Atomic Gas Research Articles

Observation of Light-Induced Dipole-Dipole Forces in Ultracold Atomic Gases

We investigate an attractive force caused by light induced dipole-dipole interactions in freely expanding ultracold 87Rb atoms. This collective, light-triggered effect results in a self-confining potential with interesting features: it exhibits nonlocal properties, is attractive for both red and blue-detuned light fields and induces a remarkably strong force that depends on the gradient of the atomic density. The experimental data are discussed in the framework of a theoretical model based on a local-field approach for the light scattered by the atomic cloud.

Splitting of topological charge pumping in an interacting two-component fermionic Rice-Mele Hubbard model

A Thouless pump transports an integer amount of charge when pumping adiabatically around a singularity. We study the splitting of such a critical point into two separate critical points by adding a Hubbard interaction. Furthermore, we consider extensions to a spinful Rice-Mele model, namely, a staggered magnetic field or an Ising-type spin coupling, further reducing the spin symmetry. The resulting models additionally allow for the transport of a single charge in a two-component system of spinful fermions, whereas in the absence of interactions, zero or two charges are pumped. In the SU(2)-symmetric case, the ionic Hubbard model is visited once along pump cycles that enclose a single singularity. Adding a staggered magnetic field additionally transports an integer amount of spin while the Ising term realizes a pure charge pump. We employ real-time simulations in finite and infinite systems to calculate the adiabatic charge and spin transport, complemented by the analysis of gaps and the many-body polarization to confirm the adiabatic nature of the pump. The resulting charge pumps are expected to be measurable in finite-pumping speed experiments in ultracold atomic gases for which the $\mathrm{SU}(2)$ invariant version is the most promising path. We discuss the implications of our results for a related quantum-gas experiment by Walter et al. [arXiv:2204.06561].

Ground states of atomic Fermi gases in a two-dimensional optical lattice with and without population imbalance

We study the ground state phase diagram of population balanced and imbalanced ultracold atomic Fermi gases with a short range attractive interaction throughout the crossover from BCS to Bose-Einstein condensation (BEC), in a two-dimensional optical lattice (2DOL) comprised of two lattice and one continuum dimensions. We find that the mixing of lattice and continuum dimensions, together with population imbalance, has an extraordinary effect on pairing and the superfluidity of atomic Fermi gases. In the balanced case, the superfluid ground state prevails the majority of the phase space. However, for relatively small lattice hopping integral $t$ and large lattice constant $d$, a pair density wave (PDW) emerges unexpectedly at intermediate coupling strength, and the nature of the in-plane and overall pairing changes from particle-like to hole-like in the BCS and unitary regimes, associated with an abnormal increase in the Fermi volume with the pairing strength. In the imbalanced case, the stable polarized superfluid phase shrinks to only a small portion of the entire phase space spanned by $t$, $d$, imbalance $p$ and interaction strength $U$, mainly in the bosonic regime of low $p$, moderately strong pairing, and relatively large $t$ and small $d$. Due to the Pauli exclusion between paired and excessive fermions within the confined momentum space, a PDW phase emerges and the overall pairing evolves from particle-like into hole-like, as the pairing strength grows stronger in the BEC regime. In both cases, the ground state property is largely governed by the Fermi surface topology. These findings are very different from the cases of pure 3D continuum, 3D lattice or 1DOL.

Fragmented atomic shell around S187 H ii region and its interaction with molecular and ionized gas

