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Supersolid phase in two-dimensional soft-core bosons at finite temperature
The supersolid phase of soft-core bosons in two dimensions is investigated using the self-consistent
Hartree-Fock and quantum Monte Carlo methods. An approximate phase diagram at finite temperatures is
initially constructed using the mean-field approach, which is subsequently validated through precise
path-integral simulations, enabling a microscopic characterization of the various phases. Superfluid
and melting/freezing transitions are analyzed through the superfluid density and the long-range behavior
of correlation functions associated with positional and orientational order, in accordance with the general
picture of Berezinskii-Kosterlitz-Thouless transitions. A broad region at low temperatures is identified
where the supersolid phase exists, separating the uniform superfluid phase from the normal quasi-crystal
phase. Additionally, a potential intermediate hexatic phase with quasi long-range orientational order is
identified in a narrow region between the normal solid and fluid phases. These findings establish
self-consistent Hartree-Fock theory beyond the local density approximation as an effective tool,
complementary to computationally intensive quantum Monte Carlo simulations, for investigating
the melting of the supersolid phase and the possible emergence of the hexatic superfluid phase
in bosonic systems with various interaction potentials.
C Sebastiano Peotta, Gabriele Spada, Stefano Giorgini, Sebastiano Pilati,
Alessio Recati, Phys. Rev. B 112, 184506 (2025)
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Lattice Bose polarons at strong coupling and quantum criticality
We develop a new theoretical framework for exploring a mobile impurity interacting strongly with a
highly correlated bath of bosons in the quantum critical regime of a Mott insulator (MI) to superfluid (SF)
quantum phase transition. Our framework is based on a powerful quantum Gutzwiller (QGW) description of the
bosonic bath combined with diagrammatic field theory for the impurity-bath interactions. By resumming a
selected class of diagrams to infinite order, a rich picture emerges where the impurity is dressed by the
fundamental modes of the bath, which change character from gapped particle-hole excitations in the MI to
Higgs and gapless Goldstone modes in the SF. This gives rise to the existence of several quasiparticle
(polaron) branches with properties reflecting the strongly correlated environment. In particular,
one polaron branch exhibits a sharp cusp in its energy, while a new ground-state polaron emerges at
the O(2) quantum phase transition point for integer filling, which reflects the nonanalytic behavior
at the transition and the appearance of the Goldstone mode in the SF phase. Smooth versions of these
features are inherited in the polaron spectrum away from integer filling because of the varying
``Mottness" of the bosonic bath. We furthermore compare our diagrammatic results with quantum Monte
Carlo calculations, obtaining excellent agreement. This accuracy is quite remarkable for such a highly
non-trivial case of strong interactions between the impurity and bosons in a maximally correlated quantum
critical regime, and it establishes the utility of our framework. Finally, our results show how impurities
can be used as quantum sensors and highlight fundamental differences between experiments performed at a
fixed particle number or a fixed chemical potential.
Ragheed Alhyder, Victor E. Colussi, Matija Čufar, Joachim Brand,
Alessio Recati, Georg M. Bruun, SciPost Phys. 19, 002 (2025)
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Sound, superfluidity and layer compressibility in a ring dipolar supersolid
We propose a protocol to excite the Goldstone modes of a supersolid dipolar Bose-Einstein condensed gas
confined in a ring geometry. By abruptly removing a properly applied periodic modulation,
we explore the resulting oscillations of the gas, by solving the extended Gross-Pitaevskii equation. The value
of the two longitudinal sound velocities exhibited in the supersolid phase are analyzed using the hydrodynamic
theory of supersolids at zero temperature. This approach allows for the determination of the layer
compressibility modulus as well as of the superfluid fraction,
in agreement with the Leggett estimate of the non-classical moment of inertia.
Marija Šindik, Tomasz Zawiślak, Alessio Recati, Sandro Stringari, Phys. Rev. Lett. 132, 146001 (2024)
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Observation of false vacuum decay via bubble formation in ferromagnetic superfluids
In quantum field theory, the decay of an extended metastable state into the real ground state is known as ``false vacuum decay'' and it takes place via the nucleation of spatially localized bubbles. Despite the large theoretical effort to estimate the nucleation rate, experimental observations were still missing. Here, we observe bubble nucleation in isolated and highly controllable superfluid atomic systems, and we find good agreement between our results,
numerical simulations and instanton theory opening the way to the emulation of out-of-equilibrium quantum field phenomena in atomic systems.
Alessandro Zenesini, Anna Berti, Riccardo Cominotti, Chiara Rogora, Ian G. Moss,
Thomas P. Billam, Iacopo Carusotto, Giacomo Lamporesi, Alessio Recati, Gabriele Ferrari, Nature Physics (2024)
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Oscillating Solitons and AC Josephson Effect in Ferromagnetic Bose-Bose Mixtures
Close to the demixing transition, the degree of freedom associated to relative density fluctuations of a two-component Bose-Einstein condensate is described by a non-dissipative Landau-Lifshitz equation. In the quasi one-dimensional weakly immiscible case,
this mapping surprisingly predicts that a dark-bright soliton should oscillate when subject to a constant force favoring separation of the two components.
We propose a realistic experimental implementation of this phenomenon which we interpret as a spin-Josephson effect in the presence of a movable barrier.
Sebastiano Bresolin, Arko Roy, Gabriele Ferrari, Alessio Recati, Nicolas Pavloff, Phys. Rev. Lett. 130, 220403 (2023)
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