (collisionless regime). Excitations are provided by ideal gas Hamiltonian. Two frequencies: Differently from superfluid the normal gas exhibits low frequency mode (crucial to ensure rigid value of moment of inertia)

8. Scissors measured at Oxford (Marago’et al, PRL 84, 2056 (2000)) Above (normal) 2 modes: Below (superfluid) : single mode:

9. oscillation-frequency Scissors in Fermi gas, hydrodynamic regime(Innsbruck 2007, to be published) below the resonance 1/kfa = 0.65 on resonance 1/kfa = 0

10. oscillation-frequencies Scissors in Fermi gas, collisionless(Innsbruck 2007, to be published) Colissionless Superfluidity is very fragile In BCS regime. Gap parameter is exponentially small above the resonance 1/kfa < -0.6

11. Scissors in superfluid Fermi gas • Is the measurement of the scissors mode at hydrodynamic • frequency a proof of superfluidity ? • - Scissors are characterized by irrotational flow • If normal gas is deeply in collisional regime dynamics is • governed by same HD equations as in the superfluid • (viscosity effects are too weak to generate rotational • components in the velocity field). • - Innsbruck experiment shows that, near resonance, HD • scissors mode persists also above Tc.

12. (From Rudy Grimm) Universal HD behavior Investigation of scissors mode, near unitarity, cannot distinguish between superfluid and normal phase (unless one considers steady rotating configurations)

13. - In order to exploit superfluidity one should study collective oscillations in the presence of rotating trap ! - In this case frequency of the scissors mode in the superfluid and normal phases behaves differently even if normal gas is in collisional regime.

14. Steady rotation of the trap: irrotational vs rotational flow In the presence of deformedrotating trap the stationary velocity field behaves differently depending on whether the system is superfluid or normal. In a superfluid the velocity velocity field is subject to constraint of irrotationality : Normal gas, in steady configuration, instead rotates in rigid way What happens to the cloud if we suddenly stop the rotation of the trap? System will be no longer in equilibrium and will start oscillate (scissors mode). If the gas is normal and collisional, the scissors mode will be described by the equations of rotationalhydrodynamics absent in superfluid hydrodynamics !!

15. Role of viscosity in rotating normal gas (two different time scales) • Viscosity is crucial to bring the system into • steady rotational flow: • spin up time (long time scale), • fixed by trap anisotropy and • collisional cross section, • Viscosity in the deep HD regime can be ignored • for describing the dynamics of the scissors • mode (short time scale) : (Guery-Odelin, 2000)

16. Scissors mode (rotation angle after stopping trap) (Cozzini and Stringari, 2003) Superfluid (T=0) Normal (collisional) beating between Normal (collisionless) beating between

17. (From Rudy Grimm) Possibility of distinguishing between irrotational (superfluid) hydrodynamics and collisional (classical) hydrodynamics is unique feature of Fermi gas near unitarity

18. Crucial condition to observe differences in the scissors mode between a rotating superfluid and a rotating collisional gas If angular velocity is too small (or if trap deformation is too large) the dynamics of the scissors is not sensitive to the presence of rotational components in the velocity field

19. Expansion of a rotating superfluid gas : consequences of irrotationality

20. In the absence of rotation the expansion of a cigar trapped gas is faster in the radial direction (consequence of HD forces) After time such that shape of the system becomes spherical (aspect ratio = 1) For longer times the density profile takes a pancake form. What happens if the gas is rotating? At t=0 superfluid gas carries irrotational angular momentum Rotating superfluid cannot appraoch spherical shape during the expansion because the moment of inertia would vanish and angular momentum would not be conserved. The gas starts rotating fast when approaches , but deformation remains finite (aspect ratio ). Angular velocity eventually decreases for larger times

21. Effect of rotation Rigid rotor decreases angular velocity during the expansion Superfluid gas increases angular velocity during the expansion (and never reaches aspect ratio=1) Theory: Edwards et al., 2002 Exp in BEC gas: Hechenblaickner et al, 2002)

22. Expansion of rotating Fermi gas recently measured at Duke (Clancy et al. 2007) Sudden tilt of the trap produces rotation in the gas Sd Aspect ratio Angle of rotation non rotating Increase of angular velocity Lines are predictions of irrotational HD equaions

23. Duke experiment shows same behavior both below and above critical temperature Surprise? • - System in both cases is deeply hydrodynamic • and expansion time is too fast to reveal effects of viscosity. • Expansion in both cases is described by irrotational • HD equations (situation similar to the scissors mode)

24. What happens if the gas, before expansion, is not superfluid and rotates in a rigid way? (collaboration with P. Pedri) • Preliminary answers: • If condition is not satisfied • initial rigid motion is soon damped out • and expansion is practically indistinguishible • from superfluid expansion (Duke scenario) • If condition is satisfield rigid • motion is preserved during the expansion • and visible effects emerge in time dependence • of aspect ratio and rotational angle

25. COLLECTIVE MODES IN THE PRESENCE OF QUANTIZED VORTICES

26. QUANTIZED VORTICES Quantized vortex is configuration characterized by velocity field where azimuth angle is corresponding phase of order parameter M=2m in a Fermi superfluid (m is atomic mass) • Velocity field gives rise to • singular vorticity • - quantization of circulation

