Super Uidity
Chapter 5SuperfluiditySuperfluidity is closely related to Bose-Einstein condensation. In a phenomenological level,superfluid can flow through narrow capillaries or slits without dissipating energy. Superfluid does not possess the shear viscosity. The superfluid of liquid 4 He, below the so-calledλ-point, was discovered by Kapitza [1] and independently by Allen and Misener [2]. Soonafter Landau explained that if the excitation spectrum satisfies certain criteria, the motionof the fluid does not cause the energy dissipation [3]. These Landau criteria are met by theBogoliubov excitation spectrum associated with the Bose-Einstein condensate consistingof an interacting Bose gas and thus establish the first connection between superfluidity andBEC. The connection between the two phenomena is further established in a deeper levelthrough the relationship between irrotationality of the superfluid and the global phase ofthe BEC order parameter. This is the first subject of this chapter. The second subject ofthis chapter is the rotational properties of the irrotational superfluid, with special focuson the quantized vortices. We will conclude this chapter with the concept of superfluidityand BEC in a uniform 2D system, known as the Berezinskii-Kousterlitz-Thouless phasetransition.5.1Landau’s criteria of superfluidityLandau’s theory of superfluids is based on the Galilean transformation of energy andmomentum. Let E and P be the energy and momentum of the fluid in a reference frameK. If we try to express the energy and momentum of the same fluid but in a movingframe K 0 , which has a relative velocity V with respect to a reference frame K, we havethe following relations:P0 P M V,(5.1)E0 P0 22M1 P M V 22M1 E P · V M V 2 ,2 0 2(5.2) where E P2M and M is the total mass of the fluid.We first consider a fluid at zero temperature, in which all particles are in the groundstate and flowing along a capillary at constant velocity v. If the fluid is viscous, the1
motion will produce dissipation of energy via friction with the capillary wall and decreaseof the kinetic energy. We assume that such dissipative processes take place through thecreation of elementary excitation, which is the Bogoliubov quasi-particle for the case ofan interacting Bose gas. Let us first describe this process in the reference frame K which,rather confusingly, moves with the same velocity v of the fluid. In this reference frame,the fluid is at rest. If a single elementary excitation with a momentum p appears in thefluid, the total energy of the fluid in the reference frame K is E0 ε(p), where E0 andε(p) are the ground state energy and the elementary excitation energy. Let us move tothe moving frame K 0 , in which the fluid moves with a velocity v but the capillary is atrest. In this moving frame K 0 which moves with the velocity v with respect to the fluid,the energy and momentum of the fluid are given, setting V v in (5.2) and (5.1), byP0 p M v.(5.3)1E 0 E0 ε(p) p · v M v 2 ,(5.4)2The above results indicate that the changes in energy and momentum caused by theappearance of one elementary excitation are ε(p) p · v and p, respectively.Spontaneous creation of elementary excitations, i.e. energy dissipation, can occur ifand only if such a process is energetically favorable. This means if the energy of anelementary excitation, in the moving frame K 0 where the capillary is at rest, so that athermal equilibrium condition is satisfied, is negative:ε(p) p · v 0,(5.5)the dissipation of energy occurs. The above condition is satisfied when v ε(p) p andp · v 0, i.e. when the elementary excitation has the momentum p opposite to the fluidvelocity v and the fluid velocity v exceeds the critical value,vc minpε(p), p (5.6)where the minimum is calculated over all the values of p. If instead the fluid velocity vis smaller than (5.6), then no elementary excitation will be spontaneously formed. Thus,the Landau’s criteria of superfluidity is summarized as the