ABSTRACT The environment of S187, a nearby H ii region (1.4 ± 0.3 kpc), is analyzed. A surrounding shell has been studied in the H i line, molecular lines, and also in infrared and radio continua. We report the first evidence of a clumpy H i environment in its photodissociation region. A background radio galaxy enables the estimation of the properties of cold atomic gas. The estimated atomic mass fraction of the shell is ∼260 M⊙, the median spin temperature is ∼50 K, the shell size is ∼4 pc with typical wall width around 0.2 pc. The atomic shell consists of ∼100 fragments. The fragment sizes correlate with mass with a power-law index of 2.39–2.50. The S187 shell has a complex kinematical structure, including the expanding quasi-spherical layer, molecular envelope, an atomic sub-bubble inside the shell and two dense cores (S187 SE and S187 NE) at different stages of evolution. The atomic sub-bubble inside the shell is young, contains a Class II young stellar object and OH maser in the centre and the associated YSOs in the walls of the bubble. S187 SE and S187 NE have similar masses (∼1200 and ∼900 M⊙, respectively). S187 SE is embedded into the atomic shell and has a number of associated objects, including high-mass protostars, outflows, maser sources, and other indicators of ongoing star formation. No YSOs inside S187 NE were detected, but indications of compression and heating by the H ii region exist.

Quantum dynamics of cold atomic gas with SU(1,1) symmetry

Motivated by recent advances in quantum dynamics, we investigate the dynamics of the system with $SU(1,1)$ symmetry. Instead of performing the time-ordered integral for the evolution operator of the time-dependent Hamiltonian, we show that the time evolution operator can be expressed as an $SU(1,1)$ group element. Since the $SU(1,1)$ group describes the "rotation" on a hyperbolic surface, the dynamics can be visualized on a Poincar\'e disk, a stereographic projection of the upper hyperboloid. As an example, we present the trajectory of the revival of Bose-Einstein condensation and that of the scale-invariant Fermi gas on the Poincar\'e disk. Further considering the quantum gas in the oscillating lattice, we also study the dynamics of the system with time-dependent single-particle dispersion. Our results are hopefully to be checked in current experiments.

On Producing Long-Lived Spin Polarized Metastable Atoms—Feasibility of Storing Electric Energy

We describe a method of producing long-lived multiply excited spin polarized atoms or ions, the decay of which is strongly delayed or even blocked by intra-ionic magnetic stabilization. Special configurations with huge internal magnetic fields capture only spin polarized electrons in collisions with spin aligned atomic hydrogen gas targets. It is expected that the spin aligned configuration yields an extremely high internal magnetic field which will effectively block spin flip transitions. By this the lifetime of inner shell vacancies is expected to strongly increase.

Doppler-dependent fluorescence spectroscopy of Yb atomic gas for trapped-ion experiments

An ion trap-based Quantum system has been one of the leading architectures toward building a scalable and practical quantum computer. The trapped ion system also has been used for precision experiments such as quantum sensing, metrology, and atomic clock. For the ion-trap experiment, searching resonant frequencies of atomic isotopes are essential for selectively ionization and trapping a specific isotope. In this work, we set up an Yb fluorescence spectroscopy for detecting 399 nm photons of 1S0 → 1P1 transition of the Yb gas from a heated oven. We observed the relative frequency differences between the Yb isotopes and calibrated an optical wavemeter comparing with previous literatures. In addition, we obtain characteristic properties of the atomic oven such as gas’ velocity and density distribution at different oven temperatures. Our experiment can offer a relatively simple and cost-efficient apparatus of spectroscopy and can be useful for designing trap devices in the trapped-ion experiment.

Frequency Conversion of Optical Vortex Arrays Through Four-Wave Mixing in Hot Atomic Gases

Arrays of multiple vortices were transferred from infrared to the blue region of the optical spectrum. This demonstration was achieved by inducing four-wave mixing in an atomic gas with a Gaussian beam and a quasi-invariant propagation beam of the Mathieu type. The latter structure was analyzed in the Fourier space for the pump and the generated light. In both cases, the phase structure can be written with a compact mathematical expression by using the same parameters within experimental error bars. A Michelson–Morley interferometer was used to confirm that a phase singularity was present at each site as predicted by the theory. These studies add to the available control over orbital angular momentum in photons generated by atoms, which has a broad span of applications in quantum and classical information management.

