On the rapid orbital expansion in the compact low-mass X-ray binary 2A 1822−-371

The neutron star low-mass X-ray binary 2A 1822$-$371 has an orbital period of 5.57 hr. Mass transfer in such short-period binaries is thought to be driven by magnetic braking with orbital shrinking. However, 2A 1822$-$371 shows a very rapid orbital expansion, implying that mass transfer occurs rapidly in this system. The accretion rate of the neutron star is observationally estimated to be higher than the Eddington limit, which is also hard to be explained by the standard magnetic braking mechanism. In this work, we construct a model to account for the peculiar properties of 2A 1822$-$371. We assume that the donor star possesses a relatively strong magnetic field, which is coupled with the stellar winds excited by the X-ray radiation from the neutron star. This would generate efficient angular momentum loss, leading to a high mass transfer rate and hence orbital expansion. We provide possible evolutionary tracks of 2A 1822$-$371 and study how the input parameters affect the results. The observational implications of the irradiation-driven mass loss are also briefly discussed in the context of evolution of low-mass X-ray binaries and millisecond pulsars.Comment: 14 pages, 10 figures, accepted for publication in Ap

Numerically Fitting The Electron Fermi Energy and The Electron Fraction in A Neutron Star

Based on the basic definition of Fermi energy of degenerate and relativistic electrons, we obtain a special solution to electron Fermi energy, $E_{\rm F}(e)$, and express $E_{\rm F}(e)$ as a function of electron fraction, $Y_{e}$, and matter density, $\rho$. Several useful analytical formulae for $Y_{e}$ and $\rho$ within classical models and the work of Dutra et al. 2014 (Type-2) in relativistic mean field theory are obtained using numerically fitting. When describing the mean-field Lagrangian, density, we adopt the TMA parameter set, which is remarkably consistent with with the updated astrophysical observations of neutron stars. Due to the importance of the density dependence of the symmetry energy, $S$, in nuclear astrophysics, a brief discussion on the symmetry parameters $S_v$ and $L$ (the slope of $S$) is presented. Combining these fit formulae with boundary conditions for different density regions, we can evaluate the value of $E_{\rm F}(e)$ in any given matter density, and obtain a schematic diagram of $E_{\rm F}(e)$ as a continuous function of $\rho$. Compared with previous study on the electron Fermi energy in other models, our methods of calculating $E_{\rm F}(e)$ are more simple and convenient, and can be universally suitable for the relativistic electron regions in the circumstances of common neutron stars. We have deduced a general expression of $E_{\rm F}(e)$ and $n_{e}$, which could be used to indirectly test whether one EoS of a NS is correct in our future studies on neutron star matter properties. Since URCA reactions are expected in the center of a massive star due to high-value electron Fermi energy and electron fraction, this study could be useful in the future studies on the NS thermal evolution.Comment: 30 pages, 14 figure

A quantum degenerate Bose-Fermi mixture of 41^{41}K and 6^6Li

We report a new apparatus for the study of two-species quantum degenerate mixture of $^{41}$K and $^6$Li atoms. We develop and combine several advanced cooling techniques to achieve both large atom number and high phase space density of the two-species atom clouds. Furthermore, we build a high-efficiency two-species magnetic transport system to transfer atom clouds from the 3D magneto-optical-trap chamber to a full glass science chamber of extreme high vacuum environment and good optical access. We perform a forced radio-frequency evaporative cooling for $^{41}$K atoms while the $^6$Li atoms are sympathetically cooled in an optically-plugged magnetic trap. Finally, we achieve the simultaneous quantum degeneracy for the $^{41}$K and $^6$Li atoms. The Bose-Einstein condensate of $^{41}$K has 1.4$\times$10$^5$ atoms with a condensate fraction of about 62%, while the degenerate Fermi gas of $^6$Li has a total atom number of 5.4$\times$10$^5$ at 0.25 Fermi temperature

Observation of the density effect on the closed-channel fraction in a 6^6Li superfluid

Atomic Fermi gases provide an ideal platform for studying the pairing and superfluid physics, using a Feshbach resonance between closed channel molecular states and open channel scattering states. Of particular interest is the strongly interacting regime. We show that the closed-channel fraction $Z$ provides an effective probe for the important many-body interacting effects, especially through its density dependence, which is absent from two-body theoretical predictions. Here we measure $Z$ as a function of interaction strength and the Fermi temperature $T_F$ in a trapped $^6$Li superfluid throughout the entire BCS--BEC crossover. Away from the deep BEC regime, the fraction $Z$ is sensitive to $T_F$. In particular, our data show $Z \propto T_F^{\alpha}$ with $\alpha=1/2$ at unitarity, in quantitative agreement with calculations of a two-channel pairing fluctuation theory, and $\alpha$ increases rapidly into the BCS regime, reflecting many-body interaction effects as predicted.Comment: Major changes have been made since last version. This submission contains supplementary information as an ancillary fil

Coupled Dipole Oscillations of a Mass-Imbalanced Bose and Fermi Superfluid Mixture

