Bose-Einstein Condensates in Optical Lattices with Higher-Order Interactions

The higher-order interactions of Bose-Einstein condensate in multi-dimensional optical lattices are discussed both analytically and numerically.It is demonstrated that the effects of the higher-order atomic interactions on the sound speed and the stabilities of Bloch waves strongly depend on the lattice strength.In the presence of higher-order effects,tighter and high-dimensional lattices are confirmed to be two positive factors for maintaining the system's energetic stability,and the dynamical instability of Bloch waves can take place simultaneously with the energetic instability.In addition,we find that the higher-order interactions exhibit a long-range behavior and the long-lived coherent Bloch oscillations in a tilted optical lattice exist.Our results provide an effective way to probe the higher-order interactions in optical lattices.     

A single-crystal neutron scattering study of lattice melting in ferroelastic [Formula: see text

We present the results of an extensive single-crystal neutron scattering study of the ferroelastic phase transition in [Formula: see text]. This material has previously been demonstrated to undergo a continuous loss of long-range order at its ferroelastic transition, which is the phenomenon known as lattice melting. We show that our data are consistent with a special form of lattice melting where the long-range order appears to be destroyed in a two-dimensional sense, but is preserved in the third dimension.     

Observation of Magnetic Excitations of Skyrmion Crystal in a Helimagnetic Insulator Cu_{2}OSeO_{3}

We have investigated the low-energy dynamics of the triangular lattice of Skyrmions in a helimagnetic insulator Cu_{2}OSeO_{3} in terms of microwave response. We have observed two elementary excitations of the Skyrmion with different polarization characteristics: the counterclockwise circulating mode at 1?GHz with the magnetic field polarization parallel to the Skyrmion plane and the breathing mode at 1.5?GHz with a perpendicular magnetic field polarization. These modes reflect the topological nature of Skyrmions and may play a central role in the Skyrmion dynamics.     

Origin of low-coherence enhanced backscattering

The origin of low-coherence enhanced backscattering (EBS) of light in random media when the spatial coherence length of illumination is much smaller than the transport mean free path has been poorly understood. We report that in weakly scattering discrete random media low-coherence EBS originates from time-reversed paths of double scattering. Low spatial coherence illumination dephases the time-reversed waves outside its finite coherence area, which isolates the minimal number of scattering events in EBS from higher-order scattering. Moreover, we show the first experimental evidence that the minimal number of scattering events in EBS is double scattering, which has been hypothesized since the first observation of EBS.     

Spin-orbit coupled weakly interacting Bose-Einstein condensates in harmonic traps

We investigate theoretically the phase diagram of a spin-orbit coupled Bose gas in two-dimensional harmonic traps. We show that at strong spin-orbit coupling the single-particle spectrum decomposes into different manifolds separated by ?ω{⊥}, where ω{⊥} is the trapping frequency. For a weakly interacting gas, quantum states with Skyrmion lattice patterns emerge spontaneously and preserve either parity symmetry or combined parity-time-reversal symmetry. These phases can be readily observed in a spin-orbit coupled gas of ^{87}Rb atoms in a highly oblate trap.     

Huang diffuse scattering from interstitials in an hcp lattice

Using the continuum theory of linear elasticity, the Huang diffuse scattering from interstitials in an hcp lattice has been calculated to distinguish between the possible interstitial configurations. The symmetry of the lattice permits four such configurations. In each case, the Huang diffuse scattering is averaged over all possible equivalent orientations (assumed to be equally populated) of the defect configuration. The limitations of Huang diffuse scattering in discriminating between defect configurations having the same long-range symmetry are discussed, considering the specia I cases of Mg and Zn.     

