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

The recent observation of dipole-allowed P -excitons up to principal quantum numbers of n = 25 in cuprous oxide has given insight into exciton states with unprecedented spectral resolution. While so far the exciton description as a hydrogen-like complex has been sufficient for cubic crystals, we demonstrate here distinct deviations: The breaking of rotational symmetry leads to mixing of high angular momentum F -and H-excitons with the P -excitons so that they can be observed in absorption. The F -excitons show a three-fold splitting that depends systematically on n, in agreement with theoretical considerations. From detailed comparison of experiment and theory we determine the cubic anisotropy parameter of the Cu2O valence band. PACS numbers:Introduction. Excitonic effects are decisive for the optical properties of semiconductors and insulators [1]. Not only leads the Coulomb interaction between an electron and a hole to a series of bound states, the excitons, with energies below the band gap, but also above the gap the Coulomb effects lead to a massive redistribution of oscillator strength towards the low-energy states compared to a free particle description. Due to this importance it has been a major goal to develop a detailed understanding of excitons on a quantitative level [1]. The description of the bound exciton states by the hydrogenic model has turned out to be extremely successful in this respect, in particular, for bulk semiconductors of cubic symmetry.For excitons with wavefunction extensions much larger than the crystal unit cell (the Mott-Wannier excitons) the hydrogen formula for their binding energy, R/n 2 with the Rydberg energy R in a state of principal quantum number n, can be simply adapted to the solid state case by (i) changing the reduced mass of electron and proton m to that of electron and hole m * , and (ii) screening the carrier interaction by the dielectric constant ε:The influence of the many-body crystal environment is thus comprised in material properties that for cubic semiconductors are, as a rule, isotropic such as the scalar dielectric constant ε, leading to a formula for excitonic energies that is identical to the one in a system with rotational symmetry. The material environment typically causes a reduction of the atomic Rydberg energy by 2 − 3 orders of magnitude into the meV range.For the hydrogen problem the spatial symmetry is determined by the continuous rotation group SO(3), where the square of the orbital momentum L 2 = l(l + 1)

The Landé or g-factors of charge carriers are decisive for the spin-dependent phenomena in solids and provide also information about the underlying electronic band structure. We present a comprehensive set of experimental data for values and anisotropies of the electron and hole Landé factors in hybrid organic-inorganic (MAPbI3, MAPb(Br0.5Cl0.5)3, MAPb(Br0.05Cl0.95)3, FAPbBr3, FA0.9Cs0.1PbI2.8Br0.2, MA=methylammonium and FA=formamidinium) and all-inorganic (CsPbBr3) lead halide perovskites, determined by pump-probe Kerr rotation and spin-flip Raman scattering in magnetic fields up to 10 T at cryogenic temperatures. Further, we use first-principles density functional theory (DFT) calculations in combination with tight-binding and k ⋅ p approaches to calculate microscopically the Landé factors. The results demonstrate their universal dependence on the band gap energy across the different perovskite material classes, which can be summarized in a universal semi-phenomenological expression, in good agreement with experiment.

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strength. Encapsulation of monolayers such as MoSe2 in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizeable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allow us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

We address spin properties and spin dynamics of carriers and charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies are performed by time-resolved and polarization-resolved photoluminescence, spin-flip Raman scattering and picosecond pump-probe Faraday rotation in magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets are negatively charged so that their photoluminescence is dominated by radiative recombination of negatively charged excitons (trions). Electron g-factor of 1.68 is measured, and heavy-hole g-factor varying with increasing magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for two-dimensional structures are calculated for various hole confining potentials for cubic- and wurtzite lattice in CdSe core. These calculations are extended for various quantum dots and nanoplatelets based on II-VI semiconductors. We developed a magneto-optical technique for the quantitative evaluation of the nanoplatelets orientation in ensemble.