First principle study of the thermal conductance in graphene nanoribbon with vacancy and substitutional silicon defect

The thermal conductance in graphene nanoribbon with a vacancy or silicon point defect (substitution of C by Si atom) is investigated by non-equilibrium Green's function (NEGF) formalism combined with first-principle calculations density-functional theory with local density approximation. An efficient correction to the force constant matrix is presented to solve the conflict between the long-range character of the {\it ab initio} approach and the first-nearest-neighboring character of the NEGF scheme. In nanoribbon with a vacancy defect, the thermal conductance is very sensitive to the position of the vacancy defect. A vacancy defect situated at the center of the nanoribbon generates a saddle-like surface, which greatly reduces the thermal conductance by strong scattering to all phonon modes; while an edge vacancy defect only results in a further reconstruction of the edge and slightly reduces the thermal conductance. For the Si defect, the position of the defect plays no role for the value of the thermal conductance, since the defective region is limited within a narrow area around the defect center.Comment: accepted by AP

Modeling the Dynamics of Desakota Regions: Global - Local Nexus in the Taipei Metropolitan Area

Since the 1970s, Asia has experienced rapid urbanization processes, which are distinct from U.S. society, and the direction of Asian urbanization is more strongly affected by economic globalization. The desakota model, proposed by McGee and Ginsburg in 1991, focuses on how internal domestic and local forces drive the specific rural-urban transformation in Asia. However, the McGee-Ginsburg model does not emphasize the importance of globalization on Asian urbanization. To fill the gap, this study develops a GIS-based CA framework based on the desakota model to not only simulate the unique urbanization processes in Asia but also integrate the influence of globalization into Asian urban dynamics. Three approaches are developed in the CA simulation: 1) physical constraints and land-use classification from remotely sensed images in 1993, 2000, and 2008, are incorporated into micro-scale transformation; 2) population dynamics, shifts of economic activities, and foreign direct investment (FDIs), a representative of the impact of globalization, are applied for multi-scale interconnection; 3) the Monte Carlo mechanism is finally introduced to combine the above two approaches and implement the simulation process. The Taipei metropolitan area, a rapid urbanizing region that highly interacts with the global economy in Asia, is chosen to examine this model. The CA simulation model establishes a strong interaction between FDIs, an indicator representing impacts of globalization, and the dazzling Asian urban model. The combination of multi-scale economic factors and micro-scale land-use transformation also reveals how urban growth of the Taipei metropolis in recent years fits the characterization of the desakota model, and how desakota regions, the growth generators, interact with city cores. As a result, the research not only successfully links the influence of globalization with the desakota model and simulates urban dynamics of Asian cities but also provides scenarios of different FDI inputs for governments to better handle urban growth with global impacts under the deep economic recession since 2007

C2H N=1-0 and N2H+ J=1-0 observations of Planck Galactic cold clumps

A survey of C2H N=1-0 and N2H+ J=1-0 toward Planck Galactic cold clumps (PGCCs) was performed using the Purple Mountain Observatory's 13.7 m telescope. C2H and N2H+ were chosen to study the chemical evolutionary states of PGCCs. Among 121 observed molecular cores associated with PGCCs, 71 and 58 are detected with C2H N=1-0 and N2H+ J=1-0, respectively. The detected lines of most sources can be fitted with a single component with compatible Vlsr and line widths, which confirms that these PGCC cores are very cold (with gas temperatures 9-21 K) and quiescent while still dominanted by turbulence. The ratio between the column densities of C2H and N2H+ (N(C2H)/N(N2H+)) is found to be a good tracer for the evolutionary states of PGCC cores. Gas-grain chemical model can reproduce the decreasing trend of N(C2H)/N(N2H+) as a function of time. The cores with the lowest abundances of N2H+ (X[N2H+] < 10^{-10}) are the youngest, and have nearly constant abundances of C2H. In evolved cores with X[N2H+] ~ 1E-9, abundances of C2H drop quickly as the exhaustion of carbon atoms. Although these PGCC cores are in different evolutionary states, they are all quite young ( N(N2H+). Mapping observations are carried out toward 20 PGCC cores. The PGCC cores in Cepheus have lower N(C2H)/N(N2H+) and larger line widths compared with those in Taurus. This implies that PGCC cores in Taurus are less chemically evolved than those in Cepheus.Comment: 23 pages, 6 figures, 5 table