Search

We demonstrate the formation of two types of symbiotic nondipolar droplet supersolid in a binary dipolar-nondipolar mixture with an interspecies atraction, where the dipolar (nondipolar) atoms are trapped (untrapped). In the absence of an interspecies attraction, in the first type, a dipolar droplet supersolid exists, whereas in the second type, there are no droplets in the dipolar component. To illustrate, we consider a $^{164}$Dy-$^{87}$Rb mixture, where the untrapped $^{87}$Rb supersolid sticks to the trapped $^{164}$Dy supersolid due to the interspecies attraction and forms a symbiotic supersolid with overlapping droplets. The first (second) type of symbiotic supersolid emerges for the scattering length $ a_1=85a_0$ ($a_1=95a_0$) of $^{164}$Dy atom, while under an appropriate trap a dipolar droplet supersolid exists (does not exist) for no interspecies interaction, where $a_0$ the Bohr radius. This study is based on the numerical solution of an improved binary mean-field model, where we introduce an intraspecies Lee-Huang-Yang interaction in the dipolar component, which stops a dipolar collapse and forms a dipolar supersolid.To observe this symbiotic droplet supersolid, one should prepare the corresponding fully trapped dipolar-nondipolar supersolid and then remove the trap on the nondipolar atoms.

We study the formation of quasi-one- (quasi-1D) and quasi-two-dimensional (quasi-2D) symbiotic solitons bound by an interspecies dipolar interaction in a binary dipolar Bose-Einstein condensate. These binary solitons have a repulsive intraspecies contact interaction stronger than the intraspecies dipolar interaction, so that they can not be bound in isolation in the absence of an interspecies dipolar interaction. These symbiotic solitons are bound in the presence of an interspecies dipolar interaction and zero interspecies contact interaction. The quasi-1D solitons are free to move along the polarization $z$ direction of the dipolar atoms, whereas the quasi-2D solitons move in the $x$-$z$ plane. To illustrate these, we consider a $^{164}$Er-$^{166}$Er mixture with scattering lengths $a$($^{164}$Er)$ =81a_0$ and $a$($^{166}$Er)$ =68a_0$ and with dipolar lengths $a_{\mathrm{dd}}$($^{164}$Er)$\approx a_{\mathrm{dd}}$($^{166}$Er)$\approx 65a_0$, where $a_0$ is the Bohr radius. In each of the two components $a> a_{\mathrm{dd}}$, which stops the binding of solitons in each component in isolation, whereas a binary quasi-1D or a quasi-2D $^{164}$Er-$^{166}$Er soliton is bound in the presence of an interspecies dipolar interaction. The stationary states were obtained by imaginary-time propagation of the underlying mean-field model; dynamical stability of the solitons was established by real-time propagation over a long period of time.

A Bose-Einstein condensate (BEC) formed on a curved surface with a distinct topology has been a hot topic of intense research, in search of new phenomena in quantum physics as well as for its possible application in quantum computing. In addition to the study of a spherical-shell-shaped BEC, we studied the formation of a cylindrical-shell-shaped harmonically-trapped dipolar BEC of $^{164}$Dy atoms theoretically using an improved mean-field model including a Lee-Huang-Yang-type interaction, meant to stop a collapse at high atom density. To test the robustness of the cylindrical-shell-shaped BEC, here we study its expansion in the same model. We find that as the harmonic trap in the $x$ and $y$ directions are removed, maintaining the axial trap, the cylindrical-shell-shaped BEC expands in the $x$-$y$ plane without deformation, maintaining its shell-shaped structure. After an adequate radial expansion, the axial trap can be relaxed for a desired axial expansion of the cylindrical-shell-shaped BEC allowing its observation.

We demonstrate the spontaneous formation of a Bose-Einstein condensate (BEC) of strongly-bound harmonically-trapped dipolar $^{164}$Dy atoms on the outer curved surface of an elliptical or a circular cylinder, with a distinct topology, employing the numerical solution of an improved mean-field model including a Lee-Huang-Yang-type interaction, meant to stop a collapse at high atom density, the axis of the cylindrical-shell-shaped BEC being aligned along the polarization direction of the dipolar atoms. These states are dynamically stable and a Gaussian initial state leads to the cylindrical-shell-shaped state in both imaginary-time and real-time propagation. The formation of the hollow cylindrical BEC by a real-time simulation starting from a solid cylindrical state demonstrate the possibility of the formation of such a condensate experimentally.

We demonstrate mini droplet, mega droplet and stripe formation in a dipolar 164Dy condensate, using an improved mean-field model including a Lee-Huang-Yang-type interaction, employing a quasi-two-dimensional (quasi-2D) trap in a way distinct from that in the pioneering experiment, M. A. Norcia et. al., Nature 596, 357 (2021), where the polarization z direction was taken to be perpendicular to the quasi-2D x-y plane. In the present study we take the polarization z direction in the quasi-2D x-z plane. Employing the same trapping frequencies as in the experiment, and interchanging the frequencies along the y and z directions, we find the formation of mini droplets for number of atoms N as small as N = 1000. With the increase of number of atoms, a spatially-periodic supersolid-like one-dimensional array of mega droplets containing 50000 to 200000 atoms are formed along the x direction in the x-y plane. These mega droplets are elongated along the polarization z direction, consequently, the spatially periodic arrangement of droplets appears as a stripe pattern in the x-z plane. To establish the supersolidity of the droplets we demonstrate continued dipole-mode and scissors-mode oscillations of the droplet-lattice pattern. The main findings of the present study can be tested experimentally with the present know-how.

