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Ultra-Dilute Gas of Polarons in a Bose–Einstein Condensates
Full text linkWe investigate the properties of a dilute gas of impurities embedded in an ultracold gas of bosons that forms a Bose–Einstein condensate (BEC). This work focuses mainly on the equation of state (EoS) of the impurity gas at zero temperature and the induced interaction between impurities mediated by the host bath. We use perturbative field-theory approaches, such as Hugenholtz–Pines formalism, in the weakly interacting regime. In turn, for strong interactions, we aim at non-perturbative techniques such as quantum–Monte Carlo (QMC) methods. Our findings agree with experimental observations for an ultra dilute gas of impurities, modeled in the framework of the single impurity problem; however, as the density of impurities increases, systematic deviations are displayed with respect to the one-body Bose polaron problem
Monte Carlo methods for impurity physics in ultracold Bose quantum gases
No full textPolarons are quasiparticles formed when electrons interact with the lattice vibrations in a polarizable crystal. They are essential to describe semiconductor transport properties and also arise in other systems, for instance, in transition metal oxides, colossal magnetoresistance, organic polymers, DNA transport. Moreover, the coupling between polarons has been one of the hallmarks of high-temperature superconductivity. However, a full microscopic description of these quasiparticles in real materials is challenging because real materials are imperfect and hard to model
Dynamical formation of polarons in a Bose-Einstein condensate: A variational approach
No full textWe investigate the nonequilibrium dynamics of an impurity coupled to a Bose-Einstein condensate, systematically compared with recent experimental results [M. G. Skou et al., Nat. Phys. (2021)]. The dynamics of the impurity is tracked down by using a time-dependent variational coherent ansatz. For weak coupling between the impurity and the bath, analytical expressions for the time-dependent contrast are derived, matching quite well with previous findings obtained within a master equation approach. For strong coupling instead, the variational ansatz provides a good quantitative description of the polaron dynamics, in particular, in signaling the transition from the few- to the many-body correlated regime where polarons are expected to form
Impurity in a Bose-Einstein condensate: study of the attractive and repulsive branch using quantum Monte-Carlo methods
No full textWe investigate the properties of an impurity immersed in a dilute Bose gas at zero temperature using quantum
Monte Carlo methods. The interactions between bosons are modeled by a hard-sphere potential with scattering
length a, whereas the interactions between the impurity and the bosons are modeled by a short-range, square-well
potential where both the sign and the strength of the scattering length b can be varied by adjusting the well depth.
We characterize the attractive and the repulsive polaron branch by calculating the binding energy and the
effective mass of the impurity. Furthermore, we investigate the structural properties of the bath, such as the
impurity-boson contact parameter and the change of the density profile around the impurity. At the unitary limit
of the impurity-boson interaction, we find that the effective mass of the impurity remains smaller than twice its
bare mass, while the binding energy scales with , where n is the density of the bath and m is the common
mass of the impurity and the bosons in the bath. The implications for the phase diagram of binary Bose-Bose
mixtures at low concentrations are also discussed
Ground-state properties of the Dipolar Bose-Polaron
No full textWe consider a quantum impurity immersed in a dipolar Bose–Einstein condensate and study the properties of the emerging polaron. We calculate the energy, effective mass and quasi-particle residue of the dipolar polaron and investigate their behaviour with respect to the strength of zerorange contact and a long-range dipolar interactions among the condensate atoms and with the impurity. While quantum fluctuations in the case of pure contact interactions typically lead to an increase of the polaron energy, dipole–dipole interactions are shown to cause a sign reversal. The described signatures of dipolar interactions are shown to be observable with current experimental capabilities based on quantum gases of atoms with large magnetic dipole moments such as Erbium or Dysprosium condensate
Measuring the single-particle density matrix for fermions and hard-core bosons in an optical lattice
No full textUltracold atoms in optical lattices provide clean, tunable, and well-isolated realizations of paradigmatic
quantum lattice models. With the recent advent of quantum-gas microscopes, they now also offer the
possibility to measure the occupations of individual lattice sites. What, however, has not yet been achieved
is to measure those elements of the single-particle density matrix, which are off- diagonal in the occupation
basis. Here, we propose a scheme to access these basic quantities both for fermions as well as hard-core
bosons and investigate its accuracy and feasibility. The scheme relies on the engineering of a large effective
tunnel coupling between distant lattice sites and a protocol that is based on measuring site occupations after
two subsequent quenches
Finite range effects in the unitary Fermi polaron
No full textQuantum Monte Carlo techniques are employed to study the properties of polarons in an ultracold Fermi gas, at T=0, and in the unitary regime using both a zero-range model and a square-well potential. For a fixed density, the potential range is varied and results are extrapolated and compared against a zero-range model. A discussion regarding the choice of an interacting potential with a finite range is presented. We compute the polaron effective mass, the polaron binding energy, and the effective coupling between them. The latter is obtained using the Landau-Pomeranchuk's weakly interacting quasiparticle model. The contact parameter is estimated by fitting the pair distribution function of atoms in different spin states
Bose Polaron Problem: effect of mass imbalance on binding energy
No full textBy means of quantum Monte Carlo methods we calculate the binding energy of an impurity immersed in a Bose-Einstein condensate at T=0. The focus is on the attractive branch of the Bose polaron and on the role played by the mass imbalance between the impurity and the surrounding particles. For an impurity resonantly coupled to the bath, we investigate the dependence of the binding energy on the mass ratio and on the interaction strength within the medium. In particular, we determine the equation of state in the case of a static (infinite mass) impurity, where three-body correlations are irrelevant and the result is expected to be a universal function of the gas parameter. For the mass ratio corresponding to
40K impurities in a gas of
87 Rb atoms, we provide an explicit comparison with the experimental findings of a recent study carried out at JILA
Quantum Droplets of Dipolar Mixtures
Full text linkRecently achieved two-component dipolar Bose-Einstein condensates open exciting possibilities for the study of mixtures of ultradilute quantum liquids. While nondipolar self-bound (without external confinement) mixtures are necessarily miscible with an approximately fixed ratio between the two densities, the density ratio for the dipolar case is free. Therefore, self-bound dipolar mixtures present qualitatively novel and much richer physics, characterized by three possible ground-state phases: miscible, symmetric immiscible, and asymmetric immiscible, which may in principle occur at any population imbalance. Self-bound immiscible droplets are possible due to mutual nonlocal intercomponent attraction, which results in the formation of a droplet molecule. Moreover, our analysis of the impurity regime shows that quantum fluctuations in the majority component crucially modify the miscibility of impurities. Our work opens intriguing perspectives for the exploration of spinor physics in ultradilute liquids, which should resemble to some extent that of 4He–3He droplets and impurity-doped helium droplet
Elastic constants of hcp 4He: path integral Monte-Carlo
No full textThe elastic constants of hcp 4He are computed using the path-integral Monte Carlo (PIMC) method. The
stiffness coefficients are obtained by imposing different distortions to a periodic cell containing 180 atoms,
followed by measurement of the elements of the corresponding stress tensor. For this purpose an appropriate
path-integral expression for the stress tensor observable is derived and implemented into the PIMC++ package.
In addition to allowing the determination of the elastic stiffness constants, this development also opens the way
to an explicit atomistic determination of the Peierls stress for dislocation motion using the PIMC technique.
A comparison of the results to available experimental data shows an overall good agreement of the density
dependence of the elastic constants, with the single exception of C13. Additional calculations for the bcc phase,
on the other hand, show good agreement for all elastic constants
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