We highlight the importance of carrier–phonon coupling in inorganic lead halide perovskite nanocrystals. The low-temperature photoluminescence (PL) spectrum of CsPbBr3 has been investigated under a nonresonant and a nonstandard, quasi-resonant excitation scheme, and phonon replicas of the main PL band have been identified as due to the Fröhlich interaction. The energy of longitudinal optical (LO) phonons has been determined from the separation of the zero phonon band and phonon replicas. We reason that the observed LO phonon coupling can only be related to an orthorhombically distorted crystal structure of the perovskite nanocrystals. Additionally, the strength of carrier–phonon coupling has been characterized using the ratio between the intensities of the first phonon replica and the zero-phonon band. PL emission from localized versus delocalized carriers has been identified as the source of the observed discrepancies between the LO phonon energy and phonon coupling strength under quasi-resonant and nonresonant excitation conditions, respectively.
We have studied the size dependence of the exciton g-factor in self-assembled InAs/InP quantum dots. Photoluminescence measurements on a large ensemble of these dots indicate a multimodal height distribution. Cross-sectional Scanning Tunneling Microscopy measurements have been performed and support the interpretation of the macro photoluminescence spectra. More than 160 individual quantum dots have systematically been investigated by analyzing single dot magnetoluminescence between 1200 nm and 1600 nm. We demonstrate a strong dependence of the exciton g-factor on the height and diameter of the quantum dots, which eventually gives rise to a sign change of the g-factor. The observed correlation between exciton g-factor and the size of the dots is in good agreement with calculations. Moreover, we find a size dependent anisotropy splitting of the exciton emission in zero magnetic field.
Koenraad, P. M. (2016). Anisotropy of electron and hole g tensors of quantum dots: An intuitive picture based on spin-correlated orbital currents. Physical Review B, 93(3), [035311]. DOI: 10.1103/PhysRevB.93.035311 General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Using single spins in semiconductor quantum dots as qubits requires full control over the spin state. As the g tensor provides the coupling in a Hamiltonian between a spin and an external magnetic field, a deeper understanding of the g tensor underlies magnetic-field control of the spin. The g tensor is affected by the presence of spin-correlated orbital currents, of which the spatial structure has been recently clarified. Here we extend that framework to investigate the influence of the shape of quantum dots on the anisotropy of the electron g tensor. We find that the spin-correlated orbital currents form a simple current loop perpendicular to the magnetic moment's orientation. The current loop is therefore directly sensitive to the shape of the nanostructure: for cylindrical quantum dots, the electron g-tensor anisotropy is mainly governed by the aspect ratio of the dots. Through a systematic experimental study of the size dependence of the separate electron and hole g tensors of InAs/InP quantum dots, we have validated this picture. Moreover, we find that through size engineering it is possible to independently change the sign of the in-plane and growth direction electron g factors. The hole g tensor is found to be strongly anisotropic and very sensitive to the radius and elongation. The comparable importance of itinerant and localized currents to the hole g tensor complicates the analysis relative to the electron g tensor.
The phonon bottleneck in self-assembled InAs/GaAs quantum dots ͑SAD's͒ is observed directly in continuous-wave photoluminescence experiments when exciting one GaAs longitudinal optical ͑LO͒-phonon energy above the ground level of the smallest dot. To overcome the phonon bottleneck, selective photoluminescence ͑PL͒ experiments are performed and multiple phonon-assisted radiative bands are observed. We found that no real crystal states are involved in the experimentally observed phonon emission. Under nonresonant excitation at 5 K, the SAD's photoluminescence band is centered at 1.315 eV. As proven by our photoluminescence experiments at high excitation densities, there are no excited states in such small dots. We interpret the phonon-assisted PL as being due to enhanced Fröhlich interaction between strain-induced polarized excitons in the SAD's and LO phonons. Further experimental support for this model is found from the cleaved-side PL measurements. A light-hole ground state is observed, instead of the theoretically predicted heavy-hole one.
We have experimentally determined the band offsets at a highly strained InAs/GaAs interface by means of coupling between two ultrathin InAs layers embedded in a GaAs matrix. When both InAs layers are separated by a 32-ML barrier, the confined electron and light-hole ͑lh͒ states are split into symmetric and antisymmetric states, whereas the heavy-hole ͑hh͒ level is not split yet. Consequently, the splitting between the hh exciton transitions, which is measured by photoluminescence excitation spectroscopy, is solely determined by the conduction-band offset ⌬E c . Knowing ⌬E c , the hh and lh band offsets ⌬E hh and ⌬E lh were subsequently determined from the coupling-induced shift and splitting in samples with 16-, 8-, and 4-ML barriers. We find a conduction-band offset of 535 meV, a conduction-band offset ratio of Q c ϭ0.58, and a strain-induced splitting between the hh and lh levels of 160 meV. This method for the direct determination of band offsets is explicitly sensitive to the band-offset ratio, and its application is not restricted to particular type-I semiconductor heterostructures as long as the effective-mass-band-offset product for the conduction and valence bands differs by at least a factor of 2. ͓S0163-1829͑99͒01715-4͔
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