Giant Skyrmions stabilized by dipole-dipole interactions in thin ferromagnetic films

Motivated by a recent magnetization reversal experiment on a TbFeCo thin film, we study a topological excitation in the anisotropic nonlinear sigma model together with the Zeeman and magnetic dipole-dipole interactions. Dipole-dipole interactions turn a ferromagnet into a frustrated spin system, which allows a nontrivial spin texture such as a giant Skyrmion. We derive an analytic formula for the Skyrmion radius. The radius is controllable by the external magnetic field. It is intriguing that a Skyrmion may have already been observed as a magnetic domain. A salient feature is that a single Skyrmion can be created or destroyed experimentally. An analysis is made also on Skyrmions in chiral magnets.     

Dynamics of Skyrmion crystals in metallic thin films

We study the collective dynamics of the Skyrmion crystal in thin films of ferromagnetic metals resulting from the nontrivial Skyrmion topology. It is shown that the current-driven motion of the crystal reduces the topological Hall effect and the Skyrmion trajectories bend away from the direction of the electric current (the Skyrmion Hall effect). We find a new dissipation mechanism in noncollinear spin textures that can lead to a much faster spin relaxation than Gilbert damping, calculate the dispersion of phonons in the Skyrmion crystal, and discuss the effects of impurity pinning of Skyrmions.     

Higher-order surface solitons in one-dimensional Bessel optical lattices

We demonstrate the existence of higher-order solitons occurring at an interface separating two one-dimensional (1D) Bessel optical lattices with different orders or modulation depths in a defocusing medium. We show that, in contrast to homogeneous waveguides where higher-order solitons are always unstable, the Bessel lattices with an interface support branches of higher-order structures bifurcating from the corresponding linear modes. The profiles of solitons depend remarkably on the lattice parameters and the stability can be enhanced by increasing the lattice depth and selecting higher-order lattices. We also reveal that the interface model with defocusing saturable Kerr nonlinearity can support stable multi-peaked solitons. The uncovered phenomena may open a new way for soliton control and manipulation.     

Observation of topologically stable 2D Skyrmions in an antiferromagnetic spinor Bose-Einstein condensate

We present the creation and time evolution of two-dimensional Skyrmion excitations in an antiferromagnetic spinor Bose-Einstein condensate. Using a spin rotation method, the Skyrmion spin textures were imprinted on a sodium condensate in a polar phase, where the two-dimensional Skyrmion is topologically protected. The Skyrmion was observed to be stable on a short time scale of a few tens of ms but to dynamically deform its shape and eventually decay to a uniform spin texture. The deformed spin textures reveal that the decay dynamics involves breaking the polar phase inside the condensate without having topological charge density flow through the boundary of the finite-sized sample. We discuss the possible formation of half-quantum vortices in the deformation process.     

Vortex-lattice dynamics in rotating spinor Bose-Einstein condensates

We observe interlaced square vortex lattices in rotating dilute-gas spinor Bose-Einstein condensates (BEC). After preparing a hexagonal vortex lattice in a one-component BEC in an internal atomic state |1, we coherently transfer a fraction of the superfluid to a different state |2. The subsequent evolution of this pseudo-spin-1/2 superfluid towards a state of offset square lattices involves an intriguing interplay of phase-separation and -mixing dynamics, both macroscopically and on the length scale of the vortex cores, and a stage of vortex turbulence. The stability of the square structure is proved by its response to applied shear perturbations. An interference technique shows the spatial offset between the two vortex lattices. Vortex cores in either component are filled by fluid of the other component, such that the spin-1/2 order parameter forms a Skyrmion lattice.     

The effect of the long-range order in a quantum dot array on the in-plane lattice thermal conductivity

Semiconductor quantum dot superlattices consisting of arrays of quantum dots have shown great promise for a variety of device applications, including thermoelectric power generation and cooling. In this paper we theoretically investigate the effect of long-range order in a quantum dot array on its in-plane lattice thermal conductivity. It is demonstrated that the long-range order in a quantum dot array enhances acoustic phonon scattering and, thus leads to a decrease of its lattice thermal conductivity. The decrease in the ordered quantum dot array, which acts as a phonon grating, is stronger than that in the disordered one due to the contribution of the coherent scattering term. The numerical calculations were carried out for a structure that consists of multiple layers of Si with layers of ordered Ge quantum dots separated by wetting layers and spacers.     