Temporal analysis of self-reversed Ag I resonant lines in LIBS experiment at different laser pulse energy and in different surrounding media

In this work we studied the temporal evolution of the resonant emission lines of neutral silver (Ag I) at wavelengths 328.9 nm and 338.2 nm, in different surrounding media (He, Ar, Xe and air) and at different laser pulse energy (15 mJ, 30 mJ and 60 mJ), using a frequency triplicated Nd:YAG laser (355 nm).We observed that the self-reversal of the Ag lines, caused by the combined effect of self-absorption and plasma inhomogeneity, is strong when the plasma expands in molecular gases (N2 and O2 in air) while is negligible in atomic noble gases (He, Ar, Xe), independently on the mass of the atoms. From the experimental data, we also conclude that the shock wave generated during the breakdown may play an important role in generating the plasma inhomogeneities which are essential conditions for the occurrence of the self-reversal effect.

Formation and fragmentation of quantum droplets in a quasi-one-dimensional dipolar Bose gas

We theoretically investigate the droplets formation in a tightly trapped one-dimensional dipolar gas of bosonic atoms. When the strength of the dipolar interaction becomes sufficiently attractive compared to the contact one, we show how a solitoniclike density profile evolves into a liquidlike droplet on increasing the number of particles in the trap. The incipient gas-liquid transition is also signaled by a steep increase of the breathing mode and a change in sign of the chemical potential. Upon a sudden release of the trap, varying the number of trapped atoms and the scattering length, the numerical solution of a time-dependent generalized Gross-Pitaevskii equation shows either an evaporation of the cloud, the formation of a single self-bound droplet, or a fragmentation in multiple droplets. These results can be probed with lanthanide atoms and help in characterizing the effect of the dipolar interaction in a quasi-one-dimensional geometry.

Unpaired topological triply degenerate point for spin-tensor-momentum-coupled ultracold atoms

The realization of triply degenerate points (TDPs) with exotic fermionic excitations has opened a new perspective for the understanding of our nature. Here we explore the coexistence of single unpaired TDP and multiple twofold Weyl points (WPs) and propose an experimental scheme with ultracold pseudospin-1 atomic gases trapped in optical lattices. We show that the predicted single unpaired TDP emerged by the interplay of quadratic spin-vector- and spin-tensor-momentum-coupling could possess a topological nontrivial middle band. This exotic TDP with mirror symmetry breaking is essential different from the recently observed TDPs that must appear in pairs due to the Nielsen-Ninomiya theorem and host the topological trivial middle band. Strikingly, the topologically protected Fermi-arc states directly connect the unpaired TDP with additional WPs, in contrast to the conventional Fermi-arc states that connect the same degeneracies of band degenerate points with opposite chirality. Furthermore, the different types of TDPs with unique linking structure of Fermi arcs can be readily distinguished by measuring spin texture along high-symmetry lines of the system. Our scheme provides a platform for emerging new fermions with exotic physical phenomena and versatile device applications.

Simulation and validation of the gas flow in close-coupled gas atomisation process: Influence of the inlet gas pressure and the throat width of the supersonic gas nozzle

The effectiveness of a close-coupled gas atomisation process largely depends on the operational and the geometric variables. In this study, Computational Fluid Dynamics (CFD) techniques are used to model and simulate the gas flow in the melt nozzle area for a convergent-divergent, close-coupled gas atomiser in the absence of the melt stream. Firstly, a reference case, in which the atomisation gas is nitrogen at 50 bar and a supersonic gas nozzle with a throat width of L0 has been modelled, is presented. Then, the influence of both the inlet gas pressure and this design parameter are investigated, comparing the numerical results provided by simulations varying the inlet pressure from 5 to 80 bar and modelling different convergent-divergent gas nozzles with throat widths of 0.29∙Lo, 0.5∙Lo, 0.77∙Lo and 2∙Lo respectively. The simulation results show how similarly these two parameters modify gas mass flow rates, gas velocity fields, aspiration pressures in the melt delivery tube or the size of the recirculation zones below the melt nozzle. Therefore, it can be stated that this geometric variable of the gas nozzle may be as relevant as the inlet pressure in the atomisation process. The most important novelty of this study is related to experimental validation of the numerical results using the Particle Image Velocimetry (PIV) technique and through direct measurements of gas mass flow rates, with a clear correlation between simulated and measured data. Moreover, some results obtained with experimental atomisations using copper and nitrogen are also presented. The experimental results show that finer powders are produced by increasing the atomising pressure or the throat width of the supersonic gas nozzle, which can be directly related to the gas flow dynamics calculated numerically.

