Quasi-1D sextic trapping potential

The sextic potentials are obtained by Taylor expanding the sin²-like potential equation (3.2) up to the sixth degree. The corresponding trap potential in c.m.-rel. coordinates (for zero trap separation) is expressed as

V(R,r) = V_c.m.(R) +V_rel.(r) +W(R,r), (7.7) with the individual components given by

V_c.m.(R) = ^X

The full six-dimensional description of the atom-ion problem is performed following the procedure outlined in Chapter 3, i. e., the trap potentials equations (7.8) and (7.9) are respectively substituted in the Hamiltonian equations (3.11) - (3.12) then transformed to spherical coordinates. The product of the solutions (orbitals) of the resulting Schr¨odinger equations are then used to form configuration for incorporating the coupling part (7.10) of the full Hamiltonian (3.10).

7.3.1.1 Eigenenergy spectrum

In Figure 7.3 the eigenenergy spectrum of the relative-motion Hamiltonian equation (3.12) as a function of the inverse scattering length a_sc (scaled by the trap length d⊥) for trap potential (7.9) is shown. The variation of the scattering length is done by continuously varying the repulsive steep inner wall part of the ungerade electronic state of ⁷Li⁺ − ⁷Li potential curve as described in Chapter 5. This variation represents a single-channel model for mimicking the variation of the s-wave scattering length at a Feshbach resonance [100].

The trap specification parameters gives an anisotropy ratio of ωx =ωy = 10ωz

which is well within the quasi-1D regime [95]. The relative-motion spectrum has a bound state (red line) bending downwards to negative infinity asasc →0⁺ and trap states represented by the almost horizontal lines. The green line in Figure 7.3 is the first trap state.

Figure 7.4 shows the energy spectrum of the full coupled Hamiltonian (3.10).

The c.m. energies are added to each of the relative-motion energies leading to an

-5 -4 -3 -2 -1 0 1 2 3 4 5 d

/a

sc

-2 -1 0 1 2 3 4 5

E / h

w

Figure 7.3: Relative-motion eigenenergy spectrum for the hybrid atom-ion system of ⁷Li⁺ - ⁷Li pair confined in sextic trap potential for a varying s-wave scattering length. The parameters for the trap potential are: wavelength λ_x = λ_y = λ_z = 1000 nm, intensity I_x =I_y = 5000 W cm⁻² andI_z = 50 W cm⁻² giving a potential depth of 29.477E_r in the x and y directions and 0.295E_r in the z direction. The basis set used for the calculation is specified in Table E.8.

The plot on the right is the magnified part showing the avoided crossing that is responsible for the inelastic CIR. The labels (a) - (b) on the zoomed part of Figure 7.4 are only used to denote the positions of the wavefunctions for the atom-ion pair to be discussed later in section 7.3.1.2. The crossings between the ground trap state and the transversely excited bound state becomes avoided at around d⊥/a_sc≈1.007, the approximate position of the ICIR. If one presumes a loss experiment involving a single ultracold neutral atom and an ion, when an inelastic CIR is observed for example when the ratio of the trap length to the scattering length is say 1.007, then the value of the scattering length can be determined using this information.

In this example calculation, an isotropic transversal confinement trapping has been used hence only a single resonance is observed due to the degeneracy of the transversal excitation. If a transition is made to the nondegenerate case, i. e., an anisotropic transverse confinement where ω_x 6= ω_y ω_z, then a splitting of the resonance is observed in the eigenenergy spectrum. This transition to anisotropic confinement can be applied in understanding the

7.3. Results and discussion

-4 -3 -2 -1 0 1 2 3 4 d_⊥/a_sc

0 1 2 3 4

E / h_ ω⊥

0.995 1 1.005 1.01 1.015 1.02 d_⊥/a_sc

2.06 2.07 2.08 2.09

(a) (d)

(b)

(c)

Figure 7.4: Eigenenergy spectrum of the full Hamiltonian for Li⁺₂ pair confined in identical sextic trapping potentials. The magnified view on the right shows the avoided crossings that causes the inelastic CIRs.

For this particular example, the most pronounced resonance occurs at d⊥/a_sc ≈1.007. Converged CI calculations were obtained using the basis specifications given in Table E.8.

Figure 7.5: Sketch illustrating the transition from an anisotropic transverse confinement (a) to the case of isotropic confinement (b).

Figure 7.6: Cuts through the wavefunction density along the x-y plane (|Ψ(z₁, z₂;x₁ =x₂ =y₁ =y₂ = 0|²) for the states labeled (a)-(d) in Figure 7.4. The plots have been scaled by the trap lengthl_z along the z direction. In (a) and (d), the atom-ion pair are unbound. When the two particles pass through the crossing adiabatically, they transform into bound states, marked (b) and (c) and they posses no c.m. excitations.

physical interpretation of the almost vertical blue line going through the avoided crossing in Figure 7.4. To visualize how the almost vertical line arise, consider the sketch shown in Figure 7.5. On the left side is a case of nondegerate transverse excitation where the transversal confinement is anisotropic. As the degeneracy is lifted, the bending curve (blue line in Figure 7.5 (a)) is squeezed together until it becomes almost vertical (Figure 7.5 (b)) in the case of full degeneracy of the transverse excitation i. e., when the particles are experience a transverse isotropic confinement.

7.3. Results and discussion

7.3.1.2 Wavefunction analysis

To conclude the discussions about the sextic trap confinement, the behavior of the ab initio wavefunctions are investigated for the states labeled (a) - (d) in Figure 7.4. These states are expressed by their corresponding six-dimensional wavefunctions in absolute coordinates as shown in Figure 7.6. The atom-ion pair in the state labeled (a) in Figure 7.4 are unbound when the ratio of the transversal confinement to scattering length d_⊥/asc = 0.994. This is seen in the cut though the trap state density along the z direction of the full 6D ab initio wavefunction in the top panel (Figure 7.6 (a)). For this interaction strength, the atom-ion pair exhibit a large probability to be off-diagonal i. e., they are separated from each other away from the trap elongation direction.

As the interaction strength reduces, the repulsion between the atom-ion pair decreases and they get closer and closer forming a molecular bound state at d⊥/a_sc = 1.013 (plot labeled (c)). The occupation of bound state is only possible because the excess binding energy can be transferred into the center-of-mass excitation energy [125, 129]. This redistribution of the binding energy is an inelastic process and that is why the resonances induced by the c.m.-rel. coupling are called inelastic CIRs [125]. From the states labeled (a) and (c), one sees how the unbound atom-ion pair transforms into a bound pair after passing through the crossing adiabatically. Similar observations hold for the state labeled (b) and (d) where d⊥/a_sc= 1.003 and d⊥/a_sc = 1.019, respectively.

In this case, the bound atom-ion pair become unbound after crossing the resonance. Noteworthy, the bound atom-ion pair (Figure 7.6 (b) and (c)) posses no center-of-mass excitations in the z direction.