Critical densities for the Skyrme type effective interactions

We use the stability conditions of the Landau parameters for the symmetric nuclear matter and pure neutron matter to calculate the critical densities for the Skyrme type effective nucleon-nucleon interactions. We find that the critical density can be maximized by adjusting appropriately the values of the enhancement factor $\kappa$ associated with isovector giant dipole resonance, the quantity $L$ which is directly related to the slope of the symmetry energy and the Landau parameter $G_0^\prime$. However, restricting $\kappa$, $L$ and $G_0^\prime$ to vary within acceptable limits reduces the maximum value for the critical density $\tilde\rho_{cr}$ by $\sim 25$. We also show that among the various quantities characterizing the symmetric nuclear matter, $\tilde\rho_{cr}$ depends strongly on the isoscalar effective mass $m^*/m$ and surface energy coefficient $E_s$. For realistic values of $m^*/m$ and $E_s$ we get $\tilde\rho_{cr} = 2\rho_0$ to $3\rho_0$ ($\rho_0 = 0.16$fm$^{-3}$).Comment: 10 pages, 3 figures. Physicsl Review C (in press

Scaling of the distribution of shortest paths in percolation

We present a scaling hypothesis for the distribution function of the shortest paths connecting any two points on a percolating cluster which accounts for {\it (i)} the effect of the finite size of the system, and {\it (ii)} the dependence of this distribution on the site occupancy probability $p$. We test the hypothesis for the case of two-dimensional percolation.Comment: 7 pages, 3 figure

Determination of the parameters of a Skyrme type effective interaction using the simulated annealing approach

We implement for the first time the simulated annealing method (SAM) to the problem of searching for the global minimum in the hyper-surface of the chi-square function which depends on the values of the parameters of a Skyrme type effective nucleon-nucleon interaction. We undertake a realistic case of fitting the values of the Skyrme parameters to an extensive set of experimental data on the ground state properties of many nuclei ranging from normal to exotic ones. The set of experimental data used in our fitting procedure includes the radii for the valence $1d_{5/2}$ and $1f_{7/2}$ neutron orbits in the $^{17}$O and $^{41}$Ca nuclei, respectively, and the breathing mode energies for several nuclei, in addition to the typically used data on binding energy, charge radii and spin-orbit splitting. We also include in the fit the critical density $\rho_{cr}$ and further constrain the values of the Skyrme parameters by requiring that (i) the quantity $P = 3\rho \frac{dS}{d\rho}$, directly related to the slope of the symmetry energy $S$, must be positive for densities up to $3\rho_0$ (ii) the enhancement factor $\kappa$, associated with the isovector giant dipole resonance, should lie in the range of $0.1 - 0.5$ and (iii) the Landau parameter $G_0^\prime$ is positive at $\rho = \rho_0$. We provide simple but consistent schemes to account for the center of mass corrections to the binding energy and charge radii.Comment: 33 pages, 4 figures, Phys. Rev. C (in press

Secure Degrees of Freedom of MIMO X-Channels with Output Feedback and Delayed CSIT

We investigate the problem of secure transmission over a two-user multi-input multi-output (MIMO) X-channel in which channel state information is provided with one-unit delay to both transmitters (CSIT), and each receiver feeds back its channel output to a different transmitter. We refer to this model as MIMO X-channel with asymmetric output feedback and delayed CSIT. The transmitters are equipped with M-antennas each, and the receivers are equipped with N-antennas each. For this model, accounting for both messages at each receiver, we characterize the optimal sum secure degrees of freedom (SDoF) region. We show that, in presence of asymmetric output feedback and delayed CSIT, the sum SDoF region of the MIMO X-channel is same as the SDoF region of a two-user MIMO BC with 2M-antennas at the transmitter, N-antennas at each receiver and delayed CSIT. This result shows that, upon availability of asymmetric output feedback and delayed CSIT, there is no performance loss in terms of sum SDoF due to the distributed nature of the transmitters. Next, we show that this result also holds if only output feedback is conveyed to the transmitters, but in a symmetric manner, i.e., each receiver feeds back its output to both transmitters and no CSIT. We also study the case in which only asymmetric output feedback is provided to the transmitters, i.e., without CSIT, and derive a lower bound on the sum SDoF for this model. Furthermore, we specialize our results to the case in which there are no security constraints. In particular, similar to the setting with security constraints, we show that the optimal sum DoF region of the (M,M,N,N)--MIMO X-channel with asymmetric output feedback and delayed CSIT is same as the DoF region of a two-user MIMO BC with 2M-antennas at the transmitter, N-antennas at each receiver, and delayed CSIT. We illustrate our results with some numerical examples.Comment: To Appear in IEEE Transactions on Information Forensics and Securit

Possible Connection between the Optimal Path and Flow in Percolation Clusters

We study the behavior of the optimal path between two sites separated by a distance $r$ on a $d$-dimensional lattice of linear size $L$ with weight assigned to each site. We focus on the strong disorder limit, i.e., when the weight of a single site dominates the sum of the weights along each path. We calculate the probability distribution $P(\ell_{\rm opt}|r,L)$ of the optimal path length $\ell_{\rm opt}$, and find for $r\ll L$ a power law decay with $\ell_{\rm opt}$, characterized by exponent $g_{\rm opt}$. We determine the scaling form of $P(\ell_{\rm opt}|r,L)$ in two- and three-dimensional lattices. To test the conjecture that the optimal paths in strong disorder and flow in percolation clusters belong to the same universality class, we study the tracer path length $\ell_{\rm tr}$ of tracers inside percolation through their probability distribution $P(\ell_{\rm tr}|r,L)$. We find that, because the optimal path is not constrained to belong to a percolation cluster, the two problems are different. However, by constraining the optimal paths to remain inside the percolation clusters in analogy to tracers in percolation, the two problems exhibit similar scaling properties.Comment: Accepted for publication to Physical Review E. 17 Pages, 6 Figures, 1 Tabl