Tunable bandgaps and excitons in doped semiconducting carbon nanotubes made possible by acoustic plasmons

Doping of semiconductors is essential in modern electronic and photonic devices. While doping is well understood in bulk semiconductors, the advent of carbon nanotubes and nanowires for nanoelectronic and nanophotonic applications raises some key questions about the role and impact of doping at low dimensionality. Here we show that for semiconducting carbon nanotubes, bandgaps and exciton binding energies can be dramatically reduced upon experimentally relevant doping, and can be tuned gradually over a broad range of energies in contrast to higher dimensional systems. The later feature is made possible by a novel mechanism involving strong dynamical screening effects mediated by acoustic plasmons.Comment: 5 pages, 4 figures, published in Phys. Rev. Lett

Atomistic study of an ideal metal/thermoelectric contact: the full-Heusler/half-Heusler interface

Half-Heusler alloys such as the (Zr,Hf)NiSn intermetallic compounds are important thermoelectric materials for converting waste heat into electricity. Reduced electrical resistivity at the hot interface between the half-Heusler material and a metal contact is critical for device performance, however this has yet to be achieved in practice. Recent experimental work suggests that a coherent interface between half-Heusler and full-Heusler compounds can form due to diffusion of transition metal atoms into the vacant sublattice of the half-Heusler lattice. We study theoretically the structural and electronic properties of such an interface using a first-principles based approach that combines {\it ab initio} calculations with macroscopic modeling. We find that the prototypical interface HfNi$_2$Sn/HfNiSn provides very low contact resistivity and almost ohmic behavior over a wide range of temperatures and doping levels. Given the potential of these interfaces to remain stable over a wide range of temperatures, our study suggests that full-Heuslers might provide nearly ideal electrical contacts to half-Heuslers that can be harnessed for efficient thermoelectric generator devices.Comment: 8 pages, 8 figure

Lifetime of dynamic heterogeneity in strong and fragile kinetically constrained spin models

Kinetically constrained spin models are schematic coarse-grained models for the glass transition which represent an efficient theoretical tool to study detailed spatio-temporal aspects of dynamic heterogeneity in supercooled liquids. Here, we study how spatially correlated dynamic domains evolve with time and compare our results to various experimental and numerical investigations. We find that strong and fragile models yield different results. In particular, the lifetime of dynamic heterogeneity remains constant and roughly equal to the alpha relaxation time in strong models, while it increases more rapidly in fragile models when the glass transition is approached.Comment: Submitted to the proceedings of the 6th EPS Liquid Matter Conference, Utrecht 2-6 July 200

Ab initio calculations of low-energy quasiparticle lifetimes in bilayer graphene

Motivated by recent experimental results we calculate from first-principles the lifetime of low-energy quasiparticles in bilayer graphene (BLG). We take into account the scattering rate arising from electron-electron interactions within the $GW$ approximation for the electron self-energy and consider several p-type doping levels ranging from $0$ to $\rho \approx 2.4\times 10^{12}$ holes/cm$^2$. In the undoped case we find that the average inverse lifetime scales linearly with energy away from the charge neutrality point, with values in good agreement with experiments. The decay rate is approximately three times larger than in monolayer graphene, a consequence of the enhanced screening in BLG. In the doped case, the dependence of the inverse lifetime on quasiparticle energy acquires a non-linear component due to the opening of an additional decay channel mediated by acoustic plasmons.Comment: 15 pages, 3 figures, accepeted for publication in Applied Physics Letter

Phase ordering and roughening on growing films

We study the interplay between surface roughening and phase separation during the growth of binary films. Already in 1+1 dimension, we find a variety of different scaling behaviors depending on how the two phenomena are coupled. In the most interesting case, related to the advection of a passive scalar in a velocity field, nontrivial scaling exponents are obtained in simulations.Comment: 4 pages latex, 6 figure

Theory of enhancement of thermoelectric properties of materials with nanoinclusions

Based on the concept of band bending at metal/semiconductor interfaces as an energy filter for electrons, we present a theory for the enhancement of the thermoelectric properties of semiconductor materials with metallic nanoinclusions. We show that the Seebeck coefficient can be significantly increased due to a strongly energy-dependent electronic scattering time. By including phonon scattering, we find that the enhancement of ZT due to electron scattering is important for high doping, while at low doping it is primarily due to a decrease in the phonon thermal conductivity

A JKO splitting scheme for Kantorovich-Fisher-Rao gradient flows

In this article we set up a splitting variant of the JKO scheme in order to handle gradient flows with respect to the Kantorovich-Fisher-Rao metric, recently introduced and defined on the space of positive Radon measure with varying masses. We perform successively a time step for the quadratic Wasserstein/Monge-Kantorovich distance, and then for the Hellinger/Fisher-Rao distance. Exploiting some inf-convolution structure of the metric we show convergence of the whole process for the standard class of energy functionals under suitable compactness assumptions, and investigate in details the case of internal energies. The interest is double: On the one hand we prove existence of weak solutions for a certain class of reaction-advection-diffusion equations, and on the other hand this process is constructive and well adapted to available numerical solvers.Comment: Final version, to appear in SIAM SIM

Theme Analysis of Flora Nwapa’s Never Again: A Systemic Functional Approach

This paper deals with theme analysis of Flora Nwapa’s Never Again. The theory that underpins this study is systemic functional linguistics, which is one of the approaches proposed by scholars such as M.A.K Halliday, S. Eggins, J. R. Martin, R. Fowler, J. D. Benson to mention but a very few, for the study of language and its function(s). In this system, the study of language involves three functional labels: experiential, interpersonal and textual meanings. This study aims at describing and analysing linguistic features which connote textual meaning in Never Again so as to have a deep understanding of the novel and provide a new interpretation of it. It also aims at uncovering the deep messages conveyed through the novel. Both quantitative and qualitative methods have been used in this study. Thus, two extracts have been selected from Never Again and systemic functional linguistic theories have been applied to each of them for the purpose of exploring how language is used to organise itself. The study of the internal organisation and communicative nature of the selected Texts has enabled the researchers to come up with valuable findings. Indeed, through Mood adjuncts that conflate interpersonal themes, participants in Text 1 have expressed particular attitudes and judgments about the limited power of women and the roles of women and children during war whereas those in Text 2 have expressed a high degree of certainty and inclination. This study enables the researchers to provide a deeper understanding of Never Again by contending that the deep messages conveyed through the novel are the Nigerian civil war and the roles of women and children in our societies especially during war time. Keywords: Metafunctions, register variables, systemic functional linguistics, theme patterns