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Abstract |
Dipolar interactions are fundamentally different from the usual van der
Waals forces in real gases. Besides the anisotropy the dipolar interaction
is nonlocal and as such allows for self organized structure formation. In
2005 the first dipolar effects in a quantum gas were observed in an
ultracold Chromium gas. By the use of a Feshbach resonance a purely dipolar
quantum gas was observed three years after [1]. Recently it became possible
to study degenerate gases of lanthanide atoms among which one finds the most
magnetic atoms. Similar to the Rosensweig instability in classical magnetic
ferrofluids self-organized structure formation was expected. In our
experiments with quantum gases of Dysprosium atoms we could observe the
formation of a droplet crystal [2]. In contrast to theoretical mean field
based predictions the super-fluid droplets did not collapse. We find that
this unexpected stability is due to beyond mean-field quantum corrections of
the Lee-Huang-Yang type [3,4]. We observe and study self-bound droplets [5]
which can interfere with each other. We also observe self-organized stripes
in a confined geometry [6] and collective scissors mode oscillations of
dipolar droplets [7Recently in the striped phase also phase coherence was
observed in Dysprosium and Erbium experiments, which is evidence for a
supersolid state of matter [8]. Upon crossing the transition to the dipolar
supersolid a Goldstone mode appears, which we have observed recently [9].
The existence of this mode proofs the superfluid stiffness or the so-called
phase rigidity of this new state of matter. Despite the lack of symmetry
protection, also a Higgs mode was predicted to be observable in a finite
system [10].
References
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[2] H. Kadau, et al., Nature 530, 194 (2016)
[3] T.D. Lee, K. Huang, and C. N. Yang, Phys. Rev. 106, 1135 (1957),
D.S. Petrov, Phys. Rev. Lett. 115, 155302 (2015).
[4] I. Ferrier-Barbut, et al., Phys. Rev. Lett. 116, 215301 (2016)
[5] M. Schmitt, et al., Nature 539, 259 (2016)
[6] M. Wenzel, et al., Phys. Rev. A 96 053630 (2017)
[7] I. Ferrier-Barbut, et al., Phys. Rev. Lett. 120, 160402 (2018)
[8] F. Böttcher, et al. Phys. Rev. X. 9, 011051 (2019), see also L. Tanzi,
et al. Phys. Rev. Lett. 122, 130405 (2019), L. Chomaz et al., Phys. Rev. X
9, 021012 (2019)
[9] M. Guo, et al. Nature. 574, 386--389 (2019)
[10] J. Hertkorn et al., Phys. Rev. Lett. 123, 193002 (2019)
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