Charged excitons, or X ± -trions, in monolayer transition metal dichalcogenides have binding energies of several tens of meV. Together with the neutral exciton X 0 they dominate the emission spectrum at low and elevated temperatures. We use charge tunable devices based on WSe2 monolayers encapsulated in hexagonal boron nitride, to investigate the difference in binding energy between X + and X − and the X − fine structure. We find in the charge neutral regime, the X 0 emission accompanied at lower energy by a strong peak close to the longitudinal optical (LO) phonon energy. This peak is absent in reflectivity measurements, where only the X 0 and an excited state of the X 0 are visible. In the n-doped regime, we find a closer correspondence between emission and reflectivity as the trion transition with a well-resolved fine-structure splitting of 6 meV for X − is observed. We present a symmetry analysis of the different X + and X − trion states and results of the binding energy calculations. We compare the trion binding energy for the n-and p-doped regimes with our model calculations for low carrier concentrations. We demonstrate that the splitting between the X + and X − trions as well as the fine structure of the X − state can be related to the short-range Coulomb exchange interaction between the charge carriers. arXiv:1705.02110v2 [cond-mat.mes-hall] 9 May 2018
One consequence of the continued downward scaling of transistors is the reliance on only a few discrete atoms to dope the channel, and random fluctuations in the number of these dopants are already a major issue in the microelectronics industry. Although single dopant signatures have been observed at low temperatures, the impact on transistor performance of a single dopant atom at room temperature is not well understood. Here, we show that a single arsenic dopant atom dramatically affects the off-state room-temperature behaviour of a short-channel field-effect transistor fabricated with standard microelectronics processes. The ionization energy of the dopant is measured to be much larger than it is in bulk, due to its proximity to the buried oxide, and this explains the large current below threshold and large variability in ultra-scaled transistors. The results also suggest a path to incorporating quantum functionalities into silicon CMOS devices through manipulation of single donor orbitals.
Liquid core capsules having a hydrogel membrane are becoming a versatile tool for three-dimensional culture of micro-organisms and mammalian cells. Making sub-millimeter capsules at a high rate, via the breakup of a compound jet in air, opens the way to high-throughput screening applications. However, control of the capsule size monodispersity, especially required for quantitative bioassays, was still lacking. Here, we report how the understanding of the underlying hydrodynamic instabilities that occur during the process can lead to calibrated core-shell bioreactors. The requirements are: i) damping the shear layer instability that develops inside the injector arising from the co-annular flow configuration of liquid phases having contrasting viscoelastic properties; ii) controlling the capillary instability of the compound jet by superposing a harmonic perturbation onto the shell flow; iii) avoiding coalescence of drops during jet fragmentation as well as during drop flight towards the gelling bath; iv) ensuring proper engulfment of the compound drops into the gelling bath for building a closed hydrogel shell. We end up with the creation of numerous identical compartments in which cells are able to form multicellular aggregates, namely spheroids. In addition, we implement an intermediate composite hydrogel layer, composed of alginate and collagen, allowing cell adhesion and thus the formation of epithelia or monolayers of cells.
As a result of the recent recommendations of the ICRP 60, and in anticipation of possible regulation on occupational exposure of Canadian-based aircrew, an extensive study was carried out by the Royal Military College of Canada over a one-year period to measure the cosmic radiation at commercial jet altitudes. A tissue-equivalent proportional counter was used to measure the ambient total dose equivalent rate on 62 flight routes, resulting in over 20,000 data points at one-minute intervals at various altitudes and geomagnetic latitudes (i.e. which span the full cut-off rigidity of the Earth's magnetic field). These data were then compared to similar experimental work at the Physikalisch Technische Bundesanstalt, using a different suite of equipment, to measure separately the low and high linear energy transfer components of the mixed radiation field, and to predictions with the LUIN transport code. All experimental and theoretical results were in excellent agreement. From these data, a semiempirical model was developed to allow for the interpolation of the dose rate for any global position, altitude and date (i.e. heliocentric potential). Through integration of the dose rate function over a great circle flight path, a computer code was developed to provide an estimate of the total dose equivalent on any route worldwide at any period in the solar cycle.
