The Photodecomposition and Photooxidation of Aliphatic Amines

The ultraviolet irradiation of ammonia-saturated liquid cyclohexane produces two products, cyclohexylamine in a photostationary state and cyclohexylcyclohexane. It is assumed that ammonia is responsible for the initial light absorption which leads to the observed reaction products. The products and rates of product formation have been determined for the ultraviolet irradiation of cyclohexylamine, cyclopentylamine, and n-hexylamine in saturated hydrocarbon solvents. The results demonstrate that (1) imines are produced during the irradiation, (2) ammonia is isolated and identified, (3) both carbon-nitrogen and nitrogen-hydrogen bonds cleave to varying degrees during the irradiation, and (4) the cleavage pattern is dependent on the stability of the alkyl radicals produced. Direct ultraviolet irradiation of the secondary amines, _i..e., di-n- hexylamine, dicyclohexylamine, and n-hexylcyclohexylamine in cyclohexane liquid solution, produces the corresponding imine in preparative yields (89-91%). The primary process of the photooxidation of cyclohexylamine is determined as charge-transfer excitation by the light absorption dependence of the reaction. The primary products are cyclohexanone oxime, N- cyclohexylidenecyclohexylamine, ammonia, and water. Synthesis and studies of 1-hydroperoxycyclohexylamine show that it is probably the precursor for N-cyclohexylidenecyclohexylamine . Cyclohexanone oxime is presumably generated from the thermal or photodecomposition of N- hydroperoxycyclohexylamine. Hydrogen peroxide is probably present in the photomixture of cyclohexylamine. When ionization potentials of aliphatic amines are plotted versus the frequency of charge-transfer bands, a linear relationship is obtained . The relative rates of oxygen uptake during the photooxidation of cyclic hydrocarbons show that the carbon-hydrogen rather than carbon- carbon bond is the donor in the contact charge-transfer process between oxygen and the hydrocarbon

Microscopic theory of quantum anomalous Hall effect in graphene

We present a microscopic theory to give a physical picture of the formation of quantum anomalous Hall (QAH) effect in graphene due to a joint effect of Rashba spin-orbit coupling $\lambda_R$ and exchange field $M$. Based on a continuum model at valley $K$ or $K'$, we show that there exist two distinct physical origins of QAH effect at two different limits. For $M/\lambda_R\gg1$, the quantization of Hall conductance in the absence of Landau-level quantization can be regarded as a summation of the topological charges carried by Skyrmions from real spin textures and Merons from \emph{AB} sublattice pseudo-spin textures; while for $\lambda_R/M\gg1$, the four-band low-energy model Hamiltonian is reduced to a two-band extended Haldane's model, giving rise to a nonzero Chern number $\mathcal{C}=1$ at either $K$ or $K'$. In the presence of staggered \emph{AB} sublattice potential $U$, a topological phase transition occurs at $U=M$ from a QAH phase to a quantum valley-Hall phase. We further find that the band gap responses at $K$ and $K'$ are different when $\lambda_R$, $M$, and $U$ are simultaneously considered. We also show that the QAH phase is robust against weak intrinsic spin-orbit coupling $\lambda_{SO}$, and it transitions a trivial phase when $\lambda_{SO}>(\sqrt{M^2+\lambda^2_R}+M)/2$. Moreover, we use a tight-binding model to reproduce the ab-initio method obtained band structures through doping magnetic atoms on $3\times3$ and $4\times4$ supercells of graphene, and explain the physical mechanisms of opening a nontrivial bulk gap to realize the QAH effect in different supercells of graphene.Comment: 10pages, ten figure

Stabilizing topological phases in graphene via random adsorption

We study the possibility of realizing topological phases in graphene with randomly distributed adsorbates. When graphene is subjected to periodically distributed adatoms, the enhanced spin-orbit couplings can result in various topological phases. However, at certain adatom coverages, the intervalley scattering renders the system a trivial insulator. By employing a finite-size scaling approach and Landauer-B\"{u}ttiker formula, we show that the randomization of adatom distribution greatly weakens the intervalley scattering, but plays a negligible role in spin-orbit couplings. Consequently, such a randomization turns graphene from a trivial insulator into a topological state.Comment: 5 pages and 3 figure

Two-Dimensional Topological Insulator State and Topological Phase Transition in Bilayer Graphene

We show that gated bilayer graphene hosts a strong topological insulator (TI) phase in the presence of Rashba spin-orbit (SO) coupling. We find that gated bilayer graphene under preserved time-reversal symmetry is a quantum valley Hall insulator for small Rashba SO coupling $\lambda_{\mathrm{R}}$, and transitions to a strong TI when $\lambda_{\mathrm{R}} > \sqrt{U^2+t_\bot^2}$, where $U$ and $t_\bot$ are respectively the interlayer potential and tunneling energy. Different from a conventional quantum spin Hall state, the edge modes of our strong TI phase exhibit both spin and valley filtering, and thus share the properties of both quantum spin Hall and quantum valley Hall insulators. The strong TI phase remains robust in the presence of weak graphene intrinsic SO coupling.Comment: 5 pages and 4 figure