27. Differences between vortices in Fermi and Bose superfluids density Reduced visibility • Density does not vanish along vortex • line,differently fromorder parameter • Size of vortex core is smaller • (healing lenght is smallest near unitarity) order parameter Bulgac and Yu, 2003 Chien et al., 2006 Sensarma et al. 2006 Factor 2 • Quantum of circulation and angular momentum per particle • of single vortex line twice smaller than for bosons

28. Vortex lattices (Mit 2001) (Jila 2002) BEC gases Fermi gas (Mit 2005)

29. Vortex density in vortex lattice When the number of vortices is large it is useful to introduce concept of diffused vorticity. Average vorticity per unit area is given by Vortex density density of vortices is uniform (even if atomic density is not uniform) Despite the discretized nature of vortex lines the superfluid, at a macroscopic level, behaves like a fluid rotating with velocity It carries rigid value of angular momentum

30. Centrifugal effect due to vortex lattice accounted for by effective trapping potential Yielding increase in radial size is upper limit for rotating gas at equilibrium in the Presence of harmomic trap (critical angular velocity)

31. Due to stiffer equation of state (quantum pressure) harmonically trapped Fermi gas can host larger number of vortices compared to BEC gas For example at unitarity: Choosing and One finds from eq. of state consequences on Tkachenko oscillations (see later)

32. Rotational hydrodynamics Collective oscillations in non rotating superfluids are described by irrotational hydrodynamics When number of vortices is large one can introduce concept of diffused vorticity and develop formalism of rotational hydrodynamics laboratory frame Includes, in particular, Thomas-Fermi result for stationary density profile with bulge effect

33. Predictions of rotational hydrodynamics: collective oscillations Collective oscillations are obtained by looking for linearized solutions of hydrodynamic equations: Both surface and compression oscillations are affected by the rotation Surface quadrupole modes Choosing one finds analytic solution (Chevy and Stringari, 2003) Degeneracy between is broken by the rotation. - Measured in BEC gase (ENS, Jila) - Useful to ‘measure’ angular velocity

34. Splitting of quadrupole modes in BEC gas Quadrupole frequency measured at JILA (Haljan et al., 2001)

35. Rotational hydrodynamics: compression modes in rotating Fermi superfluids (Antezza et al., 2007) Coupling between radial and axial motion is affected by the vortex lattice. Choosing HD equations provide two m=0 decoupled modes For radial mode in cigar trap configuration one finds • When (BEC gas) • independent of angular velocity • At unitarity frequency • evolves from • at • to as radial mode at unitarity

36. Applicability of rotational hydrodynamics Large number of vortices (or distance between vortices much smaller than radial size) At unitarity condition is satisfied also for small angular velocities Thomas-Fermi regime size of vortices should be much smaller than intervortex distance At unitarity: for rotating harmonic trap ( ) one finds: When condition is violated one enters LLL and QH regimes

37. Elastic modes of the vortex lattice (Tkachenko modes) - In addition to shape and compression oscillations vortex lattices exhibit new oscillations of elastic nature (Tkachenko modes) corresponding to deformation of vortex latticegeometry. - Tkachenko modes cannot be described by hydrodynamic theory. They require inclusion of elastic effects or full microscopic approach (Baym, Sonin, Anglin, Simula et al., Mizushima et al. etc.. ). Theory well agrees with experiments on BEC gases in TF regime deviations from TF sum rule approach Cozzini et al, 2004 Tkachenko waves at Jila (2003)

38. Tkachenko modes in unitary Fermi superfluid (G. Watanabe, M. Cozzini and S.S. in preparation) • Original Tkachenko work (1966) applies to incompressible fluids • Generalization to • compressible fluid • (Baym 1983, Sonin 1987) • Two relevant • regimes • Values of q fixed by radius of the cloud • ( for incompressible Bose fluid in cylindrical geometry, • Anglin and Crescimanno, 2002) • In trapped BEC gases Tkachenko modes measured in • intermediate regimes holds in TF regime highly compressible fluid incompressible fluid

39. In order to achieve incompressible regime one needs • to work at “small” angular velocities. • - Condition easily ensured in superfluid Fermi gases where • can be large due to large quantum pressure effects. • - For example choosing • one finds (at unitarity) • Exact determination of and hence • of lowest Tkachenko frequency • requires imposing proper boundary • conditions and knowledge of equation of state: • Bose (cigar ) • Bose (disc ) • Fermi (cigar ) • - Fermi (disc ) unitary

40. Conclusions and outlook Collective oscillations in traps: - power tool to explore superfluid features in quantum gases. - accurate test of equation of state along BEC-BCS crossover - sensitive to superfluid effect in rotating gases Some open problems: Theory: - transition from superfluid to normal phase at T=0 on BCS side - kinetic phenomena at unitarity (role of viscosity) - Collective oscillations as a function of T (second sound) - collective oscillations in spin polarized superfluids - ……… Exps: - collective oscillations in rotating configurations, in spin polarized gases and in unequal masses Fermi superfluids; - dynamic structure factor in Fermi gases; - critical velocity at unitarity - oollective oscillations in 2D Bose gase below and above Tc - ……..