The Structure of Polarized Vortices in the Unitary Fermi Gas

Project for Ph.D thesis

Fermionic superfluids do not generally support polarization, and the nature of the ground state of a slightly polarized unitary Fermi gas remains an open question. However, vortices naturally support polarization since the pairing gap vanishes in the core of superfluid vortices. The structure of a polarized vortex is not well understood and may have some interesting properties. To study the microscopic structure of a vortex, we use a density functional theory called the asymmetric superfluid local density approximation (ASLDA) to simulate how vortexes interact, evolve and how energy transfers between paired and unpaired particles. In this talk, I will discuss the structure and properties of polarized vortices using the ASLDA, and how these related to polarized phases through the Thomas-Fermi (TF) approximation.


Some Note from Dr. Forbes

We start with the issue of regularization. In our first version we use an extremely simple approach: we simply use all states in the box, and adjust the fixed coupling constant gc so as to reproduce a particular value of the gap Δ. As a baseline, we use the following known 2D solution.

n+=kF22π,ϵF=2kF22m,EFG=kF48mπ=12n+ϵF,C~=0,μϵF=12,ΔϵF=2

The T=0 equations in d dimensions can be expressed in terms of the gap Δ and effective chemical potential μeff=μ0gcn+/2 where gc<0 represents an attractive interaction:

Δ=gcνc,ϵ+=2k22mμeff,E=EV=2τc2m+gc(nanb+νcνc)=2τc2m+Δνc(μeffμ0)n+2,

through the following integrals:

νc=cddk(2π)dΔ2ϵ+2+Δ2cddk(2π)dmΔ2k2, τc=cddk(2π)dk2[1ϵ+ϵ+2+Δ2]cddk(2π)d2m2Δ24k2, n+=cddk(2π)d[1ϵ+ϵ+2+Δ2]cddk(2π)d2m2Δ24k4.

In 1D, these converge, but in 2D and higher, the quantities τc and νc diverge as ln(kc) in 2D and as kc in 3D. The density n+ converges in both 2D and 3D, although slower in 3D. Note that the divergence in τc and νc cancel in the combination

(1)κ=cddk(2π)d(2τc2m+Δνc)cddk(2π)d2m2Δ2μeff4k4,

which enters the energy density E.

To regularize the theory, we must define some finite physical quantities that we can hold fixed as we take the cutoff to infinity so as to defined the parameters of the theory in a way that the physics becomes independent of the cutoff in the large cutoff limit. In 3D we express this as follows (see Bulgac:2011 Eq. (84) but we consider only unit effective mass α+=1):

(2)C~(na,nb)=νcΔ1gc+12cddk(2π)d1ϵ++i0+Λc=1gc+Λccddk(2π)d2m3Δ26k6.

where the integral is take in the principal value sense. This is motivated by the familiar Eagles-Leggett model, where this quantity is related to the two-body scattering length as:

C~(na,nb)=m4π2as.

Strictly speaking, the denominator in the integral should have ϵ+=2k2/2m without the chemical potential because the calculation of the scattering length should take place in the vacuum. To improve convergence, however, we include the chemical potential in ϵ+=2k2/2mμ as advocated in BY:2002fk. This does not change the 3D result in the limit kc but shifts the finite portion to improve convergence. Without this shift, the integral would behave as C~cddk2m2μeff/(2π)d4k4.

The principal value integrals can now be performed analytically:

Λc1D=m212πk0lnkck0kc+k0m21πkc0, Λc2D=m214πln(kc2k021)m212πlnkck0, Λc3D=m2kc2π2(1k02kclnkc+k0kck0)m2kc2π2,

where

2k022mμ=0,2kc22mμ=Ec.

The latter equation allows us to replace the momentum cutoff kc in terms of an energy cutoff Ec which is of use in inhomogeneous systems.

Comments:

As we take the cutoff kc we notice the following:

  • In 1D, Λc0:
    • This indicates that C~=νc/Δ=1/gc is finite, and the theory is well defined in this limit with finite anomalous density ν, and finite cutoff g=v0. In this case, no cutoff is needed, but one might still like to use the cutoff procedure to improve the convergence. In particular, when working on a lattice, there is a maximum momentum that can be represented kmax which will limit the convergence roughly according to the behavior discussed above. For example, ν1/kmax will converge slowly, requiring large lattices to approach the physical limit. Using the cutoff procedure and Thomas-Fermi completion discussed below will speed this convergence.
    • Since g is finite, one must also include the Hartree corrections to the chemical potentials:

      μa,beff=μa,b+gnb,a.

      This complicates things a bit because now the effective potentials depend on the density. One can still work with fixed μ, but this means that the self-consistent solution is the solution in a funky background potential which has been chosen to absorb these corrections.

  • In 2D and 3D, Λc divergence. This means that if we fix C~ to be finite, then gc0. In this case, the Haretree corrections vanish and the effective chemical potentials do not depend on the state:

    μeff=μ=μ0V(x).

    This allows us to easily work in the grand-canonical ensemble.

  • In 3D, Λc is independent of k0 in the limit of large cutoff. This means that the including the pole, which improves the convergence, does not affect the usual identification of the scattering length.

