We propose an experimental scheme to effectively assemble chains of dipolar gases with a uniform length in a multi-layer system.The obtained dipolar chains can form a chain crystal with the system temperature easily controlled by the initial lattice potential and the external field strength during processing.When the density of chains increases, we further observe a second order quantum phase transition for the chain crystal to be dissociated toward layers of 2D crystal, where the quantum fluctuation dominates the classical energy and the compressibility diverges at the phase boundary.The experimental implication of such a dipolar chain crystal and its quantum phase transition is also discussed.
We propose an experimental scheme to effectively assemble chains of dipolar gases with a uniform length in a multi-layer system.The obtained dipolar chains can form a chain crystal with the system temperature easily controlled by the initial lattice potential and the external field strength during processing.When the density of chains increases, we further observe a second order quantum phase transition for the chain crystal to be dissociated toward layers of 2D crystal, where the quantum fluctuation dominates the classical energy and the compressibility diverges at the phase boundary.The experimental implication of such a dipolar chain crystal and its quantum phase transition is also discussed.
We predict that ultracold bosonic dipolar gases, confined within a multilayer geometry, may undergo self-assembling processes, leading to the formation of chain gases and solids. These dipolar chains, with dipoles aligned across different layers, emerge at low densities and resemble phases observed in liquid crystals, such as nematic and smectic phases. We calculate the phase diagram using quantum Monte Carlo methods, introducing a newly devised trial wave function designed for describing the chain gas, where dipoles from different layers form chains without in-plane long-range order. We find gas, solid, and chain phases, along with quantum phase transitions between these states. Specifically, we predict the existence of quantum phase transitions from gaseous to self-ordered phases, as the interlayer distance is decreased. Remarkably, in the self-organized phases, the mean interparticle distance can significantly exceed the characteristic length of the interaction potential, yielding solids and chain gases with densities several orders of magnitude lower than those of conventional quantum solids.
Ultracold polar molecules, in highly anisotropic traps and interacting via a repulsive dipolar potential, may form one-dimensional chains at high densities. According to classical theory, at low temperatures there exists a critical value of the density at which a second order phase transition from a linear to a zigzag chain occurs. We study the effect of thermal and quantum fluctuations on these self-organized structures using classical and quantum Monte Carlo methods, by means of which we evaluate the pair correlation function and the static structure factor. Depending on the parameters, these functions exhibit properties typical of a crystalline or of a liquid system. We compare the thermal and the quantum results, identifying analogies and differences. Finally, we discuss experimental parameter regimes where the effects of quantum fluctuations on the linear - zigzag transition can be observed.
We investigate the confinement effects on a two-dimensional mesoscopic (particle number N >200) self-assembled dipolar crystal trapped in a harmonic potential. The ground-state wave function and energy are calculated by diagonalizing the position fluctuations to the quadratic order within the strong-interaction limit. We further investigate the crystal-liquid domain wall boundary as well as the system temperature by changing the confinement aspect ratio and/or dipole moment strength. We find that the entropy of a trapped dipolar crystal can be much larger than the entropy in the liquid state at zero dipole moment, indicating a polarization cooling mechanism during the formation of dipolar crystal when the external field is increased adiabatically. Our results therefore have significant application to the experimental preparation of a self-assembled dipolar crystal using ultracold polar molecules.
It is well-known that the liquid properties in a strongly confined system can be very different from their ordinary behaviors in an extended system, due to the competition between the thermal energy and the interaction energy. Here we show that, in a low-dimensional self-assembled dipolar crystal, the parabolic optical confinement potential can also strongly affect the quantum many-body properties in the low temperature regime. For example, by changing the confinement aspect ratio, the bulk of the system can undergo a quantum phase transition between a liquid state and a solid state via a nonmonotonic pattern formation of the domain wall. Furthermore, the entropy of a trapped dipolar crystal can be much larger than the liquid state in the weak dipole limit, indicating an intrinsic polarization cooling mechanism via increasing the external field. These highly correlated confinement effects are very important to the experimental preparation of a self-assembled dipolar crystal using ultracold polar molecules.
Low-temperature properties of harmonically confined two-dimensional assemblies of dipolar bosons are systematically investigated by Monte Carlo simulations. Calculations carried out for different numbers of particles and strengths of the confining potential yield evidence of a quantum phase transition from a superfluid to a crystal-like phase, consistently with what is observed in the homogeneous system. It is found that the crystal phase nucleates in the center of the trap, as the density increases. Bose-Einstein condensation vanishes at T=0 upon entering the crystalline phase, concurrently with the disappearance of the superfluid response.
We investigate the finite-temperature phase diagram of polar molecules confined in a quasi-two-dimensional geometry by a harmonic potential along the polarization axis. We employ quantum Monte Carlo simulations to explore the strongly correlated regime accessible with current experimental setups. By tuning temperature and confinement strength, we identify a rich set of phases, including normal fluid, superfluid, supersolid, cluster crystal, and bilayer crystal states. Our results reveal the emergence of crystallization upon increasing temperature, highlighting the nontrivial role of thermal fluctuations in dipolar systems. In particular, we show that a bilayer crystal with one molecule per lattice site can be stabilized by varying the confinement strength at fixed interaction. Moreover, we show evidence of layering of superfluid states with phase coherence between the two layers. These findings provide insight into the interplay between interactions, confinement, and temperature in low-dimensional dipolar systems, and suggest new directions for engineering quantum phases with ultracold polar molecules.