4,709,145 research outputs found

### Quantum thermoelectrics based on 2-D Semi-Dirac materials

We show that a gap parameter can fully describe the merging of Dirac cones in
semi-Dirac materials from $K$- and $K^\prime$-points into the common $M$-point
in the Brillouin zone. We predict that the gap parameter manifests itself by
enhancing the thermoelectric figure of merit $zT$ 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

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

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 $\lambda_0$
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 $a \gg d \gg \xi$, $\lambda_0 \gg
\Delta_{\lambda}^{1/2}$, and $D>1$, where $a$ is the size of the sensor, $d$ is
its spatial resolution, $\xi$ is the correlation length of fluctuations in the
material constant, $\Delta_{\lambda}$ is the local variability of the material
constant, and $D$ 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

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

Let $f\in L^2(S^2)$ be an arbitrary fixed function with small norm on the
unit sphere $S^2$, and $D\subset \R^3$ be an arbitrary fixed bounded domain.
Let $k>0$ and $\alpha\in S^2$ be fixed.
It is proved that there exists a potential $q\in L^2(D)$ such that the
corresponding scattering amplitude
$A(\alpha')=A_q(\alpha')=A_q(\alpha',\alpha,k)$ approximates $f(\alpha')$ 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
$\{A_q(\alpha')\}_{\forall q\in L^2(D)}$ is complete in $L^2(S^2)$. 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

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 $U$ connecting the superconducting
layers for the realistic systems.Comment: 11 pages, 1 figures, 2 tables, APEX in printin

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