We study ultracold superfluid Bose-Fermi mixtures in three dimensions, with stronger confinement along one or two directions, using a non-perturbative beyond-mean-field model for bulk chemical potential valid along the weak-coupling to unitarity crossover. Although bosons are considered to be in a superfluid state, we consider two possibilities for the fermions -- spin-polarized degenerate state and superfluid state. Simplified reduced analytic lower-dimensional models are derived along the weak-coupling to unitarity crossover in quasi-one-dimensional (quasi-1D) and quasi-two-dimensional (quasi-2D) settings. The only parameters in these models are the constants of the beyond-mean-field Bose-Bose and Fermi-Fermi Lee-Huang-Yang interactions and the respective universal Bertsch parameter at unitarity. In addition to the numerical results for a fully-trapped system, we also present results for quasi-2D Bose-Fermi mixtures where one of the components is untrapped but localized due to the interaction mediated by the other component. We demonstrate the validity of the reduced quasi-1D and quasi-2D models via a comparison of the numerical solutions for the ground states obtained from the reduced models and the full three-dimensional (3D) model.

We study dipole-mode and scissors-mode oscillations of a harmonically-trapped dipolar supersolid, composed of dipolar droplets arranged on a one-dimensional (1D) or a two-dimensional (2D) lattice, to establish the robustness of its crystalline structure under translation and rotation, using a beyond-mean-field model including a Lee-Huang-Yang interaction. The dipolar atoms are polarized along the $z$ direction with the supersolid crystalline structure lying in the $x$-$y$ plane. A stable dipole-mode oscillation was possible in case of both quasi-1D and quasi-2D dipolar supersolids, whereas a sustained angular scissors-mode oscillation was possible only in the case of a quasi-1D dipolar supersolid between a maximum and a minimum of trap anisotropy in the $x$-$y$ plane. In both cases there was no visible deformation of the crystalline structure of the dipolar supersolid during the oscillation. The theoretical estimate of the scissors-mode-oscillation frequency was in good agreement with the present results and the agreement improved with an increase of the number of droplets in the supersolid and also with an increase in the confining trap frequencies. The results of this study can be tested experimentally with present knowhow.

We demonstrate spontaneous generation of spatially-periodic supersolid-like super-lattice and stripe solitons in Rashba spin-orbit (SO) coupled spin-1 and spin-2 quasi-two-dimensional nonmagnetic Bose-Einstein condensates (BECs). The solitons in a weakly SO-coupled spin-1 BEC are circularly-symmetric of $(-1, 0, +1)$ and $(0, +1, +2)$ types and have inherent vorticity; the numbers in the parentheses are the winding numbers in hyper-spin components $+1, 0, -1$, respectively. The circularly-symmetric solitons in an SO-coupled spin-2 BEC are of types $(-2, -1, 0, +1, +2)$ and $(-1, 0, +1, +2, +3)$ with the former being the ground state, where the winding numbers correspond to spin components $+2, +1, 0, -1, -2$, respectively. For stronger SO-coupling strengths, these solitons acquire a multiring structure while preserving the winding numbers. Quasi-degenerate stripe and super-lattice solitons, besides a circularly-asymmetric soliton, also emerge as excited stationary states for stronger SO-coupling strengths in spin-1 and spin-2 BECs. pla-cl.tex

Using a beyond-mean-field model including a Lee-Huang-Yang-type interaction, we demonstrate a supersolid-like spatially-periodic square- and triangular-lattice crystallization of droplets in a polarized dipolar condensate confined by an appropriate three-dimensional (3D) box trap. In this paper we consider a rectangular box (cuboid) trap, a square box (cuboid with two equal sides) trap, a cylindrical box trap and a hexagonal box (hexagonal prism) trap. The droplet lattice is always formed in the $x$-$y$ plane perpendicular to the polarization $z$ direction of dipolar atoms. In contrast to a harmonic trap, the box traps allow the formation of a large clean supersolid-like spatially-periodic crystallization in free space without any distortion. Moreover, a droplet lattice can be formed in a 3D box trap with a significantly reduced number of atoms than in a harmonic trap, which could facilitate the experimental observation of droplet lattice in a box trap. With present know-how such a supersolid-like crystallization of dipolar droplets in a 3D box trap can be realized in a laboratory thus allowing the study of a large periodic lattice of dipolar droplets in free space bounded by rigid walls.

We study the effect of beyond-mean-field quantum-fluctuation (QF) Lee-Huang-Yang (LHY) and three-body interactions, with quartic and quintic nonlinearities, respectively, on the formation of a stable self-repulsive (positive scattering length $a$) and a self-attractive (negative $a$) self-bound dipolar Bose-Einstein condensate (BEC) droplet in free space under the action of two-body contact and dipolar interactions. Previous studies of dipolar droplets considered either the LHY interaction or the three-body interaction, as either of these interactions could stabilize a dipolar BEC droplet against collapse. We find that the effect of three-body recombination on the formation of a dipolar droplet could be quite large and for a complete description of the problem both the QF LHY and three-body interactions should be considered simultaneously, where appropriate. In the self-repulsive case for small $a$ and in the self-attractive case, no appropriate LHY interaction is known and only three-body interaction should be used, otherwise both beyond-mean-field interactions should be used. We consider a numerical solution of a highly-nonlinear beyond-mean-field model as well as a variational approximation to it in this investigation and present results for size, shape and energy of a dipolar droplet of polarized $^{164}$Dy atoms. The shape is filament-like, along the polarization direction, and could be long, for a large number of atoms $N$, short for small $N$, thin for negative $a$ and small positive $a$, and fat for large positive $a$.

We demonstrate a supersolid-like spatially-periodic square- and honeycomb-lattice crystallization of droplets, in addition to the commonly-studied triangular-lattice crystallization, in a cylindrically-symmetric quasi-two-dimensional trapped dipolar condensate, using a beyond-mean-field model including a quantum-fluctuation Lee-Huang-Yang-type interaction. These three types of crystallization of droplets may appear for the same atomic interactions and the same trap frequencies. The energy $E$ of all three crystallization as a function of number $N$ of atoms satisfy the universal scaling relation $E\sim N^{0.4}$ indicating that all three arrangements of the droplets should be energetically probable processes of phenomenological interest. The state of square-lattice crystallization may have the central site occupied or unoccupied, corresponding to a parity-symmetric or parity-antisymmetric state, respectively. The state of square-lattice crystallization with the occupied central site and the state of triangular-lattice crystallization, for a fixed $N$, constitute two quasi-degenerate ground states while the other states are low-lying excited states. This makes the square-lattice crystallization with the occupied central site an ideal candidate for future experimental observation.

