[en] We show that a quantized Berry phase in Bloch momentum space can serve as a topological order parameter to the quantum phases of a gapped spin chain system with time-reversal invariance. Specifically, we study this approach analytically in a class of XY spin-1/2 chain with multiple sites interactions in a transverse field. In order to derive a proper definition of the Berry curvature in a two-dimensional parameter space, we performed a local gauge transformation to the spin chain system by a twist operator, which endows the Hamiltonian of the system with the topology of a torus T² without changing its energy spectrum. We show that a topological Z₂ order parameter can be obtained as a quantized Berry phase by a loop integral of the Berry gauge potential along quarter of the Brillouin zone, which determines the zero-temperature phase diagram of the system.

[en] An integrable lattice hierarchy is constructed from a discrete matrix spectral problem, in which one of the Suris systems is the first member of this hierarchy. Some related properties such as Hamiltonian structure of this lattice hierarchy are discussed. The Suris system is solved by the N-fold Darboux transformation. As a result, the multi-soliton solutions are derived and the soliton structures along with the interaction behaviors among solitons are shown graphically. Finally, the infinitely many conservation laws of the Suris system are given.

[en] Recently, it is shown that the Kac–Wakimoto equation associated with ${\mathfrak{e}}_6^{(1)}$ is not integrable since it does not pass Painlevé test and does not have three-soliton solution, even it has one- and two-soliton solutions. Thus in this paper, we investigate some new types of exact solutions for this equation based on its bilinear form. As a result, the rational solutions, kink-type breather solution and degenerate three-solitary wave solutions of this equation are found. The properties and space structures of these exact solutions are analyzed by displaying their profiles in (x, y)-directions. Furthermore, the Lie symmetry analysis is done to present the one-parameter group of symmetries for the KW equation. (paper)

[en] In this work, the Lie point symmetries of the inhomogeneous Toda lattice equation are obtained by semi-discrete exterior calculus, which is a semi-discrete version of Harrison and Estabrook’s geometric approach. A four-dimensional Lie algebra and its one-, two- and three-dimensional subalgebras are given. Two similarity reductions of the inhomogeneous Toda lattice equation are obtained by using the symmetry vectors. (paper)

[en] Highlights: ► We propose some exact vortex soliton solutions of the Gross–Pitaevskii equation with spatially inhomogeneous cubic–quintic nonlinearity. ► We show that both attractive cubic–quintic nonlinearity and attractive cubic plus repulsive quintic nonlinearities support exact vortex solitons. ► The number of ring structures in the vortex solitons increases by one with increasing the radial quantum number by one. -- Abstract: The exact vortex soliton solutions of the quasi-two-dimensional cubic–quintic Gross–Pitaevskii equation with spatially inhomogeneous nonlinearities are constructed by similarity transformation. It is demonstrated that spatially inhomogeneous cubic–quintic nonlinearity can support exact vortex solitons in which there are two quantum numbers S and m. The radius structures and density distributions of these vortex solitons are studied, and it is shown that the number of ring structure of the vortex solitons increases by one with increasing the “radial quantum number” m by one.

[en] Based on the Wronskian technique and Lax pair, double Wronskian solution of the nonisospectral BKP equation is presented explicitly. The speed and dynamical influence of the one soliton are discussed. Soliton resonances of two soliton are shown by means of density distributions. Soliton properties are also investigated in the inhomogeneous media. (paper)

[en] This paper deals with localized waves in the (3+1)-dimensional Boiti–Leon–Manna–Pempinelli equation in the incompressible fluid. Based on Hirota’s bilinear method, N-soliton solutions related to Boiti–Leon–Manna–Pempinelli equation are constructed. Novel nonlinear wave phenomena are obtained by selecting appropriate parameters to N-soliton solutions, and time evolutions of different kinds of solitary waves are investigated in detail. Rich elastic interactions are illustrated analytically and graphically. More specifically, the inelastic interactions, i.e., fusion and fission of solitary waves, are constructed by choosing special parameters on kink solitons and breathers. The analysis of the influence of parameters on propagation is revealed in three tables. The results have potential applications in fluid mechanics.

[en] Highlights: • We investigate the matter-wave solitons in hybrid atomic–molecular BECs. • Two groups of exact non-autonomous matter-wave soliton solutions are presented. • Three types of harmonic potentials are considered. • The time-modulated nonlinearities and external potentials can support matter-wave solitons. - Abstract: In this paper, we investigate matter-wave solitons in hybrid atomic–molecular Bose–Einstein condensates with tunable interactions and external potentials. Three types of time-modulated harmonic potentials are considered and, for each of them, two groups of exact non-autonomous matter-wave soliton solutions of the coupled Gross–Pitaevskii equation are presented. Novel nonlinear structures of these non-autonomous matter-wave solitons are analyzed by displaying their density distributions. It is shown that the time-modulated nonlinearities and external potentials can support exact non-autonomous atomic–molecular matter-wave solitons.

[en] Highlights: • The instabilities of spin-2 BEC in an optical lattice are studied theoretically and numerically. • Zeeman effects obviously affect the dynamical instability. • Fast moving spin-2 BEC has larger energetic instability region than lower one. The dynamical and energetic instabilities of the $F=2$ spinor Bose–Einstein condensates in an optical lattice are investigated theoretically and numerically. By analyzing the dynamical response of different carrier waves to an additional linear perturbation, we obtain the instability criteria for the ferromagnetic, uniaxial nematic, biaxial nematic and cyclic states, respectively. When an external magnetic field is taken into account, we find that the linear or quadratic Zeeman effects obviously affect the dynamical instability properties of uniaxial nematic, biaxial nematic and cyclic states, but not for the ferromagnetic one. In particular, it is found that the faster moving $F=2$ spinor BEC has a larger energetic instability region than lower one in all the four states. In addition, it is seen that for most states there probably exists a critical value $k_c>0$, for which $k>k_c$ causes the energetic instability to arise under appreciative parameters.