4,442 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
Ground-state entanglement spectrum of a generic model with nonlocal excitation-phonon coupling
While the concept of the entanglement spectrum has heretofore been utilized
to address various many-body systems, the models describing an itinerant
spinless-fermion excitation coupled to zero-dimensional bosons (e.g.
dispersionless phonons) have as yet not received much attention in this regard.
To fill this gap, the ground-state entanglement spectrum of a model that
includes two of the most common types of short-ranged, nonlocal
excitation-phonon interaction -- the Peierls- and breathing-mode couplings --
is numerically evaluated here. This model displays a sharp, level-crossing
transition at a critical coupling strength, which signifies the change from a
nondegenerate ground state at the quasimomentum to a
twofold-degenerate one corresponding to a symmetric pair of nonzero
quasimomenta. Another peculiarity of this model is that in the special case of
equal Peierls- and breathing-mode coupling strengths the bare-excitation Bloch
state with the quasimomentum or is its exact eigenstate. Moreover,
below a critical coupling strength this state is the ground state of the model.
Thus, the sharp transition between a bare excitation and a heavily
phonon-dressed (polaronic) one can be thought of as a transition between
vanishing and finite entanglement. It is demonstrated here that the smallest
ground-state entanglement-spectrum eigenvalue to a large extent mimics the
behavior of the entanglement entropy itself and vanishes in this special case
of the model; by contrast, all the remaining eigenvalues diverge in this case.
The implications of excitation-phonon entanglement for -state engineering in
superconducting and neutral-atom-based qubit arrays serving as analog
simulators of this model are also discussed.Comment: 13 pages, 5 figure
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
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
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