The possibility of generating stable anisotropic solitons in dipolar Bose-Einstein condensates (BECs) leads to a different scenario not possible in a nondipolar BEC with contact interaction. We study the statics and dynamics of anisotropic bright solitons in quasi-two-dimensional BECs consisting of polarized dipolar atoms.We study the collision dynamics of two such solitons at different velocities for different angles between the polarization and collision directions. The collision is found to be quasi elastic at large velocities. At small velocities the collision is inelastic leading to the formation of a coalesced soliton in an excited scissors mode, monopole mode or quadrupole mode. Also, at small velocities, after collision, a large change of direction of motion of the solitons is possible. The investigation is performed through a numerical solution of the underlying mean-field Gross-Pitaevskii equation.

We study supersolid-like crystalline structures emerging in the stationary states of a quasi-two-dimensional spin-orbit (SO)-coupled spin-2 condensate in the ferromagnetic, cyclic, and antiferro-magnetic phases by solving a mean-field model.Interplay of different strengths of SO coupling and interatomic interactions gives rise to a variety of non-trivial density patterns in the emergent solutions. For small SO-coupling strengths $\gamma$ ($\gamma \approx 0.5$), the ground state is an axisymmetric multi-ring soliton for polar, cyclic and weakly-ferromagnetic interactions, whereas for stronger-ferromagnetic interactions a circularly-asymmetric soliton emerges as the ground state.Depending on the values of interaction parameters, with an increase in SO-coupling strength, a stripe phase may also emerge as the ground state for polar and cyclic interactions. For intermediate values of SO-coupling strength ($\gamma \approx 1$), in addition to these solitons, one could have a quasi-degenerate triangular-lattice soliton in all magnetic phases. On further increasing the SO-coupling strength ($\gamma \gtrapprox 4$), a square-lattice and a superstripe soliton emerge as quasi-degenerate states. The emergence of all these solitons can be inferred from a study of solutions of the single-particle Hamiltonian.

We study the formation of spin-1 symbiotic spinor solitons in a quasi-one- (quasi-1D) and quasi-two-dimensional (quasi-2D) hyper-fine spin $F=1$ ferromagnetic Bose-Einstein condensate (BEC). The symbiotic solitons necessarily have a repulsive intraspecies interaction and are bound due to an attractive interspecies interaction. Due to a collapse instability in higher dimensions, an additional spin-orbit coupling is necessary to stabilize a quasi-2D symbiotic spinor soliton. Although a quasi-1D symbiotic soliton has a simple Gaussian-type density distribution, novel spatial periodic structure in density is found in quasi-2D symbiotic SO-coupled spinor solitons. For a weak SO coupling, the quasi-2D solitons are of the $(-1, 0, +1)$ or $(+1, 0, -1)$ type with intrinsic vorticity and multi-ring structure, for Rashba or Dresselhaus SO coupling, respectively, where the numbers in the parentheses are angular momenta projections in spin components $F_z = +1, 0, -1$, respectively. For a strong SO coupling, stripe and superlattice solitons, respectively, with a stripe and square-lattice modulation in density, are found in addition to the multi-ring solitons. The stationary states were obtained by imaginary-time propagation of a mean-field model; dynamical stability of the solitons was established by real-time propagation over a long period of time. The possibility of the creation of such a soliton by removing the trap of a confined spin-1 BEC in a laboratory is also demonstrated.

We demonstrate the formation of stable spatially-ordered states in a {\it uniform} and also {\it trapped} quasi-two-dimensional (quasi-2D) Rashba or Dresselhaus spin-orbit (SO) coupled pseudo spin-1/2 Bose-Einstein condensate using the mean-field Gross-Pitaevskii equation. For weak SO coupling, one can have a circularly-symmetric $(0,+1)$- or $(0,-1)$-type multi-ring state with intrinsic vorticity, for Rashba or Dresselhaus SO coupling, respectively, where the numbers in the parentheses denote the net angular momentum projection in the two components, in addition to a circularly-asymmetric degenerate state with zero net angular momentum projection. For intermediate SO couplings, in addition to the above two types, one can also have states with stripe pattern in component densities with no periodic modulation in total density. The stripe state continues to exist for large SO coupling. In addition, a new spatially-periodic state appears in the uniform system: a superlattice state, possessing some properties of a supersolid, with a square-lattice pattern in component densities and also in total density. In a trapped system the superlattice state is slightly different with multi-ring pattern in component density and a square-lattice pattern in total density. For an equal mixture of Rashba and Dresselhaus SO couplings, in both uniform and trapped systems, only stripe states are found for all strengths of SO couplings. In a uniform system all these states are quasi-2D solitonic states., Comment: arXiv admin note: text overlap with arXiv:2012.00872

We study supersolid-like states in a quasi-two-dimensional trapped Rashba and Dresselhaus spin-orbit (SO) coupled spin-1 condensate. For small strengths of SO coupling $\gamma$ ($\gamma \lessapprox 0.75$), in the ferromagnetic phase, circularly-symmetric $(0,\pm 1, \pm 2)$- and $(\mp 1, 0,\pm 1)$-type states are formed where the numbers in the parentheses denote the angular momentum of the vortex at the center of the components and where the upper (lower) sign correspond to Rashba (Dresselhaus) coupling; in the antiferromagnetic phase, only $(\mp 1, 0,\pm 1)$-type states are formed. For large strengths of SO coupling, supersolid-like superlattice and superstripe states are formed in the ferromagnetic phase. In the antiferromagnetic phase, for large strengths of SO coupling, supersolid-like superstripe and multi-ring states are formed. For an equal mixture of Rashba and Dresselhaus SO couplings, only a superstripe state is found. All these states are found to be dynamically stable and hence accessible in an experiment and will enhance the fundamental understanding of crystallization onto spatially periodic states in solids.