Down but Not Out: Properties of the Molecular Gas in the Stripped Virgo Cluster Early-type Galaxy NGC 4526

We present Atacama Large Millimeter/submillimeter Array data on the 3 mm continuum emission, CO isotopologues (12CO, 13CO, and C18O), and high-density molecular tracers (HCN, HCO+, HNC, HNCO, CS, CN, and CH3OH) in NGC 4526. These data enable a detailed study of the physical properties of the molecular gas in a longtime resident of the Virgo Cluster; comparisons to more commonly studied spiral galaxies offer intriguing hints into the processing of molecular gas in the cluster environment. Many molecular line ratios in NGC 4526, along with our inferred abundances and CO/H2 conversion factors, are similar to those found in nearby spirals. One striking exception is the very low observed 12CO/13CO(1−0) line ratio, 3.4 ± 0.3, which is unusually low for spirals though not for Virgo Cluster early-type galaxies. We carry out radiative transfer modeling of the CO isotopologues with some archival (2−1) data, and we use Bayesian analysis with Markov Chain Monte Carlo techniques to infer the physical properties of the CO-emitting gas. We find surprisingly low [12CO/13CO] abundance ratios of and at radii of 0.4 kpc and 1 kpc. The emission from the high-density tracers HCN, HCO+, HNC, CS, and CN is also relatively bright, and CN is unusually optically thick in the inner parts of NGC 4526. These features hint that processing in the cluster environment may have removed much of the galaxy’s relatively diffuse, optically thinner molecular gas along with its atomic gas. Angular momentum transfer to the surrounding intracluster medium may also have caused contraction of the disk, magnifying radial gradients such as we find in [13CO/C18O]. More detailed chemical evolution modeling would be interesting in order to explore whether the unusual [12CO/13CO] abundance ratio is entirely an environmental effect or whether it also reflects the relatively old stellar population in this early-type galaxy.

Predicting HCN, HCO+, multi-transition CO, and dust emission of star-forming galaxies

The interstellar medium is a turbulent, multiphase, and multi-scale medium that follows scaling relations that link the surface density, volume density, and velocity dispersion with the cloud size. Galactic clouds range from below 1 pc to about 100 pc in size. Extragalactic clouds appear to follow the same range, although they are only now becoming observable in atomic and molecular lines. Analytical models of galactic gaseous disks need to take the multi-scale and multiphase nature of the interstellar medium into account. They can be described as clumpy star-forming accretion disks in vertical hydrostatic equilibrium, with the mid-plane pressure balancing the gravity of the gaseous and stellar disk. Interstellar medium turbulence is taken into account by applying Galactic scaling relations to the cold atomic and molecular gas phases. Turbulence is maintained through energy injection by supernovae. With the determination of the gas mass fraction at a given spatial scale, the equilibrium gas temperature between turbulent heating and line cooling, the molecular abundances, and the molecular line emission can be calculated. The resulting model radial profiles of infrared, HI, CO, HCN, and HCO+ emission are compared to THINGS, HERACLES, EMPIRE, SINGS, and GALEX observations of 17 local spiral galaxies. The model free parameters were constrained for each galactic radius independently. The Toomre parameter, which measures the stability against star formation (cloud collapse), exceeds unity in the inner disk of a significant number of galaxies. In two galaxies it also exceeds unity in the outer disk. Therefore, in spiral galaxies Qtot = 1 is not ubiquitous. The model gas velocity dispersion is consistent with the observed HI velocity dispersion where available. Within our model, HCN and HCO+ is already detectable in relatively low-density gas (∼1000 cm−3). We derive CO and HCN conversion factors and molecular gas depletion times. Both conversion factors are consistent with values found in the literature. Whereas in the massive galaxies the viscous timescale greatly exceeds the star-formation timescale, the viscous timescale is smaller than the star-formation timescale within R ∼ 2 Rd, the disk scale length, in the low-mass galaxies. We suggest that massive spiral galaxies undergo starvation in the absence of gas accretion from the halo, whereas in low-mass galaxies the fuel for star formation reaches R ∼ 2 Rd from outside via a thick gas disk component with a high radial infall velocity observable in the HI line.

