Phase diagram of dipolar bosons in two dimensions with tilted polarization

Abstract

We analyze the ground state of a system of dipolar bosons moving in the XY plane and such that their dipolar moments are all aligned in a fixed direction in space. We focus on the general case where the polarization field forms a generic angle α with respect to the Z axis. We use the path-integral ground-state method to analyze the static properties of the system as both α and the density n vary over a wide range where the system is stable. We use the maximum of the static structure function as an order parameter to characterize the different phases and the transition lines among them. We find that, in addition to a superfluid gas and a solid phase, the system reaches a stripe phase at large tilting angles that is entirely induced by the anisotropic character of the interaction. We also show that the quantum phase transition from the gas to the stripe phase is of second order and report approximate values for the critical exponents. In recent years, dipolar Bose gases have received much attention. The study of quantum degenerate gases of dipolar species has become one of the most active areas of experimental and theoretical research in the field of ultracold atoms [1][2][3]. From the theoretical point of view, the anisotropic and long-range character of the interaction makes dipolar systems unique, exhibiting features such as p-wave superfluidity in two-dimensional (2D) Fermi gases [4] or roton instability [5][6][7][8].Up to now, little attention has been paid to the 2D case where dipoles are polarized along an arbitrary direction, including the analysis of scattering properties [9] or the superfluid and collapse instabilities of a quasi-two-dimensional gas of dipolar fermions aligned by an external field [4]. Experiments such as those reported in Ref. [10] have also investigated the effect of (quasi-)2D confinement of a cloud of dipolar particles. One remarkable feature induced by the anisotropy of the interaction is the emergence of a stripe phase, which has been predicted to appear in both Bose [7] and Fermi [11][12][13] systems. Some calculations, in the mean-field approximation, predict the appearance of stripes even in the isotropic case where all dipoles are polarized perpendicular to the plane of movement, although recent Monte Carlo calculations for dipolar fermions arrived at a different conclusion [14]. In Ref.[7] we used path-integral ground-state (PIGS) calculations to reveal the existence of a stripe phase at large densities and polarization angles in 2D dipolar bosonic systems. In this work we extend the previous analysis and discuss the complete phase diagram at zero temperature, characterizing the stripe phase as a function of the density and polarization angle and determining the corresponding solid and stripe transition lines together with the critical exponents.In previous works we discussed the low-density properties [15] and elementary excitation spectrum [7] of the fully anisotropic 2D dipolar interaction. In this work we extend the analysis and investigate the phase diagram of a 2D system...

Vortices and vortex lattices in quantum ferrofluids

“…The relatively recent realization of dipolar cold atoms and molecules now provides an alternative venue for investigating supersolid phases. Apart from the strongly correlated dipolar systems [70,[72][73][74][75][76] cited in section 2, extended Bose-Hubbard lattice models are also thought to support supersolidity [287][288][289][290][291][292][293][294][295][296][297]. The common ingredient in both sets of investigations is the presence of long-range interactions.…”

“…Strongly correlated dipolar gases do not yet exist in the laboratory but ideas to realize them include using ultracold molecules with very large dipole moments dressed by microwaves [70] and Rydberg dressed ultracold gases [71]. One of the main interests in these systems is the formation of so-called supersolids which are crystalline and yet also have superfluid properties [70,[72][73][74][75][76]. Unlike the density wave structures we shall study later in this review, which have many atoms per wavelength, in the strongly correlated case the periodicity can be at the single atom or few atom length scale.…”

Dipolar Bose Supersolid Stripes

“…Stripe phases have also been discussed in the context of quantum dipolar physics, including very recent theoretical and experimental analysis of metastable striped gases of 164 Dy [10]. Because of the anisotropic character of the dipolar interaction, in some regions of the phase diagram dipoles arrange in stripes, both in Fermi [11,12] and Bose [13,14] systems. In some cases the presence of this phase has been reported to exist even in the isotropic limit [15].…”

“…In a previous work we determined the phase diagram [14] of the two-dimensional system of Bose dipoles at zero temperature, tracing the transition lines between the solid, gas and stripe phases. The formation and excitation spectrum of the stripe phase, where the system acquires crystal order in one direction while being fluid on the other, was previously analyzed in Ref.…”

Two-dimensional bright solitons in dipolar Bose-Einstein condensates with tilted dipoles

“…1) on the properties and stability of 2D bright solitons in dipolar BECs. Introducing the tilting angle α with respect to the normal vector of the quasi-2D trap plane [29][30][31] disregarded regimes of interaction parameters for soliton stability as well as to manipulate the soliton anisotropy in a more controlled manner. These features may enhance the possibilities of realizing 2D BEC solitons experimentally in the stateof-the-art dipolar BECs.…”