Heat conduction in graphite has been studied for decades because of its exceptionally large thermal anisotropy. While the bulk thermal conductivities along the in-plane and cross-plane

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We have measured both spectrum- and time-resolved photoluminescence (PL) of InGaN/GaN light-emitting diode structure as a function of an external bias. From spectrum-resolved PL, we observed regions of blueshift and redshift in peak PL energies. From the bias point at which redshift begins, which we attribute to the inversion of electric field due to full compensation of the piezoelectric field (PEF), we estimate PEF to be 2.1±0.2 MV/cm. From time-resolved PL, we found the carrier lifetimes to drastically decrease (2.5 ns–2 ps) with increasing reverse bias. We attribute this decrease to escape tunneling through tilted barriers.

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS3, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures.

Large amplitude time-domain oscillations are detected in InxGa1−xN/GaN structures via femtosecond differential reflectivity spectroscopy. The oscillation amplitude increases with increasing indium fraction and abruptly disappears at a critical time that depends on GaN thickness. We show that spatially localized, coherent acoustic phonon wave packets are generated via the photoexcited carriers and propagate into the samples modulating the reflectivity. Our results show that a system with strong built-in strain can be a very effective source for ultrafast acoustic phonon wave packets which can be used as a powerful probe for nanoscale structures.