The formation of quasicondensate droplets in ultracold atomic gases is investigated as a first-order phase transition. Physical parameters and size distribution of metastable condensate droplets are derived from conditions of local equilibrium between quasicondensate and normal gas phases. Droplet characteristics are used for the quantitative description of recent measurements of enormous frequency shifts in the $1S\ensuremath{-}2S$ two-photon absorption in spin-polarized hydrogen in the normal gas phase. Theoretical evaluation of the line shape is in good agreement with experimental data.
The formation of quasicondensate droplets in ultracold atomic gases is investigated as a first-order phase transition. Physical parameters and size distribution of metastable condensate droplets are derived from conditions of local equilibrium between quasicondensate and normal gas phases. Droplet characteristics are used for the quantitative description of recent measurements of enormous frequency shifts in the 1S\ensuremath−2S two-photon absorption in spin-polarized hydrogen in the normal gas phase. Theoretical evaluation of the line shape is in good agreement with experimental data.
Bose-Einstein condensation, observed in either strongly interacting liquid helium or weakly interacting atomic Bose gases, is widely known to be a second-order phase transition. Here we predict a first-order Bose-Einstein condensation in a cloud of harmonically trapped bosons interacting with both attractive two-body interaction and repulsive three-body interaction, characterized respectively by an s-wave scattering length a<0 and a three-body scattering hypervolume D>0. It happens when the harmonic trapping potential is weak, so with increasing temperature the system changes from a low-temperature liquidlike quantum droplet to a normal gas and therefore experiences a first-order liquid-to-gas transition. At large trapping potential, however, the quantum droplet can first turn into a superfluid gas, rendering the condensation transition occurring later from a superfluid gas to a normal gas smooth. We determine a rich phase diagram and show the existence of a tricritical point, where the three phases, i.e., quantum droplet, superfluid gas, and normal gas, meet. We argue that an ensemble of spin-polarized tritium atoms could be a promising candidate to observe the predicted first-order Bose-Einstein condensation, across which the condensate fraction or central condensate density jumps to zero and the surface-mode frequencies diverge.
Given the right set of circumstances, ultracold quantum gases are able to change character and condense into a liquid state of quantum droplets. The size distribution of the droplets is determined dynamically in the condensation process. A semi-quantitative argument is presented which suggests that, at zero temperature, a multiple droplet system has is a preferred scale ∝v−1/3, where v is the rate of change of parameters at the time of droplet formation. Numerical simulations of two dimensional systems strongly support a power law v−d, with an exponent d∈(0.327,0.375).
We report observation of Bose-Einstein condensation (BEC) of a trapped, dilute gas of atomic hydrogen. The condensate and normal gas are studied by two-photon spectroscopy of the 1S- 2S transition. Interactions among the atoms produce a shift of the resonance frequency proportional to density. The condensate is clearly distinguished by its large frequency shift. The peak condensate density is $4.8\ifmmode\pm\else\textpm\fi{}1.1\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{cm}}^{\ensuremath{-}3}$, corresponding to a condensate population of 109 atoms. The BEC transition occurs at about $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}50\ensuremath{\mu}\mathrm{K}$ and $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.8\ifmmode\times\else\texttimes\fi{}{10}^{14}{\mathrm{cm}}^{\ensuremath{-}3}$.
We report the first detailed decay studies of trapped metastable (2S) hydrogen. By two-photon excitation of ultracold H samples, we have produced clouds of at least $5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ magnetically trapped 2S atoms at densities greater than $4\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\ensuremath{-}3}$ and temperatures below $100\ensuremath{\mu}\mathrm{K}.$ At these densities and temperatures, two-body inelastic collisions of metastable atoms are evident. Experimental values for the total two-body loss rate constant are ${K}_{2}{=1.8}_{\ensuremath{-}0.7}^{+1.8}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}$ at $87\ensuremath{\mu}\mathrm{K}$ and ${K}_{2}{=1.0}_{\ensuremath{-}0.5}^{+0.9}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}$ at $230\ensuremath{\mu}\mathrm{K}.$ These results are in the range of recent theoretical calculations for the total $2S\ensuremath{-}2S$ inelastic rate constant. The metastable clouds were excited in a gas of ground-state (1S) hydrogen with peak densities reaching $7\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}.$ From the one-body component of the metastable decay, we derive experimental upper limits for K12, the rate constant for loss due to inelastic $1S\ensuremath{-}2S$ collisions.