We demonstrate spontaneous spatial order in stripe and super-lattice solitons in a Rashba spin-orbit (SO) coupled spin-1 uniform quasi-two-dimensional (quasi-2D) ferromagnetic Bose-Einstein condensate. For weak SO coupling, the solitons are of the $(- 1,0,+ 1)$ or $(0,+ 1,+ 2)$ type with intrinsic vorticity, where the numbers in the parentheses denote angular momentum in spin components $F_z=+1,0,-1$, respectively. For intermediate SO coupling, three types of solitons are found: (a) circularly-asymmetric solitons, (b) circularly-symmetric $(- 1,0,+ 1)$- and (c) $(0,+ 1,+ 2)$-type multi-ring solitons maintaining the above-mentioned vortices in respective components. For large SO coupling, quasi-degenerate stripe and super-lattice solitons are found in addition to the circularly-asymmetric solitons. A super-lattice soliton forms a 2D square lattice structure in the total density as in a super-solid; in component densities it may have either (i) a 1D stripe pattern or (ii) a 2D square lattice structure., Comment: arXiv admin note: text overlap with arXiv:2012.00872

We demonstrate exotic stable quasi-two-dimensional solitons in a Rashba or a Dresselhaus spin-orbit (SO) coupled hyperfine spin-1 ($F=1$) trap-less antiferromagnetic Bose-Einstein condensate using the mean-field Gross-Pitaevskii equation. For weak SO coupling, the solitons are of the $(-1,0,+1)$ or $(+1,0,-1)$ type with intrinsic vorticity, for Rashba or Dresselhaus SO coupling, where the numbers in the parentheses denote angular momentum in spin components $F_z= +1,0,-1$, respectively. For intermediate SO coupling, the solitons have multi-ring structure maintaining the above-mentioned vortices in respective components. For larger SO coupling, superlattice solitons with a square-lattice structure in total density are found in addition to stripe solitons with stripe pattern in component densities with no periodic modulation in total density.

We study the vortex-lattice formation in a rotating {Rashba} spin-orbit (SO) coupled quasi-two-dimensional (quasi-2D) hyper-fine spin-1 spinor Bose-Einstein condensate (BEC) in the $x-y$ plane using a numerical solution of the underlying mean-field Gross-Pitaevskii equation. % The wave function for this system %has three components corresponding to the three projections of hyper-fine spin $F_z= +1,0,-1$. In this case, the non-rotating {Rashba} SO-coupled spinor BEC can have topological excitation in the form of vortices of different angular momenta in the three components, e.g. the $(0,+1,+2)$- and $(-1,0,+1)$-type states in ferromagnetic and anti-ferromagnetic spinor BEC: the numbers in the parenthesis denote the intrinsic angular momentum of the vortex states of the three components with the negative sign denoting an anti-vortex. The presence of these states with intrinsic vorticity breaks the symmetry between rotation with vorticity along the $z$ and $-z$ axes and thus generates a rich variety of vortex-lattice and anti-vortex-lattice states in a rotating quasi-2D spin-1 spinor ferromagnetic and anti-ferromagnetic BEC, not possible in a scalar BEC. {For weak SO coupling, } we find two types of symmetries of these states $-$ hexagonal and "square". The hexagonal (square) symmetry state has vortices arranged in closed concentric orbits with a maximum of $6, 12, 18...$ ($8,12,16...$) vortices in successive orbits. Of these two symmetries, the square vortex-lattice state is found to have the smaller energy.

We demonstrate the formation of multi-peak three-component stationary stripe vector solitons in a quasi-one-dimensional spin-orbit-coupled hyper-fine spin $F = 1$ polar Bose-Einstein condensate. The present investigation is carried out through a numerical solution by imaginary-time propagation and an analytic variational approximation of the underlying mean-field Gross-Pitaevskii equation. Simple analytic results for energy and component densities were found to be in excellent agreement with the numerical results for solitons with more than 100 pronounced maxima and minima. The vector solitons are one of the two types: dark-bright-dark or bright-dark-bright. In the former a maximum density in component $ F_z = 0 $ at the center is accompanied by a zero in components $F_z = \pm 1$. The opposite happens in the latter case. The vector solitons are demonstrated to be mobile and dynamically stable. The collision between two such vector solitons is found to be quasi elastic at large velocities with the conservation of total density of each vector soliton. However, at very small velocity, the collision is inelastic with a destruction of the initial vector solitons. It is possible to observe and study the predicted SO-coupled vector solitons in a laboratory., Comment: arXiv admin note: text overlap with arXiv:1911.03498

We study the phase-separation in three-component bright vector solitons in a quasi-one-dimensional spin-orbit-coupled hyper-fine spin $F=1$ ferromagnetic Bose-Einstein condensate upon an increase of the strength of spin-orbit (SO) coupling $p_x \Sigma_z$ above a critical value, where $p_x$ is the linear momentum and $\Sigma_z$ is the $z$-component of the spin-1 matrix. The bright vector solitons are demonstrated to be mobile and dynamically stable. The collision between two such vector solitons is found to be elastic at all velocities with the conservation of density of each vector soliton. The two colliding vector solitons repel at small separation and at very small colliding velocity, they come close and bounce back with the same velocity without ever encountering each other. This repulsion produced by SO coupling is responsible for the phase separation in a vector soliton for large strengths of SO coupling. { The collision dynamics is found to be completely insensitive to the relative phase of the colliding solitons.} However, in the absence of SO coupling, at very small velocity, the two colliding vector solitons attract each other and form a vector soliton molecule and the collision dynamics is sensitive to the relative phase as in scalar solitons. The present investigation is carried out through a numerical solution and an analytic variational approximation of the underlying mean-field Gross-Pitaevskii equation.

