Momentarily trapped exciton polaron in two-dimensional lead halide perovskites

Two-dimensional (2D) lead halide perovskites with distinct excitonic feature have shown exciting potential for optoelectronic applications. Compared to their three-dimensional counterparts with large polaron character, how the interplay between long- and short- range exciton-phonon interaction due to polar and soft lattice define the excitons in 2D perovskites is yet to be revealed. Here, we seek to understand the nature of excitons in 2D CsPbBr3 perovskites by static and time-resolved spectroscopy which is further rationalized with Urbach-Martienssen rule. We show quantitatively an intermediate exciton-phonon coupling in 2D CsPbBr3 where exciton polarons are momentarily self-trapped by lattice vibrations. The 0.25 ps ultrafast interconversion between free and self-trapped exciton polaron with a barrier of ~ 34 meV gives rise to intrinsic asymmetric photoluminescence with a low energy tail at room temperature. This study reveals a complex and dynamic picture of exciton polarons in 2D perovskites and emphasizes the importance to regulate exciton-phonon coupling.

Measurements of cross sections for high energy neutron induced reactions on Co and Bi

There are few experimental data for neutron cross-section libraries in (n,xn) reactions for various materials at energies above 20 MeV. For neutron energies above 20 MeV, these set of (n,xn) reactions are important for neutron fluence monitoring and spectra unfolding for future generation IV nuclear reactors. There were attempts to measure the cross-sections on natural cobalt and bismuth at incident neutron energies of about 90 MeV and 140 MeV. These measurements were made using the quasi-monoenergetic neutron facility at iThemba LABS, South Africa. In addition, at The Svedberg Laboratory facility in Sweden, similar experiments were performed on natural Yttrium. The measured cross-sections were compared with some of the few available data for neutron-induced reactions at high energies. Data collected from these two facilities, required corrections to be made for the contribution of the low energy tail (continuum) in the incident neutron spectrum. Preliminary results from iThemba LABS showed large discrepancy which we suspect was due to instability of the proton beam during the irradiations. Follow-up experiments are planned to accurately determine the uncertainty contributions, with additional data at other neutron energies.

Capturing the interplay between spin–orbit coupling and non-Condon effects on the photoabsorption spectra of Ru and Os dyes

A theoretical protocol developed to study the low-energy tail of the absorption spectrum of homoleptic biscyclometalated ruthenium and osmium complexes.

Achieving the photon up-conversion thermodynamic yield upper limit by sensitized triplet–triplet annihilation

Triplet–triplet annihilation (TTA) based up-conversion is a promising strategy for light harvesting the low-energy tail of the solar spectrum in photovoltaic technologies.

Liquid X-ray scattering with a pink-spectrum undulator

X-ray scattering from a liquid using the spectrum from the undulator fundamental is examined as a function of the bandwidth of the spectrum. The synchrotron-generated X-ray spectrum from an undulator is `pink',i.e.quasi-monochromatic but having a saw-tooth-shaped spectrum with a bandwidth from 1 to 15%. It is shown that features inS(q) are slightly shifted and dampened compared with strictly monochromatic data. In return, the gain in intensity is 250–500 which makes pink beams very important for time-resolved experiments. The undulator spectrum is described by a single exponential with a low-energy tail. The tail shifts features in the scattering function towards high angles and generates a small reduction in amplitude. The theoretical conclusions are compared with experiments. Ther-resolved Fourier transformed signals are discussed next. Passing fromq- tor-space requires a sin-Fourier transform. The Warren convergence factor is introduced in this calculation to suppress oscillatory artifacts from the finiteqMin the data. It is shown that the deformation ofr-resolved signals from the pink spectrum is small compared with that due to the Warren factor. Theq-resolved and ther-resolved pink signals thus behave very differently.

Overlapping ion structures in the mid-altitude cusp under northward IMF: signature of dual lobe reconnection?

On some rare occasions, data from the Cluster Ion Spectrometer (CIS) in the mid-altitude cusp reveal overlapping ion populations under northward interplanetary magnetic field (IMF). While the poleward part of the cusp exhibits the expected reverse dispersion due to lobe reconnection, its equatorward part shows a second ion population at higher-energy that coexists with the low energy tail of the dispersion. This second population is either dispersionless or slightly dispersed with energies increasing with increasing latitudes, indicative of lobe reconnection as well. Our analysis of a case that occurred 3 September 2002 when the IMF stayed northward for more than two hours suggests that the second population comes from the opposite hemisphere and is very likely on newly-closed field lines. We interpret this overlap of cusp populations as a clear mid-altitude signature of re-closed magnetic field lines by double lobe reconnection (reconnection in both hemispheres) under northward IMF. This interpretation is supported by modelling performed with the Cooling model and an MHD model.

RESONANT ELASTIC AND INELASTIC SCATTERING ASTROPHYSICAL APPLICATIONS NEW PARADIGM BEYOND DRIP-LINES?

Two experimental techniques have been developed at GANIL using resonant elastic and inelastic scattering reactions in inverse kinematics. These techniques were used to study the structure of unstable nuclei. A brief description of the methods is presented through two examples of application in astrophysics. Moreover, new ideas and simple questions are put forward: what happens in the low energy tail of unbound nuclei ground state resonances?