The Non-Equilibrium Green's Function Method for Nanoscale Device Simulation

Pourfath, Mahdi

doi:10.1007/978-3-7091-1800-9

Cited by 76 publications

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“…The retarded and advanced electron SEs can then be efficiently calculated using the real-time equality [74] …”

Section: Self-consistent Proceduresmentioning

confidence: 99%

“…(18). This contribution is proportional to the trace of the "lesser" electron GF, with matrix elementsFrom there, one can compute expectation values 2πn α,k,k = dω G < α,k,k (ω) as well as real space populations n α,j = c † α,j c α,j in the steady-state [74]. The GFs G r α (ω) and G < α (ω) can be related to electron and photon self-energies through Dyson and Keldysh equations.…”

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confidence: 99%

“…The dressing of the electronic bands and the cavity mode requires a self-consistent solution of the problem, which we obtain using a non-equilibrium Green's function method [67][68][69][70][71][72][73][74][75][76][77]. For g = 0, we show that T (ω) acquires a new additional transmission channel [sketched in Fig.…”

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confidence: 99%

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Cavity-Enhanced Transport of Charge

et al. 2017

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We theoretically investigate charge transport through electronic bands of a mesoscopic onedimensional system, where inter-band transitions are coupled to a confined cavity mode, initially prepared close to its vacuum. This coupling leads to light-matter hybridization where the dressed fermionic bands interact via absorption and emission of dressed cavity-photons. Using a selfconsistent non-equilibrium Green's function method, we compute electronic transmissions and cavity photon spectra and demonstrate how light-matter coupling can lead to an enhancement of charge conductivity in the steady-state. We find that depending on cavity loss rate, electronic bandwidth, and coupling strength, the dynamics involves either an individual or a collective response of Bloch states, and explain how this affects the current enhancement. We show that the charge conductivity enhancement can reach orders of magnitudes under experimentally relevant conditions. PACS numbers: 05.60. Gg, 42.50.Pq, 74.40.Gh, The study of strong light-matter interactions [1][2][3][4] is playing an increasingly crucial role in understanding as well as engineering new states of matter with relevance to the fields of quantum optics [5][6][7][8][9][10][11][12][13][14][15][16][17][18], solid state physics [19][20][21][22][23][24][25][26][27][28][29][30][31], as well as quantum chemistry [32][33][34][35][36] and material science [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]. An emerging topic of interest is the modification of material properties using either external electro-magnetic radiation [54][55][56] or spatially confined modes such as in cavity quantum electrodynamics [57][58][59][60][61][62][63]. Recent experiments with organic semiconductors have demonstrated a dramatic enhancement of charge conductivity when molecules interact strongly with a surface plasmon mode [64]. In principle, this can open up exciting new opportunities both for basic science and applications of organic electronics [65]. The microscopic mechanisms leading to charge conductivity enhancement, however, remain today largely unexplained. In this work, we propose a proof-of-principle model that sheds light on the physical mechanisms behind current enhancement due to the interaction with a confined bosonic mode. We show that our model can lead to a dramatic current enhancement by orders of magnitude for certain conditions that can be relevant to typical experiments across fields.The setup we consider [see Fig. 1(a)] consists of a mesoscopic chain of N sites with two orbitals of energy ω 1 and ω 2 ( = 1) in a 1D geometry, forming two bands in a tight-binding picture. The edges of the chain are connected to a source and a drain with a bias voltage across, respectively inserting and removing (spin-less) electrons in the two orbitals at rates Γ 1 and Γ 2 . The on-site interband transitions with energy ω 21 = ω 2 −ω 1 are resonantly coupled to a cavity mode with coupling strength g and loss rate κ. Initially, we only allow electrons in the upper band to hop with a ...