Atomic-motion-induced spectroscopic effects that are nonlinear in atomic density in a gas

The interatomic dipole–dipole interaction is commonly thought to be the main physical reason for spectroscopic effects, which are nonlinear in atomic density. However, we have found that the free motion of atoms can lead to other effects that are nonlinear in atomic density n , using a previously unknown self-consistent solution of the Maxwell–Bloch equations in the mean-field approximation for a gas of two-level atoms with an optical transition at unperturbed frequency ω 0 . These effects distort the Doppler lineshape (shift, asymmetry, broadening), but are not associated with an atom–atom interaction. In particular, in the case of n k 0 − 3 < 1 (where k 0 = ω 0 / c ) and significant Doppler broadening (with respect to collisional broadening), atomic-motion-induced nonlinear effects significantly exceed the well-known influence of the dipole–dipole interatomic interaction (e.g., Lorentz–Lorenz shift) by more than one order of magnitude. Moreover, under some conditions, a frequency interval appears in which a non-trivial self-consistent solution of the Maxwell–Bloch equations is absent due to atomic motion effects. Thus, the existing physical picture of spectroscopic effects that are nonlinear in atomic density in a gas medium should be substantially revised.

Magic Gap Ratio for Optimally Robust Fermionic Condensation and Its Implications for High-T_{c} Superconductivity.

Bardeen-Schrieffer-Cooper (BCS) and Bose-Einstein condensation (BEC) occur at opposite limits of a continuum of pairing interaction strength between fermions. A crossover between these limits is readily observed in a cold atomic Fermi gas. Whether it occurs in other systems such as the high temperature superconducting cuprates has remained an open question. We uncover here unambiguous evidence for a BCS-BEC crossover in the cuprates by identifying a universal magic gap ratio 2Δ/k_{B}T_{c}≈6.5 (where Δ is the pairing gap and T_{c} is the transition temperature) at which paired fermion condensates become optimally robust. At this gap ratio, corresponding to the unitary point in a cold atomic Fermi gas, the measured condensate fraction N_{0} and the height of the jump δγ(T_{c}) in the coefficient γ of the fermionic specific heat at T_{c} are strongly peaked. In the cuprates, δγ(T_{c}) is peaked at this gap ratio when Δ corresponds to the antinodal spectroscopic gap, thus reinforcing its interpretation as the pairing gap. We find the peak in δγ(T_{c}) also to coincide with a normal state maximum in γ, which is indicative of a pairing fluctuation pseudogap above T_{c}.

Nonlinear Quantum Optics With Structured Light: Tightly Trapped Atoms in the 3D Focus of Vectorial Waves

Atomic gases tightly trapped near the focus of an electromagnetic wave interact with photons that exhibit a complex structure, displaying strong gradients of field amplitude and local polarization that can lead to topological phase singularities. We illustrate the consequences of this structure on a paradigmatic nonlinear optical process: three-wave mixing. The process begins by proper selection of the pump field, whose spatial structure is tailored to present huge gradients of the EM field that enhance atomic excitations through forbidden transitions. Atoms can then be depopulated via two electric dipole decays in a cascade configuration, thus providing the three necessary waves. The properties of the down-converted photons are conditioned to those of the pump field through phase matching conditions. It is emphasized that the expression of the photons must incorporate both the structure of the vectorial EM modes and the spatial configuration of the atomic trap. Due to the three-dimensional focusing, the slowly varying envelope approximation becomes inadequate when describing the scattered EM field. We discuss an alternative using a Green function formalism valid for any configuration of the field that also allows to identify the phase matching conditions. Spherical vectorial waves exemplify most concepts here discussed, including the possibility of observing nonlinear quantum phenomena at the single photon level.

