7 research outputs found
Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone
Fault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging-wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP-2). We present observational evidence for extensive fracturing and high hanging-wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP-2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging-wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off-fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation
First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes
We overview nonequilibrium Green function combined with density functional
theory (NEGF-DFT) modeling of independent electron and phonon transport in
nanojunctions with applications focused on a new class of thermoelectric
devices where a single molecule is attached to two metallic zigzag graphene
nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make
possible injection of evanescent wavefunctions from ZGNRs, so that their
overlap within the molecular region generates a peak in the electronic
transmission. Additionally, the spatial symmetry properties of the transverse
propagating states in the ZGNR electrodes suppress hole-like contributions to
the thermopower. Thus optimized thermopower, together with diminished phonon
conductance through a ZGNR/molecule/ZGNR inhomogeneous structure, yields the
thermoelectric figure of merit ZT~0.5 at room temperature and 0.5<ZT<2.5 below
liquid nitrogen temperature. The reliance on evanescent mode transport and
symmetry of propagating states in the electrodes makes the
electronic-transport-determined power factor in this class of devices largely
insensitive to the type of sufficiently short conjugated organic molecule,
which we demonstrate by showing that both 18-annulene and C10 molecule
sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can
search for molecules that will further reduce the phonon thermal conductance
(in the denominator of ZT) while keeping the electronic power factor (in the
nominator of ZT) optimized. We also show how often employed Brenner empirical
interatomic potential for hydrocarbon systems fails to describe phonon
transport in our single-molecule nanojunctions when contrasted with
first-principles results obtained via NEGF-DFT methodology.Comment: 20 pages, 6 figures; mini-review article prepared for the special
issue of the Journal of Computational Electronics on "Simulation of Thermal,
Thermoelectric, and Electrothermal Phenomena in Nanostructures", edited by I.
Knezevic and Z. Aksamij
Local Optima Networks: A New Model of Combinatorial Fitness Landscapes
This chapter overviews a recently introduced network-based model of
combinatorial landscapes: Local Optima Networks (LON). The model compresses the
information given by the whole search space into a smaller mathematical object
that is a graph having as vertices the local optima and as edges the possible
weighted transitions between them. Two definitions of edges have been proposed:
basin-transition and escape-edges, which capture relevant topological features
of the underlying search spaces. This network model brings a new set of metrics
to characterize the structure of combinatorial landscapes, those associated
with the science of complex networks. These metrics are described, and results
are presented of local optima network extraction and analysis for two selected
combinatorial landscapes: NK landscapes and the quadratic assignment problem.
Network features are found to correlate with and even predict the performance
of heuristic search algorithms operating on these problems