In this paper, we bring out several potential signatures of negative mass regimes while investigating an expanding spin-orbit-coupled spinor
F=1 Bose-Einstein condensate by analyzing the dispersion relation of the single-particle quantum system. In spin-orbit-coupled spinor condensates, a negative-mass parameter generates a wave packet that propagates in the opposite direction of the momentum. We analyze the dynamics of spin waves analytically and present a simple approach to investigate the expansion of spinor condensates. In particular, we examine the dynamics when both masses are negative, which results in the spinor condensates splitting into two counterpropagating self-interfering packets. Using numerical simulations of the coupled Gross-Pitaevskii equations, we demonstrate the density expansion and self-interference patterns with and without magnetization for repulsive and attractive interactions with different coupling parameters. The highlight of our investigation is that we are able to unearth several phenomena observed in experiments, such as self-interfering packets, pileup, modulation instability, slow down, self-trapping, and gap solitons. In particular, the gap soliton exists at the gap created by the intersection of two negative masses.
A negative effective mass can be realized in quantum systems by engineering the dispersion relation. A powerful method is provided by spin-orbit coupling, which is currently at the center of intense research efforts. Here we measure an expanding spin-orbit coupled Bose-Einstein condensate whose dispersion features a region of negative effective mass. We observe a range of dynamical phenomena, including the breaking of parity and of Galilean covariance, dynamical instabilities, and self-trapping. The experimental findings are reproduced by a single-band Gross-Pitaevskii simulation, demonstrating that the emerging features-shock waves, soliton trains, self-trapping, etc.-originate from a modified dispersion. Our work also sheds new light on related phenomena in optical lattices, where the underlying periodic structure often complicates their interpretation.
The interplay of non-Hermiticity and spin-orbit (SO) coupling has attracted great interest in condensed-matter physics, ultracold atomic physics, optics, and other fields. Here we study the ground states, collective modes, and free-expansion dynamics of non-Hermitian SO-coupled Bose-Einstein condensates (BECs) with momentum-dependent gain and loss in a harmonic trap. An efficient analytical method for revealing the impact of non-Hermiticity on the ground states and dynamics of SO-coupled BECs is proposed. A dispersion relation with effective mass
m2 that can be larger than 1, less than 1, or negative is engineered by the momentum-dependent gain and loss, which is distinct from the ordinary SO-coupled BECs, where
m2>1 always holds. Negative effective mass can lead to the self-interference effect of condensates, while
m2<1 means that condensates can gain a larger acceleration when they are subjected to a constant force. Interestingly, the high-order zero-point kinetic energies are derived analytically and confirmed numerically, establishing a link between the zero-point kinetic energy and dispersion relation. In particular, the second-order zero-point kinetic energy can be controlled by the effective mass
m2, which has an important impact on many phenomena of ultracold atomic physics. In addition, another key result is that the frequencies of dipole and breathing modes can exceed the values of traditional BECs without SO coupling when the effective mass
m2<1, showing that gain (loss) is a powerful tool for probing and controlling quantum systems. Moreover, the asymmetric free expansion and self-interference effect of the condensates caused by negative effective masses are observed, and it is found that the rate of expansion is controlled by the effective mass
m2. Our results show that the controllable gain-loss engineering in SO-coupled BECs provides a prospect to explore novel quantum many-body physics.
We investigate the energy-velocity dispersion relation of spin solitons with arbitrary initial velocities. Based on the dispersion relation, we further characterize the inertial mass of a spin soliton and find two critical velocities to distinguish between the positive mass and negative mass. One critical velocity separates the initial negative and positive masses, and the other denotes the velocity for negative-positive mass transition during an acceleration process. With the aid of the two critical velocities, we further propose two possible ways to observe the pure negative mass effect of a spin soliton. Moreover, the relations between the soliton's width and its velocity are discussed according to the variational results. We report that the soliton width can become narrower with larger moving speed (when a spin soliton possesses positive mass), in sharp contrast to the dark solitons reported before. These results would motivate experiments to observe negative mass effects of solitons in Bose-Einstein condensates.
We investigate the different types of matter-wave solitons in spin-orbit-coupled spin-2 spinor Bose-Einstein condensates. Using mean-field theory and adopting the multiscale perturbation method, the original five-component Gross-Pitaevskii spin-orbit-coupled spin-2 spinor Bose-Einstein condensate model can be reduced to a single effective nonlinear Schrödinger equation, which allows us to find analytical soliton solutions of this system. In this way, for different regimes of the spin-orbit coupling, Raman coupling, and interatomic interactions, we find approximate bright and dark soliton solutions. Particularly, the type of solitons depends on the dispersion properties of the system. When the lowest-energy band has a single-well structure, we find there only exist positive mass bright or dark solitons due to the dispersion coefficient of effective nonlinear Shrödinger equation always positive. However, when the lowest-energy band has a double-well structure, there will appear positive (negative) mass bright or dark solitons because the sign of the dispersion coefficient can be positive (negative) under different momentum. We employ direct numerical simulation of the original five-component Gross-Pitaevskii equations to confirm the analytical results.
We present a unified description of different types of matter-wave solitons that can emerge in quasi one-dimensional spin-orbit coupled (SOC) Bose-Einstein condensates (BECs). This description relies on the reduction of the original two-component Gross-Pitaevskii SOC-BEC model to a single nonlinear Schrödinger equation, via a multiscale expansion method. This way, we find approximate bright and dark soliton solutions, for attractive and repulsive interatomic interactions respectively, for different regimes of the SOC interactions. Beyond this, our approach also reveals "negative mass" regimes, where corresponding "negative mass" bright or dark solitons can exist for repulsive or attractive interactions, respectively. Such a unique opportunity stems from the structure of the excitation spectrum of the SOC-BEC. Numerical results are found to be in excellent agreement with our analytical predictions.