The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase space for three-phonon scattering processes, combined with the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optic ferroelectric mode
We used X-ray/neutron diffraction to determine the low temperature (LT) structure of IrTe2. A structural modulation was observed with a wavevector of k =(1/5, 0, 1/5) below Ts≈285 K, accompanied by a structural transition from a trigonal to a triclinic lattice. We also performed the first principles calculations for high temperature (HT) and LT structures, which elucidate the nature of the phase transition and the LT structure. A local bonding instability associated with the Te 5p states is likely the origin of the structural phase transition in IrTe2.The competition between charge density wave (CDW) state and superconductivity is one of mostly interesting phenomena in transition metal dichalcogenides and has been widely studied due to possible relation to high-Tc superconductivity. [1-5] Classically, CDW transitions are second order transitions driven by Fermi surface nesting in a metal, i.e. a Kohn anomaly leading to a soft mode instability of the high tempearature structure. A key feature of this type of transition is a coupling of the electronic structure at the Fermi energy to the structural distortion leading to strong signatures of the phase transition in transport and also in some systems an interplay between the CDW and superconductivity.Recently IrTe 2 , a new member of the TX 2 family incorporating a 5d transition metal, presents the superconductivity when its first-order structural transition is suppressed through doping. [6][7][8][9][10][11] Its HT structure has a trigonal symmetry with edge-sharing IrTe 6 octahedra forming layers stacked along the c-axis with the Ir ions forming an equilateral triangular lattice ( Fig. 1(a)). The LT structure was proposed to be monoclinic based on powder X-ray diffraction.[12] Accompanied with the structural transition, the resistivity shows a hump-shaped maximum and the magnetic susceptibility drops, which is similar to that of the CDW state in other TX 2 systems.However, recent measurements for IrTe 2 imply that the physics is more complicated than a simple CDW. [6-8, 10, 13] In particular, while optical and transport measurements do imply a strong reconstruction of the electronic structure at E F through the transition, other measurements show that the transition is first order, which is not the generic behavior of a standard CDW. There are many possible origins for a first order transition. One is that the mechanism is still CDW type related to Fermi surface nesting, but that the transition becomes first order due to coupling with strain. Another is that it is driven by local ordering, such as orbital ordering on the transition metal. Finally, a transition can be driven by chemical bonding effects.Up to now, all the reported studies used a proposed LT structure model from powder X-ray diffraction [12]. Given that electron diffraction revealed the existence of superlattice peaks [6], which principally also can be from the structure, the LT structure is probably more complicated than the proposed model. The correct LT structure of IrTe 2 is essential to explore t...
A low-work-function tether is a long conductor coated with a low-work-function material that orbits around a planet with both the magnetic field and ionosphere. Depending on the work function W of the coating and the tether temperature T, the photoelectron emission can be relevant within the cathodic tether segment. Thus, this mechanism needs to be added to the thermionic emission considered in previous works. An emission model for low-work-function tethers, including a typical solar photon spectrum, a Fowler-DuBridge law for the photoelectron yield of the coating, and a Richardson-Dushman law for the thermionic emission, is presented, and used to organize the thermionic and photoelectric dominated regimes of low-work-function tethers within the W-T plane. For T ≈ 500 K and W ≈ 1.5 eV, the photoemission and thermionic emission can be of the same order and have similar efficiency as the electron collection. The emission model is combined with orbital-motion theory for all the plasma and emitted particles, and the longitudinal bias and current profiles throughout a low-work-function tether are determined for typical low-Earth-orbit environmental values. Results for the average current are presented. The study highlights the main electrical, mechanical, and optical properties that should be considered in the design of low-work-function tethers, and it briefly discusses some promising materials.
A new material, C12A7:e- electride, which might present a work function as low as 0.6 eV and moderately high temperature stability, was recently proposed as coating for floating bare tethers. Arising from heating under space operation, current is emitted by thermionic emission along a thus coated cathodic segment. A preliminary study on the space-charge-limited (SCL) double layer in front of the cathodic segment is presented using Langmuir’s SCL electron current between cylindrical electrodes and orbital-motion-limited ion-collection sheath. A detailed calculation of current and bias profiles along the entire tether length is carried out with ohmic effects and the transition from SCL to full Richardson-Dushman emission included. Analysis shows that in the simplest drag mode, under typical orbital and tether conditions, thermionic emission leads to a short cathodic section and may eliminate the need for an active cathodic device and its corresponding gas feed requirements and power subsystem, which results in a truly “propellant-less” tether system for such basic applications as de-orbiting low earth orbit satellites.
Important progress has recently been made on the Orbital Motion Theory for cylindrical emissive probes immersed at rest in collisionless and Maxwellian plasmas. However, due to the computational cost of its numerical algorithm, only solutions for specific values of the physical parameters were found, thus preventing its direct application to the interpretation of experimental current-voltage characteristics (I p − V p curves). In this work, and thanks to an analytical analysis of a Jacobian matrix appearing in the algorithm, the computational cost was reduced by a factor in the order of N r , where N r is the number of grid points. This achievement, together with the implementation of parallel programming, allowed to construct a database with more than 18,000 I p − V p curves for a broad range of physical parameters, including the emission level, the probe radius-to-Debye length, and the ion-toelectron temperature ratios. The boundaries in parameter space of the operational regimes of emissive probes, covering both orbital motion limited (OML) and space charge limited (SCL) transitions were computed. A novel OML/non-OML transition for emissive probes operating at low bias was found. The numerical results were used to propose useful analytical laws for the SCL boundary happening at negative bias, the reduction of the emitted electron current due to SCL effects, and the floating potential of emissive probes. The applications of the results to the modeling of low work function tethers and three experimental methods for measuring the plasma potential, i.e. the separation point, the inflection point, and the floating potential techniques, were discussed. The formation of an inverse sheath for strong emission was investigated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.