In this letter we present an experimental study of the collective dipole oscillation of a spin-orbit coupled Bose-Einstein condensate in a harmonic trap. Dynamics of the center-of-mass dipole oscillation is studied in a broad parameter region, as a function of spin-orbit coupling parameters as well as oscillation amplitude. Anharmonic properties beyond effective-mass approximation are revealed, such as amplitude-dependent frequency and finite oscillation frequency at place with divergent effective mass. These anharmonic behaviors agree quantitatively with variational wave-function calculations. Moreover, we experimentally demonstrate a unique feature of spin-orbit coupled system predicted by a sum-rule approach, stating that spin polarization susceptibility-a static physical quantity-can be measured via dynamics of dipole oscillation. The divergence of polarization susceptibility is observed at the quantum phase transition that separates magnetic nonzero-momentum condensate from nonmagnetic zero-momentum phase. The good agreement between the experimental and theoretical results provides a bench mark for recently developed theoretical approaches.Many interesting quantum phases can emerge in solid state materials when electrons are placed in a strong magnetic field or possess strong spin-orbit (SO) coupling, such as the fractional quantum Hall effect [1] and the topological insulator [2]. In cold atom systems, albeit neutral atoms have neither charges nor SO coupling, the recent exciting experimental progress demonstrates that artificial gauge potentials can be synthesized in laboratory by laser control technique [3][4][5][6][7][8][9][10]. Synthetic gauge potential is becoming a powerful tool for simulating real materials with cold atoms. Moreover, the system of SO coupled bosons does not have an analogy in conventional condensed matter systems, and can exhibit many novel phases [11] such as striped superfluid phase [12,13] and half vortex phase [14][15][16][17].Collective excitations play an important role in studying physical properties of trapped atomic Bose-Einstein condensates (BEC) and degenerate Fermi gases. Collective dipole oscillation is a center-of-mass motion of all atoms. For a conventional condensate, the dipole oscillation is trivial: the frequency is just the harmonictrap frequency, independent of oscillation amplitude and interatomic interaction. This is known as Kohn theorem [18,19]. For a SO coupled condensate, however, it was found [4] that the dipole-oscillation frequency deviates from the trap frequency and the experimental data thereby can be explained by effective-mass approximation. Recently, much theoretical effort has been taken to understand dynamics of a SO coupled BEC [20][21][22][23][24][25], and many predicted unconventional properties remain to be experimentally explored. In particular, the so-called sum-rule approach predicts [25] a unique feature of SO coupled condensate: spin polarization susceptibility-a static physical quantity-can be measured via dynamics of dipole oscillatio...

Spin-orbit (SO) coupling has led to numerously exciting phenomena in electron systems.Whereas the synthesized SO coupling with ultracold neutral atoms gives us an opportunity to study SO coupling in bosonic systems, which exhibit many new phenomena of superfluidity and various symmetry breaking condensate phases. A richer structure of symmetry breaking always results in a nontrivial finite-temperature phase diagram, however, the thermodynamics of the SO coupled Bose gas at finite temperature is still unknown so far either in theory or experiment. Here we experimentally determine a novel finite temperature phase transition that is consistent with a transition between the stripe ordered phase and the magnetized phase. We also observe that the magnetic phase transition and the Bose condensate transition occur simultaneously as temperature decreases.Our work determines the entire finite-temperature phase diagram of SO coupled Bose gas and demonstrates the power of quantum simulation.Superfluidity is a phenomenon known for century in physics but the study of superfluidity still keeps producing novel physics. Recently SO coupling, which has played an important role in recently discovered topological insulator 1, 2 , has also been realized in ultracold degenerate gases [3][4][5][6][7][8][9] . The SO coupled Bose gases are predicted to exhibit a host of new phenomena of superfluidity. For instance, SO coupling leads to degenerate single-particle ground states, which can result in a new type of stripe superfluid with spatial density order [10][11][12][13] . SO coupling can significantly enhance low-energy density-of-state that dramatically increases quantum and thermal fluctuation effects and also magnifies interaction effects [14][15][16][17][18][19] . The absence of Galilean 2 invariance due to SO coupling yields unconventional behavior of superfluid critical velocity 18,20,21 .In this work we generate SO coupling in 87 Rb Bose gases by two contour-propagating laser beams as described in previous works 3, 6 . In this setup only the motion along the spatial direction of Raman laser (denoted byx) is coupled to spin, and the single-particle Hamiltonian alongx is given byWe focus on the case with δ = 0 where the system has an additional Z 2 symmetry (k x → −k x and σ z → −σ z simultaneously). The single-particle dispersion is shown in Fig. 1(a). To motivate our study of finite-temperature physics, we shall first summarize what are known at zero temperature.For Ω < Ω 2 4E r (E r = k 2 r /(2m)), there are two degenerate single-particle minima denoted by ±k min and their wave functions are represented by ψ L and ψ R , respectively, and these two degenerate states have opposite magnetization. Due to this degeneracy, wave function of Bose condensation should be determined by interactions in this regime. Theoretical results 11,12 have shown that for interaction parameters of 87 Rb atoms, the condensate wave function is in a superposition state (ψ L + ψ R )/ √ 2 for Ω < Ω 1 0.2E r and bosons condensate either into ψ L or into ψ R ...

