Understanding the collective behavior of the quasiparticles in solid-state systems underpins the field of nonvolatile electronics, including the opportunity to control many-body effects for well-desired physical phenomena and their applications. Hexagonal boron nitride (hBN) is a wide-energy-bandgap semiconductor, showing immense potential as a platform for lowdimensional device heterostructures. It is an inert dielectric used for gated devices, having a negligible orbital hybridization when placed in contact with other systems. Despite its inertness, we discover a large electron mass enhancement in few-layer hBN affecting the lifetime of the π-band states. We show that the renormalization is phononmediated and consistent with both single-and multiple-phonon scattering events. Our findings thus unveil a so-far unknown manybody state in a wide-bandgap insulator, having important implications for devices using hBN as one of their building blocks.

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Magnesium borohydride (Mg(BH4)2) is an attractive compound for solid-state hydrogen storage due to its lucratively high hydrogen densities and theoretically low operational temperature. Hydrogen release from Mg(BH4)2 occurs through several steps. The reaction intermediates formed at these steps have been extensively studied for a decade. In this work, we apply spectroscopic methods that have rarely been used in such studies to provide alternative insights into the nature of the reaction intermediates. The commercially obtained sample was decomposed in argon flow during thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC) to differentiate between the H2-desorption reaction steps. The reaction products were analyzed by powder X-ray diffraction (PXRD), near edge soft X-ray absorption spectroscopy at boron K-edge (NEXAFS), and synchrotron infrared (IR) spectroscopy in mid- and far-IR ranges (SR-FTIR). Up to 12 wt% of H2 desorption was observed in the gravimetric measurements. PXRD showed no crystalline decomposition products when heated at 260–280 °C, the formation of MgH2 above 300 °C, and Mg above 320 °C. The qualitative analysis of the NEXAFS data showed the presence of boron in lower oxidation states than in (BH4)−. The NEXAFS data also indicated the presence of amorphous boron at and above 340 °C. This study provides additional insights into the decomposition reaction of Mg(BH4)2.