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Register a new userTemperature and size-dependent suppression of Auger recombination in quantum-confined lead salt nanowires
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Valery I. Rupasov
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Auger recombination (AR) of the ground biexciton state in quantum-confined lead salt nanowires (NWs) with a strong coupling between the conduction and the valence bands is shown to be strongly suppressed, and only excited biexciton states contribute to Auger decay. The AR rate is predicted to be greatly reduced when temperature or the NW radius are decreased, and the effect is explained by decrease in both the population of excited biexciton states and overlap of phonon-broadened single- and biexciton states. Suppression of AR of multiexciton states exhibiting strong radiative decay makes obviously lead salt NWs a subject of special interest for numerous lasing applications.
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Auger recombination (AR) being electron-hole annihilation with energy-momentum transfer to another carrier is believed to speed up in materials with small band gap. We theoretically show that this rule is violated in gapless three-dimensional materials with ultra-relativistic electron-hole dispersion, Weyl semimetals (WSM). Namely, AR is prohibited by energy-momentum conservation laws in prototypical WSM with a single Weyl node, even in the presence of anisotropy and tilt. In real multi-node WSM, the geometric dissimilarity of nodal dispersions enables weak inter-node AR, which is further suppressed by strong screening due to large number of nodes. While partial AR rates between the nodes of the same node group are mutually equal, the inter-group processes are non-reciprocal, so that one of groups is geometrically protected from AR. Our calculations show that geometrical protection can help prolonging AR lifetime by the two orders of magnitude, up to the level of nanoseconds.
In the framework of four-band envelope-function formalism, developed earlier for spherical semiconductor nanocrystals, we study the electronic structure and optical properties of quantum-confined lead-salt (PbSe and PbS) nanowires (NWs) with a strong coupling between the conduction and the valence bands. We derive spatial quantization equations, and calculate numerically energy levels of spatially quantized states of a transverse electron motion in the plane perpendicular to the NW axis, and electronic subbands developed due to a free longitudinal motion along the NW axis. Using explicit expressions for eigenfunctions of the electronic states, we also derive analytical expressions for matrix elements of optical transitions and study selection rules for interband absorption. Next we study a two-particle problem with a conventional long-range Coulomb interaction and an interparticle coupling via medium polarization. The obtained results show that due to a large magnitude of the high-frequency dielectric permittivity of PbSe material, and hence, a high dielectric NW/vacuum contrast, the effective coupling via medium polarization significantly exceeds the effective direct Coulomb coupling at all interparticle separations along the NW axis. Furthermore, the strong coupling via medium polarization results in a bound state of the longitudinal motion of the lowest-energy electron-hole pair (a longitudinal exciton), while fast transverse motions of charge carriers remain independent of each other.
Atomistic sp3d5s* tight-binding theory of PbSe and PbS nanocrystals is developed. It is demonstrated, that the valley splittings of confined electrons and holes strongly and peculiarly depend on the geometry of a nanocrystal. When the nanocrystal lacks a microscopic center of inversion and has T_d symmetry, the splitting is strongly suppressed as compared to the more symmetric nanocrystals with O_h symmetry, having an inversion center.
Wireless technology relies on the conversion of alternating electromagnetic fields to direct currents, a process known as rectification. While rectifiers are normally based on semiconductor diodes, quantum mechanical non-reciprocal transport effects that enable highly controllable rectification have recently been discovered. One such effect is magnetochiral anisotropy (MCA), where the resistance of a material or a device depends on both the direction of current flow and an applied magnetic field. However, the size of rectification possible due to MCA is usually extremely small, because MCA relies on electronic inversion symmetry breaking which typically stems from intrinsic spin-orbit coupling - a relativistic effect - in a non-centrosymmetric environment. Here, to overcome this limitation, we artificially break inversion symmetry via an applied gate voltage in thin topological insulator (TI) nanowire heterostructures and theoretically predict that such a symmetry breaking can lead to a giant MCA effect. Our prediction is confirmed via experiments on thin bulk-insulating (Bi$_{1-x}$Sb$_{x}$)$_2$Te$_3$ TI nanowires, in which we observe the largest ever reported size of MCA rectification effect in a normal conductor - over 10000 times greater than in a typical material with a large MCA - and its behaviour is consistent with theory. Our findings present new opportunities for future technological applications of topological devices.
We report on a detailed study of the intensity dependent optical properties of individual GaN/AlN Quantum Disks (QDisks) embedded into GaN nanowires (NW). The structural and optical properties of the QDisks were probed by high spatial resolution cathodoluminescence (CL) in a scanning transmission electron microscope (STEM). By exciting the QDisks with a nanometric electron beam at currents spanning over 3 orders of magnitude, strong non-linearities (energy shifts) in the light emission are observed. In particular, we find that the amount of energy shift depends on the emission rate and on the QDisk morphology (size, position along the NW and shell thickness). For thick QDisks (>4nm), the QDisk emission energy is observed to blue-shift with the increase of the emission intensity. This is interpreted as a consequence of the increase of carriers density excited by the incident electron beam inside the QDisks, which screens the internal electric field and thus reduces the quantum confined Stark effect (QCSE) present in these QDisks. For thinner QDisks (<3 nm), the blue-shift is almost absent in agreement with the negligible QCSE at such sizes. For QDisks of intermediate sizes there exists a current threshold above which the energy shifts, marking the transition from unscreened to partially screened QCSE. From the threshold value we estimate the lifetime in the unscreened regime. These observations suggest that, counterintuitively, electrons of high energy can behave ultimately as single electron-hole pair generators. In addition, when we increase the current from 1 pA to 10 pA the light emission efficiency drops by more than one order of magnitude. This reduction of the emission efficiency is a manifestation of the efficiency droop as observed in nitride-based 2D light emitting diodes, a phenomenon tentatively attributed to the Auger effect.
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