33,050 research outputs found
Tunable graphene bandgaps from superstrate mediated interactions
A theory is presented for the strong enhancement of graphene-on-substrate
bandgaps by attractive interactions mediated through phonons in a polarizable
superstrate. It is demonstrated that gaps of up to 1eV can be formed for
experimentally achievable values of electron-phonon coupling and phonon
frequency. Gap enhancements range between 1 and 4, indicating possible benefits
to graphene electronics through greater bandgap control for digital
applications, lasers, LEDs and photovoltaics through the relatively simple
application of polarizable materials such as SiO2 and Si3N4.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
A new approach to upscaling fracture network models while preserving geostatistical and geomechanical characteristics
A new approach to upscaling two-dimensional fracture network models is proposed for preserving geostatistical and geomechanical characteristics of a smaller-scale “source” fracture pattern. First, the scaling properties of an outcrop system are examined in terms of spatial organization, lengths, connectivity, and normal/shear displacements using fractal geometry and power law relations. The fracture pattern is observed to be nonfractal with the fractal dimension D ≈ 2, while its length distribution tends to follow a power law with the exponent 2 < a < 3. To introduce a realistic distribution of fracture aperture and shear displacement, a geomechanical model using the combined finite-discrete element method captures the response of a fractured rock sample with a domain size L = 2 m under in situ stresses. Next, a novel scheme accommodating discrete-time random walks in recursive self-referencing lattices is developed to nucleate and propagate fractures together with their stress- and scale-dependent attributes into larger domains of up to 54 m × 54 m. The advantages of this approach include preserving the nonplanarity of natural cracks, capturing the existence of long fractures, retaining the realism of variable apertures, and respecting the stress dependency of displacement-length correlations. Hydraulic behavior of multiscale growth realizations is modeled by single-phase flow simulation, where distinct permeability scaling trends are observed for different geomechanical scenarios. A transition zone is identified where flow structure shifts from extremely channeled to distributed as the network scale increases. The results of this paper have implications for upscaling network characteristics for reservoir simulation
Isospin-projected nuclear level densities by the shell model Monte Carlo method
We have developed an efficient isospin projection method in the shell model
Monte Carlo approach for isospin-conserving Hamiltonians. For isoscalar
observables this projection method has the advantage of being exact sample by
sample. The isospin projection method allows us to take into account the proper
isospin dependence of the nuclear interaction, thus avoiding a sign problem
that such an interaction introduces in unprojected calculations. We apply our
method in the calculation of the isospin dependence of level densities in the
complete shell. We find that isospin-dependent corrections to the
total level density are particularly important for nuclei.Comment: 5 pages including 4 figure
Coherent-feedback quantum control with a dynamic compensator
I present an experimental realization of a coherent-feedback control system
that was recently proposed for testing basic principles of linear quantum
stochastic control theory [M. R. James, H. I. Nurdin and I. R. Petersen, to
appear in IEEE Transactions on Automatic Control (2008),
arXiv:quant-ph/0703150v2]. For a dynamical plant consisting of an optical
ring-resonator, I demonstrate ~ 7 dB broadband disturbance rejection of
injected laser signals via all-optical feedback with a tailored dynamic
compensator. Comparison of the results with a transfer function model pinpoints
critical parameters that determine the coherent-feedback control system's
performance.Comment: 4 pages, 4 EPS figure
The question of ‘alternatives’ within food and drink markets and marketing: introduction to the special issue
The question of 'alternatives' within food and drink markets and marketing: introduction to the special issu
The nature of Ho magnetism in multiferroic HoMnO3
Using x-ray resonant magnetic scattering and x-ray magnetic circular
dichroism, techniques that are element specific, we have elucidated the role of
Ho3+ in multiferroic HoMnO3. In zero field, Ho3+ orders antiferromagnetically
with moments aligned along the hexagonal c direction below 40 K, and undergoes
a transition to another magnetic structure below 4.5 K. In applied electric
fields of up to 1x10^7 V/m, the magnetic structure of Ho3+ remains unchanged.Comment: 4 pages, 3 figures Manuscript accepted for publication in Phys. Rev.
Lett. 200
Measurements in the Turbulent Boundary Layer at Constant Pressure in Subsonic and Supersonic Flow. Part 2: Laser-Doppler Velocity Measurements
A description of both the mean and the fluctuating components of the flow, and of the Reynolds stress as observed using a dual forward scattering laser-Doppler velocimeter is presented. A detailed description of the instrument and of the data analysis techniques were included in order to fully document the data. A detailed comparison was made between the laser-Doppler results and those presented in Part 1, and an assessment was made of the ability of the laser-Doppler velocimeter to measure the details of the flows involved
The dollars and sense of bank consolidation
For nearly two decades banks in the United States have consolidated in record numbers--in terms of both frequency and the size of the merging institutions. Rhoades (1996) hypothesizes that the main motivators were increased potential for geographic expansion created by changes in state laws regulating branching and a more favorable antitrust climate. To look for evidence of economic incentives to exploit these improved opportunities for consolidation, the authors examine how consolidation affects expected profit, the riskiness of profit, profit efficiency, market value, market-value efficiencies, and the risk of insolvency. Their estimates of expected profit, profit risk, and profit efficiency are based on a structural model of leveraged portfolio production that was estimated for a sample of highest-level U.S. bank holding companies in Hughes, Lang, Mester, and Moon (1996). Here, the authors also estimate two additional measures that gauge efficiency in terms of the market values of assets and of equity. Their findings suggest that the economic benefits of consolidation are strongest for those banks engaged in interstate expansion and, in particular, interstate expansion that diversifies banks' macroeconomic risk. Not only do these banks experience clear gains in their financial performance, but society also benefits from the enhanced bank safety that follows from this type of consolidation.Bank mergers
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