Part A - Comments on the papers of Burovski et al. Part B - On Superfluid Properties of Asymmetric Dilute Fermi Systems

Transcription

1 Part A - Comments on the papers of Burovski et al. Part B - On Superfluid Properties of Asymmetric Dilute Fermi Systems

2 Part A

3 Comments on papers of E. Burovski,, N. Prokof ev ev,, B. Svistunov and M. Troyer - Phys. Rev. Lett. 96,, (2006) - cond-mat/ version 2, New Journal of Physics, in e-press, e August (2006) by A. Bulgac,, J.E. Drut and P. Magierski

4 Determinant Diagrammatic Monte Carlo The partition function is expanded in a power series in the interaction It is notoriously known that the pairing gap is a non-analytical function of the interaction strength and that no power expansion of pairing gap exists. It is completely unclear why an expansion of this type should describe correctly the pairing properties of a Fermi gas at unitarity.

5 Extrapolation prescription used by Burovski et al. Argument based on comparing the continuum and lattice T-matrix T at unitarity. However: - T-matrix is governed by the scattering length, which is infinite at unitarity - many Fermion system is governed by Fermi wave length, which is finite at unitarity - large error bars and clear non-linear dependence

6 Burovski et al. Energy at critical temperature Notice the strong size dependence! Burovski et al. Bulgac et al.

7 Single particle kinetic energy and occupation probabilities - We have found that in order to have a reasonable accuracy the highest momentum states should have an occupation probability of less than 0.01! -Notice the large difference, and the spread of values, between the kinetic energy of the free particle and the kinetic energy in the Hubbard model,, even at half the maximum momentum (one quarter of the maximum energy) Occupation probabilities are from our results, were we treat the kinetic energy exactly. Burovski et al. have not considered them this quantity. Since both groups have similar s filling factors, we expect large deviations from continuum limit.

8 Finite size scaling Condensate fraction Burovski et al. T c = 0.157(7) ε Bulgac These authors however never displayed the order parameter as function of T and we have to assume that the phase transition really exists in their simulation. F T Bulgac et al. (new, preliminary results) c 0.23(3) ε Value consistent with behavior of other thermodynamic quantities F

9 Preliminary new data! Finite size scaling consistent with our previously determined value T c 0.23 ε F Burovski et al.? However, see next slide

10 Two-body correlation function, condensate fraction System dependent scale Here the Fermi wavelength L/2 Green symbols, T=0 results of Astrakharchick et al, PRL 95, (2005) Power law critical scaling expected between the Fermi wavelength and L/2!!! Clearly L is in all cases too small!

11 The energy of a Fermi gas at unitarity in a trap at the critical temperature is determined experimentally, even though T is not. ET ( c ) E(0) 0.8 Kinast et al, Science 307, 1296(2005) 1 = 0.2 Burovski et al. cond-mat/ Bulgac et al. PRL 96, (2006) estimated Kinast et al. Science, 307,, 1296 (2005)

12 Our conclusions based on our results Below the transition temperature the system behaves as a free condensed Bose gas (!), which is superfluid at the same time! No thermodynamic hint of Fermionic degrees of freedom! Above the critical temperature one observes the thermodynamic behavior of a free Fermi gas! No thermodynamic trace of bosonic degrees of freedom! New type of fermionic superfluid.

13 Part B

14 On Superfluid Properties of Asymmetric Dilute Fermi Systems Aurel Bulgac,, Michael McNeil Forbes and Achim Schwenk Department of Physics, University of Washington

15 Outline: Induced p-wave superfluidity in asymmetric Fermi gases Bulgac,, Forbes, and Schwenk,, cond-mat/ , Phys. Rev. Lett. 97,, (2006) T=0 thermodynamics in asymmetric Fermi gases at unitarity Bulgac and Forbes, cond-mat/

16 Green spin up Yellow spin down If µ µ < the same solution as for µ = µ 2 LOFF (1964) solution Pairing gap becomes a spatially varying function Translational invariance broken Muether and Sedrakian (2002) Translational invariant solution Rotational invariance broken

17 Son and Stephanov,, cond-mat/ Pao,, Wu, and Yip, PR B 73, (2006) Parish, Marchetti, Lamacraft,, Simons cond-mat/ Sheeny and Radzihovsky,, PRL 96,, (2006)

