Measurement of scattering rate and minimum conductivity in graphene

The conductivity of graphene samples with various levels of disorder is investigated for a set of specimens with mobility in the range of 1-20x10(3) cm2/V sec. Comparing the experimental data with the theoretical transport calculations based on charged impurity scattering, we estimate that the impurity concentration in the samples varies from 2-15x10(11) cm(-2). In the low carrier density limit, the conductivity exhibits values in the range of 2-12e2/h, which can be related to the residual density induced by the inhomogeneous charge distribution in the samples. The shape of the conductivity curves indicates that high mobility samples contain some short-range disorder whereas low mobility samples are dominated by long-range scatterers.     

Neutrons and antiprotons in ultrahigh-energy cosmic rays

The neutron fraction in the very high-energy cosmic rays near the Greisen-Zatsepin-Kuzmin (GZK) cutoff energy is analyzed by taking into account the time dilation effect of the neutron decays and also the pion photoproduction behaviors above the GZK cutoff. We predict a non-trivial neutron fraction above the GZK cutoff and a negligibly small neutron fraction below. However, there should be a large antiproton fraction in the high-energy cosmic rays below the GZK cutoff in several existing models for the observed cosmic-ray events above and near the GZK cutoff. Such a large antiproton fraction can manifest itself by the muon charge ratio μ⁺/μ^- in the collisions of the primary nucleon cosmic rays with the atmosphere, if there is no neutron contribution. We suggest to use the muon charge ratio as one of the information to detect the composition of the primary cosmic rays near or below the GZK cutoff.     

Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers

Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers are investigated by tight-binding molecular dynamics (TBMD) simulations and by first principles total energy calculations. It is observed in the TBMD simulations that two single vacancies coalesce into a 5-8-5 double vacancy at the temperature of 3000 K, and it is further reconstructed into a new defect structure, the 555-777 defect, by the Stone-Wales type transformation at higher temperatures. First principles calculations confirm that the 555-777 defect is energetically much more stable than two separated single vacancies, and the energy of the 555-777 defect is also slightly lower than that of the 5-8-5 double vacancy. In TBMD simulation, it is also found that the four single vacancies reconstruct into two collective 555-777 defects which is the unit for the hexagonal haeckelite structure proposed by Terrones et al. [Phys. Rev. Lett. 84, 1716 (2000)].     

Scheduling for fabrication and assembly in a two-machine flowshop with a fixed job sequence

This paper studies a problem of scheduling fabrication and assembly operations in a two-machine flowshop, subject to the same predetermined job sequence on each machine. In the manufacturing setting, there are n products, each of which consists of two components: a common component and a unique component which are fabricated on machine 1 and then assembled on machine 2. Common components of all products are processed in batches preceded by a constant setup time. The manufacturing process related to each single product is called a job. We address four regular performance measures: the total job completion time, the maximum job lateness, the total job tardiness, and the number of tardy jobs. Several optimality properties are presented. Based upon the concept of critical path and block schedule, a generic dynamic programming algorithm is developed to find an optimal schedule in O(n ⁷) time.     

Migration of nitrogen in hexagonal Ge2Sb2Te5: An ab‐initio study

The migration barrier energies of the nitrogen atom and N₂ molecule, and the activation barriers for the dissociation and formation of N₂ in Ge₂Sb₂Te₅ were calculated by ab‐initio methods. Various transition and metastable states were found along the migration pathway. Migration barrier energies up to 1.19 eV for the nitrogen atom suggest that it is difficult for it to move from one site to any other site or diffuse out from Ge₂Sb₂Te₅ although doped nitrogen is energetically less stable with respect to N₂ in vacuum. N₂ in Ge₂Sb₂Te₅ was hardly expected to dissociate into nitrogen atoms and vice versa. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Behavior of nuclear matter under extreme conditions in fission

The spontaneous fission of ²⁵²Cf has been studied via γ-γ-γ coincidence and γ-γ light charged particle coincidence with Gammasphere. The binary fission yields of correlated Mo?Ba pairs with 0–10 neutron emission have been remeasured. The existence of “hot” fission mode with 8–10 neutron emission seen previously in the Mo?Ba split is confirmed but with lower intensities. By gating on the light charged particles detected in ΔE-E detectors and a γ ray in one partner, the relative yields of correlated pairs in alpha ternary SF with zero to 6n emission are observed for the first time with the distribution peaked at 2.5n. New correlated pairs are identified in ¹⁰Be ternary SF. We observed essentially only cold, On ¹⁰Be and little, if any, hot, xn ¹⁰Be. New γ-γ-γ data with 2.3 times the total events show weak non-Doppler broadened high energy peaks in coincidence with transitions in correlated pairs in ¹⁰Be SF shifted by the same 6,1 to 26 keV from the 2-0 energy in ¹⁰Be as seen earlier.     

Measurements of sealed radioactive sources by using isothermal microcalorimetry

Journal of Radioanalytical and Nuclear Chemistry - We have studied to validate the calorimetric method by comparing the 123I radioactivity measurement results with the national standard. The reason...     

A unified energy approach to a class of micromechanics models for composite materials

Several micromechanics models for the determination of composite moduli are investigated in this paper, including the dilute solution, self-consistent method, generalized self-consistent method, and Mori-Tanaka's method. These micromechanical models have been developed by following quite different approaches and physical interpretations. It is shown that all the micromechanics models share a common ground, the generalized Budiansky's energy-equivalence framework. The difference among the various models is shown to be the way in which the average strain of the inclusion phase is evaluated. As a bonus of this theoretical development, the asymmetry suffered in Mori-Tanaka's method can be circumvented and the applicability of the generalized self-consistent method can be extended to materials containing microcracks, multiphase inclusions, non-spherical inclusions, or non-cylindrical inclusions. The relevance to the differential method, double-inclusion model, and Hashin-Shtrikman bounds is also discussed. The application of these micromechanics models to particulate-reinforced composites and microcracked solids is reviewed and some new results are presented.