Chirality waves in two-dimensional magnets

We theoretically show that moderate interaction between electrons confined to move in a plane and localized magnetic moments leads to formation of a noncoplanar magnetic state. The state is similar to the Skyrmion crystal recently observed in cubic systems with the Dzyaloshinskii-Moriya interaction; however, it does not require spin-orbit interaction. The noncoplanar magnetism is accompanied by the ground-state electrical and spin currents, generated via the real-space Berry phase mechanism. We examine the stability of the state with respect to lattice discreteness effects and the magnitude of magnetic exchange interaction. The state can be realized in a number of transition metal and magnetic semiconductor systems.     

Double incoherent Raman scattering induced by molecular interactions in gases

A new Raman band is observed in two gases, CF4 and CH4. It is attributable to a double incoherent Raman scattering that takes place in pairs of interacting molecules. Spectral scattering intensities are given on an absolute scale. Zero order spectral moments are measured and are found higher than their long-range interaction value.     

Dynamics of moving and spinning skyrmions

We study the intermediate-and long-range forces between moving and spinning Skyrmions, employing two different methods. One uses a relativised product ansatz for the Skyrme fields, the other models Skyrmions as triplets of scalar dipoles. The methods lead to the same finite-dimensional Lagrangian dynamical system which may be interpreted as a point-particle approximation to Skyrmion dynamics. We discuss in detail the dynamics in the so-called attractive channel and the interaction between well-separated and rapidly spinning Skyrmions, and point out the resemblance between the latter and the one-pion exchange potential in nuclear physics.     

Energetically stable particlelike skyrmions in a trapped Bose-Einstein condensate

We numerically show that a topologically nontrivial 3D Skyrmion can be energetically stable in a trapped two-component atomic Bose-Einstein condensate, for the parameters of 87Rb condensate experiments. The separate conservation of the two atomic species can stabilize the Skyrmion against shrinking to zero size, while drift of the Skyrmion due to the trap-induced density gradient can be prevented by rotation or by a laser potential.     

Chiral unitary approach to the K–deuteron scattering length

Starting from a recent model where the amplitudes are evaluated from the chiral Lagrangians using a coupled-channel unitary method, we evaluate here the scattering length for K⁻–deuteron scattering. We find that the double scattering contribution is very large compared to the impulse approximation and that the charge-exchange contribution of this rescattering is as large as the sequential K⁻ scattering on the two nucleons. Higher-order rescattering corrections are evaluated using coupled channels with K⁻ and within the integral form of the fixed centre approximation to the Faddeev equations. The higher-order corrections involving intermediate pions and hyperons are found negligible.     

Disordered ground state and magnetic field-induced long-range order in an S=3/2 antiferromagnetic honeycomb lattice compound Bi3Mn4O12(NO3)

Bi3Mn4O12(NO3), in which the Mn4+ ions carry S=3/2, is the first honeycomb lattice system that shows no long-range magnetic order. Using neutron scattering, we have determined that short-range antiferromagnetic correlations develop at low temperatures. Applied magnetic fields induce a magnetic transition, in which the short-range order abruptly expands into a long-range order.     

玻色凝聚的原子自旋链中的非线性自旋波

研究了囚禁在光晶格中的旋量玻色-爱因斯坦凝聚体(BEC)形成的原子自旋链中的相干非线性自旋波的激发与调制不稳定性.通过解析分析，得到了调制不稳定性的一般判据以及其对原子自旋的长程耦合的依赖关系.在蓝失谐和红失谐光晶格的情况下，分别具体分析了长程非线性自旋耦合，包括光诱导的和静磁诱导的偶极-偶极相互作用对相干自旋波调制不稳定性的影响.     

Spin waves and revised crystal structure of honeycomb iridate Na2IrO3

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T(N)=15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90° Ir-O-Ir bonds required for dominant Kitaev exchange.