Electronic Instability in a Zero-Gap Semiconductor: The Charge-DensityWave in (TaSe4)(2)I

International audienceWe report a comprehensive study of the paradigmatic quasi-1D compound (TaSe4)(2)I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in the nondistorted structure, with non-negligible interchain coupling. Theory and experiment support a Peierls-like scenario for the charge-density wave formation below T-CDW = 263 K, where the incommensurability is a direct consequence of the finite interchain coupling. The formation of small polarons, strongly suggested by the ARPES data, explains the puzzling semiconductor-to-semiconductor transition observed in transport at T-CDW

First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes

We overview nonequilibrium Green function combined with density functional theory (NEGF-DFT) modeling of independent electron and phonon transport in nanojunctions with applications focused on a new class of thermoelectric devices where a single molecule is attached to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make possible injection of evanescent wavefunctions from ZGNRs, so that their overlap within the molecular region generates a peak in the electronic transmission. Additionally, the spatial symmetry properties of the transverse propagating states in the ZGNR electrodes suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with diminished phonon conductance through a ZGNR/molecule/ZGNR inhomogeneous structure, yields the thermoelectric figure of merit ZT~0.5 at room temperature and 0.5<ZT<2.5 below liquid nitrogen temperature. The reliance on evanescent mode transport and symmetry of propagating states in the electrodes makes the electronic-transport-determined power factor in this class of devices largely insensitive to the type of sufficiently short conjugated organic molecule, which we demonstrate by showing that both 18-annulene and C10 molecule sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can search for molecules that will further reduce the phonon thermal conductance (in the denominator of ZT) while keeping the electronic power factor (in the nominator of ZT) optimized. We also show how often employed Brenner empirical interatomic potential for hydrocarbon systems fails to describe phonon transport in our single-molecule nanojunctions when contrasted with first-principles results obtained via NEGF-DFT methodology.Comment: 20 pages, 6 figures; mini-review article prepared for the special issue of the Journal of Computational Electronics on "Simulation of Thermal, Thermoelectric, and Electrothermal Phenomena in Nanostructures", edited by I. Knezevic and Z. Aksamij

Comparative analysis of the various types of structures for the electric transmission power cables supports' static work in the software complexes implementing the finite elements method

© Published under licence by IOP Publishing Ltd. The aim of the work was to conduct a comparative analysis of the static work of two structural types of steel poles of power lines (HVPC): in the form of a closed profile rods-shells with height-varying wall thickness and in the form of a trellised trihedral design made according to the patent [1]. In this case, both types of supports were considered for three different heights-under power lines of 10 kV - 9-11 m high, 35 kV - 20.6 m high and 110 kV - 22.5 m high, respectively. The bulk of the research was carried out at the ANSYS PC, where the joint work of the structural system "steel support of an overhead power transmission cable - a prefabricated and dismantled reinforced concrete foundation of a new type [2] - soil base"was considered. For this, the authors proposed a computer simulation technique that takes into account the spatial work of the structural system elements and the physical nonlinearity of the materials they are made of. At the same time, the Mises strength theory was used for steel, Williams-Warnake yield criterion was used for concrete, and Drucker-Prager yield criterion was used for the base soil. In addition, all the necessary geometric, power, and physical characteristics of the model were obtained on the basis of the current building codes for the energy facilities' design. The auxiliary engineering calculations taking into account these codes were performed in the LIRA-SAPR 2017 PC in a linear formulation

Bulk and surface band structure of the new family of semiconductors BiTeX (X=I, Br, Cl)

We present an overview of the new family of semiconductors BiTeX (X = I, Br, Cl) from the perspective of angle resolved photoemission spectroscopy. The strong band bending occurring at the surface potentially endows them with a large flexibility, as they are capable of hosting both hole and electron conduction, and can be modified by inclusion or adsorption of foreign atoms. In addition, their trigonal crystal structure lacks a center of symmetry and allows for both bulk and surface spin-split bands at the Fermi level. We elucidate analogies and differences among the three materials, also in the light of recent theoretical and experimental work. ©2014 Elsevier B.V. All rights reserved11091sciescopu

Engineering the topological surface states in the (Sb2)m-Sb2Te3 (m=0-3) superlattice series

We investigate the evolution of both the occupied and unoccupied electronic structure in representative compounds of the infinitely adaptive superlattice series (Sb2)m-Sb2Te3 (m = 0\u20133) by means of angle-resolved photoemission spectroscopy and time-delayed two-photon photoemission, combined with first-principles band-structure calculations. We discover that the topological nature of the surface states and their spin texture are robust, with dispersions evolving from linear (Dirac-like) to parabolic (Rashba-like) along the series, as the materials evolve from semiconductors to semimetals. Our findings provide a promising strategy for engineering the topological states with the desired flexibility needed for realizing different quantum phenomena and spintronics applications