Bose 2.2 User Manual download pdf (Page 107)

In contrast, if the ﬁnal lattice depth is chosen above the stability threshold, i.e. in the

cases

= 12

and

= 8

, there is no visible decay of the atom numbers. Here,

the TOF images basically show the usual interference pattern of a coherent array of

condensates (see section 4.4.2): we observe the central zero-momentum component and

the two side peaks corresponding to the lattice recoil momenta

lat

(with

lat

π/d

lat

However, the central density peak shows a deviation with respect to the usual shape

obtained in non-dipolar BECs: it develops a

-wave shape that is similar to the one

observed in a single collapsed dBEC [36], shown in section 6.1. Due to our limited imaging

resolution, the characteristic cloverleaf shape of the central peak becomes more clear if

we expand the condensate for longer times

tof

, as shown in Fig. 6.4. For both the stable

in-trap conﬁgurations, considered here, we do not expect any in-trap evolution of the

system. Therefore, the observed

-wave collapse of the zero-momentum component must

be triggered by the time-of-ﬂight (TOF-triggered collapse). Such collapse behaviour is

fundamentally diﬀerent from the interaction-induced collapse shown in Ref. [36]. There,

the collapse happened partly during the TOF because the ramp in the scattering length

has not converged yet to its ﬁnal value due to the eddy currents.

Finally, in the case

= 6

, which is just below the stability threshold, we observe a

slow decay of the atom number with increasing

hold

, but also a

-wave density pattern

of the central cloud. The observed dynamics of the system hence shows features of

both an in-trap collapse, as well as a TOF-triggered collapse. Using numerical real-

time simulations, we will investigate the complex dynamics of the system in details in

section 6.2.3. Before that, however, we study the time scale of the collapse dynamics

by performing a quantitative analysis of the absorption images taken in the collapse

measurements.

Fig. 6.4, d-wave collapse of the zero-momentum component:

The images show

the remaining coherent atoms of a BEC that is released from a lattice of

depth 8

, for the two diﬀerent expansion times

tof

= 9

and

tof

= 14

Due to the limited resolution of our imaging system, the

-wave shape of the

central density peak is more visible at long expansion times (ﬁeld of view:

190 × 497 m

By integrating the optical density in the images shown in Fig. 6.3, we obtain the number

of remaining coherent atoms for each lattice depth as a function of

hold

. For further

processing, we calculate the remnant fraction, deﬁned as the number of coherent atoms

normalized by the total number of atoms, i.e. including the thermal cloud. Normalizing

the data in such way removes partly the shot-to-shot ﬂuctuations that occur due to the

variation of the total atom number in the experiment. We then extract an atom loss rate

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