relative velocity between thefluid and the capillary is smaller than the critical value, v vc .By looking at the Bogoliubov excitation spectrum in the previous chapter 4, one caneasily conclude that the weakly interacting Bose gas at zero temperature satisfies theLandau’s criteria of superfluidity and that the critical velocity is given by the soundvelocity as shown in Fig. 5.1(a). Strongly interacting fluids such as liquid 4 He also fulfilthe Landau criteria but in this case the critical velocity is smaller than the sound velocitydue to the complicated excitation spectrum, as shown in Fig. 5.1(b). It is easily understoodthat the critical velocity decreases with the decrease in the particle-particle interaction andp2disappears in the limit of an ideal gas because vc minp ε(p) p 0 for ε(p) 2m . Theparticle-particle interaction is a crucial requirement in the appearance of superfluidity.2
(a)(b)critical velocityphononmaxonrotoncriticalvelocityFigure 5.1: (a) The excitation spectrum of a weakly interacting Bose gas, for which thecritical velocity is equal to the sound velocity, vc c. (b) The excitation spectrum of astrongly interacting Bose liquid, for which the critical velocity is smaller than the soundvelocity, vc c.5.2Superfluidity at finite temperaturesLet us next consider a uniform Bose-Einstein condensed fluid at a finite temperature.We assume the thermodynamic properties of the system are described by the Bogoliubovquasi-particles in thermal equilibrium distributions. According to the above argument,no new excitations can be created directly by the condensate due to the motion of thesuperfluid with respect to the capillary. However, the quasi-particles are excited thermallyand the fluid associated with the quasi-particles is not superfluid but normal fluid. Theseelementary excitations can collide with the capillary walls and dissipate their energies andmomenta. Thus, we have the two fluid components at a finite temperature: a superfluidwithout viscosity and a normal fluid with viscosity. Collisions establish thermodynamicequilibrium in the normal fluid in the frame where the capillary is at rest (capillary frame).If the energy and momentum of the quasi-particle are ε(p) and p in the frame wherethe superfluid is at rest (superfluid frame), the energy of the same quasi-particle in thecapillary frame becomes ε(p) p·vs , where vs is the relative velocity of the superfluid andthe capillary. The Bogoliubov quasi-particles obey the thermal equilibrium distributionin the capillary frame (not in the superfluid frame). Thus, the quasi-particle populationis given by1hi.(5.7)Np ε(p) p·vsexp 1kB TIf ε(p) p · vs 0, i.e. vs minp ε(p) p , the quasi-particle population Np is positivefor all values of p. Therefore, we can conclude the coexistence of the two fluids becomespossible. Notice that the condition for the positive Np , vs min ε(p) p , is identical to theLandau’s criteria of superfluidity. Figure 5.2 shows the quasi-equilibrium population Npfor a positive superfluid velocity vs 0. In conclusion, co-existence of the superfluid andnormal fluid is possible at finite temperatures if the Landau’s criteria (5.6) is satisfied.3
suppressed populationenhanced population0condensate at restFigure 5.2: The quasi-particle population Np in the superfluid frame. In this frame, thecondensate (superfluid) is at rest (p 0), while the quasi-particle population becomesasymmetric, more population in a negative p region and less population in a positive pregion due to the friction with the capillary wall which moves with a velocity vs withrespect to the superfluid.5.3Superfluid velocity and phase of order parameterWe have established one important connection between BEC and superfluidity, which isthe relationship between the Landau’s criteria for the critical velocity for superfluidityand the sound velocity determined by the Bogoliubov excitation spectrum. In this sectionwe will develop one more important connection between BEC and superfluidity, whichaddresses the relationship of the superfluid velocity vs and the phase S of BEC orderparameter.The presence of a large number of particles in a ground state permits the introduction of the c number order parameter ψ0 (r, t) as discussed in Chapter 1. The quantummechanical field operator ψ̂(r, t) satisfies the Heisenberg equation of motion· 2 2hid Vext (r, t)i ψ̂(r, t) ψ̂(r, t), Ĥ dt2m Z 0000 ψ̂ (r , t)V (r r)ψ̂(r , t)dr ψ̂(r, t).