We study spontaneous-symmetry-broken phase-separated vortex lattice in a weakly interacting uniform rapidly rotating binary Bose superfluid contained in a quasi-two-dimensional circular or square bucket. For the inter-species repulsion above a critical value, the two superfluid components separate and form a demixed phase with practically no overlap in the vortex lattices of the two components, which will permit an efficient experimental observation of such vortices and study their properties. In case of a circular bucket with equal intra-species energies of the two components, the two components separate into two non-overlapping semicircular domains for all frequencies of rotation $\Omega$ generating distinct demixed vortex lattices. In case of a binary Bose superfluid in both circular and square buckets, (a) the number of vortices increases linearly with $\Omega$ in agreement with a suggestion by Feynman, and (b) the rotational energy in the rotating frame decreases quadratically with $\Omega$ in agreement with a suggestion by Fetter., Comment: arXiv admin note: text overlap with arXiv:1908.07848

We study spontaneous-symmetry-breaking circularly-asymmetric phase separation of vortex lattices in a rapidly rotating harmonically-trapped quasi-two-dimensional (quasi-2D) binary Bose-Einstein condensate (BEC) with repulsive inter- and intra-species interactions. The phase separated vortex lattices of the components appear in different regions of space with no overlap between the vortices of the two components, which will permit an efficient experimental observation of such vortices and accurate study of the effect of atomic interaction on such vortex lattice. Such phase separation takes place when the intra-species interaction energies of the two components are equal or nearly equal with relatively strong inter-species repulsion. When the intra-species energies are equal, the two phase-separated vortex lattices have identical semicircular shapes with one being the parity conjugate of the other. When the intra-species energies are nearly equal, the phase separation is also complete but the vortex lattices have different shapes. We demonstrate our claim with a numerical solution of the mean-field Gross-Pitaevskii equation for a rapidly rotating quasi-2D binary BEC., Comment: arXiv admin note: text overlap with arXiv:1811.06816

In a harmonically-trapped rotating Bose-Einstein condensate (BEC), a vortex of large angular momentum decays to multiple vortices of unit angular momentum from an energetic consideration. We demonstrate the formation of a robust and dynamically stable giant vortex of large angular momentum in a harmonically trapped rotating BEC with a potential hill at the center, thus forming a Mexican hat like trapping potential. For a small inter-atomic interaction strength, a highly controllable stable giant vortex appears, whose angular momentum slowly increases as the angular frequency of rotation is increased. As the inter-atomic interaction strength is increased beyond a critical value, only vortices of unit angular momentum are formed, unless the strength of the potential hill at the center is also increased: for a stronger potential hill at the center a giant vortex is again formed. The dynamical stability of the giant vortex is demonstrated by real-time propagation numerically. These giant vortices of large angular momentum can be observed and studied experimentally in a highly controlled fashion.

The usual treatment of a Bose-Fermi mixture relies on weak-coupling Gross-Pitaevskii (GP) and density-functional (DF) Lagrangians, often including the more realistic {perturbative} Lee-Huang-Yang (LHY) corrections. We suggest analytic {non-perturbative} beyond-mean-field Bose and Fermi Lagrangians valid along the crossover from weak- to strong-coupling limits of intra-species interactions consistent with the LHY corrections and the strong-coupling (unitarity) limit for small and large scattering lengths $|a|$, respectively, and use these to study the Bose-Fermi mixture. We study numerically mixing-demixing and spontaneous symmetry breaking in Bose-Fermi mixtures in spherically-symmetric and quasi-one-dimensional traps while the intra-species Bose and Fermi interactions are varied from weak-coupling to strong-coupling limits.The LHY correction is appropriate for medium to weak atomic interactions and diverges for stronger interactions (large scattering length $|a|$), whereas the present beyond-mean-field Lagrangian is finite in the unitarity limit ($|a|\to \infty$). We illustrate our results using the Bose-Fermi $^7$Li-$^6$Li mixture under a spherically-symmetric and a quasi-one-dimensional trap. The results obtained with the present model for density distribution of the Bose-Fermi mixture along the crossover could be qualitatively different from the usual GP-DF Lagrangian with or without LHY corrections. Specifically, we identified spontaneous symmetry breaking and demixing in the present model not found in the usual model with the same values of the parameters.

We study numerically the vortex-lattice formation in a rapidly rotating uniform quasi-two-dimensional Bose-Einstein condensate (BEC) in a box trap. We consider two types of boxes: square and circle. In a square-shaped 2D box trap, when the number of generated vortices is the square of an integer, the vortices are found to be arranged in a perfect square lattice, although deviations near the center are found when the number of generated vortices is arbitrary. In case of a circular box trap, the generated vortices in the rapidly rotating BEC lie on concentric closed orbits. Near the center, these orbits have the shape of polygons, whereas near the periphery the orbits are circles. The circular box trap is equivalent to the rotating cylindrical bucket used in early experiment(s) with liquid He II. The number of generated vortices in both cases is in qualitative agreement with Feynman's universal estimate. The numerical simulation for this study is performed by a solution of the underlying mean-field Gross-Pitaevskii (GP) equation in the rotating frame, where the wave function for the generated vortex lattice is a stationary state. Consequently, the imaginary-time propagation method can be used for a solution of the GP equation, known to lead to an accurate numerical solution. We also demonstrated the dynamical stability of the vortex lattices in real-time propagation upon a small change of the angular frequency of rotation, using the converged imaginary-time wave function as the initial state.

We study circularly-symmetric phase separation of vortex lattices in a rapidly rotating harmonically-trapped quasi--two-dimensional binary Bose-Einstein condensate (BEC) by introducing a weak quartic trap in one of the components. The increase of the rotational frequency in such a system is also found to generate a phase separation of the vortex lattices of an overlapping non-rotating BEC. The phase-separated vortex lattices have different structures for a binary BEC with inter-species repulsion and inter-species attraction. In the former case of a fully repulsive binary BEC the phase separation of the vortex-lattices is accompanied by a complete phase separation of component densities. In the latter case of inter-species attraction there is a partial phase separation of component densities, although there could be a complete phase separation of the generated vortex lattices in the two components. In the case of inter-species attraction, we need to have different intra-species repulsion in the two components for an efficient phase separation. We compare and contrast our results with the phase separation obtained in a harmonically-trapped binary BEC without any quartic trap.