Abstract We explore the ground state phase diagram and nonequilibrium dynamics of genuine two-component particle-imbalanced droplets in both isotropic and anisotropic three-dimensional confinements. A gradual transition from mixed droplet-gas to gas configurations is revealed as the average intercomponent attraction decreases or the transverse confinement becomes tighter. Within the mixed structures, a specific majority fragment binds to the minority droplet, satisfying the density ratio locking condition, while the remaining atoms are in a gas state. Our extended Gross–Pitaevskii numerical results are corroborated by a suitable variational approximation capturing the shape and characteristics of droplet-gas fragments. The tunability of the relatively low gas fraction is showcased through parametric variations of the atom number, the intercomponent imbalance, the trap aspect ratio, or the radius of a box potential. To validate the existence and probe the properties of these exotic phases, we simulate the standard time-of-flight and radio frequency experimental techniques. These allow to dynamically identify the resilience of the droplet fragment and the expansion of the gas fraction. Our results, amenable to current experimental cold atom settings, are expected to guide forthcoming investigations aiming to reveal unseen out-of-equilibrium droplet dynamics.
Ultra-dilute self bound droplets in Bose-Einstein condensates have been of great interest since their first prediction in 2015.The self-bound nature of these bosonic gases combined with their extremely low densities make them a prime target of investigation for both experimentalists and theoreticians.In this thesis work dipole-dipole interacting gases are simulated using the Gross-Pitaevskii equation applying a fourth order, split-operator imaginary time method in two dimensions.First, the systems are studied at collapse for dipoles aligned with a magnetic field that tilts the dipoles in the two-dimensional plane.For some angles the dipole interaction becomes partly attractive which can lead to the collapse of the mean field solution.However, quantum fluctuations can stabilize the gas.Introducing the so-called Lee-Huang-Yang (LHY) correction, the system can be stabilized and self-bound droplets may form.Here, we investigate this stabilization for a quasi two-dimensional set-up. Popular science Ultra-cold droplets intrigue physicistsA hot topic within ultra-cold atom physics has been the recent discovery of droplets formed out of Bose-Einstein condensates, a specific form of a super cold and very dilute gas.The condensate is however very different from any gas encountered in everyday life.A normal gas can be thought of as atoms or molecules moving around in all directions.If you cool this gas, all the particles move slower until they either form a liquid or freeze and become a solid.This is not the case for the Bose-Einstein condensate.Instead they spread through space due to quantum mechanics and together they form one entity.This entity, the so called condensate, behaves like no classical gas, as it is mainly governed by quantum mechanics and not by the random chance of a lot of small particles moving about.To add to this strangeness, droplets now seem to form out of these condensates.The existence of these droplets was quite a surprise.Among experimentalists working with creating Bose-Einstein condensates the expected result was a collapse of the system if forces pushing the condensate together won against the forces pushing it apart.However, experimentalists revealed that some other internal force stabilized drops inside the condensate.Not only that, this force seemed to be able to create drops that could exist outside a "trap", which usually is a number of lasers and magnets that keep the condensate together.Some droplets could not be formed without a trap, but instead formed crystal structures, a very intriguing result.This has led to an excited mood within the field of cold atom physics.But what to expect from this anti-collapse force, mainly being described as quantum fluctuations, and the droplets it creates needs further study to find out.In this thesis work we simulate the anti-collapse force and possible droplet systems solving the underlying quantum-mechanical many-particle equations numerically.One system of interest is the very thin disc of condensate, or a quasi two-dimensional condensate.This system is excellent for rotating the condensate in.Why? Imagine having a piece of pizza dough that in this case represents a condensate.If you spin this dough as a skilled pizza maker does, the pizza will become flat due to rotational forces.The same goes for the condensate.As you rotate it it will become flat anyways and therefore having a model where our very cold gas is already flat makes it easier to simulate rotating systems.But before you start rotating your drops, you have to find the drops.And to find the drops you have to study the forces pushing the condensate together and the forces pushing it apart.Therefore we presently aim at studying possible droplet formation in these pizza crust shaped condensates, to make way for the study of rotational excitations.Initial results show some promise that droplets can be found in these quasi two-dimensional systems.