Efficacy of crustal superfluid neutrons in pulsar glitch models

In order to assess the ability of purely crust-driven glitch models to match the observed glitch activity in the Vela pulsar, we conduct a systematic analysis of the dependence of the fractional moment of inertia of the inner crustal neutrons on the stiffness of the nuclear symmetry energy at saturation density $L$. We take into account both crustal entrainment and the fact that only a fraction $Y_{\rm g}$ of the core neutrons may couple to the crust on the glitch-rise timescale. We use a set of consistently-generated crust and core compositions and equations-of-state which are fit to results of low-density pure neutron matter calculations. When entrainment is included at the level suggested by recent microscopic calculations and the core is fully coupled to the crust, the model is only able to account for the Vela glitch activity for a 1.4$M_{\odot}$ star if the equation of state is particularly stiff $L>100$ MeV. However, an uncertainty of about 10\% in the crust-core transition density and pressure allows for the Vela glitch activity to be marginally accounted for in the range $L\approx30-60$MeV consistent with a range of experimental results. Alternatively, only a small amount of core neutrons need be involved. If less than 50\% of the core neutrons are coupled to the crust during the glitch, we can also account for the Vela glitch activity using crustal neutrons alone for EOSs consistent with the inferred range of $L$. We also explore the possibility of Vela being a high-mass neutron star, and of crustal entrainment being reduced or enhanced relative to its currently predicted values.Comment: 10 pages, 6 figure

Probing the high-density behavior of symmetry energy with gravitational waves

Gravitational wave (GW) astronomy opens up an entirely new window on the Universe to probe the equations of state (EOS) of neutron-rich matter. With the advent of next generation GW detectors, measuring the gravitational radiation from coalescing binary neutron star systems, mountains on rotating neutron stars, and stellar oscillation modes may become possible in the near future. Using a set of model EOSs satisfying the latest constraints from terrestrial nuclear experiments, state of the art nuclear many-body calculations of the pure neutron matter EOS, and astrophysical observations consistently, we study various GW signatures of the high-density behavior of the nuclear symmetry energy, which is considered among the most uncertain properties of dense neutron-rich nucleonic matter. In particular, we find the tidal polarizability of neutron stars, potentially measurable in binary systems just prior to merger, is more sensitive to the high density component of the nuclear symmetry energy than the symmetry energy at nuclear saturation density. We also find that the upper limit on the GW strain amplitude from elliptically deformed stars is very sensitive to the density dependence of the symmetry energy. This suggests that future developments in modeling of the neutron star crust, and direct gravitational wave signals from accreting binaries will provide a wealth of information on the EOS of neutron-rich matter. We also review the sensitivity of the $r$-mode instability window to the density dependence of the symmetry energy. Whereas models with larger values of the density slope of the symmetry energy at saturation seem to be disfavored by the current observational data, within a simple $r$-mode model, we point out that a subsequent softer behavior of the symmetry energy at high densities (hinted at by recent observational interpretations) could rule them in.Comment: 14 pages, 11 figures, 3 tables; submitted to EPJA Special Volume on Nuclear Symmetry Energ

Electron-Phonon Interactions for Optical Phonon Modes in Few-Layer Graphene

We present a first-principles study of the electron-phonon (e-ph) interactions and their contributions to the linewidths for the optical phonon modes at $\Gamma$ and K in one to three-layer graphene. It is found that due to the interlayer coupling and the stacking geometry, the high-frequency optical phonon modes in few-layer graphene couple with different valence and conduction bands, giving rise to different e-ph interaction strengths for these modes. Some of the multilayer optical modes derived from the $\Gamma$-$E_{2g}$ mode of monolayer graphene exhibit slightly higher frequencies and much reduced linewidths. In addition, the linewidths of K-$A'_1$ related modes in multilayers depend on the stacking pattern and decrease with increasing layer numbers.Comment: 6 pages,5 figures, submitted to PR