The perturbative Lee-Huang-Yang (LHY) interaction proportional to $n^{3/2}$, where $n$ is the density, creates an infinitely repulsive potential at the center of a Bose-Einstein condensate (BEC) with net attraction, which stops the collapse to form a self-bound state in a dipolar BEC and in a binary BEC. However, recent microscopic calculations of the non-perturbative beyond-mean-field (BMF) interaction indicate that the LHY interaction %with the $n^{3/2}$ term is valid only for very small values of gas parameter $x$. We show that a realistic non-perturbative BMF interaction can stop collapse and form a self-bound state only in a weakly attractive binary BEC with small $x$ values ($x\lessapprox 0.01$), whereas the perturbative LHY interaction stops collapse for all attractions. We demonstrate these aspects using an analytic BMF interaction with appropriate weak-coupling LHY and strong coupling limits.

We study the formation of a stable self-trapped spherical quantum ball in a binary Bose-Einstein condensate (BEC) with two-body inter-species attraction and intra-species repulsion employing the beyond-mean-field Lee-Huang-Yang and the three-body interactions. We find that either of these interactions or a combination of these can stabilize the binary BEC quantum ball with very similar stationary results, and for a complete description of the problem both the terms should be considered. These interactions lead to higher-order nonlinearities, e.g. quartic and quintic, respectively, in a nonlinear dynamical equation compared to the cubic nonlinearity of the two-body contact interaction in the mean-field Gross-Pitaevskii equation. The higher-order nonlinearity makes the energy infinitely large at the center of the binary ball and thus avoids its collapse. In addition to the formation of stationary binary balls, we also study a collision between two such balls. At large velocities, the collision is found to be elastic, which turns out to be inelastic as the velocity is lowered. We consider the numerical solution of a beyond-mean-field model for the binary ball as well as a single-mode variational approximation to it in this study.

The textbook effective-range expansion of scattering theory is useful in the analysis of low-energy scattering phenomenology when the scattering length $|a|$ is much larger than the range $R$ of the scattering potential: $|a|\gg R$. Nevertheless, the same has been used for systems where the scattering length is much smaller than the range of the potential, which could be the case in many scattering problems. We suggest and numerically study improved two-parameter effective-range expansions for the cases $|a| > R$ and $|a| < R$. The improved effective-range expansion for $|a| > R$ reduces to the textbook expansion for $|a|/R \gg 1$.

The formation of a regular lattice of quantized vortices in a fluid under rotation is a smoking-gun signature of its superfluid nature. Here we study the vortex lattice in a dilute superfluid gas of bosonic atoms at zero temperature along the crossover from the weak-coupling regime, where the inter-atomic scattering length is very small compared to the average distance between atoms, to the unitarity regime, where the inter-atomic scattering length diverges. This study is based on high-performance numerical simulations of the time-dependent nonlinear Schrodinger equation for the superfluid order parameter in three spatial dimensions, using a realistic analytic expression for the bulk equation of state of the system along the crossover from weak-coupling to unitarity. This equation of state has the correct weak-coupling and unitarity limits and faithfully reproduces the results of an accurate multi-orbital microscopic calculation. Our numerical predictions of the number of vortices and root-mean-square sizes are important benchmarks for future experiments., Comment: 23 pages, 4 figures, accepted for publication in Scientific Reports

We demonstrate the possibility of creating a self-bound stable three-dimensional matter-wave spherical boson-fermion quantum ball in the presence of an attractive boson-fermion interaction and a small repulsive three-boson interaction. The three-boson interaction could be attractive or repulsive whereas the fermions are taken to be in a fully-paired super-fluid state in the Bardeen- Cooper-Schreifer ( quasi-noninteracting weak-coupling) limit. We also include the Lee-Huang-Yang (LHY) correction to a repulsive bosonic interaction term. The repulsive three-boson interaction and the LHY correction can stop a global collapse while acting jointly or separately. The present study is based on a mean-field model, where the bosons are subject to a Gross-Pitaevskii (GP) Lagrangian functional and the fully-paired fermions are described by a Galilean-invariant density functional Lagrangian. The boson-fermion interaction is taken to be the mean-field Hartree interaction, quite similar to the interaction term in the GP equation. The study is illustrated by a variational and a numerical solution of the mean-field model for the boson-fermion $^7$Li- $^6$Li system.

We demonstrate stable and metastable vortex-bright solitons in a three-dimensional spin-orbit-coupled three-component hyperfine spin-1 Bose-Einstein condensate (BEC) using numerical solution and variational approximation of a mean-field model. The spin-orbit coupling provides attraction to form vortex-bright solitons in both attractive and repulsive spinor BECs. The ground state of these vortex-bright solitons is axially symmetric for weak polar interaction. For a sufficiently strong ferromagnetic interaction, we observe the emergence of a fully asymmetric vortex-bright soliton as the ground state. We also numerically investigate moving solitons. The present mean-field model is not Galilean invariant, and we use a Galilean-transformed mean-field model for generating the moving solitons., Comment: arXiv admin note: text overlap with arXiv:1612.03264

We study spontaneous symmetry breaking (SSB), Josephson oscillation, and self-trapping in a stable, mobile, three-dimensional matter-wave spherical quantum ball self-bound by attractive two-body and repulsive three-body interactions. The SSB is realized by a parity-symmetric (a) one-dimensional (1D) double-well potential and (b) a 1D Gaussian potential, both along the $z$ axis and no potential along the $x$ and $y$ axes. In the presence of each of these potentials, the symmetric ground state dynamically evolves into a doubly-degenerate SSB ground state. If the SSB ground state in the double well, predominantly located in the first well ($z>0$), is given a small displacement, the quantum ball oscillates with a self-trapping in the first well. For a medium displacement one encounters an asymmetric Josephson oscillation. The asymmetric oscillation is a consequence of SSB. The study is performed by a variational and numerical solution of a non-linear mean-field model with 1D parity-symmetric perturbations.

