Origin of intense magnetic fields near black holes due to non-minimal gravitational-electromagnetic coupling

The origin of magnetic fields in astrophysical objects is a challenging problem in astrophysics. Throughout the years, many scientists have suggested that non-minimal gravitational-electromagnetic coupling (NMGEC) could be the origin of the ubiquitous astrophysical magnetic fields. We investigate the possible origin of intense magnetic fields by NMGEC near rotating black holes, connected with quasars and gamma-ray bursts. Whereas these intense magnetic fields are difficult to explain astrophysically, we find that they are easily explained by NMGEC.     

Self-trapping of one-dimensional and two-dimensional optical beams and induced waveguides in photorefractive KNbO(3)

We demonstrate self-trapping of one-dimensional and two-dimensional optical beams in a photorefractive KNbO(3) crystal. We study the waveguides induced by the self-trapped beams for prospective applications of tunable nonlinear frequency conversion in soliton-induced waveguides.     

Percolation theory and fragmentation measures in social networks

We study the statistical properties of a recently proposed social networks measure of fragmentation F after removal of a fraction q of nodes or links from the network. The measure F   is defined as the ratio of the number of pairs of nodes that are not connected in the fragmented network to the total number of pairs in the original fully connected network. We compare this measure with the one traditionally used in percolation theory, _P∞$P_{\infty }$, the fraction of nodes in the largest cluster relative to the total number of nodes. Using both analytical and numerical methods, we study Erd?s–Rényi (ER) and scale-free (SF) networks under various node removal strategies. We find that for a network obtained after removal of a fraction q   of nodes above criticality, _P∞≈(1-F)^1/2$P_{\infty }\approx (1-F{)}^{1/2}$. For fixed _P∞$P_{\infty }$ and close to criticality, we show that 1-F$1-F$ better reflects the actual fragmentation. For a given _P∞$P_{\infty }$, 1-F$1-F$ has a broad distribution and thus one can improve significantly the fragmentation of the network. We also study and compare the fragmentation measure F   and the percolation measure _P∞$P_{\infty }$ for a real national social network of workplaces linked by the households of the employees and find similar results.     

Two-dimensional multipole solitons in nonlocal nonlinear media

We present the experimental observation of scalar multipole solitons in highly nonlocal nonlinear media, including dipole, tripole, quadrupole, and necklace-type solitons, organized as arrays of out-of-phase bright spots. These complex solitons are metastable, but with a large parameters range where the instability is weak, permitting their experimental observation.     

Bi2Sr2CaCu2O8 crystal with a region of periodic blind antidots – transition between bulk and interface vortex pinning

This work focuses on differential magneto-optical two-dimensional imaging of the current distribution in a high-T_c superconducting Bi₂Sr₂CaCu₂O₈ crystal having a region of 170 × 170 μm² patterned with periodic blind antidots. By measuring the self-induced field of an applied current, we can map the current flow distribution within and at the edges of the patterned region. We detected three separate types of current flow within the patterned region, which correspond to three different arrangements of pancake vortices within and at the edges of the patterned region. At high temperatures the vortices delocalize from the antidots, presumably due to thermal fluctuations. However, at lower temperatures vortex pinning mechanisms become prominent, lowering the vortex mobility in the patterned region. There are two contributing mechanisms: bulk pinning by the patterned antidots and interface pinning due to an entry barrier into the patterned region. Each mechanism is dominant at different temperatures. From our experiments we see that the T–H transition line from the bulk to the interface pinning and the vortex melting line in the pristine region coincide.     

Design of shallow donor levels in diamond by isovalent-donor coupling

Using the first-principles pseudopotential method, we have studied simultaneous isovalent and n-type doping in diamond. We show that Si induces fully occupied isovalent levels near the valence band maximum. The Si levels interact with N donor levels, making them much shallower. The donor transition energy level of the N + 4Si defect complexes is found to be 0.09 eV below the conduction band minimum, which is the shallowest level found thus far for this system. The binding energy of the N + 4Si complex is also large enough to insure its stability.