Browsing by Subject "Dipolar gases"

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Open Access
Temperature dependence of the density and excitations of dipolar droplets
(American Physical Society, 2020) Öztürk, S. F.; Enes, Aybar; Oktel, M. Özgür
Droplet states of ultracold gases which are stabilized by fluctuations have recently been observed in dipolar and two-component Bose gases. These systems present an alternate form of equilibrium where an instability at the mean-field level is arrested by higher-order correlations, making the droplet states sensitive probes of fluctuations. In a recent paper, we argued that thermal fluctuations can play an important role for droplets even at low temperatures where the noncondensed density is much smaller than the condensate density. We used the Hartree-Fock-Bogoliubov theory together with the local density approximation for fluctuations to obtain a generalized Gross-Pitaevskii (GP) equation and solved it with a Gaussian variational ansatz to show that the transition between the low density and droplet states can be significantly modified by the temperature. In this paper, we first solve the same GP equation numerically with a time-splitting spectral method to check the validity of the Gaussian variational ansatz. Our numerical results are in good agreement with the Gaussian ansatz for a large parameter regime and show that the density of the gas is most strongly modified by temperature near the abrupt transition between a pancake-shaped cloud and the droplet. For cigar-shaped condensates, as in the recent Er experiments, the dependence of the density on temperature remains quite small throughout the smooth transition. We then consider the effect of temperature on the collective oscillation frequencies of the droplet using both a time-dependent Gaussian variational ansatz and real-time numerical evolution. We find that the oscillation frequencies depend significantly on the temperature close to the transition for the experimentally relevant temperature regime (≃100nK).
Open Access
Temperature-dependent density profiles of dipolar droplets
(American Physical Society, 2019) Aybar, Enes; Öktel, M. Özgür
Recently, trapped dipolar gases were observed to form high-density droplets in a regime where mean-field theory predicts collapse. These droplets present a form of equilibrium where quantum fluctuations are critical for stability. So far, the effect of quantum fluctuations has only been considered at zero temperature through the local chemical potential arising from the Lee-Huang-Yang correction. Here, we extend the theory of dipolar droplets to nonzero temperatures using Hartree-Fock-Bogoliubov theory (HFBT) and show that the equilibrium is strongly affected by temperature fluctuations. HFBT, together with local density approximation for excitations, reproduces the zero-temperature results and predicts that the condensate density can change dramatically even at low temperatures where the total depletion is small. In particular, we find that typical experimental temperatures (T∼100 nK) can significantly modify the transition between low-density and droplet phases.