4,046 research outputs found
Polaronic signatures and spectral properties of graphene antidot lattices
We explore the consequences of electron-phonon (e-ph) coupling in graphene
antidot lattices (graphene nanomeshes), i.e., triangular superlattices of
circular holes (antidots) in a graphene sheet. They display a direct band gap
whose magnitude can be controlled via the antidot size and density. The
relevant coupling mechanism in these semiconducting counterparts of graphene is
the modulation of the nearest-neighbor electronic hopping integrals due to
lattice distortions (Peierls-type e-ph coupling). We compute the full momentum
dependence of the e-ph vertex functions for a number of representative antidot
lattices. Based on the latter, we discuss the origins of the previously found
large conduction-band quasiparticle spectral weight due to e-ph coupling. In
addition, we study the nonzero-momentum quasiparticle properties with the aid
of the self-consistent Born approximation, yielding results that can be
compared with future angle-resolved photoemission spectroscopy measurements.
Our principal finding is a significant e-ph mass enhancement, an indication of
polaronic behavior. This can be ascribed to the peculiar momentum dependence of
the e-ph interaction in these narrow-band systems, which favors small phonon
momentum scattering. We also discuss implications of our study for recently
fabricated large-period graphene antidot lattices.Comment: published versio
Economic feasibility of second generation ethanol with and without indirect greenhouse gas reduction benefits : a simulation for Brazil
The aim of this study is to determine the economic feasibility of second generation ethanol from sugar cane, whereby traditional ethanol production is combined with the use of lignocellulosic biomass for ethanol production. By applying cost-benefit analysis, this study evaluated the viability of the second generation ethanol technology as an alternative to conventional sugarcaneto- ethanol, both in terms of processing technology, and of land use impacts. Furthermore, an attempt is made to analyze impacts on CO2 mitigation and land use in economic. The research results indicate that: i) from an economic point of view, the first generation plant is clearly preferable. With IRR of 18.7%, Minimum selling price of US 213.0 million, first generation ethanol production from sugar cane has a large economic advantage compared to the second generation plant (IRR of 13.5%, Minimum selling price of US 78.5 million). ii) from an environmental point of view, a second generation biofuel that makes use of lignocellulosic biomass plant is clearly preferable. The second generation plant uses 49.6% less land and avoids a CO2 debt average of 942,282 ton per year throughout the life of the project. iii) Productivity gains improve profitability (IRR) and reduce biofuel prices (Minimum selling prices). Increasing the yearlt Ethanol and sugar cane productivityâs growth rate from 0.5% to 4.0% leads to a range of IRR from 17.5% to 21.5%, and of price from 0.29 US/l for first generation plant, and from 13.2% to 14.2% and of price from 0.39 US/l for second generation plant. iv) Process improvement shows little economic impact but matters on environmental side because less land is needed. Up to 10% more land can be saved compared to least advanced technology. v) Energy conversion development can improve income of the plant, especially for the first generation plant. Each 5% improvement can lead to 0.6% change in IRR project, and a reduction of 1.1% in the Minimum selling price. vi) Equipment investment is the most sensitive parameter to alter biofuel prices and profitability. The conventional plant is more sensitive to equipment investment, land prices and trash costs in this order while second generation plant is sensitive to equipment investment and almost insensitive to land prices and trash costs changes. vii) Assuming an average payment of US 27.7 million). viii) Productivity gains reduce the repayment time of CO2 debt, with ethanol productivity having a stronger contribution. Besides, from a growth rate of ethanol and sugar cane productivity from 0.5% to 4.0% per year, the repayment time changes from 11.8 years to a range between 6.5 years and 5.5 years and 13 and 9.5, respectively. In conclusion, the appraisal model represents a useful tool for analyzing many issues related with the dilemmas involved in biofuel production
Bare-excitation ground state of a spinless-fermion -- boson model and W-state engineering in an array of superconducting qubits and resonators
This work unravels an interesting property of a one-dimensional lattice model
that describes a single itinerant spinless fermion (excitation) coupled to
zero-dimensional (dispersionless) bosons through two different
nonlocal-coupling mechanisms. Namely, below a critical value of the effective
excitation-boson coupling strength the exact ground state of this model is the
zero-quasimomentum Bloch state of a bare (i.e., completely undressed)
excitation. It is demonstrated here how this last property of the lattice model
under consideration can be exploited for a fast, deterministic preparation of
multipartite states in a readily realizable system of inductively-coupled
superconducting qubits and microwave resonators.Comment: final, published versio
Extracting spectral properties of small Holstein polarons from a transmon-based analog quantum simulator
The Holstein model, which describes purely local coupling of an itinerant
excitation (electron, hole, exciton) with zero-dimensional (dispersionless)
phonons, represents the paradigm for short-range excitation-phonon
interactions. It is demonstrated here how spectral properties of small Holstein
polarons -- heavily phonon-dressed quasiparticles, formed in the
strong-coupling regime of the Holstein model -- can be extracted from an analog
quantum simulator of this model. This simulator, which is meant to operate in
the dispersive regime of circuit quantum electrodynamics, has the form of an
array of capacitively coupled superconducting transmon qubits and microwave
resonators, the latter being subject to a weak external driving. The magnitude
of -type coupling between adjacent qubits in this system can be tuned
through an external flux threading the SQUID loops between those qubits; this
translates into an {\em in-situ} flux-tunable hopping amplitude of a fictitious
itinerant spinless-fermion excitation, allowing one to access all the relevant
physical regimes of the Holstein model. By employing the kernel-polynomial
method, based on expanding dynamical response functions in Chebyshev
polynomials of the first kind and their recurrence relation, the relevant
single-particle momentum-frequency resolved spectral function of this system is
computed here for a broad range of parameter values. To complement the
evaluation of the spectral function, it is also explained how -- by making use
of the many-body version of the Ramsey interference protocol -- this
dynamical-response function can be measured in the envisioned analog simulator.Comment: 17 pages, 7 figure
Robust Detection of Point Correspondences in Stereo Images
A major challenge in 3D reconstruction is the computation of the fundamental matrix. Automatic computation from uncalibrated image pairs is performed from point correspondences. Due to imprecision and wrong correspondences, only an approximation of the true fundamental matrix can be computed. The quality of the fundamental matrix strongly depends on the location and number of point correspondences.Furthermore, the fundamental matrix is the only geometric constraint between two uncalibrated views, and hence it can be used for the detection of wrong point correspondences. This property is used by current algorithms like RANSAC, which computes the fundamental matrix from a restricted set of point correspondences. In most cases, not only wrong correspondences are disregarded, but also correct ones, which is due to the criterion used to eliminate outliers. In this context, a new criterion preserving a maximum of correct correspondences would be useful.In this paper we introduce a novel criterion for outlier elimination based on a probabilistic approach. The enhanced set of correspondences may be important for further computation towards a 3D reconstruction of the scene.
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