(5.8)We replace ψ̂(r, t) with ψ0 (r, t) and use the effective soft potential Veff for V (r0 r) (seeChapter4). The c number order parameter ψ0 (r, t) varies slowly over the inter-particleinteraction range, and so we can substitute r0 for r in (5.8) and finally obtain· 2 2 d 2i ψ0 (r, t) Vext (r, t) g ψ0 (r, t) ψ0 (r, t).(5.9)dt2mThis is the famous nonlinearR Schrödinger equation, which is also referred to as GrossPitaevskii equation, and g Veff (r)dr [4, 5]. Equation (5.9) plays a crucial role in bothnonlinear optics and BEC physics. In the field of nonlinear optics, a photon (Maxwellfield) acquires an effective mass via material’s dispersion and interacts with each other4
via optical Kerr effect [6]. For example, optical solitons and modulational instability innonlinear optical media are properly described by (5.9). Thus, it is not hard to anticipatethe physics of BEC has many common features with the physics of nonlinear optics [6].In the case of BEC, the local density of particles is related to the squared order parameter byn(r, t) ψ0 (r, t) 2 ,(5.10)Rand thus the total number of particles is equal to N ψ0 (r) 2 dr. Here we assume thenegligible quantum and thermal depletion, which is used already to derive (5.9). If wemultiply (5.9) by ψ0 (r, t) and subtract the complex conjugate of the resulting equation,we obtain the following continuity equation under superfluidity:dn(r, t) div [j(r, t)] 0dt(5.11)where the particle current density isj(r, t) Here the Laplacian 2 U 2U x2 i (ψ ψ0 ψ0 ψ0 ) .2m 0 2U y 2(5.12) Ay Az 2Ux, the divergent · A A x y z z 2 U z ez are defined in this way. From (5.11) it can Uand the gradient U U x ex y ey be concluded that the RGross-Pitaevskii equation guarantees the conservation of the totalparticle number N n(r)dr. If we express the c number order parameter ψ0 (r, t) interms of its amplitude and phase,p(5.13)ψ0 (r, t) n(r, t)eiS(r,t) , S(r, t). This resultthe particle current density (5.12) is rewritten as j(r, t) n(r, t) mmeans that the superfluid velocity vs of the condensate particle is related to the gradientof the phase S of order parameter:vs (r, t) S(r, t).m(5.14)The phase of the order parameter plays the role of a velocity potential and vs is referredto as a velocity field.Inserting (5.13) into (5.9), one can derive on explicit equation for the phase S(r, t);µ¶d1 222 n 0. S mv Vext gn (5.15)dt2 s2m nThe particle continuity equation (5.11) and the phase equation (5.15) provide a closedset of coupled equations, which is fully equivalent to the Gross-Pitaevskii equation (5.8).The term containing the gradient of the particle density in (5.15) implements the Heisenberg uncertainty relationship between particle number and phase, and is called ”quantumpressure”.The stationary solution of (5.9) has the form ofψ0 (r, t) ψ0 (r) exp ( iµt) ,5(5.16)
where µ E Nis the chemical potential and the total energy of the system E is¶Z µ 2 g224E ψ0 Vext (r) ψ0 ψ0 (r) dr.2m2(5.17)Using (5.16) in (5.9), one obtains the time independent Gross-Pitaevskii equation¶µ 2 22 Vext (r) µ g ψ0 (r) ψ0 (r) 0.