Superfluidity of Bose-Einstein condensates in ultracold atomic gases

Liquid helium 4 had been the only bosonic superfluid available in experiments for a long time. This situation was changed in 1995, when a new superfluid was born with the realization of the Bose-Einstein condensation in ultracold atomic gases. The liquid helium 4 is strongly interacting and has no spin; there is almost no way to change its parameters, such as interaction strength and density. The new superfluid, Bose-Einstein condensate (BEC), offers various aspects of advantages over liquid helium. On the one hand, BEC is weakly interacting and has spin degrees of freedom. On the other hand, it is convenient to tune almost all the parameters of a BEC, for example, the kinetic energy by spin-orbit coupling, the density by the external potential, and the interaction by Feshbach resonance. Great efforts have been devoted to studying these new aspects of superfluidity, and the results have greatly enriched our understanding of superfluidity. Here we review these developments by focusing on the stability and critical velocity of various superfluids. The BEC systems considered include a uniform superfluid in free space, a superfluid with its density periodically modulated, a superfluid with artificially engineered spin-orbit coupling, and a superfluid of pure spin current. Due to the weak interaction, these BEC systems can be well described by the mean field Gross-Pitaevskii theory and their superfluidity, in particular critical velocities, can be examined with Landau's theory of superfluid. Experimental proposals to observe these new aspects of superfluidity are discussed.Comment: review article for Chinese Physics B, 15 papes, 9 figure

Extended Bose-Hubbard model with pair tunneling: spontaneous symmetry breaking, effective ground state and fragmentation

The extended Bose-Hubbard model for a double-well potential with pair tunneling is studied through both exact diagonalization and mean field theory (MFT). When pair tunneling is strong enough, the ground state wavefunction predicted by the MFT is complex and doubly degenerate while the quantum ground state wavefunction is always real and unique. The time reversal symmetry is spontaneously broken when the system transfers from the quantum ground state into one of the mean field ground states upon a small perturbation. As the gap between the lowest two levels decreases exponentially with particle number, the required perturbation inducing the spontaneous symmetry breaking (SSB) is infinitesimal for particle number of typical cold atom systems. The quantum ground state is further analyzed with the Penrose-Onsager criterion, and is found to be a fragmented condensate. The state also develops the pair correlation and has non-vanishing pair order parameter instead of the conventional single particle order parameter. When this model is generalized to optical lattice, a pair superfluid can be generated. The mean field ground state can be regarded as effective ground state in this simple model. The detailed computation for this model enables us to offer an in-depth discussion of the relation between SSB and effective ground state, giving a glimpse on how nonlinearity arises in the SSB of a quantum system.Comment: 6 pages, 6 figure

Exotic superfluidity in spin-orbit coupled Bose-Einstein condensates

We study the superfluidity of a spin-orbit coupled Bose-Einstein condensate (BEC) by computing its Bogoliubov excitations, which are found to consist of two branches: one is gapless and phonon-like at long wavelength; the other is typically gapped. These excitations imply a superfluidity that has two new features: ({\it i}) due to the absence of the Galilean invariance, one can no longer define the critical velocity of superfluidity independent of the reference frame; ({\it ii}) the superfluidity depends not only on whether the speed of the BEC exceeds a critical value, but also on {\it cross helicity} that is defined as the direction of the cross product of the spin and the kinetic momentum of the BEC.Comment: 6 pages, 5 figure

Ordovician paleomagnetism of eastern Yunnan, China

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95476/1/grl4830.pd

Paleomagnetism and electron microscopy of the Emeishan Basalts, Yunnan, China

Many normal and a few reversed characteristic directions of magnetization have been obtained by predominantly thermal demagnetization from ten sites of the Late Permian Emeishan Basalts collected near Kunming, Yunnan Province. The normal magnetization directions pass a fold test at the 99% confidence level and yield declination/inclination = 26[deg]/-12[deg], k = 46, [alpha]95 = 6[deg] and a paleopole at 50[deg]N, 241[deg]E. However, the reversed-polarity directions, with declination/inclination at 244[deg] / + 3[deg] are not antipodal to the normal ones, which is also noted in other studies of the Emeishan Basalts of the Yangtze Paraplatform of the South China Block. Speculations about the cause of this lack of antipodality center on: 1. (1) local relative rotations.2. (2) incomplete demagnetization.3. (3) unusually large non-dipole fields, secular variation or asymmetric reversals in the Late Permian.4. (4) errors in sample orientation.5. (5) later remagnetization represented by one or the other polarity group. The first two causes are ruled out by our observations, and the third cannot be tested with data from China alone but is unlikely because it has not been observed elsewhere. Errors in sample orientation may be present in those studies that used a magnetic compass, because sample intensities of magnetization are on the high side. However, for the normal-polarity directions, a solar compass has been used in some of the studies and all normal-polarity directions observed in the area are in agreement. Thus, sample orientation errors can be invoked only for the reversed-polarity directions. In order to test cause (5), we have examined the magnetic carriers in eight samples with scanning electron microscopy. Titanium-poor to titanium-rich magnetite, commonly cruciform in crystal habit, indicates primary igneous crystallization in two samples that have NE and WSW declinations. In contrast, six normal-polarity samples with NNE declinations show pervasive replacement of original titanomagnetite by titanium-free magnetite and sphene, indicating a high degree of alteration. We ascribe this alteration to late hydrothermal circulation and argue that it has caused remagnetization in post-Permian times. This study suggests, therefore, that the NE-SW directions are more likely to be representative of the Late Permian paleomagnetic field than the NNE directions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30785/1/0000438.pd