  • In 2D, however, there remains a dependence on k0 in the large cutoff limit. This means that our procedure for specifying the functional through C~ depends on k0. This is not ideal for a density function because it means that the physical interpretation of the results depend on the external potential - i.e. the DFT is not independent of the potential as it should be. The previous point that Hartree terms vanish helps a bit since k0 then depends only on the external trap and bare chemical potentials. (Otherwise, the DFT would be state-dependent which would be even worse!)

    The results of this simple approach must be interpreted instead as the results of a DFT fixing the following combination:

    C~m2π2lnk0

    which becomes independent of kc in the limit of large cutoff. Note that this diverges at the classical turning points where k0=0. I am not sure if this poses any issues, and perhaps is expected and related to some of the divergent behaviour in 2D where results depend on the boundary conditions quite strongly. (One should include a dimensionless factor in the log to render k0/k~ dimensionless, but this will just be absorbed into the constant value.) In any case, this simple regularization procedure should be fine for qualitative and exploratory work keeping in mind that the results might depart strongly from “physical” 2D results near the turning points.

    Example: 2D

    As an example, consider homogeneous states in 2D. Suppose one knows Δ and μ+, how should C~ be determined? In principle, one can simply compute:

C~(na,nb)=limkc(νcΔ+120kcdk2πkϵ++i0+)=120dk2π(kϵ+2+Δ2+kϵ++i0+).

However, numerically computing this integral can be challenging because of the pole. We can proceed as we would were we performing the computation in a discrete basis.

Thomas Fermi Completion

To facilitate convergence in the case when the solutions vary slowly, we can include states with E>Ec by assuming that on length-scales smaller than 1/kc the solution is homogeneous and performing the integrals. This can be done exactly (numerically completing the integrals) or as a series approximation in 1/kc.

As an example, consider the computation of the gap equation. We will illustrate this with symmetric homogeneous matter in 2D. As discussed above, this presents some difficulties due to the divergent behaviour of Λc. In a typical calculation, this would proceed as follows:

  1. First compute the anomalous density νc by diagonalizing the Hamiltonian and include only states with E<Ec = 2kc2/2m. For 2D homogeneous matter this would be

    νc=0kckdk2πΔ2ϵ+2+Δ2.
  2. We would then use the regulated equation for C~ to compute the coupling constant:

    gc=1C~Λc.
  3. Finally, we would use the gap equation to find the self-consistency condition Δ=gcνc.

Putting these together, we have:

C~(na,nb)=νcΔ+Λc+O(kc4)=0kckdk2πΔ2ϵ+2+Δ2+120kckdk2π1ϵ++i0++O(kc4).

This is missing the contributions from the states above kc which would be corrected by including the full integrals:

C~(na,nb)=νcΔ+Λc+12kckdk2π(1ϵ++i0+1ϵ+2+Δ2).

We call this latter correction the “Thomas-Fermi Completion”. The idea is to assume that on length scales shorter than 1/kc, the system is approximately homogeneous and so one can use the homogeneous equations to complete the contributions to various pieces from the missing states. One either computes the integrals above kc numerically, or performs a series expansion in kc1 to compute the corrections at each point in space. (These corrections will in general depend on position because the quantities in the integrand such as Δ(x) will depend depend on position.)

Aside: this procedure also allows us to compute C~ in two dimensions. Although the integral is finite, numerically computing through the pole is a challenge since one really needs a PV integral. If we follow this procedure, however, we can compute νc numerically, Λc analytically, then apply the TF completion.

n+=dk(1ϵk+Ek(f(ω)f(ω+))fν)f+ n=dk(f(ω+)f(ω))f

τ+=dkk2f+,τ=dkk2f,ν=dkΔ2Ek(f(ω)f(ω+))fν κ=dk(2k22m+f+Δ22Ekfν),C~=dk12(1ϵ++i0+fνEk). n+3DE<Ecddk(2π)d[1ϵ+ϵ+2+Δ2]+m2Δ2kc4π2+4m3Δ2μ36π2kc3+O(kc5), κ3DE<Ecddk(2π)d(2τc2m+Δνc)+m2μΔ24π2kc+m3Δ2(8μ2Δ2)66π2kc3+O(kc5),

3D

Here we check the results for homogeneous matter in various dimensions. In 3D can compare these with the results we know for the BdG equation (Eagles-Leggett model) at unitarity:

n+=na+nb=kF33π2,ϵF=2kF22m,EFG=kF510mπ2=35n+ϵF,C~=0, ξ=EEFG=μϵF=0.59060550703283853378393810185221521748413488992993 ΔϵF=0.68640205206984016444108204356564421137062514068346

Δμ=1.162200561790012570995259741628790656202543181557689.

2D

In 2D, we must compute results but we seem to have:

n+=kF22π,ϵF=2kF22m,EFG=kF48mπ=12n+ϵF,C~=0,μϵF=12,ΔϵF=2

1D

In 1D, we have the following:

n+=kFπ,ϵF=2kF22m,EFG=kF33mπ=23n+ϵF, C~=m2kF,μϵF=0.28223521359741266,ΔϵF=0.41172622996179004.