Ultracold Fermi gases subject to tight transverse confinement offer a highly controllable setting to study the two-dimensional (2D) BCS to Berezinskii-Kosterlitz-Thouless superfluid crossover. Achieving the 2D regime requires confining particles to their transverse ground state which presents challenges in interacting systems. Here, we establish the conditions for an interacting Fermi gas to behave kinematically 2D. Transverse excitations are detected by measuring the transverse expansion rate which displays a sudden increase when the atom number exceeds a critical value N2D signifying a density driven departure from 2D kinematics. For weak interactions N2D is set by the aspect ratio of the trap. Close to a Feshbach resonance, however, the stronger interactions reduce N2D and excitations appear at lower density.PACS numbers: 03.75. Ss, 03.75.Hh, 05.30.Fk, 67.85.Lm Fermions confined to two-dimensional (2D) planes represent an important paradigm in many-body physics in settings ranging from thin films of superfluid helium-3 [1, 2] to the superconducting planes in high-T c cuprates [3]. Ultracold atomic gases confined in oblate potentials allow access to the 2D regime [4][5][6][7][8][9][10][11][12][13][14][15] where interactions between particles can be controlled using a Feshbach resonance [16]. In 2D Fermi gases, one can realize the BCS to Berezinskii-Kosterlitz-Thouless (BKT) superfluid crossover [17][18][19][20][21][22][23][24][25] by tuning the attractive interaction between particles in different spin states. Of particular interest is the enhanced pairing due to the transverse confinement [26][27][28][29][30] and the consequences this has for the phase diagram of the crossover [15,[31][32][33].Theoretical studies of the BCS-BKT crossover generally assume only two spatial dimensions, however, all atomic gases exist in 3D environments. Lower dimensional behaviour can be realized by freezing out dynamics along one or more directions. For atoms in a harmonic potential, with frequencies ω x , ω y and ω z , the 2D regime is achieved when the transverse (z) confinement is strong enough that occupation of transverse excited states is energetically forbidden. When a gas is frozen in the transverse ground state, dynamics in the x-y plane become decoupled from z and the gas is kinematically 2D. In an ideal gas this requires the thermal energy and chemical potential be much smaller than the transverse confinement energy k B T, µ ω z , where k B is Boltzmann's constant, T is the temperature and µ the chemical potential. When interactions are present, however, these can provide another means for generating transverse excitations which go beyond purely 2D models.In this Rapid Communication, we examine the criteria for an interacting Fermi gas to behave kinematically 2D.By measuring the transverse cloud width after time of flight we observe a rapid growth in the expansion rate when transverse excitations are present. Both the trap geometry and interaction strength are seen to limit the parameter space where interacting sys...
Abstract. Global policies that regulate anthropogenic mercury emissions to the environment require quantitative and comprehensive source-receptor relationships for mercury emissions, transport and deposition among major continental regions. In this study, we use the GEOS-Chem global chemical transport model to establish source-receptor relationships among 11 major continental regions worldwide. Source-receptor relationships for surface mercury concentrations (SMC) show that some regions (e.g., East Asia, the Indian subcontinent, and Europe) should be responsible for their local surface Hg(II) and Hg(P) concentrations due to near-field transport and deposition contributions from their local anthropogenic emissions (up to 64 and 71 % for Hg(II) and Hg(P), respectively, over East Asia). We define the region of primary influence (RPI) and the region of secondary influence (RSI) to establish intercontinental influence patterns. Results indicate that East Asia is the SMC RPI for almost all other regions, while Europe, Russia, and the Indian subcontinent also make some contributions to SMC over some receptor regions because they are dominant RSI source regions. Source-receptor relationships for mercury deposition show that approximately 16 and 17 % of dry and wet deposition, respectively, over North America originate from East Asia, indicating that transpacific transport of East Asian emissions is the major foreign source of mercury deposition in North America. Europe, Southeast Asia, and the Indian subcontinent are also important mercury deposition sources for some receptor regions because they are the dominant RSIs. We also quantify seasonal variation on mercury deposition contributions over other regions from East Asia. Results show that mercury deposition (including dry and wet) contributions from East Asia over the Northern Hemisphere receptor regions (e.g., North America, Europe, Russia, the Middle East, and Middle Asia) vary seasonally, with the maximum values in summer and minimum values in winter. The opposite seasonal pattern occurs on mercury dry deposition contributions over Southeast Asia and the Indian subcontinent.
We observe spin squeezing in three-component Bose gases where all three hyperfine states are coupled by synthetic spin-orbit coupling. This phenomenon is a direct consequence of spin-orbit coupling, as can be seen clearly from an effective spin Hamiltonian. By solving this effective model analytically with the aid of a Holstein-Primakoff transformation for spin-1 system in the low excitation limit, we conclude that the spin-nematic squeezing, a novel category of spin squeezing existing exclusively in large spin systems, is enhanced with increasing spin-orbit intensity and effective Zeeman field, which correspond to Rabi frequency ΩR and two-photon detuning δ within the Raman scheme for synthetic spin-orbit coupling, respectively. These trends of dependence are in clear contrast to spin-orbit coupling induced spin squeezing in spin-1/2 systems. We also analyze the effects of harmonic trap and interaction with realistic experimental parameters numerically, and find that a strong harmonic trap favors spin-nematic squeezing. We further show spin-nematic squeezing can be interpreted as two-mode entanglement or two-spin squeezing at low excitation. Our findings can be observed in 87 Rb gases with existing techniques of synthetic spin-orbit coupling and spin-selectively imaging.
Characterizing quantum phase transitions through quantum correlations has been deeply developed for a long time, while the connections between dynamical phase transitions (DPTs) and quantum entanglement is not yet well understood. In this work, we show that the time-averaged two-mode entanglement in the spin space reaches a maximal value when it undergoes a DPT induced by external perturbation in a spin-orbit-coupled Bose-Einstein condensate. We employ the von Neumann entropy and a correlation-based entanglement criterion as entanglement measures and find that both of them can infer the existence of DPT. While the von Neumann entropy works only for a pure state at zero temperature and requires state tomography to reconstruct, the experimentally more feasible correlation-based entanglement criterion acts as an excellent proxy for entropic entanglement and can determine the existence of entanglement for a mixed state at finite temperature, making itself an excellent indicator for DPT. Our work provides a deeper understanding about the connection between DPTs and quantum entanglement, and may allow the detection of DPT via entanglement become accessible as the examined criterion is suitable for measuring entanglement.
The spontaneous formation of lattice structure of quantized vortices is a characteristic feature of superfluidity in closed systems under thermal equilibrium. In exciton-polariton Bose-Einstein condensate, which is a typical example of macroscopic quantum state in open systems, spontaneous vortex lattices have also been proposed by not yet observed. Here, we take into account the finite decay rate of exciton reservoir, and theoretically investigate the vortex structures in circularly pumped polariton Bose-Einstein condensate. Our results show that a decreasing reservoir decay rate can reduce the number of vortices and destabilize the lattice structure, hence is unfavorable to the formation and observation of vortex lattices. These detrimental effects can be prevailed by applying an external angular momentum.
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