Acute and chronic health impacts of PM2.5 in China and the influence of interannual meteorological variability

High concentrations of PM2.5 in China have an adverse impact on human health and present a major problem for air quality control. Here we evaluate premature deaths attributable to chronic and acute exposure to ambient PM2.5 at different scales in China over 2013-2017 with an air quality model at 5 km resolution and integrated exposure-response methods. We estimate that 1,210,000 (95% Confidence Interval: 720,000-1,750,000) premature deaths annually are attributable to chronic exposure to PM2.5 pollution. Chongqing exhibits the largest chronic per capita mortality (1.4‰) among all provinces. A total of 116,000 (64,000-170,000) deaths annually are attributable to acute exposure during pollution episodes over the period, with Hubei province showing the highest acute per capita mortality (0.15‰). We also find that in urban areas premature deaths are 520,000 (320,000-760,000) due to chronic and 55,000 (3,000-81,000) due to acute exposure, respectively. At a provincial level, the annual mean PM2.5 concentration varies by ±20% due to interannual variability in meteorology, and PM2.5-attributable chronic mortality varies by ±8%, and by >±5% and ±1% at a national level. Meteorological variability shows larger impacts on interannual variations in acute risks than that in chronic exposure at both provincial (>±20%) and national (±4%) levels. These findings emphasize that tighter controls of PM2.5 and precursor emissions are urgently needed, particularly under unfavorable meteorological conditions in China

Global-regional nested simulation of particle number concentration by combing microphysical processes with an evolving organic aerosol module

Aerosol microphysical processes are essential for the next generation of global and regional climate and air quality models to determine particle size distribution. The contribution of organic aerosols (OAs) to particle formation, mass, and number concentration is one of the major uncertainties in current models. A new global–regional nested aerosol model was developed to simulate detailed microphysical processes. The model combines an advanced particle microphysics (APM) module and a volatility basis set (VBS) OA module to calculate the kinetic condensation of low-volatility organic compounds and equilibrium partitioning of semi-volatile organic compounds in a 3-D framework using global–regional nested domain. In addition to the condensation of sulfuric acid, the equilibrium partitioning of nitrate and ammonium, and the coagulation process of particles, the microphysical processes of the OAs are realistically represented in our new model. The model uses high-resolution size bins to calculate the size distribution of new particles formed through nucleation and subsequent growth. The multi-scale nesting enables the model to perform high-resolution simulations of the particle formation processes in the urban atmosphere in the background of regional and global environments. By using the nested domains, the model reasonably reproduced the OA components obtained from the analysis of aerosol mass spectrometry measurements through positive matrix factorization and the particle number size distribution in the megacity of Beijing during a period of approximately a month. Anthropogenic organic species accounted for 67 % of the OAs of secondary particles formed by nucleation and subsequent growth, which is considerably larger than that of biogenic OAs. On the global scale, the model well predicted the particle number concentration in various environments. The microphysical module combined with the VBS simulated the universal distribution of organic components among the different aerosol populations. The model results strongly suggest the importance of anthropogenic organic species in aerosol particle formation and growth at polluted urban sites and over the whole globe.Aerosol microphysical processes are essential for the next generation of global and regional climate and air quality models to determine particle size distribution. The contribution of organic aerosols (OAs) to particle formation, mass, and number concentration is one of the major uncertainties in current models. A new global-regional nested aerosol model was developed to simulate detailed microphysical processes. The model combines an advanced particle microphysics (APM) module and a volatility basis set (VBS) OA module to calculate the kinetic condensation of low-volatility organic compounds and equilibrium partitioning of semi-volatile organic compounds in a 3-D framework using global-regional nested domain In addition to the condensation of sulfuric acid, the equilibrium partitioning of nitrate and ammonium, and the coagulation process of particles, the microphysical processes of the OAs are realistically represented in our new model. The model uses high-resolution size bins to calculate the size distribution of new particles formed through nucleation and subsequent growth. The multi-scale nesting enables the model to perform high-resolution simulations of the particle formation processes in the urban atmosphere in the background of regional and global environments. By using the nested domains, the model reasonably reproduced the OA components obtained from the analysis of aerosol mass spectrometry measurements through positive matrix factorization and the particle number size distribution in the megacity of Beijing during a period of approximately a month. Anthropogenic organic species accounted for 67 % of the OAs of secondary particles formed by nucleation and subsequent growth, which is considerably larger than that of biogenic OAs. On the global scale, the model well predicted the particle number concentration in various environments. The microphysical module combined with the VBS simulated the universal distribution of organic components among the different aerosol populations. The model results strongly suggest the importance of anthropogenic organic species in aerosol particle formation and growth at polluted urban sites and over the whole globe.Peer reviewe

A Method Combining Multi-Feature Fusion and Optimized Deep Belief Network for EMG-Based Human Gait Classification

In this paper, a gait classification method based on the deep belief network (DBN) optimized by the sparrow search algorithm (SSA) is proposed. The multiple features obtained based on surface electromyography (sEMG) are fused. These functions are used to train the model. First, the sample features, such as the time domain and frequency domain features of the denoised sEMG are extracted and then the fused features are obtained by feature combination. Second, the SSA is utilized to optimize the architecture of DBN and its weight parameters. Finally, the optimized DBN classifier is trained and used for gait recognition. The classification results are obtained by varying different factors and the recognition rate is compared with the previous classification algorithms. The results show that the recognition rate of SSA-DBN is higher than other classifiers, and the recognition accuracy is improved by about 2% compared with the unoptimized DBN. This indicates that for the application in gait recognition, SSA can optimize the network performance of DBN, thus improving the classification accuracy