4,709,145 research outputs found

    Quantum thermoelectrics based on 2-D Semi-Dirac materials

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    We show that a gap parameter can fully describe the merging of Dirac cones in semi-Dirac materials from KK- and KK^\prime-points into the common MM-point in the Brillouin zone. We predict that the gap parameter manifests itself by enhancing the thermoelectric figure of merit zTzT as the chemical potential crosses the gap followed by a sign change in the Seebeck coefficient around the same point. Subsequently, we show that there is also a trade-off feature between the maximum power delivered and the efficiency when the chemical potential crosses the gap parameter. An optimal operating point that minimizes the power-efficiency trade-off is consequently singled out for the best thermoelectric performance. Our work paves the way for the use of 2D semi-Dirac materials for thermoelectric applications.Comment: 5 pages, 5 figure

    Chalcogenide Glass-on-Graphene Photonics

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    Two-dimensional (2-D) materials are of tremendous interest to integrated photonics given their singular optical characteristics spanning light emission, modulation, saturable absorption, and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. In this paper, we present a new route for 2-D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material which can be directly deposited and patterned on a wide variety of 2-D materials and can simultaneously function as the light guiding medium, a gate dielectric, and a passivation layer for 2-D materials. Besides claiming improved fabrication yield and throughput compared to the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light-matter interactions in the 2-D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared (mid-IR) waveguide-integrated photodetectors and modulators

    Physical limits to sensing material properties

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    Constitutive relations describe how materials respond to external stimuli such as forces. All materials respond heterogeneously at small scales, which limits what a localized sensor can discern about the global constitution of a material. In this paper, we quantify the limits of such constitutional sensing by determining the optimal measurement protocols for sensors embedded in disordered media. For an elastic medium, we find that the least fractional uncertainty with which a sensor can determine a material constant λ0\lambda_0 is approximately \begin{equation*} \frac{\delta \lambda_0}{\lambda_0 } \sim \left( \frac{\Delta_{\lambda} }{ \lambda_0^2} \right)^{1/2} \left( \frac{ d }{ a } \right)^{D/2} \left( \frac{ \xi }{ a } \right)^{D/2} \end{equation*} for adξa \gg d \gg \xi, λ0Δλ1/2\lambda_0 \gg \Delta_{\lambda}^{1/2}, and D>1D>1, where aa is the size of the sensor, dd is its spatial resolution, ξ\xi is the correlation length of fluctuations in the material constant, Δλ\Delta_{\lambda} is the local variability of the material constant, and DD is the dimension of the medium. Our results reveal how one can construct microscopic devices capable of sensing near these physical limits, e.g. for medical diagnostics. We show how our theoretical framework can be applied to an experimental system by estimating a bound on the precision of cellular mechanosensing in a biopolymer network.Comment: 33 pages, 3 figure

    Ab-initio quantum transport simulation of self-heating in single-layer 2-D materials

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    Through advanced quantum mechanical simulations combining electron and phonon transport from first-principles self-heating effects are investigated in n-type transistors with a single-layer MoS2, WS2, and black phosphorus as channel materials. The selected 2-D crystals all exhibit different phonon-limited mobility values, as well as electron and phonon properties, which has a direct influence on the increase of their lattice temperature and on the power dissipated inside their channel as a function of the applied gate voltage and electrical current magnitude. This computational study reveals (i) that self-heating plays a much more important role in 2-D materials than in Si nanowires, (ii) that it could severely limit the performance of 2-D devices at high current densities, and (iii) that black phosphorus appears less sensitive to this phenomenon than transition metal dichalcogenides

    Completeness of the set of scattering amplitudes

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    Let fL2(S2)f\in L^2(S^2) be an arbitrary fixed function with small norm on the unit sphere S2S^2, and DR3D\subset \R^3 be an arbitrary fixed bounded domain. Let k>0k>0 and αS2\alpha\in S^2 be fixed. It is proved that there exists a potential qL2(D)q\in L^2(D) such that the corresponding scattering amplitude A(α)=Aq(α)=Aq(α,α,k)A(\alpha')=A_q(\alpha')=A_q(\alpha',\alpha,k) approximates f(α)f(\alpha') with arbitrary high accuracy: \|f(\alpha')-A_q(\alpha')_{L^2(S^2)}\|\leq\ve where \ve>0 is an arbitrarily small fixed number. This means that the set {Aq(α)}qL2(D)\{A_q(\alpha')\}_{\forall q\in L^2(D)} is complete in L2(S2)L^2(S^2). The results can be used for constructing nanotechnologically "smart materials"

    General Rule and Materials Design of Negative Effective U System for High-T_c Superconductivity

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    Based on the microscopic mechanisms of (1) charge-excitation-induced negative effective U in s^1 or d^9 electronic configurations, and (2) exchange-correlation-induced negative effective U in d^4 or d^6 electronic configurations, we propose a general rule and materials design of negative effective U system in itinerant (ionic and metallic) system for the realization of high-T_c superconductors. We design a T_c-enhancing layer (or clusters) of charge-excitation-induced negative effective UU connecting the superconducting layers for the realistic systems.Comment: 11 pages, 1 figures, 2 tables, APEX in printin
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