(5.18) 2mThe solution of (5.18) with the lowest energy defines the order parameter of the groundstate, which is a real function. The excited states are, however, complex functions, thequantized vortex state being the most famous example of such excited states.For a uniform gas, in the absence of the external potential Vext (r) 0, 2 ψ0 0 and(5.17) is reduced to E 21 gn2 V and the chemical potential is equal to µ gn. Theseresults are identical to those of the Bogoliubov theory, developed in Chapter 4 in the caseof the weakly interacting uniform Bose gas.5.4Quantized vortices in superfluidsThe story of quantized vortices provides an important insight into the problem of rotationsin superfluids. Quantized vortices were first predicted by Onsager [7] and Feynman [8]. Infact, it is well known that a superfluid cannot rotate. In usual rigid systems, the tangentialvelocity corresponding to a rotation is given by v Ω r, where Ω is the angular velocityvector and r is the distance vector from the origin of rotation. Such a system has a diffusedvorticity curl(v) (Ω r) 2Ω 6 0. On the other hand, the superfluid velocity givenby (5.14) satisfies (5.19)curl(vs ) S.mwhich means a superfluid turns out to be irrotational and is thus expected to rotate in acompletely differentway from a rigid³ rotator. Here³ the rotation is defined by A ¡ A Ay Ay Az Az Axxex z x ey x y ez y zLet us consider a superfluid confined in a macroscopic cylinder of radius R and lengthL (Fig. 5.3). The solution of the Gross-Pitaevskii equation for a rotation around thez axis of the cylinder isψ0 (r) eisϕ ψ0 (r) ,(5.20)pwhere we have used the cylindrical coordinates (r, ϕ and z) and ψ0 (r) n(r). Due tothe symmetry of the problem, the modulus of the order parameter ψ0 (r) depends onlyon the radial variable r. The parameter s should be an integer to ensure that the orderparameter ψ0 (r) is single valued. From (5.14) and (5.20), the tangential velocity isvs 1 s S S .mm r ϕmr(5.21)This result is completely different from the tangential velocity v Ω r of the rigidrotator whose modulus increases with r (Fig. 5.4). At large distances from the z axis,the tangential velocity vs of the superfluid approaches to zero and the irrotationality of6
contourFigure 5.3: A cylindrical vessel containing a superfluid. The circulation of the velocity vsis quantized in units of h/m and is independent of the radius of the contour.superfluids (5.19) is satisfied. The circulation of the tangential velocity over a closedcontour around the z axis is given byI (5.22)vs dl 2πs ,mwhich is quantized in units of h/m, independent of the radius of the contour. This is called”Onsager-Feynman quantization condition”.Substituting (5.20) into (5.18), one obtains the following equation for the modulus ofthe order parameter:µ¶ 2 1 dd 2 s2 r ψ0 ψ0 g ψ0 3 µ ψ0 0.(5.23)2m r drdr2mr2We assume the solution for the above equation has the form ψ0 nf (η),(5.24) where η r/ξ and ξ / 2mgn is the healing length introduced in the previous chapter.The real function f (η) then satisfies the equationµ¶ µ¶1 ddfs2η 1 2 f f 3 0,(5.25)η dηdηηwith the constraint f ( ) 1. This is because the density must approach its unperturbed value n and hence ψ0 n where the superfluid velocity vs approaches to zero at η .In Fig. 5.5 the function f (η) is shown for the values of s 1 and s 2. As η 0, the7
543210013245Figure 5.4: Tangential velocity field for irrotational (virr ) and rigid rotational (vrig )flow.The irrotational velocity field diverges like 1/r as r 0. Here r and v are measured inarbitrary units (a.u.)function f (η) tends to zero as f η s , so that the superfluid density n(r) ψ0 (r) 2 tendsto zero on the axis of the vortex. The perturbation of the density exists in a spatial regionof the healing length ξ from the vortex line (z axis). Of course, the ground state solutioncorresponding to s 0 has the uniform density f (η) 1.10.80.60.40.200132456Figure 5.5: Vortical solutions (s 1, solid line; s 2, dashed line) of the Gross-Pitaevskiiequation as a function of the radial coordinate η r/ξ. The density of the gas is givenby n(r) nf 2 , where n is the density of the uniform gas.The energy of the quantized vortex Ev is calculated as E(s 6 0) E(s 0), wherethe total energy E of the superfluid is given by (5.17), where the order parameter is givenby (5.20). In terms of the dimensionless function f (η), one obtains"µ ¶#Z 2Lπ 2 n R/ξdf 2 s2 2 1 ¡ 2Ev E(s 6 0) E(s 0) 2f f 1ηdη. (5.26)mdηη208