Recent experimental realizations of superfluid mixtures of Bose and Fermi quantum gases provide a unique platform for exploring diverse superfluid phenomena. We study dipole oscillations of a double superfluid in a cigar-shaped optical dipole trap, consisting of $^{41}$K and $^{6}$Li atoms with a large mass imbalance, where the oscillations of the bosonic and fermionic components are coupled via the Bose-Fermi interaction. In our high-precision measurements, the frequencies of both components are observed to be shifted from the single-species ones, and exhibit unusual features. The frequency shifts of the $^{41}$K component are upward (downward) in the radial (axial) direction, whereas the $^{6}$Li component has down-shifted frequencies in both directions. Most strikingly, as the interaction strength is varied, the frequency shifts display a resonant-like behavior in both directions, for both species, and around a similar location at the BCS side of fermionic superfluid. These rich phenomena challenge theoretical understanding of superfluids

Oscillatory-like Expansion of a Fermionic Superfluid

We study the expansion behaviours of a Fermionic superfluid in a cigar-shaped optical dipole trap for the whole BEC-BCS crossover and various temperatures. At low temperature ($0.06(1) T_F$), the atom cloud undergoes an anisotropic hydrodynamic expansion over 30~ms, which behaves like oscillation in the horizontal plane. By analyzing the expansion dynamics according to the superfluid hydrodynamic equation, the effective polytropic index $\bar{\gamma}$ of Equation-of-State of Fermionic superfluid is extracted. The $\bar{\gamma}$ values show a non-monotonic behavior over the BEC-BCS crossover, and have a good agreement with the theoretical results in the unitarity and BEC side. The normalized quasi-frequencies of the oscillatory expansion are measured, which drop significantly from the BEC side to the BCS side and reach a minimum value of 1.73 around $1/k_Fa=-0.25$. Our work improves the understanding of the dynamic properties of strongly interacting Fermi gas

Strongly Interacting Bose Gases near a dd-wave Shape Resonance

Many unconventional quantum matters, such as fractional quantum Hall effect and $d$-wave high-Tc superconductor, are discovered in strongly interacting systems. Understanding quantum many-body systems with strong interaction and the unconventional phases therein is one of the most challenging problems in physics nowadays. Cold atom systems possess a natural way to create strong interaction by bringing the system to the vicinity of a scattering resonance. Although this has been a focused topic in cold atom physics for more than a decade, these studies have so far mostly been limited for $s$-wave resonance. Here we report the experimental observation of a broad $d$-wave shape resonance in degenerate ${}^{41}$K gas. We further measure the molecular binding energy that splits into three branches as a hallmark of $d$-wave molecules, and find that the lifetime of this many-body system is reasonably long at strongly interacting regime. From analyzing the breathing mode excited by ramping through this resonance, it suggests that a quite stable low-temperature atom and molecule mixture is produced. Putting all the evidence together, our system offers great promise to reach a $d$-wave molecular superfluid

Feshbach spectroscopy of an ultracold 41^{41}K-6^6Li mixture and 41^{41}K atoms

We have observed 69 $^{41}$K-$^6$Li interspecies Feshbach resonances including 13 elastic p-wave resonances and 6 broad d-wave resonances of $^{41}$K atoms in different spin-state combinations at fields up to 600~G. Multi-channel quantum defect theory calculation is performed to assign these resonances and the results show perfect agreement with experimental values after improving input parameters. The observed broad p- and d- wave resonances display a full resolved multiplet structure. They may serve as important simulators to nonzero partial wave dominated physics

Observation of state-to-state hyperfine-changing collision in a Bose-Fermi mixture of 6^6Li and 41^{41}K atoms

Hyperfine-changing collisions are of fundamental interest for the studying of ultracold heteronuclear mixtures. Here, we report the state-to-state study of the hyperfine-changing-collision dynamics in a Bose-Fermi mixture of $^6$Li and $^{41}$K atoms. The collision products are directly observed and the spin-changing dynamics is measured. Based on a two-body collision model, the experimental results are simultaneously fitted from which the spin-changing rate coefficient of $1.9(2)\times 10^{-12}~\rm{cm^3\cdot s^{-1}}$ is gained, being consistent with the multi-channel quantum defect theory calculation. We further show that the contact parameter of $^6$Li-$^{41}$K mixture can be extracted from the measured spin-changing dynamics. The obtained results are consistent with the first order perturbation theory in the weakly-interacting limit. Our system offers great promise for studying spin-changing interactions in heteronuclear mixtures

A 30 W, sub-kHz frequency-locked laser at 532 nm

We report on the realization of a high-power, ultranarrow-linewidth, and frequency-locked 532~nm laser system. The laser system consists of single-pass and intra-cavity second harmonic generation of a continuous-wave Ytterbium doped fiber laser at 1064~nm in the nonlinear crystal of periodically poled lithium niobate and lithium triborate, respectively. With 47~W infrared input, 30~W green laser is generated through the type I critical phase matching in the intracavity lithium triborate crystal. The laser linewidth is measured to be on the order of sub-kHz, which is achieved by simultaneously locking the single-pass frequency doubling output onto the iodine absorption line R69 (36-1) at 532~nm. Furthermore, the phase locking between the laser system and another slave 1064~nm laser is demonstrated with relative frequency tunability being up to 10~GHz. Our results completely satisfy the requirements of 532~nm laser for quantum simulation with ultracold atoms