No abstract

We study the decay behaviors of ultracold atoms in metastable states with spin-orbit coupling (SOC), and demonstrate that there are two SOC-induced decay mechanisms. One arises from the trapping potential and the other is due to interatomic collision. We present general schemes for calculating decay rates from these two mechanisms, and illustrate how the decay rates can be controlled by experimental parameters. We experimentally measure the decay rates over a broad parameter region, and the results agree well with theoretical calculations. This work provides an insight for both quantum simulation involving metastable dressed states and studies on few-body problems with SO coupling.

Pickering emulsions stabilized by food-grade particles have garnered increasing interest in recent years due to their promising applications in biorelated fields such as foods, cosmetics, and drug delivery. However, it remains a big challenge to formulate nanoscale Pickering emulsions from these edible particles. Herein we show that a new Pickering nanoemulsion that is stable, monodisperse, and controllable can be produced by employing the spherical micellar nanoparticles (EYPNs), self-assembled from the food-derived, amphiphilic egg yolk peptides, as an edible particulate emulsifier. As natural peptide-based nanoparticles, the EYPNs have a small particle size, intermediate wettability, high surface activity, and deformability at the interface, which enable the formation of stable Pickering nanodroplets with a mean dynamic light scattering diameter below 200 nm and a polydispersity index below 0.2. This nanoparticle system is versatile for different oil phases with various polarities and demonstrates the easy control of nanodroplet size through tuning the microfluidization conditions or the ratio of EYPNs to oil phase. These food-grade Pickering nanoemulsions, obtained when the internal phase is an edible vegetable oil, have superior stability during long-term storage and spray-drying based on the irreversible and compact adsorption of intact EYPNs at the nanodroplet surface. This is the first finding of a natural edible nano-Pickering emulsifier that can be used solely to make stable food Pickering nanoemulsions with the qualities of simplicity, versatility, low cost, and the possibility of controllable and mass production, which make them viable for many sustainable applications.

A new class of food‐grade foams that are ultrastable, thermostimulable, and processable can be created simply by using the naturally occurring saponin glycyrrhizic acid (GA) as the sole stabilizer. The creation of this “superfoam” is based on the spatially controllable self‐assembly of supramolecular GA nanofibril hydrogelators at the air–water interface and in the continuous phase. The rapid adsorption of GA nanofibrils at the bubble surface, forming a multilayer interfacial network, combined with the formation of viscoelastic fibrillar hydrogel networks in the continuous phase, enables the foams having ultrastability over months or years without the water drainage induced phase separation, which have been evidenced using small angle X‐ray scattering and microscopy techniques. Such ultrastable foams can be rapidly destabilized on demand by heating, which induces the melting of the fibrillar networks. These thermoresponsive foams can be reversibly switched between stable and unstable by simply changing the temperature, based on the reversible gel–sol phase transition of the supramolecular hydrogel inside the foam. This is the first finding of a natural edible surfactant system that foams very well and can be used solely to make advanced foams with the qualities of simplicity, ultrastability, stimulability, and processability, which make them viable for many sustainable applications.

No abstract

Enhancing the enzymatic activity inside metal–organic frameworks (MOFs) is a critical challenge in chemical technology and bio-technology, which, if addressed, will broaden their scope in energy, food, environmental, and pharmaceutical industries. Here, we report a simple yet versatile and effective strategy to optimize biocatalytic activity by using MOFs to rapidly “lock” the ultrasound (US)-activated but more fragile conformation of metalloenzymes. The results demonstrate that up to 5.3-fold and 9.3-fold biocatalytic activity enhancement of the free and MOF-immobilized enzymes could be achieved compared to those without US pretreatment, respectively. Using horseradish peroxidase as a model, molecular dynamics simulation demonstrates that the improved activity of the enzyme is driven by an opened gate conformation of the heme active site, which allows more efficient substrate binding to the enzyme. The intact heme active site is confirmed by solid-state UV–vis and electron paramagnetic resonance, while the US-induced enzyme conformation change is confirmed by circular dichroism spectroscopy and Fourier-transform infrared spectroscopy. In addition, the improved activity of the biocomposites does not compromise their stability upon heating or exposure to organic solvent and a digestion cocktail. This rapid locking and immobilization strategy of the US-induced active enzyme conformation in MOFs gives rise to new possibilities for the exploitation of highly efficient biocatalysts for diverse applications.