Bose 2.2 User Manual download pdf (Page 99)

states emerge mainly, if only few lattice sites

are populated [28], as shown in Fig. 5.12(b).

Loading many lattice sites, the roton instability may not be distinguished anymore from

the long-wavelength phonon instability, as the characteristic roton wavelength exceeds the

radial size of the system. This argument applies to our stability measurements presented in

section 5.3, since we load around 15 lattice sites. The inter-site coupling of the excitations

thus explains why the Gaussian variational calculations are able to reproduce the measured

critical scattering length in the regime of very deep lattices.

Conclusion

In this chapter, we have presented the results of our experimental investigation of the

stability of a dipolar Bose-Einstein condensate in a one-dimensional optical lattice. In

contrast to the case of purely contact interacting systems, an optical lattice changes

drastically the stability properties of a dipolar BEC. In particular in deep lattices, we have

shown that a condensate can be stabilized even for negative

-wave scattering lengths by

the repulsive on-site dipolar interactions. In this regime, we have furthermore identiﬁed a

signiﬁcant destabilization of the system by the dipolar inter-site interactions.

The determination of the stability threshold is fundamental for any further investigations

of lattice gases with strong dipolar interactions. It sets the basis, for example, for our

studies of the instability dynamics of a dBEC in the lattice, presented in the next chapter.

The number of ﬁlled lattice sites that leads to a visible modulation of the ground-state, depends crucially

on the lattice spacing

lat

. The calculations of Ref. [28] are performed with parameters close to our

experimental ones, using d

lat

= 560 nm.

1 2 ... 94 95 96 97 98 99 100 101 102 103 104 ... 154 155

No comments

Bose 2.2 User Manual Page 99