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“…The retarded and advanced electron SEs can then be efficiently calculated using the real-time equality [74] …”

Section: Self-consistent Proceduresmentioning

confidence: 99%

mentioning

confidence: 99%

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Cavity-Enhanced Transport of Charge

et al. 2017

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“…For the transport calculations, we use here the NEGF approach, including the effect of electron scattering with acoustic and optical phonons [31][32]. The system is treated within the effective mass approximation with uniform mass m* = m0, where m0 is the rest mass of an electron.…”

Section: Approachmentioning

confidence: 99%

On the effectiveness of the thermoelectric energy filtering mechanism in low-dimensional superlattices and nano-composites

Thesberg

Kosina

Neophytou

2016

Journal of Applied Physics

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Electron energy filtering has been suggested as a promising way to improve the power factor and enhance the ZT figure of merit of thermoelectric materials. In this work we explore the effect that reduced dimensionality has on the success of the energyfiltering mechanism for power factor enhancement. We use the quantum mechanical nonequilibrium Green's function (NEGF) method for electron transport including electronphonon scattering to explore 1D and 2D superlattice / nanocomposite systems. We find that, given identical material parameters, 1D channels utilize energy filtering more effectively than 2D as they: i) allow one to achieve maximal power factor for smaller well sizes / smaller grains (which is needed to maximize phonon scattering), ii) take better advantage of a lower thermal conductivity in the barrier/boundary materials compared to the well/grain materials in both: enhancing the Seebeck coefficient; and in producing a system which is robust against detrimental random deviations from optimal barrier design. In certain cases we find that the relative advantage can be as high as a factor of 3. We determine that energy-filtering is most effective when the average energy of carrier flow varies the most in the wells and the barriers along the channel, an event which appears when the energy of the carrier flow in the host material is low and when the energy relaxation mean-free-path of carriers is short. Although the ultimate reason these aspects, which cause a 1D system to see greater relative improvement than a 2D, is the 1D system's van Hove singularity in the density-of-states, the insights obtained are general and inform energy-filtering design beyond dimensional considerations.

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“…The basic conclusions of this study can be applied generically to all 1D systems. We employ the NEGF method [65,66], which can take into account the exact geometry of the roughness without any underlying assumptions, while we describe the phonon spectrum atomistically using force constants.…”

Section: Introductionmentioning

confidence: 99%

Low-dimensional phonon transport effects in ultranarrow disordered graphene nanoribbons

et al. 2015

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We investigate the influence of low-dimensionality and disorder in phonon transport in ultra-narrow armchair graphene nanoribbons (GNRs) using non-equilibrium Green's function (NEGF) simulation techniques. We specifically focus on how different parts of the phonon spectrum are influenced by geometrical confinement and line edge roughness. Under ballistic conditions, phonons throughout the entire phonon energy spectrum contribute to thermal transport. With the introduction of line edge roughness, the phonon transmission is reduced, but in a manner which is significantly non-uniform throughout the spectrum. We identify four distinct behaviors within the phonon spectrum in the presence of disorder: i) the low-energy, low-wavevector acoustic branches have very long mean-free-paths and are affected the least by edge disorder, even in the case of ultra-narrow W=1nm wide GNRs; ii) energy regions that consist of a dense population of relatively 'flat' phonon modes (including the optical branches) are also not significantly affected, except in the case of the ultra-narrow W=1nm GNRs, in which case the transmission is reduced because of band mismatch along the phonon transport path; iii) 'quasi-acoustic' bands that lie within the intermediate region of the spectrum are strongly affected by disorder as this part of the spectrum is depleted of propagating phonon modes upon both confinement and disorder (resulting in sparse E(q) phononic bandstructure), especially as the channel length increases; iv) the strongest reduction in phonon transmission is observed in energy regions that are composed of a small density of phonon modes, in which case roughness can introduce transport gaps that greatly increase with channel length. We show that in GNRs of widths as small as W=3nm, under moderate roughness, both the low-energy acoustic modes and dense regions of optical modes can retain semi-ballistic transport properties, even for channel lengths up to 2 L=1μm. These modes tend to completely dominate thermal transport. Modes in the sparse regions of the spectrum, however, tend to fall into the localization regime, even for channel lengths as short as 10s of nanometers, despite their relatively high phonon group velocities.

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The Non-Equilibrium Green's Function Method for Nanoscale Device Simulation

Cited by 76 publications

References 0 publications

Cavity-Enhanced Transport of Charge

Cavity-Enhanced Transport of Charge

On the effectiveness of the thermoelectric energy filtering mechanism in low-dimensional superlattices and nano-composites

Low-dimensional phonon transport effects in ultranarrow disordered graphene nanoribbons

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