On H-Linked Graphs

For a fixed multigraph H, possibly containing loops, with V(H)={h₁, . . . , h_k}, we say a graph G is H-linked if for every choice of k vertices v₁, . . . , v_k in G, there exists a subdivision of H in G such that v_i represents h_i (for all i). This notion clearly generalizes the concept of k-linked graphs (as well as other properties). In this paper we determine, for a connected multigraph H and for any sufficiently large graph G, a sharp lower bound on δ(G) (depending upon H) such that G is H-linked.     

1,3,5,7-Tetraammonium adamantyl tetranitrate (TAATN) crystal structure

Tetraammonium adamantyl tetranitrate (TAATN) crystallizes in the tetragonal space groupP4^–2₁,c witha=10.046(1),c=8.980(2)Å, andZ=2. The positions of the heavy atoms were determined with direct methods and the hydrogen atoms were located with difference Fourier methods. The structure was refined to a finalR value of 0.038. Each tetravalent tetraammonium adamantyl cation is surrounded by twelve hydrogen-bonded nearest-neighbor nitrate anions while each anion in turn is surrounded by three nearest-neighbor tetravalent cations.     

Stimulated photoluminescence of CdTe

Emission spectra of CdTe at low temperature under high intensity optical pumping by means of a tunable dye laser are reported.We show that the optical gain process is enhanced when a high exciton density is achieved by means of quasi-resonant pumping. The mechanism responsible for the amplification process is found to be the well-known exciton-exciton scattering.     

Supergravity,R invariance and spontaneous supersymmetry breaking

We show that in order for a U(1) gauge theory with a Fayet-Illiopoulos term to be consistently coupled to supergravity, preserving gauge invariance, the superpotential must be R invariant. A supersymmetric cosmological term and therefore an explicit mass-like term for the gravitino is forbidden by gauge invariance. This result severely constrains the possible models for non-gravitational interactions. We comment on possible mass term the gauginos induced by gravitational effects.     

Deep inelastic structure functions from collinear SU(1,1) symmetry

We exhibit a model of deep inelastic processes, based on conformal symmetry arguments, in which the structure functions for the scattering and annihilation channels both scale and are related by the Gribov-Lipatov relation. Analytic continuation, however, is not valid in general.     

Canonical dimensions in asymptotically conformal invariant theories

It is shown that canonical dimensions in the product of currents near the light-cone imply a free field structure for the Wightman functions of currents in the underlying skeleton theory.     

AC conductivity of porous silicon: A fractal and surface transport mechanism?

Summary In this paper we report on the frequency dependence of the AC conductivity of porous silicon in the range 10 Hz-100 kHz. Two types of testing devices have been fabricated on three different series of samples formed electrochemically using as a starting materialptype,n ⁻-type andn ⁺-type silicon substrates. For frequencies less than 20-40 kHz the conductivity is found to follow a sublinear frequency dependence. This behaviour is typical of a carrier transport mechanism determined by an anomalous diffusion process. At higher frequencies we find that surface states influence the transport mechanism. This suggests a double-channel transport mechanism: one related to porous-silicon “volume” properties and the other more connected to the “surface” itself. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettiore di Luce) Torino, 12–13 October 1995.     

Off-shell N = 8 supersymmetry with central charges

Using a new technique of dimensional reduction by Legendre transformation we derive an off-shell formulation of the N = 8 extended supersymmetry algebra, which is realised in the maximally extended supergravity theory. The theory has an Sp(8) manifest global invariance and some of the fields satisfy differential constraints.