For the s 6 0 vortex, the excitation energy is 2 s2Ev LπnlnmµRrc¶,(5.27)where rc is the size of the vortex core determined by (5.25). For s 1, rc is equal toξ/1.46. The main contribution to Ev comes from the kinetic energy (the first term ofthe integrand in (5.17). The energy Ev increases with the size of the sample as L lnR. However, the relative energy Ev /E(s 0) with respect to the ground state energy isvanishingly small in a large sample, since E(s 0) increases as V LπR2 .The above result has been derived in the laboratory (rest) frame. If a cylinder isrotating with angular velocity Ω around z axis, we must evaluate the energy in a rotatingframe, in which a cylinder is at rest and thus thermal equilibrium is expected to take place,using the Galilean transformations, the excess energy in the rotating frame is written asEv0 Ev Ω·Lz ,(5.28)where Lz is the angular momentum of the superfluid in the laboratory frame. The orderparameter (5.20) is an eigenstate of the angular momentum with an eigenvalue of lz s per particle, so that the vortex solution (s 6 0) carries a total angular momentum equalto Lz N s , where N nπR2 L is the total number of particles, while the ground state(s 0) carries no total angular momentum Lz 0. It is then easy to see that in therotating frame the vortex solution with Ω · Lz 0 becomes energetically more favorablecompared to the ground state with Lz 0 if the angular velocity Ω is large enough. Onthe other hand, at a low enough angular velocity, the superfluid part remains at rest whilethe normal component is brought into rotation. The critical angular velocity Ωc for thevortex solution with s 1 to be preferred is given byµ ¶ (2s 1)REv .(5.29)lnΩc 2LzmRrcWith the increase in the angular velocity Ω, the higher-order vortex solution with s 2acquires also a lower energy than the ground state with s 0. Since a superfluid cannotrotate in a rigid way, the rotation will eventually be realized through the creation ofquantized vortices.Notice that the dependence of (5.27) on rc is logarithmic and hence the energy ofthe vortex depends very weakly on the actual value of the core size. Since the angularmomentum Lz is proportional to s and the energy Ev is proportional to s2 , the vorticeswith s 2 are energetically unstable. At a very large angular velocity, the state withmultiple s 1 vortices is preferred to the state with single s 2 vortex. Since the numberof s 1 vortices created in this system, Nv , should be equal to s in (5.29), the vortexdensity per unit area in a very large angular velocity is given bynv Nvmm Ω Ω,2πRπ h(5.30)where ln (R/rc ) ' 1 in the limit of large s is used.It is worth stressing that vortices can exist as a stationary configuration only in asuperfluid. In the presence of a small viscosity, the vortex will diffuse from the z axis9
and eventually spreads over the whole volume of the cylinder, which is indistinguishablefrom a rigid rotator.Quantized vortices in atomic BEC have been experimentally observed by use of asuitable rotating modulation of the trap to stir the condensate [9, 10]. Above the criticalangular velocity (5.29) one observes the formation of the vortex, as shown in Fig. 5.6.At sufficiently high angular velocities, array of more vortices are formed in a triangularlattice, which is similar to those is superconductors [11].(a)(b)Figure 5.6: The observed array of quantized vortices in the ENS experiment (a) and inthe MIT experiment (b).5.5Berezinskii-Kosterlitz-Thouless (BKT) phase transitionand quantized vortex-pairsIn uniform two-dimensional systems, conventional off-diagonal long range order is destroyed
Super uidity is closely related to Bose-Einstein condensation. In a phenomenological level, super uid can ow through narrow capillaries or slits without dissipating energy. Super- uid does not possess the shear viscosity. The super uid of liquid 4He, below the so-called
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