Understanding Antiferromagnetic Coupling in Lead-Free Halide Double Perovskite Semiconductors.

Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed. Here, we combine the complementary magnetometry and electron-spin-resonance experiments, together with first-principles calculations to study the antiferromagnetic coupling in stable Cs2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method. We show the importance of nonmagnetic monovalence ions at the BI site (Na/Ag) in facilitating the superexchange interaction via orbital hybridization, offering the tunability of the Curie-Weiss parameters between -27 and -210 K, with a potential to promote magnetic frustration via alloying the nonmagnetic BI site (Ag:Na ratio). Combining our experimental evidence with first-principles calculations, we draw a cohesive picture of the material design for B-site-ordered antiferromagnetic halide double perovskites.

Near-Ergodic CsPbBr3 Perovskite Nanocrystal with Minimal Statistical Aging.

Optical robustness, uniformity, ergodicity, statistical aging, etc. dictate the applicability of nanocrystals. Based on a series of multimodal statistical analyses such as the Kolmogorov-Smirnov test, Lévy statistics, etc., we demonstrate that for CsPbBr3 perovskite nanocrystals (PNCs): (a) the extent of heterogeneity in the quality and associated physical processes is minimal; (b) the optical robustness is very high, and (c) indeed, a single PNC can depict optical behavior of its ensemble. In addition, toward prospective applications, an optically robust CsPbBr3 PNC exhibits (i) near-ergodicity and (ii) minimal statistical aging, which are extremely vital and complementary to its high defect tolerance.

Highly Sensitive Upconverting Nanoplatform for Luminescent Thermometry from Ambient to Cryogenic Temperature.

Precise assessment of temperature is crucial in many physical, technological, and biological applications where optical thermometry has attracted considerable attention primarily due to fast response, contactless measurement route, and electromagnetic passivity. Rare-earth-doped thermographic phosphors that rely on ratiometric sensing are very efficient near and above room temperature. However, being dependent on the thermally-assisted migration of carriers to higher excited states, they are largely limited by the quenching of the activation mechanism at low temperatures. In this paper, we demonstrate a strategy to pass through this bottleneck by designing a linear colorimetric thermometer by which we could estimate down to 4 K. The change in perceptual color fidelity metric provides an accurate measure for the sensitivity of the thermometer that attains a maximum value of 0.86 K-1 . Thermally coupled states in Er3+ are also used as a ratiometric sensor from room temperature to ∼140 K. The results obtained in this work clearly show that Yb3+ -Er3+ co-doped NaGdF4 microcrystals are a promising system that enables reliable bimodal thermometry in a very wide temperature range from ultralow (4 K) to ambient (290 K) conditions.

Experimental determination of the bare energy gap of GaAs without the zero-point renormalization.

The energy gap of simple band insulators like GaAs is a strong function of temperature due to the electron-phonon interactions. Interestingly, the perturbation from zero-point phonons is also predicted to cause significant (a few percent) renormalization of the energy gap at absolute zero temperature but its value has been difficult to estimate both theoretically and, of course, experimentally. Given the experimental evidence (Bhattacharya et al 2015 Phys. Rev. Lett. 114 047402) that strongly supports that the exponential broadening (Urbach tail) of the excitonic absorption edge at low temperatures is the manifestation of this zero temperature electron-phonon scattering, we argue that the location of the Urbach focus is the zero temperature unrenormalized gap. Experiments on GaAs yield the zero temperature bare energy gap to be 1.581 eV and thus the renormalization is estimated to be 66 meV.

Classification of Transitions in Upconversion Luminescence of Lanthanides by Two-Dimensional Correlation Analysis.

Upconversion luminescence bands from Yb3+/Er3+ codoped into a matrix such as NaGdF4 can show a very complex structure on account of multiple intra-f shell transitions occurring in the presence of random crystal fields. We demonstrate that two-dimensional correlation analysis, applied to such time-integrated luminescence spectra measured as a function of excitation power, allows us to gain substantial information about the states involved in transitions, without any additional theoretical input. The detailed correlation analysis allows us not only to identify the location of various transitions but further to club them into groups on the basis of their quantum mechanical origin, and finally subclassify the transitions with each group depending on whether they have a common initial or final state.