Linear and nonlinear Bragg diffraction by electromagnetically induced gratings with PT symmetry and their active control in a Rydberg atomic gas

Due to its myriad applications in many fields of science and technology, novel light diffraction from specially designed optical gratings is a subject of great interest. Here, we propose a scheme to realize the linear and nonlinear Bragg diffractions from an electromagnetically induced grating (EIG) with parity-time $(\mathcal{PT})$ symmetry in a cold Rydberg atomic gas, where the Rydberg-Rydberg interaction between atoms are mapped to strong and long-range interaction between photons, characterized by a giant nonlocal Kerr nonlinearity. We show that a probe laser beam with very low light intensity incident upon the $\mathcal{PT}$-symmetric EIG can display distinctive asymmetric diffraction patterns, which can be actively manipulated through tuning the gain-absorption coefficient of the EIG and the input power of the laser beam. We also show that the intensity distribution among different diffraction orders depends significantly on the $\mathcal{PT}$-symmetry property of the EIG and on the magnitude and nonlocality degree of the Kerr nonlinearity. In addition, we demonstrate that such Bragg diffraction patterns can be controlled by an external gradient magnetic field, which provides a different way of diffraction control. The research results reported here are not only useful for understanding the unique properties of linear and nonlinear Bragg diffractions by $\mathcal{PT}$-symmetric gratings but may also be promising for designing optical devices applicable in optical information processing and transmission.

Photodissociation and X-Ray-Dominated Regions

The radiation from stars and active galactic nuclei (AGNs) creates photodissociation regions (PDRs) and X-ray-dominated regions (XDRs), where the chemistry or heating is dominated by far-ultraviolet (FUV) radiation or X-ray radiation, respectively. PDRs include a wide range of environments, from the diffuse interstellar medium (ISM) to dense star-forming regions. XDRs are found in the center of galaxies hosting AGNs, in protostellar disks, and in the vicinity of X-ray binaries. In this review, we describe the dominant thermal, chemical, and radiation transfer processes in PDRs and XDRs, as well as give a brief description of models and their use for analyzing observations. We then present recent results from Milky Way, nearby extragalactic, and high-redshift observations. Several important results include the following: ▪ Velocity-resolved PDR lines reveal the kinematics of the neutral atomic gas and provide constraints on the stellar feedback process. Their interpretation is, however, in dispute, as observations suggest a prominent role for stellar winds, whereas they are much less important in theoretical models. ▪ A significant fraction of molecular mass resides in CO-dark gas especially in low-metallicity and/or highly irradiated environments. ▪ The CO ladder and [Ci]/[Cii] ratios can determine if FUV or X rays dominate the ISM heating of extragalactic sources. ▪ With Atacama Large Millimeter/submillimeter Array, PDR and XDR tracers are now routinely detected on galactic scales over cosmic time. This makes it possible to link the star-formation history of the Universe to the evolution of the physical and chemical properties of the gas.

Charge ambiguity and splitting of monopoles

This paper focuses on topological defects, appearing in mean-field-theory treatments of physical systems such as ultracold atomic gases and gauge field theories. We begin by investigating the ambiguity and addition of topological charges using the mathematical formalism of covering spaces, which clarifies many aspects these phenomena. Subsequently, we classify topological-defect configurations consisting of several monopoles and unknotted ring defects in terms of homotopy groups and fundamental-group actions on them, thus generalizing the previous classifications of a single monopole and a single unknotted ring defect. Finally, we examine the decay of multiply charged topological monopoles under small perturbations of the physical system, and analyze the conditions under which multiply charged monopoles are inclined to split into several singly charged monopoles.