Transport properties of the complex oxide LaAlO3/SrTiO3 interface are investigated under high magnetic field (55T). By rotating the sample with respect to the magnetic field, the two-dimensional nature of charge transport is clearly established. Small oscillations of the magnetoresistance with altered periodicity are observed when plotted versus inverse magnetic field. We attribute this effect to Rashba spin-orbit coupling which remains consistent with large negative magnetoresistance when the field is parallel to the sample plane. A large inconsistency between the carrier density extracted from Shubnikov-de Haas analysis and from the Hall effect is explained by the contribution to transport of at least two bands with different mobility.Oxide interfaces constitute a rapidly developing field of research, with potential applications in electronics [1, 2] or solar energy harvesting [3]. There is currently a focus on the band-insulators LaAlO 3 (LAO) and SrTiO 3 (STO), which host a conducting two-dimensional electron gas (2DEG) at their interface [4]. It is mainly believed to originate from the polar catastrophe [5], which results in a charge transfer between the polar oxide [100] LAO and the nonpolar oxide [100] STO. This charge transfer prevents a divergence of the electrostatic potential associated with the intra-layer built-in electric field. Charge accumulation is therefore predicted at the interface with intriguing consequences such as magnetism [6] and superconductivity [7]. In LAO/STO heterostructures, symmetry-lowering at the interface raises the Ti t 2g band degeneracy so that the d xy orbital is lower in energy than the d xz and d yz orbitals. Depending on the total two-dimensional carrier density, the band occupation and the spatial distribution of the carriers [8] [12,14,15], opening new perspectives for the investigation of the charge carriers' properties in relation to their band-structure. However, quantum transport studies remain scarce in the literature and the large variability in the results (originating from the large range of samples studied) does not yet offer a clear picture of electron transport in LAO/STO. In this context, we make use of very large magnetic field (55 T) to extend the range of magnetoresistance measurements and enhance the visibility of SdH oscillations. We interpret our experimental data by the presence of low and high mobility electrons both contributing to transport, and confirm the role of Rashba spin-orbit coupling.Two samples named S 1 and S 2 were obtained by depositing 10 unit cells of LAO on TiO 2 -terminated (100)-oriented STO substrates using pulsed laser deposition (PLD) [16]. Since both samples displayed similar results, we shall mainly discuss sample S 1 and make reference to sample S 2 only when relevant (full data for sample S 2 are available in the Supplemental Information). The LAO was grown at T=740 o C in oxygen partial pressure of P O2 = 2 × 10 −3 Torr. During the deposition, in-situ reflection high energy electron diffraction (RHEED) was used to preci...
We extend a simple model of a charge trap coupled to a single-electron box to energy ranges and parameters such that it gives new insights and predictions readily observable in many experimental systems. We show that a single background charge is enough to give lines of differential conductance in the stability diagram of the quantum dot, even within undistorted Coulomb diamonds. It also suppresses the current near degeneracy of the impurity charge, and yields negative differential lines far from this degeneracy. We compare this picture to two other accepted explanations for lines in diamonds, based respectively on the excitation spectrum of a quantum dot and on fluctuations of the density-of-states in the contacts. In order to discriminate between these models we emphasize the specific features related to environmental charge traps. Finally we show that our model accounts very well for all the anomalous features observed in silicon nanowire quantum dots.
An on-going investigation using a tissue-equivalent proportional counter (TEPC) has been carried out to measure the ambient dose equivalent rate of the cosmic radiation exposure of aircrew during a solar cycle. A semi-empirical model has been derived from these data to allow for the interpolation of the dose rate for any global position. The model has been extended to an altitude of up to 32 km with further measurements made on board aircraft and several balloon flights. The effects of changing solar modulation during the solar cycle are characterised by correlating the dose rate data to different solar potential models. Through integration of the dose-rate function over a great circle flight path or between given waypoints, a Predictive Code for Aircrew Radiation Exposure (PCAIRE) has been further developed for estimation of the route dose from galactic cosmic radiation exposure. This estimate is provided in units of ambient dose equivalent as well as effective dose, based on E/H x (10) scaling functions as determined from transport code calculations with LUIN and FLUKA. This experimentally based treatment has also been compared with the CARI-6 and EPCARD codes that are derived solely from theoretical transport calculations. Using TEPC measurements taken aboard the International Space Station, ground based neutron monitoring, GOES satellite data and transport code analysis, an empirical model has been further proposed for estimation of aircrew exposure during solar particle events. This model has been compared to results obtained during recent solar flare events.
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