Short-range approximation

In order to solve Eq. (3.2), one has to find simplified expressions for the matrix ele-ments hΦ_bg|W|Φˆ _bi, hΦ_E|W|Φˆ _bi, and hΦ_bg|Φ_Ei. To this end one can exploit that the interaction acts only in some small ranger < r0. As discussed in Sec. 1.1, for almost all atomic systems and optical traps, the interaction ranger0 is much smaller than the extensions of the trapa_ho. In a harmonic trap the open-channel solution|Φ_Eiis known analytically for r > r0. Denoting again the asymptotic behaviour of Φ_E(r) for r > r0

by ˜Φ_E(r) one has

Φ˜_E(r)≡ lim

r→∞Φ_E(r) =A_νD_ν(ρ), (3.3) whereDν(ρ) is the parabolic cylinder function, ρ =√

2r/aho, ν =E/(~ω)−1/2, and Aν is a normalization constant. Fig. 3.2 (a) shows a plot of ˜Φ_E(ρ) for different energies E.

The short-range behavior of the parabolic cylinder function is given as [58]

D_ν(ρ) =D_ν(0) 1− ρ

Using Eq. (1.10) one obtains the scattering length a(E) = a_hof(E) of the open-channel solution as a function of the eigenenergy E. The like holds for |Φ_bgi, i.e the background scattering length is given asabg =ahof(Ebg).

The function f(E) contains many important information about the wave function in an isotropic harmonic trap. For example, if one neglected at this point the RBS admixture and the energy dependence of the scattering lengtha, the eigenenergies and thus the asymptotic wave function ˜Φ_E(r) would be already fully specified by roots of a=ahof(E), which is equivalent to the result derived by Busch et al. [54].

The function f(E) can be well approximated forE >~ω/2 and E <−~ω/2 by

1The asymptotic behaviorf(E)→f<(E) forE→ −∞follows directly from the asymptotic behavior of the ratio of gamma functions Γ(z+a)/Γ(z+b)→z^a−bforz→ ∞[59]. Using the reflection prop-erty of the gamma functions Γ(z)Γ(1−z) =π/sin(πz) [59] one findsf(E) = tan ₂^πE

~ω+^π₄ f(−E).

Hence,f(E)→f>(E) forE→+∞.

3. Two-channel solution in a harmonic trap (a)

0 1 2 3 4 5 6 7

-0.5 0.0 0.5

interatomic distancer@units ofahoD FŽ EHrL@unitsofaho-12D

E=1.5ÑΩ

E=2.0ÑΩ

E=2.5ÑΩ

(b) (c)

-2 0 2 4 6

-4 -2 0 2 4

EÑΩ f,f>,f<

-6 -4 -2 0 2 4 6

0.01 0.1 1 10

EÑΩ Hf<-fLf,Hf>-fLf

Figure 3.2:(a) Asymptotic behaviour of the open-channel wave function ˜Φ_E(r) for E = 1.5~ω (scattering length a= 0), E = 2.0~ω (a= 0.49a_ho), and E = 2.5~ω (|a| =∞). (b) Comparison of the functionf(E) [blue solid] to its approximations f>(E) [red dashed] andf<(E) [green dotted]. (c)Relative difference between f(E) and f_>(E) or f_>(E) defined in Eqs. (3.5), (3.6), and (3.7).

expect, for the l= 0 harmonic oscillator eigenenergies E = (3/2 + 2n)~ω of the non-interacting system the scattering length vanishes. On the other hand, the scattering length diverges approaching an energyE = (1/2 + 2n)~ω.

In the case of free-space scattering the energy dependence of hΦ_b|W|Φˆ _regi was eval-uated by considering the energy behavior of the regular solution |Φ_regi for some small internuclear distance r0 for which the interaction could be neglected. A similar ap-proach is taken here, i.e. it is assumed that the matrix elementhΦ_E|W|Φˆ _bi is approxi-mately equal toγΦ˜_E(r₀)≈γ^hΦ˜_E(0) +r₀Φ˜⁰_E(0)ⁱ, whereγ is a proportionality constant parametrizing the coupling strength. In order to be able to accurately describe a large class of MFRs, the interaction ranger0 is replaced by a lengtha^∗ that is allowed to take arbitrary positive and negative values. Accordingly, the matrix elementhΦ_E|W|Φˆ _bi is

26

3.1. Derivation of the analytic model

The parametrization of Eq. (3.8) can be interpreted in a different way by considering the wave functions (and not the radial wave functions) of the system. Be Ψ_b(r) = Φ_b(r)/(√