The statics and dynamics of a stable, mobile three-dimensional (3D) spatiotemporal vortex light bullet in a cubic-quintic nonlinear medium with a focusing cubic nonlinearity above a critical value and any defocusing quintic nonlinearity is considered. The present study is based on an analytic variational approximation and a full numerical solution of the 3D nonlinear Schr\"odinger equation. The 3D vortex bullet can propagate with a constant velocity. Stability of the vortex bullet is established numerically and variationally. The collision between two vortex bullets moving along the angular momentum axis is considered. At large velocities the collision is quasi elastic with the bullets emerging after collision with practically no distortion. At small velocities two bullets coalesce to form a single entity called a breather., Comment: arXiv admin note: text overlap with arXiv:1701.03763

We study the statics and dynamics of a stable, mobile, self-bound three-dimensional dipolar matter-wave droplet created in the presence of a tiny repulsive three-body interaction. In frontal collision with an impact parameter and in angular collision at large velocities {along all directions} two droplets behave like quantum solitons. Such collision is found to be quasi elastic and the droplets emerge undeformed after collision without any change of velocity. However, in a collision at small velocities the axisymmeric dipolar interaction plays a significant role and the collision dynamics is sensitive to the direction of motion. For an encounter along the $z$ direction at small velocities, two droplets, polarized along the $z$ direction, coalesce to form a larger droplet $-$ a droplet molecule. For an encounter along the $x$ direction at small velocities, the same droplets stay apart and never meet each other due to the dipolar repulsion. The present study is based on an analytic variational approximation and a numerical solution of the mean-field Gross-Pitaevskii equation using the parameters of $^{52}$Cr atoms.

We consider the statics and dynamics of a stable, mobile three-dimensional (3D) spatiotemporal light bullet in a cubic-quintic nonlinear medium with a focusing cubic nonlinearity above a critical value and any defocusing quintic nonlinearity. The 3D light bullet can propagate with a constant velocity in any direction. Stability of the light bullet under a small perturbation is established numerically.We consider frontal collision between two light bullets with different relative velocities. At large velocities the collision is elastic with the bullets emerge after collision with practically no distortion. At small velocities two bullets coalesce to form a bullet molecule. At a small range of intermediate velocities the localized bullets could form a single entity which expands indefinitely leading to a destruction of the bullets after collision. The present study is based on an analytic Lagrange variational approximation and a full numerical solution of the 3D nonlinear Schr\"odinger equation.

We study the vortex-bright solitons in a quasi-two-dimensional spin-orbit-coupled (SO-coupled) hyperfine spin-1 three-component Bose-Einstein condensate (BEC) using variational method and numerical solution of a mean-field model. The ground state of these vortex-bright solitons is radially symmetric for weak ferromagnetic and polar interactions. For a sufficiently strong ferromagnetic interaction, we observe the emergence of an asymmetric vortex-bright soliton as the ground state. We also numerically investigate stable moving solitons and binary collision between them. The present mean-field model is not Galilean invariant, and we use a Galilean-transformed model for generating the moving solitons. At low velocities, the head-on collision between two {\em in-phase} solitons results either in collapse or fusion of the soliton pair. On the other hand, in head-on collision, the two {\em out-of-phase} solitons strongly repel each other and trace back their trajectories before the actual collision. At low velocities, in a collision with an impact parameter, the {\em out-of-phase} solitons get deflected from their original trajectory like two rigid classical disks. These {\em out-of-phase solitons} behave like classical disks, and their collision dynamics is governed by classical laws of motion. However, at large velocities two SO-coupled spinor solitons, irrespective of phase difference, can pass through each other in a head-on collision like two quantum solitons.

We study the statics and dynamics of a stable, mobile, three-dimensional matter-wave spherical quantum ball created in the presence of an attractive two-body and a very small repulsive three-body interaction. The quantum ball can propagate with a constant velocity in any direction in free space and its stability under a small perturbation is established numerically and variationally. In frontal head-on and angular collisions at large velocities two quantum balls behave like quantum solitons. Such collision is found to be quasi elastic and the quantum balls emerge after collision without any change of direction of motion and velocity and with practically no deformation in shape. When reflected by a hard impenetrable plane, the quantum ball bounces off like a wave obeying the law of reflection without any change of shape or speed. However, in a collision at small velocities two quantum balls coalesce to form a larger ball which we call a quantum-ball breather. We point out the similarity and difference between the collision of two quantum and classical balls. The present study is based on an analytic variational approximation and a full numerical solution of the mean-field Gross-Pitaevskii equation using the parameters of $^7$Li atoms.

We study the statics and dynamics of anisotropic, stable, bright and dark-in-bright dipolar quasi-two-dimensional Bose-Einstein condensate (BEC) solitons on a one-dimensional (1D) optical-lattice (OL) potential. These solitons mobile in a plane perpendicular to the 1D OL trap can have both repulsive and attractive contact interactions. The dark-in-bright solitons are the excited states of the bright solitons. The solitons, when subject to a small perturbation, exhibit sustained breathing oscillation. The dark-in-bright solitons can be created by phase imprinting a bright soliton. At medium velocities the collision between two solitons is found to be quasi elastic.The results are demonstrated by a numerical simulation of the three-dimensional mean-field Gross-Pitaevskii equation in three spatial dimensions employing realistic interaction parameters for a dipolar $^{164}$Dy BEC.

We classify all possible fractional charge vortices of charge less than unity in spin-1 and spin-2 polar and cyclic Bose-Einstein condensates (BECs) with zero magnetization. Statics and dynamics of these vortices in quasi-two-dimensional spinor BECs are studied employing accurate numerical solution and a Lagrange variational approximation. The results for density and collective-mode oscillation are illustrated using fractional-charge BEC vortex of $^{23}$Na and $^{87}$Rb atoms with realistic interaction and trapping potential parameters.

