Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The low-energy Goldstone mode in a trapped dipolar supersolid   A supersolid is a counter-intuitive state of matter that combines the
frictionless flow of a superfluid with the crystal-like periodic density
modulation of a solid. Since the first prediction in the 1950s, experimental
efforts to realize this state have focussed mainly on Helium, where
supersolidity remains elusive. Recently, supersolidity has also been studied
intensively in ultracold quantum gases, and some of its defining properties
have been induced in spin-orbit coupled Bose-Einstein condensates (BECs) and
BECs coupled to two crossed optical cavities. However, the periodicity of the
crystals in both systems is fixed to the wavelength of the applied periodic
optical potentials. Recently, hallmark properties of a supersolid -- the
periodic density modulation and simultaneous global phase coherence -- have
been observed in arrays of dipolar quantum droplets, where the crystallization
happens in a self-organized manner due to intrinsic interactions. In this
letter, we prove the genuine supersolid nature of these droplet arrays by
directly observing the low-energy Goldstone mode. The dynamics of this mode is
reminiscent of the effect of second sound in other superfluid systems and
features an out-ofphase oscillation of the crystal array and the superfluid
density. This mode exists only due to the phase rigidity of the experimentally
realized state, and therefore confirms the genuine superfluidity of the
supersolid.

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The categories of this paper are: patt-sol
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The categories of this paper are: solv-int
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The categories of this paper are: comp-gas
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The categories of this paper are: funct-an
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The categories of this paper are: plasm-ph
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The categories of this paper are: cs
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The categories of this paper are: cmp-lg
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The categories of this paper are: quant-ph
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The categories of this paper are: supr-con
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The categories of this paper are: q-fin
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The categories of this paper are: stat
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The categories of this paper are: ao-sci
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The categories of this paper are: hep-ex
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The categories of this paper are: nucl-th
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The categories of this paper are: alg-geom
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The categories of this paper are: astro-ph