4πr) the wave function describing the RBS and ˜Ψ_E(r) = ˜Φ_E(r)/(√

4πr) the asymptotic wave function of the open channel then

γ ^hΦ˜_E(0) +a^∗Φ˜⁰_E(0)ⁱ= parametriza-tion of Eq. (3.8) is equivalent to replacing the short-range coupling to the RBS by a zero-range coupling, i.e.W(r)Ψb(r)→√

4πγ(r+a^∗)δ(~r), that acts not on the full wave function Ψ_E(r) but only on its asymptotic form ˜Ψ_E(r). Although, only two parame-ters are used, the parametrization of the short-range coupling by a zero-range coupling is already quite general. While the asymptotic form of the wave function ˜Ψ_E(r) can exhibit a 1/rdivergence for r →0 ( ˜Φ_E(0)6= 0 for a6= 0), such that^Rr²drΨ˜_E(r)rδ(~r) is non-zero, all higher order couplings proportional tor²δ(~r), r³δ(~r), . . . automatically vanish for any asymptotic solution of the harmonic oscillator ˜Ψ_E(r).

Within the TC model the RBS is assumed to be constant. Therefore, within the zero-range coupling approximation the parametersa^∗ and γ describing the coupling to the RBS should be also constant. However, considering realistic MC solutions, both the RBS wave function and the open-channel wave function at the resonance differ slightly from the off-resonant ones (see Fig. 3.3). To account for this effect one can introduce background-coupling parameters a^∗_bg and γ_bg in order to parametrize the coupling to the background state, i.e. hΦ_bg|W|Φˆ _bi = γ_bg1−a^∗_bg/a_bg. Since the variations should be small, it suffices to introduce only a different background-coupling strengthγ_bg. Ifa^∗_bg =a^∗+δa^∗, then δa^∗ can be absorbed into the definition of γ_bg, i.e.

γ_bg1−a^∗_bg/a_bg → γ_bg(1−a^∗/a_bg) for γ_bg → γ_bg(1 +δa^∗/[a_bg−a^∗]). Accordingly, the coupling between the bound state and the background state is parametrized as

hΦ_bg|W|Φˆ _bi=γbgΦ˜_Ebg(0) 1− a^∗ a_bg

!

. (3.10)

Finally, one has to evaluate the overlap hΦ_bg|Φ_Ei appearing in Eq. (3.2). Since the

3. Two-channel solution in a harmonic trap

interatomic distancerin a.u.

wavefunctionina.u.

interatomic distancerin a.u.

wavefunctionina.u.

off-resonant´18000 resonant

Figure 3.3: Channel wave functions for ⁶Li-⁸⁷Rb scattering in the atomic basis at an off-resonant magnetic field ofB = 1000 Gauss and close to the resonance at B = 1066.92 Gauss (see also Fig. 2.2). Left: Open-channel wave function (|f₁, mf1i|f₂, mf2i = |1/2,1/2i|1,1i). Right: Exemplary closed-channel wave function (|3/2,3/2i|1,0i). Clearly, the resonant wave functions are similar but not equal to the scaled off-resonant ones.

integral is predominantly determined by the asymptotic form of |Φ_Ei and |Φ_bgi, one has to a very good approximationhΦ_bg|Φ_Ei=^DΦ˜_bgΦ˜_E^E. Only if|Φ˜_Ei and|Φ˜_bgiare almost orthogonal, the overlap for small internuclear distancesr < r0 is decisive, such that in general hΦ_bg|Φ_Ei 6=^DΦ˜_bgΦ˜_E^E. As all background solutions form a complete set of orthogonal functions the proximate orthogonality would imply that|Φ˜_Eiis itself almost identical to a different background solution |Φ⁰_bgi with energy E_bg⁰ ≈ E and E_bg⁰ 6= E_bg. In this case one is free to chose |Φ⁰_bgi instead of |Φ_bgi as a background solution such that any part of the energy spectrum can be described. Using the analytic properties of the integralhD_ν|Dν⁰i of two parabolic cylinder functions [60] one finds

hΦ_E|Φ_bgi=AνbgAν Plugging Eqs. (3.8), (3.10), and (3.11) into Eq. (3.2) and using

Φ˜_E(0) = 2^ν/2A_ν√ π Γ¹₂− ^ν₂

(3.12)

28

3.1. Derivation of the analytic model Dividing both sides by (E−E_bg) yields an eigenenergy equation

E−E_b = 2γγ_bg

which finally depends only on constants and analytic functions.