We demonstrate robust, stable, mobile two-dimensional (2D) dipolar ring-dark-in-bright (RDB) Bose-Einstein condensate (BEC) solitons for repulsive contact interaction, subject to a harmonic trap along the $y$ direction perpendicular to the polarization direction $z$. Such a RDB soliton has a ring-shaped notch (zero in density) imprinted on a 2D bright soliton free to move in the $x-z$ plane. At medium velocity the head-on collision of two such solitons is found to be quasi elastic with practically no deformation. The possibility of creating the RDB soliton by phase imprinting is demonstrated. The findings are illustrated using numerical simulation employing realistic interaction parameters in a dipolar $^{164}$Dy BEC., Comment: arXiv admin note: text overlap with arXiv:1409.5145

We analyze the localization of a Bose-Einstein condensate (BEC) in a one-dimensional bichromatic quasi-periodic optical-lattice potential by numerically solving the 1D Gross-Pitaevskii equation (1D GPE). We first derive the 1D GPE from the dimensional reduction of the 3D quantum field theory of interacting bosons obtaining two coupled differential equations (for axial wavefuction and space-time dependent transverse width) which reduce to the 1D GPE under strict conditions. Then, by using the 1D GPE we report the suppression of localization in the interacting BEC when the repulsive scattering length between bosonic atoms is sufficiently large., Comment: 10 pages, 2 figures, presented at the 7th Workshop on Quantum Chaos and Localisation Phenomena, May 29-31, 2015 - Warsaw, Poland; to be published in a special issue of Acta Physica Polonica A

We demonstrate a robust, stable, mobile, two-dimensional (2D) spatial and three-dimensional (3D) spatiotemporal optical soliton in the core of an optical vortex, while all nonlinearities are of the cubic (Kerr) type. The 3D soliton can propagate with a constant velocity along the vortex core without any deformation. Stability of the soliton under a small perturbation is established numerically. Two such solitons moving along the vortex core can undergo a quasi-elastic collision at medium velocities. Possibilities of forming such a 2D spatial soliton in the core of a vortical beam are discussed.

We demonstrate that the ground state of a trapped spin-1 and spin-2 spinor ferromagnetic Bose-Einstein condensate (BEC) can be well approximated by a single decoupled Gross-Pitaevskii (GP) equation. Useful analytic models for the ground-state densities of ferromagnetic BECs are obtained from the Thomas-Fermi approximation (TFA) to this decoupled equation. Similarly, for the ground states of spin-1 anti-ferromagnetic and spin-2 anti-ferromagnetic and cyclic BECs, some of the spin component densities are zero which reduces the coupled GP equation to a simple reduced form. Analytic models for ground state densities are also obtained for anti-ferromagnetic and cyclic BECs from the TFA to the respective reduced GP equations. The analytic densities are illustrated and compared with the full numerical solution of the GP equation with realistic experimental parameters.

We demonstrate a stable, mobile, dipolar or nondipolar three-dimensional matter-wave soliton in the vortex core of a uniform nondipolar condensate. All intra- and inter-species contact interactions can be repulsive for a strongly dipolar soliton. For a weakly dipolar or nondipolar soliton, the intra-species contact interaction in the soliton should be attractive for the formation of a compact soliton. The soliton can propagate with a constant velocity along the vortex core without any deformation. Two such solitons undergo a quasi-elastic collision at medium velocities. We illustrate the findings using realistic interactions in a mean-field model of binary $^{87}$Rb-$^{85}$Rb and $^{87}$Rb-$^{164}$Dy systems.

Many of the static and dynamic properties of an atomic Bose-Einstein condensate (BEC) are usually studied by solving the mean-field Gross-Pitaevskii (GP) equation, which is a nonlinear partial differential equation for short-range atomic interaction. More recently, BEC of atoms with long-range dipolar atomic interaction are used in theoretical and experimental studies. For dipolar atomic interaction, the GP equation is a partial integro-differential equation, requiring complex algorithm for its numerical solution. Here we present numerical algorithms for both stationary and non-stationary solutions of the full three-dimensional (3D) GP equation for a dipolar BEC, including the contact interaction. We also consider the simplified one- (1D) and two-dimensional (2D) GP equations satisfied by cigar- and disk-shaped dipolar BECs. We employ the split-step Crank-Nicolson method with real- and imaginary-time propagations, respectively, for the numerical solution of the GP equation for dynamic and static properties of a dipolar BEC. The atoms are considered to be polarized along the z axis and we consider ten different cases, e.g., stationary and non-stationary solutions of the GP equation for a dipolar BEC in 1D (along x and z axes), 2D (in x-y and x-z planes), and 3D, and we provide working codes in Fortran 90/95 and C for these ten cases (twenty programs in all). We present numerical results for energy, chemical potential, root-mean-square sizes and density of the dipolar BECs and, where available, compare them with results of other authors and of variational and Thomas-Fermi approximations., Comment: To download the programs click other and download source

State-selected total cross sections and thermal rate coefficients are computed for the HD + $ortho$-/$para$-H$_2$ rotational energy transfer collision at low temperatures: 2 K $\lesssim$ T $\lesssim$ 300 K. A modified H$_2$-H$_2$ potential energy surface (PES) devised by Hinde is used for this pure quantum-mechanical dynamical computation. A comparison of the new results for the HD + $ortho$-/$para$-H$_2$ scattering problem and previous calculations computed with the use of other older PESs is presented and discussed., Comment: Revtex preprint: 34 pages, 3 tables and 16 figures

We study the formation of bound states and three-component bright vector solitons in a quasi-one-dimensional spin-orbit-coupled hyperfine spin $f=1$ Bose-Einstein condensate using numerical solution and variational approximation of a mean-field model. In the antiferromagnetic domain, the solutions are time-reversal symmetric, and the component densities have multi-peak structure. In the ferromagnetic domain, the solutions violate time-reversal symmetry, and the component densities have single-peak structure. The dynamics of the system is not Galelian invariant. From an analysis of Galelian invariance, we establish that the single-peak ferromagnetic vector solitons are true solitons and can move maintaining constant component densities, whereas the antiferromagnetic solitons cannot move with constant component densities.