18 Sedrakian, Mur-Petit, Polls, Muether Phys. Rev. A 72, (2005)

19 What we predict? Induced p-wave superfluidity in asymmetric Fermi gases Two new superfluid phases where before they were not expected Bulgac,, Forbes, Schwenk One Bose superfluid coexisting with one P-wave P Fermi superfluid Two coexisting P-wave P Fermi superfluids

20 BEC regime all minority (spin-down) fermions form dimers and the dimers organize themselves in a Bose superfluid the leftover/un-paired majority (spin-up) fermions will form a Fermi sea the leftover spin-up fermions and the dimers coexist and, similarly to the electrons in a solid, the leftover spin-up fermions will experience an attraction due to exchange of Bogoliubov phonons of the Bose superfluid

21 p-wave gap Bulgac, Bedaque,, Fonseca, cond-mat/ n n b f ε exp 0.44, if ka 1 n f nb p F F 2 2 6π n nf 2 b k F p εf exp, if ka, 2 2 F x α n fb f nb mc b 2 = ( ka F ) ln ( x ) p max!!! 5.6 nf εf exp, if 0.44ka F 1 ka F n!!! b

22 BCS regime: The same mechanism works for the minority/spin-down component

23 = 2 1 π p εf exp εf exp NU F p 2 k F 4kkaL F F p k F z 2 2 z z z + 2 Lp() z = ln1 z ln z 30z 1 + z 15z π 0.11( 2ka F ) 2 p εf exp 2, for kf 0.77 kf and fixed kf max p ε F 2 3 F exp π k 2 k k F F 22 ( ka F ) ln k F for k F k F p ε F 18π k exp 2 F 2 kf ( 2ka F )

24 T=0 thermodynamics in asymmetric Fermi gases at unitarity What we think is going on: At unitarity the equation of state of a two-component fermion system is subject to rather tight theoretical constraints, which lead to well defined predictions for the spatial density profiles in traps and the grand canonical phase diagram is: In the grand canonical ensemble there are only two dimensionfull quantities

25 x We use both micro-canonical canonical and grand canonical ensembles nb µ b = 1, y = 1 n µ a a 3 5/3 E( na, nb) = α [ nag( x) ] P h y n n E n n E n n 5 3 5/2 ( µ, µ ) = β [ µ ( )] = µ + µ (, ) = (, ) a b a a a b b a b a b y x g'( x) 1 =, h( y) = gx ( ) xg'( x) gx ( ) xg'( x) h'( y) 1 =, g( x) = hy () yh'() y hy () yh'() y The functions g(x) ) and h(y) ) determine fully the thermodynamic properties and only a few details are relevant

26 Both g(x) ) and h(y) ) are convex functions of their argument. Non-trivial regions exist! Bounds given by GFMC 1 if y y0 hy () = 1 + y if y y 3/5 1,1 (2 ξ) h"( y) c 1 [ ] 3/5 1, c (2 ξ) 1 y Y < y < Y y = g(0) = 1, g( x) = (2 ξ) 3/5 g"( x) 0 g(1) g(1) g'(0) Y0 and g'(1), 1 1+ Y1 2 Bounds from the energy required to add a single spin-down particle to a fully polarized Fermi sea of spin-up particles

27 Now put the system in a trap µ ( r) = λ V( r), y( r) = ab, ab, 2µ = λ λ a b µ b() r µ () r a n() r = β µ ()(()) rhyr hyr (()) yrh () '(()) yr a 3/2 [ ] [ ] a = β [ µ a 3/2 ] n () r ()(()) r h y r h'(()) y r b

28 blue - green - red - P = 0 region 0 < P < 1 region P= 1 region

29 Column densities (experiment) Normal Superfluid Zweirlein et al. cond-mat/

30 y 1 R R γ = = y R R vac vac 0.70(5) Experimental data from Zwierlein et al. cond-mat/

31 Main conclusions: At T=0 a two component fermion system is always superfluid, irrespective respective of the imbalance and a number of unusual phases should exists. At T=0 and unitarity an asymmetric Fermi gas has non-trivial partially polarized phases